Amorphous Electrode: From Synthesis to Electrochemical Energy Storage

Electrochemical batteries and supercapacitors are considered ideal rechargeable technologies for next‐generation energy storage systems. The key to further commercial applications of electrochemical energy storage devices is the design and investigation of electrode materials with high energy density and significant cycling stability. Recently, amorphous materials have attracted a lot of attention due to their more defects and structure flexibility, opening up a new way for electrochemical energy storage. In this perspective, we summarize the recent research regarding amorphous materials for electrochemical energy storage. This review covers the advantages and features of amorphous materials, the synthesis strategies to prepare amorphous materials, as well as the application and modification of amorphous electrodes in energy storage fields. Finally, the challenges and prospective remarks for future development in amorphous materials for electrochemical energy storage are concluded.


Introduction
The ongoing energy crisis and environmental issues have spurred us to develop green, clean, and renewable energy, such as solar, wind, and tide.However, the intermittent nature of the above renewable energy limits the scope of its application.It is believed that integrating renewable energy with an energy storage system can effectively store natural energy for unexpected needs. [1,2]Recent developed electrochemical batteries (including alkali ion batteries and Li-S batteries) and supercapacitors (SCs) are considered ideal rechargeable technologies for next-generation energy storage systems. [3,4]In particular, since the commercial success of lithium-ion batteries (LIBs), the science community is taking steps to promote the development of other batteries (employing earth-abundant elements as a carrier such as Na, K, and Zn) to resolve the rarity and high cost of lithium resources.With this in mind, sodium-ion batteries (SIBs), potassium-ion batteries (KIBs), and zinc-ion batteries (ZIBs) have garnered attention as competitive alternatives to LIBs. [5,6]Besides, Li-S batteries with high energy density are one of the most promising devices for long-range electric vehicles.Mg ion batteries and Al ion batteries which are multiple-electron redox reaction processes and employ aqueous electrolytes with high safety have also attracted extensive attention in energy storage fields.These energy storage devices are an indispensable part of green energy in the future so it is an emergency to develop highperformance, low-cost, and environmentally friendly electrode materials to meet the high energy density and long cycling life.Although the charge carriers for energy storage are different (Li + , Na + , K + , Zn 2+ or OH − , PF 6− , Cl − . ..) in various devices, the internal configuration is similar, that is the negative electrode, positive electrode, separator, and electrolyte.Moreover, the energy storage mechanism of these electrochemical energy storage technologies are very similar and can be simply described as follows: charge carriers extracted from one electrode across the electrolyte pass through the separator and recombine with electrons (or holes) in the counter electrode.The theoretical energy density of the above energy storage technologies can be deduced from their redox mechanisms and voltage platforms.But the practical performance of these technologies is determined by many other factors, such as working conditions, energy storage kinetics, and structural durability.Among these factors, the energy storage kinetics and structural durability, which are greatly dependent on the electrode and electrolyte's physical/chemical/electrochemical properties, have accounted for the electrochemical performance of energy storage devices. [7]s an essential component of batteries and SCs, electrode and electrolyte materials modification is an effective strategy to realize an energy storage system with high power/energy density, output voltage, and lifespans.Until now, a variety of crystalline electrodes has been thoroughly investigated, including carbon-based (e.g., graphite and CNTs), alloy-based (e.g., Sn, Sb, and P), and organic-based (e.g., vitamin K and terephthalates). [8]The strategies for improving the electrochemical performance of these electrodes are summarized as follows: 1) unique architecture design (e.g., nanocages, nanorods, and nanowires) to balance the morphology and surface area; 2) downsizing the active materials (thickness and size) to buffer volume change and exposure more active sites; 3) doping heteroatoms (e.g., S, N, P, Fe, Mo, and Al)/introducing vacancies to regulate the inherent characteristics, including electrical conductivity, redox/catalytic activity, and energy barriers; 4) free-standing substrates to enhance the structural stability; and 5) constructing heterostructure to improve reaction kinetics and reduce resistance.Even though these methods have greatly improved the electrochemical properties of various energy storage systems, the pursuit of low-cost, facile, and scalable electrode materials should continue development.
Amorphous materials are different from the crystal in physical and chemical nature, showing great potential as excellent electrode materials.Generally, amorphous materials possess a short-range orderly arrangement at a few atoms level, and these orderly regions are randomly linked (long-range disorder).Thus, amorphous and crystalline materials can be easily distinguished by basic characterization techniques, such as X-ray diffraction (XRD): X-rays will be scattered in many directions for the amorphous phase leading to a large bump distributed in a wide range (2 Theta) instead of high intensity narrower peaks (X-rays will be scattered only in certain directions), and highresolution transmission electron microscopy (HRTEM): amorphous material not shows lattice fringe and no diffraction rings, and crystals are the opposite.Due to their unique structure, amorphous materials possess significant advantages, such as isotropic, remarkable elasticity and tensile strength, superior structural strength, and chemical stability, which benefit to optimization of electrochemical properties.Based on these features, amorphous materials have already been confirmed for their excellent energy storage properties relative to crystalline electrodes in energy storage devices.
Compared to their crystalline counterparts, the advantages of amorphous electrodes are as follows.1) Reduced bandgap and improved conductivity.Qin and co-workers used ultraviolet-visible diffuse reflectance spectroscopy to quantitatively compare the bandgap value of amorphous and crystalline Zn 2 V 2 O 7 electrodes.The results showed that the faster electrons transfer in amorphous materials was beneficial in enhancing conductivity and reducing bandgap. [9]2) Lower energy barrier.Benefiting from the less rigid structure of amorphous relative to its crystalline counterpart, it would reveal a lower energy barrier during repetitive charge-discharge processes.3) Increasing electrochemical active sites and diffusion pathways of ions.Ion storage sites mainly originated from crystallographic sites that ions can reach and contact.Thus, active sites are confined only to the edge of crystalline electrodes.In contrast to crystal, the amorphization of materials is always accompanied by structural defects and atom vacancies.These defects and vacancies can serve as active sites and redox centers to improve electrochemical performance, including specific capacity, initial Coulombic efficiency, rate performance, and cycling stability. [10]4) Higher reversibility.The reversibility of the electrochemical reaction is strongly dependent on the crystal structure and conductivity.Nam et al. used extended X-ray absorption fine structure analyses to investigate the conversion reaction during lithium/sodium insertion/extraction processes toward amorphous and crystalline Sb 2 Se 3 electrodes, respectively. [11]Their results showed that the crystalline Sb 2 Se 3 electrode would transfer to Sb + Li 2 Se/Na 3 Sb + Na 2 Se after the first cycle.However, amorphous Sb 2 Se 3 would undergo conversion and full recovery during lithiation/delithiation or sodiation/desodiation processes, showing higher reversibility.5) Buffering volume expansion.Different from the rigid structure of their crystalline counterparts, the amorphous structure can alleviate the mechanical strain from the ion's intercalation and deintercalation.Moreover, the effect of shape rearranges, and volume changes are less for the amorphous electrode due to their inherent irregular and disordered structure.Thus, constructing amorphous materials is an effective method of achieving infusive electrochemical energy storage performance (Figure 1).
With continuous effort, enormous amorphous materials have explored their potential in various electrochemical energy storage devices, and these attractive materials' superiorities and energy storage mechanisms have been in-depth understood (Figure 2).Although some reviews regarding amorphous materials have been reported, such as amorphous catalysts for water spitting, [17] amorphous metal oxides for energy storage, [18] and amorphous materials for SIBs, [19] a systematic review including various amorphous electrode and electrolyte materials in terms of fundamental natures, synthesis methods, electrochemical performances, and energy storage mechanism is absent.Considering this, the review comprehensively discusses the recent trends and momentous achievements of amorphous materials in the electrochemical energy storage fields.The physical/chemical properties and phase transformation related to amorphous structure and corresponding fundamental understanding are first introduced.Then, we thoroughly summarize the synthesis methods of amorphous materials and their application in an energy storage system, including LIBs/SIBs/KIBs, ZIBs, SCs, and Li-S batteries.Some other aspects of the optimized amorphous electrode and the enhanced mechanism are also discussed with the suggestion for future research on the amorphous electrode for energy storage.This review can guide the fundamental and experimental investigation of amorphous materials for developing advanced electrode materials.

The Physical/Chemical Properties of Amorphous Materials
The properties of materials are up to the arrangements and distribution of atoms or ions, which are bonded with covalent or ionic bonds.Thus, the positions of these atoms/ions and their neighbors are heavily considered in investigating material properties.In general, the arrangements of atoms/ions can be classified into two categories.One is a regular packing to form a repeating 3D structure (crystal lattice), called a crystal.The other is random packing to generate a disordered structure, namely amorphous materials.
For crystals, the atoms are organized using the standard rules to form a higher-order microscopic structure.The structure that repeats periodically can form lattices and extend in all directions.The properties of crystals are intrinsically correlated with the packing ways of atoms.Moreover, distinctive crystalline faces are generated due to their regular   [16,20-28]   Energy Environ.Mater.2023, 6, e12573 3 of 28 internal structure.The crystal is easier to break along with some faces, called the Cleavage plane.Flat and new faces intersect at the same angles as those in the original crystal, such as diamond and graphite, are formed.The angles between different faces in the various crystal are always different, so the XRD technique based on the Bragg formula can be employed to detect the components and structures of crystals.Crystals tend to show relatively sharp, narrow, and complicated peaks in the XRD pattern.Meanwhile, the local environments inside the crystal are the same due to the regularity of the crystalline lattice, including all the component's atoms/ions being the same distance, number, and species of neighbors.The regular packing of atoms/ions in crystals leads to well-defined melting points.
For amorphous, also called non-crystalline, the properties are very different from crystalline ones.The flat crystal faces cannot be observed due to the random packing of atoms/ions, so the fracture surface is generally irregular and curved, resulting in a poor XRD response.Moreover, the local environments are various because the distances to neighboring units and the numbers of neighbors are different so amorphous materials are melting over a wide temperature range instead of well-defined melting points.The unique crystalline structure makes amorphous to be more flexible and malleable than crystal ones.In some cases, the transformation between amorphous and crystal can be controlled.For example, crystals tend to be amorphization when the external pressure is strong enough.In contrast, amorphous materials can be rearrangement into crystals after undergoing annealing at the proper temperature.
In a way, the arrangement of atoms in amorphous is similar to liquid due to the random and arbitrary organization.The specific properties of amorphous materials can be concluded as follows:

Unique Band Structures
Although the long-range order ceases to appear in amorphous semiconductors, the short-range order dominates its internal atomic structure, leading to a band-like structure of electron energy states.Unlike the crystal with a smooth and distinct band edge, the amorphous shows an indistinct and wrinkle band edge due to the atom-atom distance.Bond energy is not a constant that depends on the coordinate in amorphous materials.Moreover, the band gap no longer applies to the amorphous, but the mobility gap is defined to describe the energy gap between electrons and holes' mobility edge.The difference in band structure leads to the migration of electrons ceasing to follow the band mechanism and being replaced by the hopping mechanism.

No Definite Melting Point
The crystalline materials consist of repeated unit cells, which means that the atom-atom distance and atom coordination environment are almost identical.Hence, the chemical bonds in a crystal structure present constant bond energy and can be broken at a fixed temperature so that the crystal will be transformed into liquid at a sharp melting point.In contrast, the bond length of atom-atom in amorphous varies; thus, the bond energy will be higher or lower depending on the specific local environment.Hence, the amorphous materials will be melted over a long temperature range to break all bonds, showing no definite melting point.Due to the melting curves of amorphous and crystal being recognizable, a differential scanning calorimeter is an effective technique to explore the crystalline feature of unknown materials.

Transformation into a Crystalline Structure
Whether the amorphous solid can be converted to a crystalline one or not?It depends on intrinsic characteristics and external factors.For the inherent characteristics, the microstructure of amorphous materials must have the ability and potential to transfer into crystalline.Typically, soft carbon, formed with moderately disordered structures, can be converted to graphite around 2300 °C.However, due to the cross-link and highly disordered lattice distribution, hard carbon cannot be converted to graphite even after annealing at 3000 °C.For external factors, the melted amorphous is cooling down very slowly, enough for the rearrangement and diffusion of molecules, and the recrystallization will occur.

Isotropic
Isotropic indicates that the physical and chemical properties measured in all directions are the same; that is, the direction is irrespective of the physicochemical properties of amorphous.The distributions of various atoms in amorphous materials are very different along with each axis due to the highly random arrangement.Therefore, an average measurement value is taken, and the properties show isotropic characteristics.
These unique properties of amorphous make it a promising electrode material for electrochemical energy storage.For example, eliminating grain boundaries due to the isotropic in all directions is conducive to ions transport and lowers the internal resistance.Moreover, the isotropic makes amorphous solid atomic-scale flexibility, and it is hard to definite the "collapse" of the crystalline structure due to the irregularity that will conquer and alleviate the volume changes during the insertion/extraction of ions.The indistinct and wrinkle band edges lead to a narrow bandgap, which conduces to enhance the electrical conductivity and promotes the reaction kinetics.Kim et al. also demonstrated that amorphous FePO 4 allows substantially smaller barriers than maricite NaFePO 4 for Na to hop from site to site. [29]Specifically, the hopping mechanism of charge transport can increase the ion dynamics in the polymer matrix, which helps to improve the electrochemical performance of solid-state electrolytes. [30]Further, because of the heterogeneous bond lengths, the amorphous electrode can be fully activated even deep in the electrode without the pulverization of materials, unlike crystalline electrodes.The appearance of rich dangling bonds generated by the random packing of many atoms effectively increases the active sites and improves the intrinsic activity, achieving better energy storage properties.Based on these positive effects (Figure 3), amorphization open up a new design strategy to circumvent unsatisfactory and nonideal energy storage.

The Phase Transformation Related to the Amorphous Structure
The electrochemical reaction can induce an irreversible phase transformation and structure reconstruction of active materials, leading to the succeeding energy storage provided by the transition products. [34]The reconstructed products are used as new active materials to power the energy storage devices, as shown in Figure 4. Amorphous electrode materials can maintain the amorphous feature or be reconstructed into a crystalline structure or amorphous/crystalline composite after a repeat charging-discharge process.Crystalline electrode materials are similar to amorphous electrode materials; they can retain their initial structure Energy Environ.Mater.2023, 6, e12573 or be transformed into amorphous materials or amorphous/crystalline composites after cycling.For example, Xiong et al. first demonstrated that the irreversible phase transformation occurs in amorphous TiO 2 NTs (TiO 2 nanotubes) anode for LIBs due to its self-organization during repeated charge-discharge processes. [35]Moreover, the transformation of amorphous to crystal, [35] crystal to amorphous, and even crystalamorphous composites may be generated as electrochemical reaction products. [36]In this section, we will mainly discuss the phase changes regarding the disappearance and appearance of amorphous structures in electrode materials, including: 1) How do we determine whether the electrode material's irreversible phase transition occurs?2) What kinds of electrode materials are prone to phase transitions during the electrochemical reaction?3) Are phase transitions beneficial or harmful?
The characteristic peaks of cyclic voltammetry (CV) curves and the shape of galvanostatic charge-discharge (GCD) plots are related to the electrochemical reaction of electrode materials.The disappearance/appearance of redox peaks in CV curves and potential plateau in GCD plots may portend the presence of irreversible phase transition.Du's group observed a long charge plateau at 1.7 V of the VS 2 cathode for zinc ion storage.Subsequently, the charge curves displayed a noticeable difference, indicating the electrochemical behavior has been altered due to the phase transformation of electrode materials. [37]Encouragingly, Barnes et al. reported that an amorphous Nb 2 O 5 anode could spontaneously convert to a crystalline Nb 2 O 5 when discharged to 0.5 V by employing Li foil as the counter electrode. [38]They found a more significant hysteresis in GCD plots and apparent jitter below 1.1 V in CV curves, caused by the transformation of the amorphous electrode into a crystalline one.To elucidate the molecular structure of these crystalline products, a series of advanced characterization techniques were carried out, including extended X-ray absorption fine structure, transmission electron microscopy (TEM), and synchrotron XRD, and the results pointed to a crystalline Nb 2 O 5 with a rock-salt structure that previously has not been reported.The new rock-salt Nb 2 O 5 electrode exhibits multi-electron redox reaction processes and a lower energy barrier of lithium insertion, leading to excellent rate performance and superior cycling stability.In addition, the increasing integral area of CV curves and enhancing specific capacity in the first several cycles indicate the presence of irreversible phase transition of the active electrode. [39]hang et al. reported that an ultra-low specific capacity of 16.9 mA h g −1 delivered by the V x O y @C electrode for zinc ions storage could significantly increase to 399.1 mA h g −1 after taking 20 GCD processes. [40]The irregular lumpy V x O y @C is transferred into amorphous layer-structured vanadium oxide fibers.Meanwhile, the valence of V is increasing from V 3+ /V 4+ to V 4+ /V 5+ due to the extra electro-oxidation reaction processes.
Under certain conditions, not all kinds of electrode materials undergo a phase transition.On the contrary, the occurrence of phase transition or not depends on the working environments of electrode materials, including the kind of electrolyte and the choice of potential window.Thus, we summarize the condition to induce the phase transition of electrode materials.First, electrode materials exhibit high oxidation activity in the electrolyte.Some research assumed that it is similar to the Kirkendall or anion exchange processes, and others concluded that there is a redox reaction between the electrode and oxygen or water. [41,42]For example, VS 2 is not stable in aqueous electrolytes and can be easier oxidized to vanadium oxides in water, forming VS 2 /VO x heterostructures. [43]Moreover, the self-oxidized reaction was discovered in NiCoSe, [44,45] NiCoS, [46] Ni 2 P, [47,48] and so on, when the aqueous solution was an electrolyte.Second, a reconstruction process occurred on the surface of the electrode due to it being a little soluble in electrolyte, resulting in phase transformation near the surface.Third, the working voltage window of the electrode is beyond its stable potential.In other words, the irreversible phase changes turned out to be highly dependent on the cutoff volt.The situation was often observed in vanadium-based cathode materials for ZIBs. [49,50]For example, For V 2 O 3 @C composites as ZIBs cathode, the characteristic peaks of V 2 O 3 crystal still can be detected in ex situ XRD pattern when it is charged to 1.5 V.However, amorphous V 2 O 5 @C hybrid is generated accompanied by the disappear of crystalline V 2 O 3 when the working potential is extended to 1.9 V. [51] Fourth, many publications have proposed that a recrystallized phenomenon of amorphous materials was discovered under electron-beam irradiation (EBI) caused by high current density.The fundamental properties difference between amorphous materials and their crystal counterparts.Reproduced with permission from ref. [31-33]   Energy Environ.Mater.[54] For example, Chen et al. claimed that amorphous MoO x thin films could transfer into monoclinic MoO 2 and orthorhombic MoO 3 under the influence of EBI, lighting up an investigated ways for another amorphous electrode in LIBs. [55]Fifth, the molecule structure is unstable under special electrochemical conditions, so it tends to form a more stable phase during repeated charge-discharge processes.Typically, amorphous Li 3 P with low stability as LIBs anode spontaneously crystallizes into Li 3 P with a crystalline structure during lithiation/ delithiation processes. [56]n some cases, phase transformation of electrode materials is beneficial to electrochemical energy storage due to defects, internal active sites, and generated heterostructures.Thus, it can be used as an effective strategy to optimize the energy storage capability of electrode materials.Especially for low-valent oxides and hydroxides, capacity increases in the first several cycles accompanied by a multi-step transformation. [57,58]However, the structural collapse and electrode cracks caused by phase transition are observed, leading to performance deterioration rapidly.We cannot predict whether the phase transition toward being beneficial or harmful and the corresponding conclusions should be verified by experimental or theoretical calculations.

Synthesis of Amorphous Materials
In recent years, various preparation methods for amorphous materials have been developed, including hydrothermal, atomic layer deposition (ALD), microwaves, sol-gel strategies, electrodeposition, and electrospinning techniques.Based on the different synthesis processes, the preparation methods of amorphous materials can be divided into two categories: direct synthesis and the amorphization of crystals.

Direct Synthesis
At low temperatures, the adatoms do not have enough mobility to form crystalline structures.Thus, adatoms will gradually depose randomly to form amorphous materials.Due to the one-step reaction, direct synthesis methods are always time-saving and facile.We highlight some crucial methods for synthesizing amorphous materials.First is the ALD technique, a promising way to prepare amorphous metal oxides (e.g., TiO 2 , MoO 3 , Al 2 O 3 , etc.).Compared with other methods, the thickness and composition of amorphous materials can be controlled by tuning the cycle numbers and organometallic sources in an ALD process.Recently, Sinha et al. [59] prepared a ZnOS thin film by ALD technique with alternative ZnO and ZnS deposition.Thus, ALD is not only limited to prepared metal oxides but also can synthesize amorphous metal sulfides.Second is chemical vapor deposition (CVD).In a typical CVD process, the precursors supplied in gaseous form are transported to the heated substrate to be deposited and coated.Then, it will generate a chemical reaction at the surface of the substrate to produce a thin film.The amorphous materials will be obtained by controlling the types of precursors, temperature, and vacuum degree. [60]Third, solution-processed, such as hydrothermal, solvothermal, sol-gel, and coprecipitation, is the most common method to prepare amorphous materials.Compared with ALD, CVD, and other methods, solution-processed methods are efficient in preparing amorphous materials with various morphology.For example, the hydrothermal method often observes the formation of amorphous nanowires and hydrangea-like spheres. [61,62]Amorphous FeP encapsulated in porous carbon nanosheets have been produced by a coprecipitation reaction combined with an annealing process. [63]However, it depends on the prepared environment, especially temperature, pressure, and concentration.Fourth is ultrasound sonochemical synthesis.The precursor solution will generate massive bubbles under the condition of continuous ultrasonic waves.Then, these bubbles grow, gather, and implosively collapse, releasing a lot of heat to provide the need for amorphized reactions.Transient high temperature (~5000 K) and high pressure (~1000 bar) are obtained at a specific spot, promoting the process of chemical reactions. [64]Besides the aforementioned synthetic methods, other methods, including electrostatic spray deposition, [65] electrochemical oxidation, [66] electrospinning, [67] and so on, can generate amorphous materials.As illustrated in Figure 5, we have briefly summarized some typical methods to directly synthesize amorphous material.

Amorphization of Crystals
It is possible to convert a crystal into an amorphous structure by bombarding it with external force (pressure, heat, irradiation, doping, etc.) to break the lattice (physical) or induce defects (chemical), leading to partial or complete amorphization.Ball milling, ion exchange, microwave, and so on are common strategies to transform crystals into amorphous materials.We highlight several representative methods as follows.

Thermal Treatment
The thermal treatment method can be divided into two categories: thermal reduction and thermal oxidation.Thermal reduction is a promising technique because it is time-saving, does not require complex reactants, and is usually applied to prepare amorphous metal materials with the assistance of some reductive substances (the most common is carbon).So far, various amorphous metal has been successfully synthesized by employing thermal reduction.In practical application, the thermal reduction technique is usually coupled with inert gases and high temperatures.In a thermal reduction process, calcination temperature and time are important parameters to achieve desirable amorphous materials.At low temperatures, partial phase transformation can be observed to obtain a composite of metal oxides and amorphous metals.However, crystal metals will be generated at high temperatures, indicating a structural change of amorphous to crystal has been involved.As such, a suitable reaction time is crucial to fabricate pure amorphous materials completely.For example, Yang et al. prepared the ultrafine amorphous Sb nanoparticles/N-doped layered carbon by reducing Sn 2 O 3 with carbon as a reducing agent. [68]They found that a completely amorphous structure can be obtained at 550 °C for 6 h.If shorting the reaction time, the nucleation and migration of adatoms or reaction species are not enough, and the amorphous-crystalline composites will be generated.If increasing the time, the re-crystallization will be induced to form a crystalline Sb phase.
Different from thermal reduction, thermal oxidation is the most typical way to fabricate amorphous metal oxides.Metal hydroxide easily transforms into metal oxide due to the thermal decomposition of metal hydroxide at high temperatures, based on the equation: However, the crystallinity of final products can be affected by many factors, including annealing temperature, atmosphere, time, and even precursor.For example, low crystallinity and amorphous structure of NiO dominate the materials below 300 °C.Still, the strong and prominent NiO characteristic peak can be observed in the XRD pattern of samples annealed at 400 and 500 °C, indicating that it is inclined to the formation of crystal NiO above 300 °C. [69]Mara et al. also demonstrated that amorphous TiO 2 formed at low temperature and anatase TiO 2 formed at high temperature, suggesting that the oxidation temperature needs to be tuned to obtain ideal amorphous structured materials. [70]Moreover, metal can be oxidized to metal oxide according to the reaction formula: 2xM + yO 2 → 2 M x O y .However, the reaction conditions are more stringent and difficult relative to the decomposition of metal hydroxide due to the superior stability of metal elements.

Electrochemical Reaction
In contrast to the other methods, electrochemical strategies for preparing amorphous materials are superior because electrode materials can be reused from electrochemical devices, such as metal ion batteries and electrocatalyst systems.Recently, two kinds of electrochemical techniques for the synthesis of amorphous electrodes have been proposed.One is the repetitive insertion/extraction of metal ions accompanied by volume expansion/shrink (e.g., Li + , Na + , K + , etc.) in the organic electrolytes, leading to the breaking of the crystal lattices and hindering further crystallization from forming an amorphous phase.The other one is electrochemical amorphization, which can be achieved by employing multiple oxidation/reduction cycles in aqueous mediums, leading to the formation of oxygen vacancies in the reduction process and oxides during the oxidation process.Notably, a fast scan rate is necessary during the oxidation/reduction cycles because the nucleation and growth of crystalline oxides will be hindered, resulting in the formation of an amorphous structure on the surface materials. [71]Pala's group employed the above-mentioned electrochemical reaction routes to amorphization of Co 3 O 4 , and both amorphous samples showed an enhanced oxygen evolution reaction performance. [72]

Heteroatom Doping
Heteroatom doping is an efficient method to prepare partial or complete amorphous materials because it can modulate the intrinsic crystal structure and affect the distribution of charges/ions. [73]For example, surface-amorphous Li 3 VO 4 anode is synthesized by inducing oxygendeficient.Oxygen quickly escapes from the surface due to the highly activated, reducing partial V 5+ to V 4+ .The increase in lattice stress and the rearrangement of structures reduced by oxygen vacancies result in gradual amorphization of the surface of materials. [74]

Physical Strategies
For physical strategies, the mechanism of amorphization is that the crystal's lattice will be broken by applying external stress (e.g., mechanical force, irradiation, and heat).As a result, materials will no longer have long-range orders but short-range orders that are amorphization.In general, the amorphization mechanism can be divided into two categories: the collapse of the crystal structure usually caused by high pressure or mechanical process. [75,76]The other one is generated from thermal agitation, such as microwave and X-ray. [77,78]For example, ball-milling, a facile, and low-cost method, is usually used to grind or blend materials to reduce the size or obtain uniform composites.Crystalline/amorphous composite of Sn/P/C was prepared by the ballmilling technique, showing superior capacity retention of 86% after 300 cycles in SIBs. [79] Application of Amorphous Materials

Amorphous Materials in Alkali Metal Ion Batteries
As suitable energy storage devices, alkali metal ion batteries (LIBs, SIBs, and KIBs) play a crucial role in various application fields, ranging from small electronic equipment to large grid storage systems. [80]Electrode materials are critical factors in the electrochemical performance of batteries.Numerous materials have been designed, developed, and assessed as an electrode for alkali metal ion batteries. [81,82]Amorphous materials with abundant defects and vacancies reveal remarkable specific capacity (even higher than the theoretical capacity of the corresponding crystal), which helps improve the insertion/extraction of alkali ions (Li + , Na + , and K + ).For example, the classic carbon-based, Si-based, and P-based were reported to be amorphized by a facile ball-milling method, [83] revealing improving electrochemical storage performance.Besides, amorphous materials were also demonstrated to be significant solidstate electrolytes with enhanced ion conductivity relative to the traditional organic electrolyte.This section mainly focuses on the application of amorphous materials in alkali ion batteries, including anode material, cathode materials, and solid-state electrolytes.

Anode Materials
The amorphous electrode can provide more electrochemical active sites for ion storage and higher conductivity to facilitate electrons/ions transportation for achieving remarkable rate performance.Further, multiple ion diffusion pathways have been expounded due to the disordered lattice structure.As one of the potential anode catalogs, metal vanadates with significant theoretical specific capacities have been heavily estimated to be used for energy storage electrodes. [84,85]However, the cycling stability and rate performance are inferior.To make up for this shortcoming, amorphous metal vanadates stand out from the crowd, such as amorphous Ni 3 V 2 O 8 wire, [86] amorphous FeVO 4 /carbon composites, [87] and amorphous LiV 3 O 8 @carbon fibers. [10]Qin et al. designed and estimated amorphous Zn 2 V 2 O 7 (ZVO) materials as the anode of SIBs, as shown in Figure 6a. [9]Compared with a crystalline structure, the amorphous ZVO electrode showed improved rate performance (at 10 A g −1 : 150 mA h g −1 for the amorphous electrode; <50 mA h g −1 for the crystalline electrode, as shown in Figure 6b) and cycling stability (at 5 A g −1 : 89.1% capacity retention for amorphous electrode, a sharp decrease in the first 10 cycles for crystalline one).The amorphization of ZVO could induce more defects evidenced by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectra (Figure 6c) and adjust the value of bandgap (calculated from ultraviolet-visible diffuse reflectance spectroscopy, as shown in Figure 6d).In addition, Niu et al. proposed an amorphous FeVO 4 as an extraordinary anode-active material for rechargeable potassium ion batteries. [87]An exceptional reversible capacity over 180 mA h g −1 at 2 A g −1 was observed in the as-prepared anode, suggesting the excellent possibility of the potassium ion storage materials.
The potassiation process involves the reduction of Fe ions and V ions from Fe 3+ and V 5+ to Fe 2+ and V 4+ /V 3+ , respectively, and leads to the formation of small K 2 O crystallites and the precipitation of tiny crystallites of VO 2 , V 2 O 3 , and FeO.Notable, the K ions storage mechanism of amorphous and crystalline FeVO 4 is similar, indicating the improved specific capacity mainly originates from better reaction kinetics.It has been demonstrated that amorphous materials reveal much better reaction kinetics due to their isotropic characterization and multiple channels.Besides, the amorphization of materials can affect the electrochemical reaction process.Compared to the CV curves of crystalline and amorphous SnSe electrodes (Figure 6e,f), the results showed that different conversion reactions are generated during the charge-discharge process. [88]For the crystalline SnSe electrode, a significant peak at 0.55 V disappeared after the first cycle due to forming the solid electrolyte interphase (SEI) layer, resulting in a largely irreversible redox reaction.And the decreasing of the peak at 1.1 V during the anodic scan revealed that the conversion from SnSe was partly irreversible.However, the CV curves of amorphous SnSe (a-SnSe) are drastically different from them.The CV curves of a-SnSe showed more redox peaks.Especially, the peak at 1.8 V during charge processes was attributed to the de-intercalation of Na + , which cannot be observed at crystalline SnSe, indicating the complicated and unique electrochemical processes of the amorphous electrode.Meanwhile, the serve volume expansion of Energy Environ.Mater.2023, 6, e12573 the Sn-based anode can be effectively buffered by the amorphous structure, reducing the degree of particle pulverization. [88]Moreover, Ye et al. have concluded that there are different electrochemical processes for crystalline MoS 2 and amorphous MoS 3 electrodes during the first cycle, as presented in Figure 6g. [89]Density functional theory (DFT) analysis results verified that the absorption and diffusion of Na + are more energetically stable and faster in amorphous MoS 3 electrodes due to the stronger adsorption energy and smaller diffusion energy barrier (Figure 6h), presenting a more severe radical structural rearrangement in crystalline MoS 2 electrode compared to amorphous MoS 3 electrode, resulting in the different electrochemical behavior at the first cycle.The second CV curves of both electrodes are consistent, confirming the similar sodium ion storage mechanism.
In addition, there are substantial volume changes for alloy-based anodes (Si, Ge, Sn, and P) during repeated discharge-charge processes, resulting in the collapse of the structure.Thus, due to the disordered structure and plentiful defects, alloy-based materials with low crystallinity or amorphous phase are beneficial for structural stability and electrochemical performance.Moreover, the lack of grain boundaries in amorphous materials is conducive to recovering the structure, releasing the strain stress caused by the volume expansion during the insertion/ extraction of alkali ions.For example, amorphous SnO x showed enhanced reversibility of electrochemical reactions and improved capacity retention and rate performance due to defects and vacancies. [91]Subsequently, Huang's group demonstrated that the amorphous P 4 SSe 2 compounds are more suitable for Li-ions storage than their crystalline counterparts. [90]The electrochemical superiority of as-prepared amorphous electrodes can be attributed to the strong chemical bond among P, S, and Se, which can be conducive to inhibiting the dissolution and the shuttle effect of polysulfide/polyselenide.The slight volume vibration and lower Li ion migration energy benefit from the isotropic nature of the amorphous P 4 SSe 2 electrode, promoting the promising Li-ion storage properties, delivering excellent rate performance (850 mA h g −1 at 2 A g −1 ) and exceptional cycling ability.Ex situ XRD (Figure 6i) and TEM were carried out to confirm the Li-ion storage mechanism.Results suggested that the amorphous P 4 SSe 2 was reversibly converted to crystalline Li 2 S, Li 2 Se, and Li 3 P during charging-discharging processes, which may respond to the remarkable cycling lifespans of the as-prepared P 4 SSe 2 electrode.
Shi et al. used first-principles calculations to provide insight into the enhanced mechanism of amorphous Fe 2 O 3 electrodes to explain its improved energy storage performance. [61]According to their results, amorphous Fe 2 O 3 (1.82eV) exhibited lower Na + insertion energy than that of crystalline Fe 2 O 3 (2.81eV), indicating Na + is much easier to insert into an amorphous structure (Figure 7a,b).Thus, optimized electrochemical behavior would be expected for using an amorphous Fe 2 O 3 anode in SIBs.Furthermore, Yi et al. reported an amorphous Ge anode achieved by acid-etching the Mg 2 Ge crystal. [92]Raman peaks of crystalline Ge show negligible peak shifts and peak intensity changes, suggesting that the Na ions are hard to incorporate into the lattice of Ge crystal during the sodiation process.Compared with its crystalline counterpart, the Raman peaks of amorphous Ge particles are disrupted, gradually decreasing and eventually disappearing characteristic peaks.Thus, Ge-Ge bonds rupture to form Na x Ge compounds due to the insertion of Na ions.Moreover, they also verified that K was harder to alloy with crystalline Ge anode than amorphous Ge anode by ex situ XRD.Their results provide a perspective to investigate the fundamental difference in the electrochemical storage mechanism between crystal and amorphous electrodes.
Although electron/ions transportation kinetics and electrochemical reactivity of amorphous materials are better than their crystal materials, the intrinsic low conductivity still exists in amorphous electrodes, limiting the kinetics of electrochemical performances.Constructing heterostructure with conductive substrates is an effective strategy to enhance the electrical conductivity of electrodes and further optimize the cycling stability and rate properties. [95]Encouragingly, Xu et al. first reported that 2D heterostructure consisting of amorphous MoO 3-x and MXene (aMoO 3-x @MXene, Figure 7c) was achieved for the capacitorlike anode of LIBs. [93]The amorphous MoO 3-x was anchored on the surface of MXene through a Ti-O-Mo covalent bond, which was more beneficial to maintaining the structural stability and restraining exfoliation of the amorphous layer.The non-van der Walls 2D heterostructure with an amount of O-vacancies demonstrated an abnormal capacity and excellent rate performance of 500 C g −1 at 1 A g −1 .Through DFT calculation, they concluded that the formation energy of aMoO 3-x @M-Xene (E form = −8.312eV) is lower than that of vdW heterostructure (E form = −2.151eV) and provides more diffusion pathways for Liions, indicating the higher stability and reaction kinetics (Figure 7d).In addition, Liu et al. also verified that the active materials hybrid with carbon was an effective strategy to improve the diffusion coefficient of Li ions and decrease the SEI layer resistance (R f ) and electrode-electrolyte interface (R ct ) through the galvanostatic intermittent titration technique and electrochemical impedance spectroscopy measurements. [96]Similarly, Zhang et al. presented that the sodium ion storage properties of amorphous Co-Sn-S@C (A-CSS@C) electrode were significantly superior to amorphous Co-Sn-S without carbon and crystalline Co-Sn-S@C, including specific capacity, cycling ability, and rate performance. [97]The high storage capacity of sodium ions was interpreted by the reversible formation/decomposition of the SEI layer, lower charge transfer resistance, and accelerated sodium ion diffusion due to the synergistic effect between an amorphous structure and carbon coating.
For expanding applications, amorphous materials can also be employed as a coating layer to protect the crystalline electrode and provide more diffusion paths and surface vacancies.Peng et al. reported an amorphous Fe 2 O 3 film-coated crystalline Fe 2 O 3 core-shell structure, revealing improved capacity retention of 70.1% after 600 cycles at 0.5 A g −1 . [98]To verify the effect of amorphous coating, Wang's group Reproduced with permission. [61]Copyright 2018, Elsevier.c) Schematic illustration of the synthesis route of 2D aMoO 3−x @MXene non-vdW heterostructures.d) Illustration of facile capacitor-like interlayer diffusion and diffusion-controlled intralayer diffusion.Reproduced with permission. [93]Copyright 2021, Elsevier.e) HRTEM images of R-TiO 2-x -S.Cyclic voltammograms (CVs) for the first 3 cycles of f) R-TiO 2 and g) R-TiO 2-x at a scan rate of 0.2 mV s −1 .h) Cycling performances of all the TiO 2 electrodes at 50 mA g −1 .Reproduced with permission. [94]Copyright 2018, Wiley.
Energy Environ.Mater.2023, 6, e12573 designed S-doped TiO 2 with a thin amorphous shell (around 3 nm) on the surface (denoted as R-TiO 2-x -S, as illustrated in Figure 7e) electrode using Ar/H 2 plasma treatment. [94]They proposed that the loose structure of amorphous materials was more accessible to dopant heteroatom with high concentration and deep doping, leading to a high S doping (4.68%).Moreover, compared to the pure TiO 2 electrode, the asprepared TiO 2 electrode with an amorphous surface shows a lower voltage offset (Figure 7f,g), suggesting diminished polarization and facilitated sodium ion storage kinetics.Besides, the amorphization of crystalline materials can increase the surface area, resulting in abundant diffusion pathways for sodium ions.Both surface amorphization and S doping have positive effects on improving the sodium ion storage performance of TiO 2 , delivering remarkable discharge capacities and rate performance (Figure 7h).Additionally, the amorphous electrode is more suitable for inserting larger ions, K + and Na + , due to their loosely packing structure and excellent volume vibration tolerance.Thus, Hsieh et al. designed ultrafine SnSb particles embedded in amorphous carbon (SnSb@MAC) as a promising anode of potassium ion batteries. [99]The anode shows significant K + storage properties by combining the stable structure of amorphous carbon with the ultra-high theoretical capacity of SnSb alloys.

Cathode Materials
Similar to anode materials, cathodic electrode materials are the necessary components of metal-ion batteries.An eligible cathode should have high energy/power density, be non-toxic, have the high specific capacity, electrochemical stability, and be environmental friendliness.So far, the cathode materials of commercial LIBs main focus is on the LiFePO 4 and ternary cathode materials.They all belong to lithium-containing oxides, showing inferior power density and lower capacity. [100]anadium-based cathodes stand out from the crowd because they have a variety of stable oxidation states (+5, +4, and +3), indicating multielectron reactions and considerable theoretical capacity.However, during repetitive charge-discharge processes, crystalline vanadium-based materials have problems, including severe structural degradation and low discharge voltage platform (~2.5 V). [101] Some studies have focused on decreasing the particle size of vanadium-based cathodes or hybrid them with carbons to buffer the structural changes.Another promising strategy is constructing materials with amorphous structures, such as glasses or glass ceramics.
Amorphous glass with more open frameworks can resist structural collapse due to its disordered and loose network structure.[104] Kong et al. prepared a series of V 2 O 5 -Li 3 PO 4 glass through multiple-electron reaction and estimated their electrochemical performance as the cathode of LIBs. [105]heir results have demonstrated that the ion storage properties of amorphous vanadium oxides decreased with the increasing nanocrystalline of LiV 2 O 5 .However, the vanadium and vanadophosphate glass family still exhibits limited development during long-term cycling.If the cycle performance can be significantly enhanced, the vanadium-based cathode can be a promising alternative to lithium cobalt oxide.Thus, Kindle et al. prepared some vanadate glasses and glasses-ceramic cathodes (50-LBV-A) to clarify the degradation mechanism during long-term cycling by employing EDS analysis and Raman spectra. [106]The EDS mapping of Li foil indicated that cathode materials of vanadium-based could be dissolved by electrolyte and transported to the anode, similar to the shuttle effects of polysulfide observed in Li-S batteries.Thus, the electrolyte will impact the performance of the amorphous vanadate glass electrode.The optical microscope of cycled cathode showed the segregated area, which was dark and light spots (Figure 8b).Interestingly, the Raman spectra (Figure 8a) indicated the complex components of the cycled amorphous vanadate electrode.In comparison with the dark spot, there was a disappearance D band at 1350 cm −1 and the borovanadate peak, and diminished vanadium Raman peaks.Their results demonstrated that the capacity decline of vanadate glass originated from the dissolution of cathode materials in electrolytes even though the structural stability was better.Meanwhile, Cao et al. have demonstrated that the amorphous V 2 O 5 exhibited superior cycling performance than the corresponding crystal in SIBs because the isotropic characteristic of amorphous materials could lead to faster faradaic reactions. [107]Notably, the (001) direction with the lowest diffusion energy barrier favored lithium ions diffusion due to the misalignment of V 2 O 5 pyramids. [108]Thus, exposing more (001) directions was an effective strategy to realize better ion storage performance.A composite amorphous and crystalline V 2 O 5 cathode was proposed by Lv 0 s group. [109]A variety of samples with different ratios of (001) to (110)  was prepared by tuning the concentration of intercalation ions (NaCl and KCl, denoted as N x K y ).It could be found that exposing (100)  planes increase the biggest value at N 1 K 2 , delivering a twice higher capacity than that of commercial V 2 O 5 powder.
Polyanion-type materials, such as LiFePO 4 and NaFePO 4 , are known for their low cost, high safety, significant specific capacity, and environment-friendliness.They have been extensively investigated as cathode materials for alkali ion batteries in recent years. [110]There are two classes of crystalline NaFePO 4 with maricite and olivine phases, respectively.In general, crystalline FePO 4 has a rigid crystal structure, while it appears impossible to provide an effective pathway for Na + diffusion and defects as ion storage sites. [113]Also, the excellent thermodynamical stability of maricite NaFePO 4 makes it hard to release the Na ions from its orthorhombic.Consequently, its intrinsic structure characteristics make it electrochemically inactive.Although olivine NaFePO 4 shows considerable electrochemical activity during the Na ion storage process, olivine NaFePO 4 is metastable even at room temperature and thus can only be synthesized by employing LiFePO 4 as the precursor.This shortcoming dramatically restricts the application of NaFePO 4 as a cathode candidate for SIBs.Recently, amorphous polyanion cathode materials have been applied to lithium and other alkali-metal ion batteries, and the energy storage performance is quite attractive.For example, Okada et al. have proposed that the reversible capacity of amorphous FePO 4 is higher than that of crystalline FePO 4 . [114]Wang et al. have reported an amorphous FePO 4 cathode of LIBs revealed electrochemical with a specific capacity of 175.6 mA h g −1 at 0.1 C and 139.1 mA h g −1 at 5 C. Besides, considering a conceptually defect-free phase, amorphous FePO 4 exhibits an uninterrupted charge-discharge voltage curve, indicating the continuous insertion/extraction of Na ions. [115]owever, amorphous FePO 4 still suffers many challenges in practical application, including low electronic conductivity and volume expansion (over 22%) after full charge, resulting in a rapid capacity decline in the long-term cycles.Zhang et al. reported a sagacious multi-step templating process to produce yolk-shell FePO 4 nanospheres for SIBs (Figure 8c). [110]The amorphous FePO 4 cathode provided a porous shell which was a benefit to the permeation of electrolytes and easier for the insertion/extraction of Na ions.Moreover, the robust and Energy Environ.Mater.2023, 6, e12573 mesoporous yolk could be used as the host site to accommodate more Na ions.The obtained yolk-shell nanospheres cathodes showed remarkable capacity retention of 91.3% after 1000 cycles at 100 mA g −1 and attractive rate properties of 97.1 mA h g −1 at 1 A g −1 (Figure 8d).Besides, amorphous FePO 4 nanosheets@GO and FePO 4 /CNTs nanoparticles have also been investigated as Li/Na ion storage systems, effectively improving the electronic conductivity and structural durability of FePO 4 . [116,117]ased on the significant electrochemical properties of iron phosphates, efforts have been focused on exploring the cathode materials with an increasing redox voltage, resulting in wider potential windows and specific capacity.Since fluorine possesses higher electronegativity and smaller molecules than oxygen, fluoride-based cathodes can provide higher voltage and energy density.In LIBs, the amorphization or disordering of fluoride-based materials is an effective method to optimize their ion storage performance.Thus, this enhanced strategy led to a new arrangement being rebuilt from the amorphous structure, resulting in multiple pathways for ion diffusion in the cathode materials. [118]or SIBs, amorphous FeF 3 nanoparticles anchored in porous carbon network C were proposed by Liu's group, presenting exceptional sodiumion storage capacity and cycling ability. [119]A drawback of iron fluoride is the limited electronic conductivity, which can be improved by hybrid with carbon-based materials, such as carbon nanotubes and graphene.Besides, reducing band-gap is another effective method to enhance conductivity by replacing a part of fluoride with oxygen or hydroxyl, exhibiting an enhanced electrochemical energy storage performance, such as FeF 2.2 O 0.4 and FeF 2.2 (OH) 0.8 . [120,121]A series of amorphous bimetallic oxyfluorides were prepared via suitable thermal treatments, M 2+ M 3+ 2 F 8-2x O x (M 2+ = Mn, Fe, Co, Ni, Cu; M 3+ = V, Fe). [111]The obtained materials exhibit fruitful pores to reduce the diffusion distance of the pathway, leading to superior lithium-ion storage performance compared to the crystalline counterparts (Figure 8e).Their results indicated that amorphous oxyfluorides are better for accessing high-performance cathode candidates for LIBs.However, the synthesis processes of oxyfluorides are often accompanied by high reaction temperature or high pressure and corrosive HF acid or toxic F 2 .Thus, new synthesis methods with features of low-cost and nontoxic to prepare amorphous oxyfluorides are highly encouraged, such as the plasma technique and microwave strategy.
In addition to the above-summarized cathode materials, various other amorphous compounds can be used as superior cathodes for ion storage.Manganese oxides show significant advantages as cathode Reproduced with permission. [106]Copyright 2021, American Chemical Society.c) TEM images of FePO 4 YSNSs.d) Rate properties of FePO 4 YSNSs (yolkshell), FePO 4 HNSs (hollow), and FePO 4 SNSs (solid).Reproduced with permission. [110]Copyright 2020, Wiley.e) Illustration for the features of an amorphous bimetallic oxyfluoride.Reproduced with permission. [111]Copyright 2019, American Chemical Society.f) SEM image and g) TEM images of M4-4.h) The cycling ability and i) rate performance of M4-4.j) TEM images of 75Mo:25P electrode.k) Cycling performance of the various electrodes with different Mo/P ratios.Reproduced with permission. [112]Copyright 2019, Elsevier.
candidates.However, they are easy to form crystalline structures at high temperatures. [122]To conquer this challenge, Wang et al. prepared a K + -stabilized nanostructured amorphous MnO 2 through liquid-phase co-precipitation, denoted as M4-4 (Figure 8f,g). [123]The doped K + with the larger radius at the interlayer can stable the tunnels of MnO 2 , remaining in an amorphous state even at higher temperatures.As a result, the as-procured materials showed a significantly dischargespecific capacity of 118.1 mA h g −1 and excellent cycling stability after 200 cycles (Figure 8h,i).Amorphous MoO 3 has also been investigated in recent years. [124]Further, molybdenum-based glass, MoO 3 -P 2 O 5 with different Mo/P ratios, was first reported as a cathode by Zhao's group, as shown in Figure 8j. [112]Different from the regular channels of crystalline MoO 3 electrode, the open network of amorphous MoO 3 -P 2 O 5 glass could accept more Li + , delivering a much higher discharge capacity (291 mA h g −1 ) than the crystalline MoO 3 (213.5 mA h g −1 ), as shown in Figure 8k.

Solid-State Electrolyte
The electrolyte is an essential part of secondary ion batteries and dramatically affects electrochemical performances.The most employed electrolytes in alkali metal ion batteries are organic electrolytes, which are flammable and toxic, limiting the further development of alkali metal ion batteries. [125]Substituting organic electrolytes with nonleakage, thermal stability, and nonflammable solid-state electrolytes have been considered a promising strategy to enhance the safety of alkali metal ion batteries.Nonetheless, many intractable challenges, such as poor interfacial charge transport arising from inferior contact with the electrodes (compared with liquid electrolytes), the risk of metal dendrite growth and proliferation along grain boundaries at low current densities, undesirable electrode side reactions, and uneven deposition of Li metal, are inefficiently addressed, limiting the applications of solid-state batteries. [126,127]130] The uniform deposition of Li on the electrode and current collector can provide continuous pathways for transferring ions and electrons.To optimize the interface, Hu's group developed an amorphous carbon nanocoating that enhanced the electro-chemomechanical stability of the interface between electrode and electrolyte, as shown in Figure 9a,b. [131]he amorphous carbon layers acted as a robust bonding layer to provide continuously connected pathways for ion and electron conductivity during the Li stripping and plating process, benefitting from the stability and insolubility of carbon coating (Figure 9c,d).The compound electrolyte exhibited ultrahigh stability for 500 h at 3 mA cm −2 .Except for the challenges of interface contact, Li + ion conductivity is another limiting factor for the application of solidstate electrolytes.To resolve that, surface engineering by introducing the coating layer or transition layer on the electrolyte's surface is a reasonable design to optimize the properties of the solid-state electrolyte.Inspired by the above consideration, Gao et al. presented a novel LLZTO composite electrolyte with two amorphous coating layers (LiBO 2 and LiBH 4 as the inner and outer modification layer, respectively, as shown in Figure 9e) by a facile ball milling method. [132]The as-prepared solid-state electrolyte could have up to a significant Li + ion conductivity if 8.02 × 10 −5 S cm −1 even at ambient temperature (30 °C).The impressive cycling stability could also be obtained, which were 1000 h at 60 °C and 300 h at 30 °C at 0.15 mA cm −2 for Li¦LLZTO-4LiBH 4 ¦SUS symmetric cell, and the capacity retention of 95% after 21 cycles for full cell (Figure 9f).To investigate the function of LiBH 4 and LiBO 2 layer, the samples removed LiBH 4 were prepared, denoted as LRMO.The results showed that the LRMO (under 300 MPa) was more difficult to press into electrolyte pellet than that of LLZTO-xLiBH 4 samples (30 GPa), indicating that the LiBH 4 layer was conducive to filling the open pore and promoting the transport of Li + ions.Besides, the LiBH 4 coating layer could realize the conversion of the contact between electrode and electrolyte from point contact to face contact (Figure 9g,h).LiBO 2 could be acted as a separation layer to prevent the reaction between LiBH 4 and LLZTO, exhibiting a sustainably elevated cycling stability.
Nevertheless, introducing an inactive modification layer intends to lower the energy density of the full cell.Hence, Liang et al. designed amorphous electrode-electrolyte interphase via the in situ growing method, realizing a dual-interface amorphous cathode electrolyte interphase (CEI)/SEI protection (DACP) layer. [133,134]During the electrochemical process, lithium difluoro(oxalato)borate (LiDFOB), a component of electrolyte, oxidizes on the cathode to form amorphous CEI (around 4 nm) and generates LiBF 4 salt.Furthermore, the amorphous SEI was constructed on the anode side by reducing LiBF 4 (Figure 9i).The high plasticity CEI was the help of the contact between the cathode and electrolyte and alleviated lattice distortion, severe volume expansion, and structural collapse during the repeated discharge-charge process (Figure 9j,k).Meanwhile, the rigidification SEI was promising for inhibiting Li dendrite, leading to uniform Li plating and stripping.Under the influence of DACP, a wide voltage window as high as 4.6 V could be achieved, which was higher than that of a traditional liquid electrolyte and a single LiPF 6 salt electrolyte without LiDFOB.Encouraging, Lai et al. proposed in situ growth of Li 2 S interfacial layer on the Li anode side via filling amorphous 3Li 2 S•2P 2 S 5 powders into polyvinylidene difluoride (PVDF) polymer. [135]The generation of the Li 2 S layer not only buffered the decompositions of PVDF and LiTFSI to Li 2 F but also improved the compatibility between Li anode and solid electrolyte.With the help of an in situ Li 2 S layer, the composite electrode effectively prohibited the formation of Li dendrite and electrode polarization during continuous electrochemical processes.
[138] For example, Tian et al. embedded LLZTO (Li 6.75 La 3 Zr 1.75 Ta 0.25 O 12 ) into an amorphous Li 3 OCl matrix to prepare a composite solid electrolyte, which delivers astonishing ionic conductivity of 2.27 × 10 −4 S cm −1 . [139]The amorphous Li 3 OCl not only could be regarded as a binder but also could provide a connection among LLZTO particles for continuous ionic transport.Moreover, the amorphous interfacial layer could be connected to the lithium metal anode on one side and the electrolyte on the other, decreasing the interfacial resistance and restraining the growth of lithium dendrite during electrochemical reaction processes.Subsequently, Song et al. incorporated crystalline cubic Li-La-Zr-Ta-O (LLZTO) particles into an amorphous LLZTO matrix. [140]The composite electrolyte revealed an excellent ionic conductivity of 0.8 × 10 −5 S cm −1 at 30 °C due to supplementary and synergistic effects between the amorphous network and crystalline nanofillers.The previous amorphous materials in alkali metal ion batteries are summarized in Table 1.Amorphous components are a considerable strategy to optimize the properties of rechargeable batteries for further application.

Zinc Ion Batteries
Zinc ion batteries (ZIBs) catch ever-increasing attention due to their high safety, natural abundance, and low cost.Compared with rechargeable ion batteries employing organic electrolyte, ZIBs with near-neutral aqueous electrolyte is competitive thanks to their significant advantages, including superior ionic conductivity, environmentally friendly, and easier preparation technology.[165] So far, there are a few materials that can be applied to the cathode of ZIBs, such as manganese-based, [166] vanadium-based, [167,168] Prussian blue analogs, [169,170] and polyanion compounds. [171,172]Considering the poor electric conductivity of these active materials, a hybrid with conductive additives is an effective method to improve the electrochemical performance.Besides, defects engineering is another attractive technique to enhance the ability of ion storage by increasing the electronic conductivity and adsorption/desorption reversibility.However, the phase transition of crystalline cathode materials during the discharge/ charge process will induce a severe volume variation, limiting the development of crystalline electrode materials.Recently, amorphous structures have been employed as active materials to store zinc ions.
Amorphous MnO 2 has been widely investigated as a cathode material for zinc ion batteries due to their high theoretical capacity and high working voltage; for example, mesoporous amorphous manganese oxide delivered a capacity of 226 mA h g −1 at 0.1 A g −1 . [173]Freestanding amorphous MnO 2 @CNT foams have been introduced by Niu's group. [174]In their study, they stated the preparation of amorphous MnO 2 with different loading masses by using in situ deposited method (Figure 10a).The as-prepared amorphous electrode delivered a maximum specific capacity of up to 308.5 mA h g −1 (0.97 C, 1.0-1.8V) and 100% capacity retention after 1000 cycles at 32.5 C.Moreover, they found two different energy storage mechanisms in amorphous MnO 2 electrodes due to GCD profiles showing two discharge  and d) with carbon coating after cycling.Reproduced with permission. [131]Copyright 2021, American Chemical Society.e) TEM image of images of the as-milled LLZTO-4LiBH 4 sample.f) cycling performance of a LiCoO 2 ¦LLZTO-4LiBH 4 ¦Li full cell.The inset displays a digital photograph of an LED array powered by the full cell.Schematic illustration of contact and the Li + diffusion path between particles in the pellets of g) pristine LLZTO sample, with h) LiBH 4 coating layer.Reproduced with permission. [132]Copyright 2021, Wiley.i) Illustration of the detailed chemical configuration of amorphous CEI/SEI in a hybrid solid-liquid Li-metal battery.SEM images of the NCM622 cathode in j) DACP and k) the liquid cell after 100 cycles at 0.2 C. Reproduced with permission. [133]Copyright 2021, American Chemical Society.
Energy Environ.Mater.2023, 6, e12573 platforms (Figure 10b).Using ex situ XRD (Figure 10c) and XPS analysis (Figure 10d), it was demonstrated that the formation of (Zn (OH) 2 ) 3 (ZnSO 4 ) (H 2 O) 4 and zinc hydroxide due to the consumption of H + .Meanwhile, the XPS spectra of the full discharge electrode demonstrated that zinc ions existed in cathode materials, indicating that Zn + was inserted into the amorphous MnO 2 electrode.
Further, defect engineering enhances the zinc ion storage properties of amorphous manganese oxides, especially oxygen vacancies. [177,178]he oxygen vacancies can be used as active sites to adsorb the Zn 2+ , optimizing the properties of Zn ion storage.As a typical example, Tong et al. obtained the oxygen vacancy amorphous MnO 2 on the CNTs by a simple coprecipitation approach (Figure 10e), denoted as V o -MnO 2 / CNTs. [175]As shown in Figure 10f,g, the two peaks of Mn 2p 1/2 and Mn 2p 3/2 in V o -MnO 2 /CNT shifted to higher binding energy and increased peak intensity of oxygen vacancies in the O 1s spectrum, indicating the defective oxygen is increasing in V o -MnO 2 /CNTs electrode.The introduced oxygen vacancy can reduce the band gaps and enhance the electronic conductivity and the ion adsorption ability, resulting in a superior zinc storage capacity of 314 mA h g −1 at 0.2 A g −1 (Figure 10h).Meanwhile, the energy storage mechanism and structural evolution of amorphous MnO 2-δ with abundant oxygen defects have been demonstrated by Srinivasan's group. [179]The in situ XRD pattern indicated four reaction processes for amorphous MnO 2-δ .At stage I, the structure and composition of the amorphous electrode do not change, indicating that the insertion of cation will not generate a new phase and lead to the collapse of the electrode structure.The specific peaks of Zn 4 SO 4 (OH) 6 Á5H 2 O appeared and gradually became stronger at stage II (1.3-1.0V), suggesting the insertion of Zn 2+ .The charge process is in contrast with stage I and stage II.Meanwhile, ex situ XPS and Raman further demonstrated the combined insertion of H + and Zn 2+ with the enhanced pseudocapacitive contribution, revealing excellent cycling ability and rate performance.
Among various transition metal oxides, V-based oxides have attracted substantial attention due to their higher theoretically specific capacity.It has been confirmed that the zinc ion storage properties of vanadiumbased oxides are related to their degree of crystallinity and composite state.The de-/intercalation of Zn 2+ in crystalline vanadium oxides electrode is often accompanied by intrinsic sluggish kinetics, irreversible redox reactions, and structural collapse due to its large ion radius, resulting in inferior energy storage performances. [180]In contrast, amorphous vanadium-based compounds can tolerate serious volume expansion during charge-discharge processes, which benefits cycling stability.Moreover, an irregular internal ions arrangement provides more routes for ions diffusion and generates a high concentration of vacancies, which can optimize reaction kinetics and active sites of V-based materials, presenting infusive rate performance and specific capacity. [181]ng et al. employed a novelty in situ electrochemical induction method to prepare amorphous V 2 O 5 and carbon composites (a-V 2 O 5 @C) derived from V-MOF. [16]Compared with its crystalline counterpart, a-V 2 O 5 @C with isotropic Zn 2+ diffusion feature possessed more charge transfer routes, showing an excellent rate performance (72.8 mA h g −1 at an ultra-high current density of 200 A g −1 ).Moreover, first-principles calculations suggested that the a-V 2 O 5 @C electrode exhibited a lower Zn 2+ (de)intercalation energy relative to that of crystalline V 2 O 5 , which was conducive to the diffusion of Zn 2+ . [16]imultaneously and independently, Wang et al. developed an ionadsorption approach to fabricate amorphous vanadium pentoxide/graphene heterostructured electrode (A-V 2 O 5 /GO), which  and d) the XPS spectra of Zn at charged to 1 V. Reproduced with permission. [174]Copyright 2020, Elsevier.e) SEM image, f) Mn 2p spectral, g) O 1s spectral, and h) rate performance of V o -MnO 2 /CNTs.Reproduced with permission. [175]Copyright 2020, Wiley.i) The corresponding specific capacities and j) cycling performance of C-NiCoP//Zn, A-NiCoPO 4 //Zn, and C-NiCoP@A-NiCoPO 4 //Zn; and k) the intuitionistic schematic of the synergistic mechanism between C-NiCoP and A-NiCoPO 4 .Reproduced with permission. [13]Copyright 2021, The Royal Society of Chemistry.l) Ex situ XRD patterns corresponding to the curves of original, 1st, 2nd, 10th, and 20th full charged/discharged states at 0.1 A g −1 .TEM images with SAED patterns at (m) fully discharged state and (n) fully charged state of CaVO/CNTs electrode.Reproduced with permission. [176]Copyright 2022, Elsevier.
Energy Environ.Mater.2023, 6, e12573 guaranteed promising zinc ions storage performances, including a delightful specific capacity (489 mA h g −1 at 1 A g −1 ), remarkable rate capability (123 mA h g −1 at 20 A g −1 ). [182]Furthermore, the insertion of Zn 2+ was confirmed after being discharged to 0.2 V by ex situ XPS.And XRD and TEM after 5 cycles were performed to indicate that no crystalline V 2 O 5 component formed, revealing a highly stable structure of amorphous materials.
Constructing hierarchical electrodes combined with the merits of amorphous and crystal are prone to superior cycling stability and high specific capacity.The coexistence of order/disorder structure gives the presence of irregular bonds and breaks the space constraints of crystal electrodes, resulting in a more stable structure and open frameworks with more active sites.For example, Zhao et al. reported that the zinc ion storage ability is strongly influenced by the composition and structure of the electrode. [13]They compared the electrochemical properties of crystalline NiCoP (C-NiCoP), amorphous NiCoPO 4 (A-NiCoPO 4 ), and hierarchical crystalline NiCoP coated with amorphous NiCoPO 4 (C-NiCoP@NiCoPO 4 ), as shown in Figure 10i,j.Among the three samples, the heterostructure electrode that contained a crystalline Ni-Co phosphide core surrounded by an amorphous phosphate shell revealed the highest specific capacity of 350.6 mA h g −1 and ultra-stable cycling life with 92.6% retention after continuous 10 000 discharge-charge processes.Due to the short-range order and loose structure, the amorphous shell, in this case, facilitated the diffusion of OH − and buffered the volume vibration of crystalline NiCoP during charge storage processes.Furthermore, DFT calculation results indicated that the electrons at the interface were spontaneous transport from the NiCoP side to the NiCoPO 4 side, and the adsorption energies for OH − were lower to promote the adsorption of OH − on the electrode.The excellent electrochemical performances stem from the synergistic mechanism between the amorphous shell and crystalline core, as displayed in Figure 10k.
Crystalline electrodes can also generate the amorphous phase during charge-discharge processes in some cathodes, benefiting the structural stability and cycling performance in subsequent electrochemical reactions.Du et al. found that the CV curves of the first cycle and the next four cycles were not overlapped for the CaV 4 O 9 electrode, and the specific capacity of the initial discharge was lower than that of 5th cycle, indicating that there were irreversible reactions during the first charge-discharge processes. [176]Ex situ XRD (Figure 10l), ex situ XPS, and ex situ TEM (Figure 10m,n) were conducted to further validate the phase transition during electrochemical reactions.Combined with the results of XRD and XPS, they confirmed that the amorphous V 2 O 5 •nH 2 O was generated as a resultant phase to participate in the subsequent Zn ions storage, providing abundant ion diffusion channels and facilitating the rapid transport of ions/electrons.Due to the appearance of the amorphous phase, the as-prepared electrode delivered a high specific capacity of 209 mA h g −1 at 1 A g −1 even with the large loadings of 50 mg cm −2 .

Others Batteries
Beyond Zn ion batteries, many other multivalent ion batteries, such as magnesium ion batteries (MgIBs) and aluminum ion batteries (AlIBs), have been extensively investigated due to their high volumetric capacity and exemplary safety.For MgIBs, although many types of research have been applied to optimize the energy storage ability, few breakthroughs have been made in the field.Mizuno et al. were encouraged by the superior electrochemical reversibility of the MnO 2 amorphous mixture and proposed that amorphization could be a promising strategy to overcome the sluggish diffusion of Mg 2+ in cathode materials and enhance the structural durability of cathode frameworks during magnesium insertion/extraction. [183] They prepared amorphous V 2 O 5 -P 2 O 5 cathodes via planetary ball-milling, revealing the significantly improved electrochemical performance compared to the crystalline V 2 O 5 powder.Mortazavi et al. also demonstrated that amorphous Mn 2 Sn alloys were elastically softer than that crystalline Mg 2 Sn alloys by DFT calculation, suggesting the amorphous materials were more helpful in buffering the volume changes and preventing structural collapse. [184]Pan et al. introduced amorphous phase and grain refinement into spinel Mn 3 O 4 by pre-embedding of sodium ions, presenting an excellent rate performance of 33.2 mA h g −1 at 5 A g −1 . [185]The superior electrochemical performance could be attributed to the abundant ion diffusion channels generated by amorphous structure and pseudocapacitance contribution without any phase transformation.
As another promising rechargeable battery, aluminum ion batteries catch much attention because of their high volumetric capacity (8.04 Ah cm −3 is 4 times as high as LIBs), abundant resources, and ultra-stability in air and water.Due to trivalent cations, Al 3+ strongly interacts with the positive electrode, leading to a slow diffusion rate, lowering the rate performance, and cycling stability.Inspired by previous work, Chiku et al. investigated the potential application of amorphous V 2 O 5 /C composite cathode in aluminum batteries. [27]Their results indicated that there were reversible oxidation/reduction reactions of V 5+ ↔V 3+ /V 2+ for V 2 O 5 as a rechargeable Al batteries cathode, delivering a high specific capacity of 200 mA h g −1 at the current density of C/40.Recently, Lin et al. prepared amorphous anion-rich titanium polysulfides via high-energy ball-milling and verified the electrochemical performance of AlIBs. [186]The amorphous TiS 4 showed the best energy storage properties with 206 mA h g −1 after 1000 cycles.The flexible amorphous structure was conducive to releasing the strain due to the insertion of large Al ions and provided more storage sites and diffusion pathways, resulting in significant multivalent metalions storage.
The iron-based compounds are rather cheap and easy preparation, revealing great promise in commercialization for the large-scale energy storage system.During the past decades, iron-based alkaline batteries (Ni-Fe batteries, all iron batteries) based on multi-electron redox behaviors (Fe 0 , Fe 2+ , and Fe 3+ ) have been regarded as promising candidates for aqueous batteries. [187]There remain key issues and challenges on the way to achieving the commercial application of Fe-based batteries, which urgently need discussing to put forwards possible solutions.A massive of ferruginous anodes have been applied to achieve large capacity, such as FeO, Fe 2 O 3 , FeS, ZnFe 2 O 4 , and FeF 3 .However, a well-crystallized structure has difficulty in expanding or contracting, limiting the permeation and diffusion of ions.The amorphous counterpart is flexible that is conducive to accommodating more active ions, resulting in a high capacity and long lifespan device.Recently, amorphous ferruginous species is exposed as the active electrode for Febased aqueous batteries.For instance, amorphous FeOOH nanorods, [188] and amorphous Co-Fe-B nanosheets. [189]Xia's group first investigated the energy storage properties of amorphous FeOOH quantum dots/graphene nanofilm, exhibiting an outstanding cycling stability (89.7% capacitance retention after 20 000 cycles with the potential window of −0.8-0 V vs Ag/AgCl). [190]Sun et al. proposed that the electrochemical performance (including capacity can cycling lifespan) can be enhanced by inducing crystalline/amorphous interface. [191]ITT results indicated that the calculated D li value of amorphous@crystalline Fe 2 O 3 is almost one order of magnitude higher than that of Energy Environ.Mater.2023, 6, e12573 crystalline counterparts, suggesting the improved electrolyte ions diffusion rate and electrochemical reaction kinetics.The defective characteristics and disordered structure of amorphous coating relative to crystalline one can provide more percolation pathways for ions diffusion, indicating low-crystalline or amorphous phase materials has the potential to exhibit excellent electrochemical performance because of its high structural disorder.Overall, the design of amorphous electrodes with a loose structure and isotropy nature will open up a promising avenue for developing high-energy density multivalent metal-ion batteries.

Amorphous Materials for Li-S Batteries
Although incremental optimized strategies have made LIBs dominate the power market, from portable electronic devices to electric vehicles, the energy density of LIBs still falls far short of the future energy demand.197] Many efforts have been made to overcome these challenges, including replacing sulfur cathode with Li 2 S, encapsulating sulfur in a functionalized host, and employing solid-state electrolytes. [198]mong them, using host materials to immobilize sulfur cathode has been regarded as an effective way to address the LiPS shuttling problem, and substantial works have been devoted to exploring advanced host materials. [199,200]p to now, many materials have explored their potential as sulfur hosts, such as carbons (e.g., porous carbon, graphene, CNTs, carbon fibers, and carbon nitride), and metal compounds (e.g., metal oxides, sulfides, nitrides, and carbides).It is found that carbon hosts can provide a substantial electron transportation pathway to accelerate the sulfur redox kinetics and trap LiPSs through physical blocking/ adsorption. [201]Still, the physical interaction between nonpolar carbon and polysulfides is insufficient to anchor LiPSs during long-term cycling. [202,203]Accordingly, metal compounds can provide excellent chemical adsorption ability to effectively anchor LiPSs and catalyze the conversion reaction between soluble LiPSs and solid Li 2 S 2 /Li 2 S, efficiently prohibiting the shuttle effects and sulfur redox kinetics.Based on the state-of-the-art understanding of Li-S chemistry, a superior sulfur cathode should possess the following features: 1) high conductivity, 2) moderate LiPS adsorption/desorption ability, 3) excellent LiPS catalytic activity, and 4) chemical/electrochemical stable in electrolytes. [204,205]In this regard, amorphous host materials with unique disordered arranged structures, abundant defects, and substantial dangling bonds possess the potential to anchor polysulfides effectively.Li et al. introduced low-proportional Se into S/C composites to prepare amorphous S 1-x Se x /C cathodes. [206]It is well known that the conductivity of Se was better than that of S. Thus, it is a practical and feasible method to replace S with an appropriate proportion of Se to improve the electrical conductivity, which is consistent with the experiment results.Besides, Se-S bonds in S 1-x Se x /C composites could restrict the intermediate dissolution, leading to excellent cycling performance and rate properties.Guan et al. proposed the amorphous Co 2 B@graphene composite cathode for Li-S batteries. [207]Both Co and B could be regarded as active sites to anchor polysulfides, revealing superior cycling stability and rate performance.Combined with enhanced adsorption ability and high catalytic of metal compounds with excellent conductivity carbon networks, Yu et al. designed N-doped carbon/ amorphous MoS 3 hollow nano boxes (NC/MoS 3 NBs) for sulfur host materials. [208]By comparing the electrochemical properties and DFT results, they found that the nucleation barrier of NC/MoS 3 NBs (9.5 mV) and the energy barrier of conversion among various sulfides was lower than NC NBs (Figure 11a).As expected, the NC-S NBs cathode-produced Li-S batteries exhibited poor cycling performance due to insufficient ability to prohibit polysulfide shuttling.In contrast, the NC/MoS 3 NBs could accelerate the conversion between LiPSs and Li 2 S and suppress the shuttle effect due to the function of amorphous MoS 3 , achieving a Li-S battery with a high specific capacity and superior cycle ability (Figure 11b).
To uncover the effect of the crystallinity properties of the electrode on electrochemical performance, Sun et al. systematically investigated the energy storage behavior of amorphous CoP and crystalline CoP in Li-S batteries. [209]First, the adsorption ability with LiPSs, as shown in Figure 11c, illustrated that the amorphous structure was more conducive to the adsorb polysulfides than the crystalline structure.Second, the XPS analysis reflected a stronger interaction between rGO-CNT-CoP(A) and Li 2 S 6 (Figure 11d), revealing that the amorphous CoP was more conducive to anchor polysulfides due to the abundant dangling bonds.Third, during the Li 2 S nucleation experiments, a higher capacity and shorter time were obtained by rGO-CNT-CoP(A) than that of rGO-CNT-CoP(C) and rGO-CNT, suggesting the improved conversion from LiS 4 to LiS 2 and optimizing the electrochemical reaction kinetics, as displayed in Figure 11e,f.Similar conclusions were obtained by Xu's group.They employ amorphous V 2 O 5 and crystalline V 2 O 5 as separators for lithium-sulfur batteries. [212]Less resistance and superior electrochemical performance were achieved by the amorphous V 2 O 5 separator compared to the crystalline counterpart, indicating that the amorphous with disordered structure effectively facilitated ions/electrons diffusion, immobilizing polysulfides, and enhancing the catalytic ability of polysulfides conversion.Xia et al. also confirmed that the crystallization property strongly affects the performance of Li-S batteries. [213]This was contributed by the significantly reduced HOMO-LUMO gap of the aFeP-Li 2 S 4 system with a lower LUMO level and the enhanced electron exchange between Fe-S and P-S atoms.
Although many efforts have been made to confirm the advantages and energy storage mechanism of amorphous materials, crystalline materials should not be ignored due to their strong physical confinement to sulfur.Theoretically, rational design of the crystal-amorphous heterostructured materials to achieve multifunctional electrode integrated improved conductivity, lower catalytic barrier, and enhanced LiPSs diffusions were highly desirable and feasible.Yang et al. prepared MnO-TiO 2 heterostructure with core-shell nano-boxes structure. [214]he amorphous TiO 2 shell was conducive to dissolving polysulfide and anchoring the lithium polysulfide ions due to the strong interaction between TiO 2 and sulfur species.Moreover, the crystalline MnO in the core provides enhanced physical confinement and chemical adsorption, which acts on the polysulfides, resulting in long-term cycling stability (0.05% capacity loss per cycle) and rate performance (436 mA h g −1 at 5 C).Constructing heterostructure materials should effectively optimize the electrochemical energy storage properties in Li-S batteries.Different from the core-shell heterostructure prepared by Yang's group, Qin et al. proposed an island-like heterostructure consisting of amorphous MoO 3 and crystalline Mo 2 C, as shown in Figure 11g. [210]RTEM image indicated that the amorphous MoO 3 was randomly Energy Environ.Mater.2023, 6, e12573 distributed in the crystalline matrix to form the island-like heterostructured electrode.Relative to core-shell structure and hierarchical structure, the island-like heterostructure tended to construct more heterointerface, which was crucial for the performance of the heterostructure electrode.In general, abundant interfaces promoted charge redistribution.Compared with crystalline MoO 3 , amorphous MoO 3 with a lower diffusion barrier helped transport and convert LiPSs (Figure 11h,j).Combined with the crystalline Mo 2 C component, both the high conductivity and catalytic activity could be realized in an integrated electrode, resulting in the smooth traping-diffusion-conversion of LiPSs toward superior Li-S batteries.DFT simulations (Figure 11j) were carried out to confirm the synergistic effect between amorphous MoO 3 and crystalline Mo 2 C. The excellent electrochemical performance could be attributed to 1) the immobilization LiPSs ability of amorphous MoO 3 was stronger than that of crystalline MoO 3 and Mo 2 C, suggesting that the as-prepared heterostructure presented improved anchoring capability relative to the one-component Mo 2 C electrode.2) The lower Li 2 S 6 diffusion barrier on amorphous MoO 3 (0.40 eV) as compared to crystalline MoO 3 (0.52 eV), evidencing that the amorphous structure was conducive to the diffusion of Li 2 S 6 and improved reaction kinetics.3) The Gibbs free energy for Li 2 S 4 to Li 2 S reduction on the surface of Mo 2 C is 0.98 eV, which was much lower than amorphous MoO 3 (1.32 eV) and crystalline MoO 3 (2.11eV), revealing the best catalytic activity of Mo 2 C component.Based on the excellent anchoring ability of amorphous MoO 3 and the high catalytic activity of crystalline Mo 2 C, the heterostructured electrode delivered a significant reversible capacity of 1500 mA h g −1 and a rate performance of 870 mA h g −1 at 5 C.
Beyond employing an advanced host to trap LiPSs, constructing an all-solid-state Li-S battery is also a reasonable way to cope with the Reproduced with permission. [208]Copyright 2020, Wiley.c) Ultraviolet/visible absorption spectra of the Li 2 S 6 electrolyte before and after contact with rGO-CNT, rGO-CNT-CoP(C), and rGO-CNT-CoP(A).Inset: graphs of Li 2 S 6 solutions with different absorbers.d) Co 2p3/2 XPS spectra of rGO-CNT-CoP(A) and rGO-CNT-CoP(C).Illustration of the catalytic conversion of LiPSs on e) amorphous CoP and f) crystalline CoP.Reproduced with permission. [209]Copyright 2021, American Chemical Society.g) HRTEM image of Mo 2 C-MoO 3 .Schematics of LiPSs anchoring-diffusion-conversion processes on h) crystalline Mo 2 C-crystalline MoO 3 and i) crystalline Mo 2 Camorphous MoO 3 binary heterostructure surfaces, respectively.j) Binding energies of LiPSs on crystalline MoO 3 , Mo 2 C, and amorphous MoO 3 .Reproduced with permission. [210]Copyright 2022, Elsevier k) SEM image of the amorphous rGO@S-40 composite.l) XRD pattern of the amorphous rGO@S-40, rGO, and sublimed sulfur.m) Schematic diagram of an all-solid-state lithium-sulfur battery.n) Cycling performances of amorphous rGO@S-40, amorphous rGO@S-50, and crystal rGO@S-40 under 0.5 C at 60 °C.Reproduced with permission. [211]Copyright 2017, Wiley.
Energy Environ.Mater.2023, 6, e12573 dendrite problem and LiPS shuttling.Unfortunately, new issues are brought by the incompatibility between the solid-solid interface of electrolyte and cathode, resulting in severe stress, larger resistance, and sluggish diffusion kinetics.Yao et al. designed an internal pathway in an amorphous rGO@S cathode (Figure 11k) for the diffusion of ionic and electronic by a homogeneous distribution of the rGO@S active materials into the Li 10 GeP 2 S 12 solid-state electrolyte, as shown in Figure 11l,m. [211]The effects of sulfur mass-loading and crystal structure on energy storage performance have been investigated (Figure 11n).The amorphous rGO@S-40 delivered the highest specific capacity among various samples.The crystal sulfur tended to increase the thickness of the composite electrode, and the excessive sulfur mass-loading decreased the electronic and ionic conductivity.

Amorphous Materials for Supercapacitors
The utilization of SCs is impeded by insufficient energy density.One effective way to improve energy density is to regulate the crystal structure and design the micro-morphology of active materials.Numerous materials applied to SCs have demonstrated that amorphous electrode materials reveal some advantageous features that can greatly improve the double-layer capacitance and widen the work potential range and structural stability. [215,216]For example, an amorphous MnO 2 electrode has been reported, which exhibits excellent energy storage properties over the crystalline one. [217]Amorphous V 2 O 5 can act as a promising SCs electrode in neutral KCl electrolyte. [23]Besides metal oxides, some amorphous sulfides, hydroxides, and carbonaceous materials also show ideal SC behavior.This section mainly discusses amorphous electrodes for SCs, which are crucial for developing high energy density devices.
Beyond carbonaceous actives materials, amorphous metal oxides, most belonging to pseudocapacitance electrode materials, open up a way to develop SCs.However, their limited intrinsic conductivities incur high electrochemical resistances and poor rate performances, hindering further improvements in energy storage capability.Combining carbon-based materials to construct a heterostructured electrode is an efficient and facile strategy to resolve the above issues.The interface generated by the heterostructure electrode can improve the electrons/ ions diffusion kinetics, accelerate charge transfer and enhance the reversible redox reaction during the charge-discharge process. [218]For example, Wu et al. employed a facile excessive metal-ion-induced selfassembly method to uniformly and densely anchor amorphous Fe 2 O 3 nanodots on graphene (a-Fe 2 O 3 /RGO). [219]Compared with crystalline Fe 2 O 3 /RGO (c-Fe 2 O 3 /RGO), the amorphous material provided more active sites and interfacial areas for electrolyte ions to be intercalated/ de-intercalated.Moreover, the unique porous network structure was suitable for electrons/ion transfer and electrochemical reactions.The a-Fe 2 O 3 /RGO electrode revealed a high specific capacity of 347.4 F g −1 at 1 A g −1 .Even at a large current density of 10 A g −1 , a capacitance of 184 F g −1 could still obtain, much higher than that of c-Fe 2 O 3 /RGO and Fe 2 O 3 electrodes.In addition, Meng et al. prepared an amorphous nickel sulfides/N-doped graphene nanotubes integrated composites via the electrochemical-deposition method, which exhibited excellent energy storage performance, including a significant capacitance of 2160 F g −1 at 6 A g −1 , prominent capacity retention of 95.8% after continuous charge-discharge for 12 000 cycles, and outstanding rate performance of 1650 F g −1 at 40 A g −1 . [220]Subsequently, Kim et al. reported an amorphous cobalt phosphate/carbon hybrid through a one-step carbonization strategy. [221]Their results indicated that the amorphous structure of the materials had no connection with the massloading, but it does depend on the concentration of cobalt salt.Benefiting from the amorphous nature and the involvement of carbon materials, the hybrid electrode delivered an impressive reversible areal capacitance of 2.15 F cm −2 at 4 mA cm −2 with delightful cycling stability (95% after 10 000 cycles).Besides, many kinds of carbonaceous materials have been applied to the amorphous compounds/carbon hybrid electrode, such as multiwalled carbon nanotubes, [217] amorphous carbon, [222] reduced graphene oxides, [223] and metal-organic frameworks derived carbon [224] resulting in ameliorating electrochemical and structural stability.Moreover, metal nitrides with high conductivity are an excellent choice for growing amorphous metal oxides.Typically, Tu et al. designed a NiMnO x /TiN/CC electrode with a unique branch-leaf structure (Figure 12a,b).Benefiting from the high conductive TiN nanowires frame and the branch-leaf structure, a high mass-loading (10.12 mg cm −2 ) electrode could be obtained, and it delivered a significant areal capacitance of 1224.14 mF cm −2 at 5 mA cm −2 , as shown in Figure 12c.The presence of the TiN nanowires and branch-leaf structure could reduce the resistance of the electrode, restrain the stacking of NiMnO x nanosheets and provide a highspeed charge transport channel. [225]he electrochemical performance can be further enhanced by combining amorphous structure with defect engineering, including vacancies and heteroatom doping.An attractive superiority of amorphous materials is that they can generate more unsaturated dangling bonds with a high concentration of non-metallic vacancies, such as oxygen vacancy, sulfur vacancy, and phosphorus vacancy. [199]Thus, the dualenhancement mechanism can be achieved, that is, more active sites and improved electrical conductivity.Based on DFT calculations, Chen et al. demonstrated that amorphous V 2 O 5 nanosheets (Figure 12d) transited from semiconductor to metallicity by generating a high concentration of oxygen vacancies, as shown in Figure 12e. [226]Besides, the lower ion diffusion energy barriers of defective V 2 O 5 are 0.25 and 0.32 eV, which is beneficial to the insertion of electrolytes, resulting in a high specific capacitance and rate performance.It has been claimed that the P atom is difficult to dope in transition metal sulfides at high concentrations due to its large radius. [229,230]However, Zhao et al. reported a CoS doped with a high P concentration (as high as 18 at%) as the active electrode for SCs. [227]They used hydrothermal by controlling the reaction time to synthesize amorphous shells on the surface of the crystalline CoS core, as shown in Figure 12f,g.They found that the doped P was mainly distributed on the amorphous CoS layer, demonstrating that the amorphous layer was conducive to incorporating heteroatoms.The possible reason may be related to the disordered and defective features of the amorphous phase that could release the strains caused by doping, making introducing large radius heteroatoms easier.The Pdoped CoS electrode displayed an improved specific capacity of 536 C g −1 at 5 A g −1 and optimized rate performance of 502 C g −1 at 20 A g −1 (Figure 12h,i).
Similar to crystalline materials, structural water has a positive effect on the electrochemical performance of amorphous electrodes, delivering less resistance for ions intercalation and keeping structure stability during the charge-discharge process.Lu 0 s group designed flower-like amorphous cobalt hydrogen phosphate (ACHP) by a harmless and environmental method, as shown in Figure 12j,k. [228]To investigate the function of structural water, various ACHP electrode with different treatment temperature was prepared, and the SC's performances were studied.As shown in Figure 12l, the ACHP-400 and ACHP-500 revealed low capacitance due to the escape of structural water at high Energy Environ.Mater.2023, 6, e12573 temperatures.In contrast, ACHP showed excellent cycling stability of 97.6% capacitance retention after 10 000 continuous charge-discharge processes (Figure 12m).
In addition to the amorphous transition metal compounds, metalorganic frameworks (MOFs), constructed by metal ions and organic ligands through coordination bonds, have also been regarded as potential candidates for SC electrodes.Numerous kinds of research focused on crystalline MOFs (cMOFs) show superior SC energy storage performance.Thus, it is reasonable to deduce that excellent SC properties can also be obtained using amorphous MOFs (aMOFs) electrodes.cMOFs can be easily converted into aMOFs by applying external pressure, high temperature, or ball milling.Yang et al. synthesized amorphous UiO-66 MOFs via a facile solution method, and the asprepared amorphous MOFs revealed excellent life-span due to the metastable feature of the glassy state and high specific capacitance due to more active sites provided by disorder structure. [231]Subsequently, Hu et al. designed a crystalline ZIF-67@amorphous ZIF core-shell for a high-performance SC. [232] The amorphous coating layer with a disordered structure provided more storage sites, which was conducive to reversibility and optimizing the reaction kinetics.Moreover, the amorphous surface was easier to contact with the electrolyte, facilitating electrolyte infiltration and improving the utilization rate of active sites.The previous amorphous materials works in SCs were summarized in Table 2.  [225] Copyright 2021, The Royal Society of Chemistry.d) SEM images and e) schematic diagram for regulating the V 3d band edge with high electrical conductivity by defect engineering of H-VO x -500.Reproduced with permission. [226]Copyright 2021, Elsevier.f) SEM image and g) TEM image of P-CoS 1.097 -6.h) The current density-dependent specific capacities and i) cycling performance at the current density of 10 A g −1 , of Co 1−x S-6, P-CoS 1.097 -6, CoS 1.097 -48, and P-CoS 1.097 -48.Reproduced with permission. [227]Copyright 2021, Elsevier.j) SEM image and k) TEM image of ACHP electrode.l) The plots of specific capacitance for ACHP, ACHP-200, ACHP-400, and ACHP-500 electrodes under various current densities.m) Cycling behavior of ACHP electrode at 2 A g −1 .Reproduced with permission. [228]Copyright 2019, Elsevier.
Energy Environ.Mater.2023, 6, e12573 far from the actual structure due to defects and disordered domains.Thus, it is hard to predict the position of every atom and the number of near atoms in the amorphous materials based on the geometrical crystallography theory.That is, the atomic arrangements in amorphous materials remain mysterious at present.So far, the amorphous structure can be obtained via ab initio molecular dynamics methods.The obtained amorphous structure is random and inaccurate.Hence, the state-of-the-art theoretical calculation and analog simulation cannot efficiently illustrate what is going on inside amorphous materials, resulting in an inadequate understanding of mechanisms and electrochemical reactions.Although many notable contributions have been made to clarify atoms' packing in amorphous, the ideal models to describe the atomic arrangements are still lacking due to the extensive structure distribution caused by the random packing of atoms.The statistic, theoretical calculation, and high-resolution imaging technology may pave the way for confirming the crystalline structure of amorphous materials.

Synthesis Method Improvement and Innovation
To date, many methods have been employed to fabricate amorphous materials, including plasma, hydrothermal, sonication, and so on.Most of them are hard to commercialize.Although ball-milling is a promising approach that is expected to achieve low-cost and large-scale production, the morphology of as-prepared electrode materials is irregular and hard to control due to the strong destructive power of the ballmilling technique.So far, all synthesis methods cannot allow controlling the internal crystalline structure to form an ideal amorphous structure, which is crucial to the energy storage performance and achieving precise theoretical calculation.Moreover, the influence of synthesis methods on the electrochemical activity of amorphous electrodes is still under the cover.Thus, the development of innovative and modified synthesis methods that achieve precise control over micro-morphology and atomic arrangement will undoubtedly provide the opportunity to optimize the properties and overcome the production constraints, realizing the practical application of amorphous materials in energy storage fields.

Improving Energy Storage Performance
Many efforts have been done to improve the energy storage performance of amorphous electrodes, such as constructing heterostructure (amorphous/crystal composites and hybrid with carbon), defects engineering (vacancies and heteroatoms doping), and morphological control (3D architecture and 2D ultra-thin nanosheets).There is no doubt that the abovementioned strategies are promising methods to enhance electrochemical performance, however, the cost is increasing and the large scale is hard to achieve.The development of new amorphous materials with excellent cycling stability and high capacity is necessary but a challenge.The energy density of an energy storage device greatly depends on the proportion of active materials.Thus, the high mass-loading of active materials is a challenge that must be overcome to achieve commercialization.For example, the areal capacity of commercial LIBs is up to 4 mAh cm −1 even at the high mass-loading of 10 mg cm −2 .Currently, the active mass loading in the laboratory is <5 mg cm −2 (even lower than 2 mg cm −2 for arrays electrode), indicating that there is a challenge for further enhancement in optimizing the amorphous materials for energy storage.

Updating and Optimizing the Energy Storage Mechanism
The phase transformation and coordination environment variation of crystalline electrodes are easier to explore due to the fixed/regular atoms arrangement and distinguishable XRD/TEM signals, which provide powerful evidence for electrochemical reaction processes.Although many signs of progress have been made in the amorphous electrode for energy storage, the coordination environment is very different for each atom, and it is difficult to discover the effect of the transformation on electrochemical behavior by employing existing characterization techniques.Thus, it is a priority to illuminate the intrinsic structure and atoms arrangement by developing advanced simulation technology and statistics.Moreover, the precise structural evolution of amorphous electrodes/electrolytes is not clear enough due to their irregular and nonstandard molecular structure.Some optimized and enhanced mechanisms have been proposed.Nevertheless, the electrochemical reaction and energy storage process in the interface of amorphous coating and amorphous-crystalline heterostructure is still unclear.Combined with advanced operando characterization techniques Energy Environ.Mater.2023, 6, e12573 with a higher spatial and temporal resolution to explore the evolution of the interface is necessary but challenging.
All in all, the development and research of amorphous materials still face many challenges, but the unique physical-chemical properties of amorphous materials are desirable in energy storage fields.We hope this review can provide insight into the state-of-the-art amorphous materials and push forward the development of these attractive materials toward practical applications in electrochemical energy storage and conversion.

Figure 2 .
Figure 2. The representative examples and timeline of amorphous materials in energy storage devices.Reproduced with permission from ref.[16,20-28]

Figure 3 .
Figure3.The fundamental properties difference between amorphous materials and their crystal counterparts.Reproduced with permission from ref.[31-33]

Figure 4 .
Figure 4. Irreversible phase transformation and structure reconstruction of active materials.

Figure 5 .
Figure 5.The typical synthetic methods of amorphous materials.

Figure 7 .
Figure 7.The optimized geometry structures and the insertion of Na + models of a) crystalline Fe 2 O 3 and b) amorphous Fe 2 O 3 .Reproduced with permission.[61]Copyright 2018, Elsevier.c) Schematic illustration of the synthesis route of 2D aMoO 3−x @MXene non-vdW heterostructures.d) Illustration of facile capacitor-like interlayer diffusion and diffusion-controlled intralayer diffusion.Reproduced with permission.[93]Copyright 2021, Elsevier.e) HRTEM images of R-TiO 2-x -S.Cyclic voltammograms (CVs) for the first 3 cycles of f) R-TiO 2 and g) R-TiO 2-x at a scan rate of 0.2 mV s −1 .h) Cycling performances of all the TiO 2 electrodes at 50 mA g −1 .Reproduced with permission.[94]Copyright 2018, Wiley.

Figure 9 .
Figure 9. a) The smooth surface of electrolyte without carbon coating.b) SEM image of the carbon coating.The schematic shows the Li deposition c) without carbon coating andd) with carbon coating after cycling.Reproduced with permission.[131]Copyright 2021, American Chemical Society.e) TEM image of images of the as-milled LLZTO-4LiBH 4 sample.f) cycling performance of a LiCoO 2 ¦LLZTO-4LiBH 4 ¦Li full cell.The inset displays a digital photograph of an LED array powered by the full cell.Schematic illustration of contact and the Li + diffusion path between particles in the pellets of g) pristine LLZTO sample, with h) LiBH 4 coating layer.Reproduced with permission.[132]Copyright 2021, Wiley.i) Illustration of the detailed chemical configuration of amorphous CEI/SEI in a hybrid solid-liquid Li-metal battery.SEM images of the NCM622 cathode in j) DACP and k) the liquid cell after 100 cycles at 0.2 C. Reproduced with permission.[133]Copyright 2021, American Chemical Society.

Figure 10 .
Figure 10.a) SEM image, b) GCD curves, c) in situ XRD of MnO 2 @CNT electrode, and d) the XPS spectra of Zn at charged to 1 V. Reproduced with permission.[174]Copyright 2020, Elsevier.e) SEM image, f) Mn 2p spectral, g) O 1s spectral, and h) rate performance of V o -MnO 2 /CNTs.Reproduced with permission.[175]Copyright 2020, Wiley.i) The corresponding specific capacities and j) cycling performance of C-NiCoP//Zn, A-NiCoPO 4 //Zn, and C-NiCoP@A-NiCoPO 4 //Zn; and k) the intuitionistic schematic of the synergistic mechanism between C-NiCoP and A-NiCoPO 4 .Reproduced with permission.[13]Copyright 2021, The Royal Society of Chemistry.l) Ex situ XRD patterns corresponding to the curves of original, 1st, 2nd, 10th, and 20th full charged/discharged states at 0.1 A g −1 .TEM images with SAED patterns at (m) fully discharged state and (n) fully charged state of CaVO/CNTs electrode.Reproduced with permission.[176]Copyright 2022, Elsevier.

Figure 12 .
Figure 12. a) Schematic diagram of the synthesis mechanism of NiMnO x /CC, NiMn 2 O 4 /CC, and NiMnO x /TiN/CC electrode materials and b) illustration of the branch-leaf structure.c) Areal capacitances of NiMnO x /TiN/CC electrodes as a function of current density under different mass loadings.Reproduced with permission.[225]Copyright 2021, The Royal Society of Chemistry.d) SEM images and e) schematic diagram for regulating the V 3d band edge with high electrical conductivity by defect engineering of H-VO x -500.Reproduced with permission.[226]Copyright 2021, Elsevier.f) SEM image and g) TEM image of P-CoS 1.097 -6.h) The current density-dependent specific capacities and i) cycling performance at the current density of 10 A g −1 , of Co 1−x S-6, P-CoS 1.097 -6, CoS 1.097 -48, and P-CoS 1.097 -48.Reproduced with permission.[227]Copyright 2021, Elsevier.j) SEM image and k) TEM image of ACHP electrode.l) The plots of specific capacitance for ACHP, ACHP-200, ACHP-400, and ACHP-500 electrodes under various current densities.m) Cycling behavior of ACHP electrode at 2 A g −1 .Reproduced with permission.[228]Copyright 2019, Elsevier.

Figure 13 .
Figure 13.The challenges of amorphous materials in energy storage fields.

Table 1 .
The electrochemical performance of previous amorphous materials in alkali metal ion batteries.