Field‐assisted electrocatalysts spark sulfur redox kinetics: From fundamentals to applications

The chief culprit impeding the commercialization of lithium–sulfur (Li–S) batteries is the parasitic shuttle effect and restricted redox kinetics of lithium polysulfides (LiPSs). To circumvent these key stumbling blocks, incorporating electrocatalysts with rational electronic structure modulation into sulfur cathode plays a decisive role in vitalizing the higher electrocatalytic activity to promote sulfur utilization efficiency. Breaking the stereotype of contemporary electrocatalyst design kept on pretreatment, field‐assisted electrocatalysts offer strategic advantages in dynamically controllable electrochemical reactions that might be thorny to regulate in conventional electrochemical processes. However, the highly interdisciplinary field‐assisted electrochemistry puzzles researchers for a fundamental understanding of the ambiguous correlations among electronic structure, surface adsorption properties, and catalytic performance. In this review, the mechanisms, functionality explorations, and advantages of field‐assisted electrocatalysts including electric, magnetic, light, thermal, and strain fields in Li–S batteries have been summarized. By demonstrating pioneering work for customized geometric configuration, energy band engineering, and optimal microenvironment arrangement in response to decreased activation energy and enriched reactant concentration for accelerated sulfur redox kinetics, cutting‐edge insights into the holistic periscope of charge‐spin‐orbital‐lattice interplay between LiPSs and electrocatalysts are scrutinized, which aspires to advance the comprehensive understanding of the complex electrochemistry of Li–S batteries. Finally, future perspectives are provided to inspire innovations capable of defeating existing restrictions.


| INTRODUCTION
The contradiction between the deepening global energy crisis and ever-growing demands for energy consumption has spurred the development of sustainable energy technologies. To be concrete, the prevalence of electric vehicles and the implementation of wind and solar energies are driving an upgrade in high-performance energy storage systems. [1,2] Among the alternatives, lithium-sulfur (Li-S) batteries, as one of the core battery technologies of the post-lithium-ion batteries era, have showcased attractive application prospects since their emergence in the 1960s, abounded in gifts of high theoretical energy density (2600 Wh/kg), environmental benignity, and low cost. [3][4][5] Despite the overwhelming merits mentioned above, the commercialization of Li-S batteries remains inextricable plights, which is plagued by the sulfur cathode's own set of technical drawbacks, mainly pertaining to the parasitic shuttle effect and restricted redox kinetics of lithium polysulfides (LiPSs). [6,7] To circumvent the aforementioned dilemma, oriented by the rational electronic structure modulation of electrocatalysts, research activities on incorporating multifunctional electrocatalysts into sulfur cathode to prolong cyclic durability and bolster sulfur utilization efficiency of Li-S batteries have recently surged. [8] On the basis of multitudinous explorations of transition metal compounds, [9] burgeoning strategies including defect manipulation, facet regulation, interface modulation, singleatom customization, and alloy configuration vitalize the higher electrocatalytic activity, thereby paving a promising avenue toward practical applications of Li-S batteries. [10][11][12][13] Whilst the present research on electronic structure modulation of electrocatalysts mainstreams the exquisite control of pretreatment parameters and artificial architectures, their capability to accelerate the sulfur redox kinetics is constrained by their intrinsic electrocatalytic properties. [14][15][16][17] Therefore, to enhance the comprehensive electrocatalytic performance toward sulfur redox, field-assisted electrocatalysts might exhibit considerable breakthroughs in multiplicity and functionalization, which require equivalent efforts to exploit. In particular, advanced electrocatalytic systems integrated with multiple fields offer many innately strategic advantages in quantitatively, precisely, self-regulation, and dynamically controllable electrochemical reactions that might be thorny to regulate in conventional electrochemical processes. [18][19][20] For instance, the introduction of an external or built-in electric field (EF) to manipulate interfacial electron transport or LiPSs migration in a predictable manner brings new twists to Li-S batteries at this nascent stage. [21,22] Admittedly, concomitant with complicated LiPSs conversion, research on field-assisted electrocatalysts is highly interdisciplinary. With the preliminary explorations of electrochemistry coupled with multiphysics fields, the mechanism elucidation for field-triggered conversion provides a fresh platform for decoding the electrocatalytic origin of stepwise/ bidirectional conversion of LiPSs, while the integrated electrocatalytic system referring to electrochemistry, hydrodynamics, thermodynamics, magnetism, and light put forward strict requirements on component/device customization, which jointly pushes advances in electrochemistry and triggers technology innovations. Therefore, it is of paramount significance and highly anticipated to comprehensively understand the functions of fields in facilitating sulfur redox kinetics.
In this review, the pioneering work of electrocatalysis coupled with electric, magnetic, light, thermal, and strain fields in Li-S batteries is systematically investigated ( Figure 1). Meanwhile, various tactics for customized geometric arrangements, component engineering, and device technologies to couple the specific field conditions are interspersed. Subsequently, profound insights into structure-property relationships for the charge-spinorbital-lattice interplay between LiPSs and electrocatalysts are scrutinized to demystify the activity origin of enhanced electrocatalysis at the atomic scale. Finally, future guidance and perspectives of field-assisted electrocatalysts for developing practical Li-S batteries are presented. We hope that the new research prospects in field-assisted electrocatalysts will inspire more innovations for the flexible manipulation of electronic properties and rational design of elaborate structure.

| The principle of field-assisted electrocatalysis
Although different field-assisted electrocatalysts have respective mechanisms and advantages, they all substantially affect the sulfur redox kinetics. Generally, electrochemical kinetics depend on the nature of the electrocatalysts and the localized concentration of the reactants at the electrocatalytic interface. [23] The simplified kinetic equation can be described as follows: LiPSs Li 2 B where r is the reaction rate, A is the pre-exponential factor, E a is the activation energy, k B is the Boltzmann constant, T is the temperature, C LiPSs and C Li correspond to the respective concentration, α is the transfer coefficient, e is the elementary charge, and η is the overpotential. The introduction of fields shows a prominent effect on manipulating interfacial ion/electron motion and intrinsic geometrical-site-dependent reactivity. [18,24,25] The former could enhance mass transfer to realize the enrichment of reactants (C LiPSs and C Li ), while the latter could regulate the intermediate configuration or mediate the thermodynamic pathways to reduce E a . Additionally, the redox kinetics could be dynamically regulated by changing the field conditions, such as direction, magnitude, and action objects, to precisely dictate the multistep reactions of Li-S batteries. [26] Classified by the origin of fields and resulting electrocatalytic mechanisms, the typical representatives for field-assisted electrocatalysts emerging in Li-S batteries are introduced in sequence.

| EF-assisted electrocatalysis
In an electrochemical system, EF exists anywhere. The applications of EF are categorized into four main aspects in the subsection, with a focus on the respective strategies for charge transfer enhancement.

| Ion spatial motion behaviors modulated by an external EF
An EF is spontaneously formed between the positive and negative electrodes, induced by the directional migration of cations and anions in electrolytes. During the discharge process of Li-S batteries, EF points from the Li anode to the sulfur cathode. Such a driving force simultaneously impels the parallel migration of positively charged Li + in the same direction. Generally, it is regarded that the Li + migration rate in the liquid electrolytes is so sufficient that it is not a rate-limiting step for the sulfur reduction process. Despite no room for further improvements in the Li + migration rate, less attention has been paid to the potential advantages of EF in regulating ion spatial motion behaviors, yet it might be promising.
To this end, the dynamic polarization stemming from the EF in electrolytes results in a spatial progression of isolated Li between electrodes, thereby realizing highly reversible Li stripping/deposition, as first demonstrated by Cui's group (Figure 2A). [28] Such reformation of activating isolated Li by EF liberates us from the commonly accepted presumption that isolated Li is supposed to be completely dead. In retrospect of the notorious shuttle effect of LiPSs, where EF drives the reduced short-chain polysulfides from the anode to the cathode during the charge process, [29,30] it is anticipated that EF is highly responsive to negatively charged polysulfides analogous to positively charged Li + , making it a possible brand-new paradigm for optimizing battery operation conditions such as EF orientation and intensity to tame the shuttle effect of LiPSs. Notwithstanding the ambiguous existing form of LiPSs in electrolytes, [31][32][33][34][35] the external EF effect also manifests the potential for sufficiently manipulating the motion behaviors of ionic additives. [36,37] On top of that, on-site electrostatic polarization induced by an external EF offers new opportunities to controllably adjust the frontier orbitals of reactant intermediates, thus significantly modifying the E a or reaction pathways during the electrocatalytic process. [38][39][40][41] 2.2.2 | Aggregation of targeted reactants induced by a localized EF Once Li + directly migrates to the sulfur cathode, the localized EF distribution near the reaction surface determines the concentration and generation state of reactants. [42][43][44] Concretely, to afford highly efficient  [22] Copyright 2021, Wiley-VCH. (F) In-situ visual confirmation of LiPSs entrapment at specific discharge depths for S + BTO cathode and corresponding schematic illustration of the ferroelectric-enhanced adsorption mechanism. Reproduced with permission. [27] Copyright 2017, Wiley-VCH. sulfur redox kinetics at the triple-phase interface among electrocatalysts, conductive networks, and infiltrative electrolytes, the geometrical morphology of electrocatalysts plays a pivotal role in regulating the surface EF distribution. [45] As a representative study proposed by Sargent et al., the detailed simulation associated with persuasive experimental observation confirms the universality of the tip-enhanced localized EF effect in needle-like electrocatalysts, where a high-curvature structure is known to concentrate EF and cation concentration, showing spatially specified electrocatalytic capability appreciably ( Figure 2B). [42] Impressed by the prevailing synthesis of hierarchical electrocatalysts, [46][47][48] the preembedded high-curvature domains in skeletons not only endow many transport pathways for Li + but also render ample electrocatalytic sites with customized distribution, enabling the flexible manipulation of space-specific sulfur redox.
Similarly, Cui and colleagues reported that the nonuniform distribution of localized EF might also exist on two-dimensional (2D) flakes ( Figure 2C). [43] They deserve credit for identifying distinct sulfur growth behaviors, namely, liquid droplets grown on the basal plane of flakes and solid crystals growing from the edges of thick flakes. Such a phenomenon is partially accredited to the strongly localized partial charges and an enhanced EF at the edges of MoS 2 flakes, which facilitate localized sulfur supersaturation and drive the wetting of the liquid sulfur droplets to synergistically promote the crystallization of sulfur at the edges. As indicated in the resultant performance, controlling the growth of liquid sulfur on 2D materials by the EF effect sheds light on the adjustable form of sulfur redox, which helps tremendously advance the field of low-temperature or fast-charging batteries. [49,50] Of note, in parallel with an elaborate sculpture for space-specific active sites induced by local highconcentration EF, Lu and co-workers did the opposite to construct a highly uniform EF distribution for fluent ionic flux in expectation. [51] By employing a highly ordered one-dimensional nanoarray electrode with a uniform gap smaller than 0.05 μm, a dense and flat surface for unifying the EF was attained, which promotes the formation of an equipotential surface to inhibit charge accumulation.

| Interfacial charge transfer elevated by a built-in EF
To smoothly bolster the adsorption-diffusion-conversion process of LiPSs at the interface of the heterostructure, [15,52] the formed built-in EF is usually intertwined with the charge redistribution caused by spontaneous electron transport at the interface, exerting an additional degree of freedom to tune the interfacial electron density, manipulate the charge transfer, and modulate the targeted ion adsorption. [25,[53][54][55] As a representative, when p-type (a semiconductor with holes as majority carriers) and n-type (a semiconductor with electrons as majority carriers) semiconductors have a favorable lattice match and different Fermi energy levels (E F ), the electrons and holes would reversely diffuse and cross the interface until the two levels are aligned, thus generating a built-in EF ( Figure 2D). [25,56] In concomitance with favored interfacial chargetransfer kinetics, the anisotropic characteristics are empowered to different components to fully ignite the consecutive adsorption-diffusion-catalysis process of LiPSs on the interface of heterojunctions. [22,57] For instance, established on a strong donor-acceptor interaction between MoS 2 with good electrocatalytic activity and MoO 3 with strong adsorption ability, the derived built-in EF is capable of bridging the adsorption and electrocatalytic sites simultaneously to boost the interfacial charge transfer, which could synergistically impulse the sulfur redox. [57] As a result, the Li−S batteries assembled with the MoS 2 -MoO 3 /CS-modified separator deliver superior cycling stability with only 0.0135% capacity decay per cycle after 600 cycles at 1 C. Escorted by the well-known promotion of interfacial charge transfer, the electronic redistribution and agminated active sites located at the interface of p-n junctions are critical to strengthen the adsorption ability of LiPSs and reduce the overpotential of Li 2 S nucleation as well. [58,59] Notably, the concept of spontaneous formation of built-in EF could be further extended to Mott-Schottky, p-p, and n-n junctions with a semiconductive substance, and thereinto, the applicability of Mott-Schottky electrocatalysts in the Li-S realm has been continually testified. [60,61] Identically, to diminish the insurmountable kinetic energy barrier required for LiPSs to cross the grain boundaries inside the heterostructure, [62] a built-in EF was recruited to directionally propel the spatially stepwise sulfur redox on the surface of different components, as first proposed by our group ( Figure 2E). [22] To be specific, the built-in EF forming at the compact interface between p-type Co 3 O 4 and ntype TiO 2 induces directional migration of negatively charged polysulfides, and therefore, stepwise sulfur redox is subtly triggered by conjugating with their selective catalysis in the reduction of long-chain polysulfides and Li 2 S deposition, respectively. Consequently, the resultant cathode (S@p-Co 3 O 4 /n-TiO 2 -HPs) harvests long-term cycling stability with a capacity decay rate of 0.07% per cycle after 500 cycles at 10 C. Notably, although the previous studies for built-in EF are merely the tip of an iceberg, profound viewpoints buried inside ingenious experimental design lay a solid knowledge foundation on the functional explorations of built-in EF, contributing to a periscope reference for the feasibility of spatial sulfur conversion. Attributed to the considerable scope of builtin EF from polar surface termination to heterostructure, [25] it is envisaged that the in-depth explorations of built-in EF in space-specific sulfur electrocatalysis can be accelerated in an unimaginable manner.
Apart from great endeavors for constructing the builtin EF via experiments, it is of paramount significance to systematically characterize the properties of built-in EF by distinguishing the orientation and quantifying the strength. The work function is regarded as a preliminary descriptor to estimate the migration orientation of electrons between two components, where electrons are prone to migrate from the surface of one component with a lower work function (higher E F ) to the other. [63] Advanced technologies such as electron-holography transmission electron microscopy (TEM), spatially resolved electron energy-loss spectroscopy, Kelvin probe force microscopy (KPFM), and differential phase contrast TEM are viable for reconstructing EF vector maps and visualizing the spatial distribution of interfacial built-in EF in electrocatalysts. [21,[64][65][66][67][68] Regrettably, how to characterize potential changes caused by internal EF embedded in bulk electrocatalysts sounds momentous but remote by the existing technologies.
Additionally, the strength of built-in EF should be quantitative to guide its further functional improvements. [69] As proposed by Zhu and coworkers, the strength of the built-in EF could be experimentally determined by measuring the surface potential (V S ) and surface charge density (ρ). [70] The built-in EF of materials can be calculated with the Kanata model equation: [71] where E is the value of the built-in EF, Ɛ is the lowfrequency dielectric constant, and Ɛ 0 is the vacuum dielectric constant. Among them, V S can be detected by KPFM. According to the Gouy-Chapman model, ρ can be tested and calculated by the zeta potential as follows: [72]  where c is the ion concentration and V i is the zeta potential. Thus, from the above equations, the built-in EF can be approximately concluded to be a function of V s and V i , and the increased V s and V i will contribute to a strengthened built-in EF. Strikingly, suffering from postmodern analysis and low spatial resolution of builtin EF analysis, the real-time decipherment of the actual role for space-specific built-in EF in working conditions is extremely difficult and requires arduous efforts to advance the in-situ/operando observation.

| Spontaneous polarization switched by an external EF
Triggered by an external EF, materials with high dielectric polarization tend to align themselves to spontaneously form a reverse EF inside. [73] Thus, the derived ferroelectric "spontaneous polarization" offers new opportunities for lowering the localized EF gradient to regulate the targeted ion motion behaviors, such as Li + and O n− . [74][75][76][77] Especially for Li-S batteries, the concept of ferroelectric-enhanced LiPSs adsorption appearing in the public vision was first introduced by Wei et al.
( Figure 2F). [27] Attributed to the spontaneously polarized BaTiO 3 (BTO), the heteropolar polysulfides could be strongly anchored on the polarized surface of ferroelectrics, thus combating the shuttle effect of LiPSs for cycle lifespan amelioration. Although a distinct phenomenon for the ferroelectric switching of BTO before and after cycling was seized by piezoresponse force microscopy, an in-depth understanding of the entrapment mechanisms for LiPSs is still ambiguous. Admittedly, it is well established that the spring of research on ferroelectric materials such as sulfur electrocatalysts has come. [78,79] In contrast, investigations into pyroelectric fields formed upon temperature fluctuation and triboelectric fields induced in the presence of contact and friction have yet to be reported, which merits a comprehensive exploration in future. [80]

| Magnetic field (MF)-assisted electrocatalysis
By means of permanent magnets, incorporating an external MF is another representative noncontact energy transfer method to memorably impact the electrocatalytic proprieties by MF-induced effects, such as spin polarization and spin-selective effect. [81][82][83][84][85] Additionally, the Lorentz force (F ⃗ ) can directly act on targeted ions in convective flow to regulate their motion behaviors for enhanced mass transfer or optimal redistribution. [37,[86][87][88] Other magnetic hyperthermia effects caused by alternating MF may lead to localized intense heating, also facilitating electrochemical reactions. [89][90][91] Despite seldom research on magnetic hyperthermia-promoted electrocatalysis reported in the Li-S realm, spin state engineering and magnetohydrodynamic promotion under an external MF are successively introduced in the following section. In addition, magnetism is an inherent attribute of all materials and can be further classified into diamagnetism, paramagnetism, ferromagnetism, and antiferromagnetism according to their magnetic properties. [92] Especially for ferromagnetic electrocatalysts, the self-contained "built-in MF" paves another path to enhance LiPSs adsorption capacity.

| External MF-engineered spin state
MF is exceedingly responsive to spin-dependent electrochemistry. In Li-S batteries, the spin-orbital coupling between electrocatalysts and LiPSs dictates the resultant adsorption capacity, thus affecting the redox kinetics. [93,94] However, research on spin alignment/density to modify the orbital symmetry and reduce/increase orbital overlap with LiPSs remains in its infancy ( Figure 3A). In view of LiPSs with spin-polarized electrons, [96,97] the MFengineered spin-aligned principle was pioneered by Cabot and colleagues to enhance the pinning effect between CoS x and LiPSs for higher intrinsic redox activity ( Figure 3B). [95] Ascribed to the flexible spin state manipulation of Co atoms, in the presence of MF, the Co atoms can be transferred from a low-to high-spin state with more unpaired electrons in their 3d orbitals. With systematic validation by experimental and theoretical analyses, the increased spinpolarized electrons in CoS x could strengthen the 3d-3p hybridization with LiPSs, thus promoting interfacial charge transfer and decreasing LiPSs conversion barrier. In particular, they also proved that the strength of the external MF has a certain effect on battery performance, where the spin effect might saturate at relatively low MF. Under an external MF of 360 mT, the CNF/CoS x /S cathode delivers unprecedented cycling stability with a capacity decay rate of only 0.0084% per cycle for 8150 cycles at 2 C.
Inspiringly, MF-assisted electrocatalysis presents a self-regulation spin polarization by changing the strength/orientation of the external MF, which sheds light on a new strategy to dynamically manipulate the electrocatalytic activity of electrocatalysts and encourages more stimulus-responsive electrocatalysts to couple with diverse requirements for stepwise/ bidirectional sulfur redox.

| External MF-controlled magnetohydrodynamics
In the presence of MF, mobile ions in electrolytes will be subjected to F ⃗ perpendicular to the direction of motion according to the following equation: where q is the charge quantity; v⃗ is the velocity; and B ⃗ is the magnetic field strength. [98] As mentioned above, optimizing the interfacial reactant concentration represents a promising strategy to modulate the sulfur redox kinetics by controllably regulating the ion motion behaviors.
For the Li anode, the requirements for homogenizing positively charged Li + are perceived by plenty of pioneering works. [37,86,88,99] Impressively, aiming at several representative models, Xu's group explicitly depicted the obscure role of the external MF orientation in regulating Li + migration, as discussed below ( Figure 3C). [86] In the absence of an MF, the acceleration behaviors of Li + depend on the localized high current, resulting in an uneven flux of Li + at the interface followed by the growth of Li dendrites. In contrast, under an applied vertical MF, Li + is forced to deviate from the MF line and rotate in a circular motion around the Li tip owing to F ⃗ . As a result, the velocity in the horizontal direction could slightly resist the attraction of Li dendrites. However, the sparse deposition and irregular spheres formed around the current collector persist. After further modification, a toroidal MF is tailored to transfer the Li + motion behaviors from circular to spiral, inducing the planarized ultra-dense growth of Li metal attributed predominately to the disorder homogenization effect. Impressively, once the stable deposition interface is successfully fabricated in the initial cycles, the cycled batteries still retain robustness toward durable Li deposition/stripping with the removal of toroidal MF.
It is well known that the velocity of Li + is simultaneously determined by the EF and MF forces with a nonparallel orientation, resulting in complicated motion behaviors beyond experimental confirmation at present. As expected, the phase-field simulation has emerged expeditiously to forecast the changes in physical fields and convective flow of ions during the electrochemical process. [100,101] In the sulfur cathode, suffering from polysulfides with a larger volume attached with much greater resistance from electrolytes than Li + , rational MF-assisted regulation of the motion behaviors of negatively charged polysulfides has yet to be disclosed. Along the line that magnetic separation is a promising tool to cycle micro-magnetic particles from mixed agglomerates, Cui and co-workers spruced up a new concept of ferrofluid LiPSs entrapment in the presence of Fe 2 O 3 nanoparticles (NPs) for the first time ( Figure 3D). [87] Taking full advantage of the strong LiPSs adsorption and superparamagnetic property of Fe 2 O 3 , most of LiPSs together with Fe 2 O 3 are attracted to the magnet close to the cathode, yielding a high concentration of LiPSs near the sulfur cathode. The concentrated LiPSs immeasurably suppressed the shuttle, endowed with improved coulomb efficiency (CE) and cyclability.
Despite seldom discussing the concomitant Maxwell stress effect and Kelvin forces when the electrode is paramagnetic, [18] their importance in accelerating interfacial mass transfer should not be neglected.

| Built-in MF-promoted LiPSs immobilization
The origin of an external MF always comes from permanent magnets or electromagnets. [89,95] Unconventional battery operation hinders the universal applications of MF-assisted electrocatalysts. [101] In this regard, based on the particularity of ferromagnetic NPs, a built-in MF can be formed near the cathode once ferromagnetic electrocatalysts are incorporated into sulfur composites, largely broadening the scope of MFactivated sulfur redox ( Figure 4A).
For instance, impelled by F ⃗ near the cathode, the motion behaviors of negatively charged polysulfides dissolved in electrolytes will be altered and thereby reinforce the entrapment of polysulfides, which sweeps the monotonous statement for in-situ immobilization. The novel concept of the ferromagnetic-induced shielding effect was first proposed by Li's group ( Figure 4B). [79] Specifically, once ferromagnetic Fe/Fe 3 C NPs are embedded in sulfur composites, an annular built-in MF is formed around the cathode, which is enabled by the spontaneous magnetization of Fe/Fe 3 C NPs. During the discharge process, the polysulfides are forced to be chained in the near cathode by the F ⃗ pointing to the cathode, leading to the partial aggregation of LiPSs around the cathode instead of shuttle. Convincingly, the effective inhibition of the shuttle effect of LiPSs is also assigned to the improved CE together with the significantly enhanced cyclic lifespan. More recently, the strong interplay between electrocatalysts and LiPSs was further intuitively verified by magnetic hysteresis loops, shedding new insights into magnetic adsorbents. [104] Currently, the applications of magnetic electrocatalysts mainly fasten on ferromagnets. How to distinguish the detailed mechanisms of electrocatalytic process for different magnetic properties, especially for their quantum spin-exchange interaction, is strategic yet remains an uncultivated land. [105][106][107][108]

| Spin polarization reinforcing LiPSs interaction
Spin alignment in domains dictates the ferromagnetic properties, otherwise regarded as active sites with spin polarization, which is in essence manipulated by the highest occupied state. According to the preceding contributions by Cheng's group, Fe-N 4 with more spin electrons among the three typical single-atom models (Fe-N 4 , Co-N 4 , and Ni-N 4 ) reduces the antibonding orbital occupation in Li 2 S 2 -Fe-N 4 , thereby attenuating the strength of the S-S bond in Li 2 S 2 and eventually facilitating the solid conversion from Li 2 S 2 to Li 2 S ( Figure 4C). [102] The resulting cathode (S@HP-SAFe) shows a reversible specific capacity of 578 mAh/g after 200 cycles, prevailing against the counterparts. Logically, the different d electron numbers account for the discrepancy in magnetic moments among the three metal sites with identical planar configuration. More recently, versatile spin electrocatalysis has been in full swing and helped integrate a comprehensive understanding of sulfur redox. [109][110][111][112] Meanwhile, our group took full advantage of spinel-type electrocatalysts with tetrahedral (Td) and octahedral (Oh) configuration simultaneously, plowing a universal concept of geometrical-site-dependent electrocatalytic activity to enrich the scope of applications for spin manipulation ( Figure 4D). [103] Once Mn 3+ is incorporated into Oh sites in antiferromagnetic Co Oh -O-Co Td backbones, the originally located Co 3+ Oh is driven into Td sites to construct ferromagnetic Mn 3+ Oh -O-Co 3+ Td backbones instead. Ridding of the localized electronic structure brought by low-spin Co 3+ Oh in Co Oh -O-Co Td backbones, the high-spin Co 3+ Td not only delocalizes the electrons to facilitate oriented electron hopping but also strengthens LiPSs adsorption via the spin pinning effect. In conjunction with orbital-specific catalysis by Jahn-Teller-active Mn 3+ Oh , the consecutive "adsorptioncatalysis" process of LiPSs is smoothly conducted on Mn Oh -O-Co Td backbones. Strikingly, the opposite spin states of Mn Oh and Co Td in Mn Oh -O-Co Td backbones also sheds light on the spin-selective effect toward LiPSs conversion for the first time.

| Light field (LF)-assisted electrocatalysis
The correlation established between the reaction barrier and energy band structure for reaction intermediates is substantial yet scarcely executed. [121][122][123] Enlightened by the separation of photogenerated electron-hole pairs in photocatalysis, dynamically modifying the energy band structure of electrocatalysts can partake of the optimal manipulation of LiPSs reduction energy barrier, which was preliminarily verified by Yu and co-workers. [26] In addition, harnessing the inherent photothermal effects to Reproduced with permission. [102] Copyright 2022, Wiley-VCH. (D) Schematic illustration of the mechanism for the consecutive "adsorptionconversion" process of LiPSs on the surface of CoFeMnO YSNCs. Reproduced with permission. [103] Copyright 2022, Wiley-VCH. achieve internal heating paves another way to accelerate LiPSs conversion. [124][125][126] Apart from the externally powered system mentioned above, Gao groups innovated self-driven Li-S batteries by integrating perovskite solar cells and conventional Li-S batteries into a threeelectrode system. [127] 2.4.1 | Photoelectric effect It is perceived that the appropriate energy band couple between LiPSs and electrocatalysts favors ample electron transport, which is responsible for accelerated redox kinetics. [128][129][130] In an ideal scenario of the discharge process under photo-assistance, where the electrons in the valence band (VB) are excited to the conductive band (CB), leaving holes in the VB, the energy level of CB in non-electrochemically active electrocatalysts must be higher than the lowest unoccupied molecular orbital (LUMO) of Li 2 S in terms of the continual rise of LUMO to a higher position from sulfur to Li 2 S. This energy band configuration ensures spontaneous electron injection into LiPSs to trigger their reduction. Conversely, in the charge process, the energy level of holes in electrocatalysts must be lower than the highest occupied molecular orbital (HOMO) of S 8 as well ( Figure 5A). Accordingly, the elaborate heterostructure of CdS-TiO 2 / CC with CB at 1.88 V and VB at 3.95 V is expected to couple the reaction potentials of S 8 and Li 2 S with 1.9-2.3 V, implying the anticipated LF-assisted electrocatalysts for bidirectional sulfur conversion. [26] As a result, the cathode with CdS-TiO 2 /CC under light illumination shows an ultrahigh initial specific capacity of more than 1500 mAh/g and reversibly remains at 1225 mAh/g after approximately 50 cycles, which is superior to the counterpart without light irradiation. Very recently, in-situ irradiated X-ray photoelectron spectroscopy is developed to intuitively observe the transfer routes of photogenerated electrons and holes inside heterogeneous electrocatalytic pair sites in Li-S electrocatalysis, which was first reported by Zhang et al. [131] In contrast, there is a lack of separation of electrons and holes in the discharge process without photoassistance. If the VB of the electrocatalysts is lower than the LUMO of LiPSs, the driving force stemming from the F I G U R E 5 (A) Schematic illustration of charge transfer between LiPSs and electrocatalysts with different energy band configurations. (B) Synthesis schematic of three-dimensional (3D) Cu/Si-Cu nanowires and working mechanism of batteries. Reproduced with permission. [124] Copyright 2022, Wiley-VCH. (C) Schematic illustration of integrated solar storable batteries and corresponding cycling stability for the last 40 power-supply cycles. Reproduced with permission. [127] Copyright 2019, Wiley-VCH. external circuit stimulates nonspontaneous electron transport from electrocatalysts to LiPSs, which directly correlates with the polarization potential. [132] Specifically, the smaller energy-level difference between the LUMO of LiPSs and the VB of electrocatalysts determines the lower polarization potential that needs to be overcome, thus correlating the smaller Gibbs free energy. According to the Bulter-Volmer equation, enhanced charge transfer kinetics are acquired. That is, band engineering must be placed in a prominent position for electronic structure modulation of electrocatalysts to a certain extent.
Compared with nonelectrochemically active electrocatalysts with immutable energy band configuration in the sulfur redox voltage region, exogenous electrocatalysts with electrochemical activity are naturally regarded as attractive candidates for stimulus-responsive sulfur redox. Established on highly developed synchrotron-based spectroscopy technologies, [133] the rational design of multicomponent or electrochemically active electrocatalysts with self-alterable energy band configuration has been ongoing. [121,122,129,130,[134][135][136] The most conventional strategy is strengthening the ionic conductivity resulting from the continuous rise of E F when Li + is injected into electrocatalysts in the discharge process, namely hybrid intercalation-conversion. [4] As a special case, through dynamically and harmonically coupling the HOMO and LUMO of LiPSs and the VB and CB of lithiated electrocatalysts, spontaneous and rapid electron/Li + transfer based on chemical reactions between lithiated electrocatalysts and LiPSs is attained, namely Li + transfer bridging agent. [129] In principle, electrochemically active electrocatalysts with a lower reduction onset potential (positively correlated with the VB position) than the oxidation onset potential of LiPSs (positively correlated with the LUMO position) are reasonable candidates. [136] In parallel with exogenous electrocatalysts with a decreased Li + diffusion barrier, [8] the endogenous in the form of electrocatalystincorporated intermediates can lower the corresponding LUMO to increase the output voltage, delivering an enlarged discharge capacity. [128,137] Exception of the enhanced charge transfer kinetics, the smaller voltage polarization is conquered, which is ascribed to the compensation of photoelectrons/holes in the cathode reactions. [138] Furthermore, as inspired by research on Li-O 2 batteries, [139][140][141] the origin of intensified electrocatalytic reactivity might also be expanded to regulate Li 2 S growth/deposition or mediate thermodynamic pathways. Regrettably, the small window for accepting light is always arranged near the back of the cathode, [26] and the resultant limited visible light utilization and inhomogeneous light intensity across the cathode make it remain at the conception stage. In this sense, conspicuously for those air-insensitive sulfides, the exploration of all solid-state photo-assisted batteries might be a promising recipe for bypassing the use of a transparent window to pass light into the cathode directly. [142]

| Photothermal effect
The photothermal effect of solar energy can achieve internal cathode heating to accelerate the charge transfer and broaden the operating temperature. [126] Attributed to the unprecedented light-harvesting capability of welldesigned Cu/Si uniformly grown on the Cu foam substrate (3D Cu/Si-Cu), the cathode side of the 3D Cu/Si-Cu-based battery achieves a temperature of 61.8°C after simulated sunlight irradiation (250 mW/cm 2 ), which could realize the normal operation of PEO-based all-solid-state Li-S batteries at room temperature ( Figure 5B). [124] As a consequence, the batteries harvest a high initial reversible capacity of 1089.9 mAh/g at 0.2 C, immensely exceeding the poor performance in the absence of LF assistance. Coincidentally, taking full advantage of the photothermal Cu-Si nanowire, Chen et al. performed explorations of an all-solid-state Li-S battery operating at extremely low-temperature (−60°C), enabling broad solar spectrum absorption (>93%) with a wide incident angle (0-70°). [125] A feasible benchmark for efficient conversion of light-to-heat and temperature monitoring in the entire battery should be established in response to the demands of practical applications with adequate safety.
The band gap and CB/VB positions determine the light absorption property and redox capability of photocatalysts, respectively. In this regard, desired photocatalysts should be empowered with sufficient photocatalytically active areas and an appropriate band structure to meet the requirements of broad light absorption. [139] On top of that, the selective adsorption of the specific solar spectrum to flexibly regulate the electronic structure of heterophotocatalysts sounds promising as well. [143]

| Solar-driven Li-S batteries
To date, solar cells and rechargeable batteries are two representative technologies for energy conversion and storage, respectively. [127] As requested by the voltage matching principle, the high charge voltage of Li-ion batteries (3.0-4.2 V) is adverse to the combination with solar cells. Instead, the lower charge voltage of Li-S batteries makes them suitable to cobuild integrated solar storable batteries. [144] Incorporating them into one cathode realizes continuous transformation from solar to chemical energy stored in the sulfur cathode, followed by delivering electrical energy. For tentative attempts, though, Gao's group resorted to perovskite solar cells, while Lu et al. utilized dye-sensitized solar cells, both of them coincidentally realized stable operation for many cycles ( Figure 5C). [127,144] Pushing into a new stride forward, an upsurge of interest in highly integrated power systems has quietly risen. [145][146][147][148]

| Thermal field (TF)-assisted electrocatalysis
It is imperative to disclose the fundamental understanding of the thermodynamics and kinetic process of TF-assisted electrocatalysis to improve the performance of Li-S batteries at elevated temperature. [149] Although most attention is naturally paid to enhanced sulfur redox kinetics, endeavors should also be allotted to neglected concerns, such as surface passivation and battery safety, which could increase the potential for major breakthroughs.

| Double-edged sword of elevated temperature
It is expected that increasing the temperature results in a smaller E a . Some recent studies have demonstrated that the faster redox kinetics of LiPSs at a high-temperature increase the sulfur utilization of Li-S batteries. [149][150][151][152] However, the solubility and shuttle effect of LiPSs are synchronously aggravated, which can offset the benefit. [153] Plenty of reports have coincidentally revealed the impact of elevated temperature on the positive side for higher reversible capacities and the negative side for lower CE and deteriorative capacity retention. [154][155][156][157] At an elevated temperature, despite the slight enhancement of adsorption capacity of adsorbents, [158,159] more severe shuttle effect and faster capacity attenuation still occur. Under the inspiration of intensified sulfur redox by electrocatalysts, an impressive attempt could be traced back to the first stable cycling of Li-S batteries at 100°C achieved by Ye's group ( Figure 6A). [160] Specifically, the as-fabricated porous carbon/cuprous phosphide (CPC/ Cu 3 P/S) cathode delivers a high initial capacity of 545.9 mAh/g with a low capacity decay rate of 0.061% per cycle after 1000 cycles at 1 C.

| Li 2 S deposition regulation
Temperature also affects the distribution and morphology of discharged products, such as Li 2 S growth. [163,164] The proof of concept work was first comprehensively demonstrated by Yushin and colleagues. [163] They deciphered that an elevated temperature of 90°C could induce the 3D deposition of Li 2 S 2 /Li 2 S attributed to the fast diffusion of LiPSs within the carbon nanotube walls F I G U R E 6 (A) Long-term cycle performance of CPC/Cu 3 P/S at 1 C from 25°C to 150°C. Reproduced with permission. [160] Copyright 2021, Wiley-VCH. (B) Schematic illustrations of interfacial processes at room temperature and 60°C, respectively. Reproduced with permission. [161] Copyright 2017, Wiley-VCH. (C) Photographs of the burning test of f-CNTs and f-BNNSs/f-CNTs with electrolytes and corresponding infrared thermal images. Reproduced with permission. [162] Copyright 2021, Elsevier. and the prevention of preferential re-deposition of LiPSs at the electrode surface. Thus, the conformal and uniform deposition of Li 2 S effectively withstands the passivation of conductive matrix, which greatly improves sulfur utilization during cycling. Coincidentally, Wen et al. exerted their unique advantages of in-situ atomic force microscopy and painted a full picture of the interfacial Li-S electrochemical process at elevated temperature ( Figure 6B). [161] It easily comes to the conclusion that LiF NPs produced by LiFSI decomposition can form a net with both physical confinement and chemical anchoring effects on LiPSs, which leads to an in-situ formed film after Li 2 S nucleation upon discharge at 60°C.

| Thermal runaway
Potential thermal runaway might occur if the heat generation rate is higher than the heat dissipation rate. To the best of our knowledge, although an unprecedented research opportunity exists in the imperative requirements for thermal safety evaluation of batteries, only a few pioneering works have noticed and emphasized the role of assessing the safety features of Li-S batteries. [165][166][167][168] Indeed, a considerable thermal risk in batteries mainly results from the chemically active Li anode and sulfur cathode, highly flammable organic electrolytes, and nitrates formed from additives in terms of gassing, burning, or even explosion during battery thermal runaway. [169,170] Because of the complexity of the concurrent reactions and the shortcomings in existing characterization methods, its explicit influence on the thermal runaway behaviors of Li-S batteries is still unknown until it is filled with a comprehensive investigation of the thermal runaway behaviors of Li-S pouch cells depicted by Cui's group using accelerating rate calorimetry. [165] A universal phenomenon of thermal runaway appears in Li-S batteries coupled with electrolytes with different thermal stabilities, even inorganic allsolid-state electrolytes. Beginning from the cathodeinduced reactions, the thermal runaway is then accelerated by reactions from the anode. Of note, solvent vaporization plays a dominant role in pressure build-up during thermal runaway.
In the domain of thermal safety enhancement, the method of salvation encompasses thermally conductive materials to tolerate heat accumulation and fireextinguishing materials to suppress combustion, which was systematically introduced by the tutorial review from Yang's group. [152] Impressively, flame-retardant hosts with a hierarchical cellular architecture composed of boron nitride nanosheets and functional CNTs (f-BNNSs/ f-CNTs) manage to cycle over 300 times even at a high temperature of 60°C without suffering thermal runaway, inseparably attributed to the homogeneous TF with smooth and ultrafast heat-conduction channels ( Figure 6C). [162] 2.6 | Strain field (SF)-assisted electrocatalysis Strain engineering is a universal scheme that can finely tailor the electronic structure of electrocatalysts, hence paving a novel avenue to boost their electrocatalytic activity. [171][172][173][174] In detail, when the metal d orbitals in electrocatalysts hybridize with the 3p orbitals in LiPSs, the relative filling of the thus-formed bonding and antibonding states is crucial to the bond strength and resultant electrocatalytic activity. In terms of the antibonding states pertaining to the d-band center of electrocatalysts, the higher d-band center not only decreases the energy barrier for charger transfer as mentioned above, but also brings stronger metal-S covalency between electrocatalysts and LiPSs ( Figure 7A). [177][178][179] Except for the d-band model, the strain effect in metal-based electrocatalysts can be explained by crystal field theory as well ( Figure 7B). [93,103,111] Beneficial from the versatile strategies (e.g., external force, defect, and lattice mismatch) for the introduction of strain in electrocatalysts, which is in response to flexible manipulation of tensile/ compressive strain, strain engineering flourishes in the exploration of SF-assisted electrocatalysts in Li-S batteries. [180][181][182][183][184][185][186] 2.6.1 | d-band center modulation Tensile strain and a lower coordination number result in less wave-function overlap and a narrowed metal d-band, thus resulting in a higher d-band center, followed by strengthened LiPSs adsorption ( Figure 7C). [171] First, strain can be implanted into electrocatalysts when combined with an external force during the synthetic procedure. Chen et al. creatively designed tensile-strained MXene/CNT porous microspheres in a facile spray-drying process. [182] The in-situ formed O-Ti-C interface induces lattice distortion and enlarges the Ti-Ti bond, which upshifts the d-band center of Ti atoms toward E F , and therefore a stronger sulfur adsorption ability. Second, surface strain is generated through lattice mismatch, which extensively exists in core-shell nanostructure or metal-substrate interface ( Figure 7F). [176,184,186] For instance, Cabot and co-workers deciphered the prominent role of different strain-inducing cores on rational tensile strain generated in the MoS 2 shell. [186] In detail, they particularized the growth of 2D MoS 2 on the surface of different metal sulfide NPs, including Co 9 S 8 , Ni 9 S 8 , and  [175] Copyright 2023, American Chemical Society. (E) Charge density differences of Fe-N 3 C 2 -C-Li 2 S and Fe-N 4 -C-Li 2 S to decipher the orbitalspecific electrocatalysis. Reproduced with permission. [94] Copyright 2022, American Chemical Society. (F) Schematic illustration of representative strain engineering strategies. Reproduced with permission. [176] Copyright 2022, Springer Nature. (G) Theoretical understanding of the enhanced LiPSs adsorption by amorphization-induced surface electronic structure modulation of CoO. Reproduced with permission. [111] Copyright 2021, Springer Nature. obtained. Third, a more general strategy by incorporating defect or doping has been widely adopted for the synthesis of electrocatalysts and proven to play an effective role in the enhancement of sulfur electrocatalytic activity. [94,175,180,187] To this end, Zhang et al. explicated the appropriate energy level configuration of exposed orbitals for strengthened TM-S covalency ( Figure 7D) while our group rooted in asymmetric coordination engineering to activate horizontal d orbitals ( Figure 7E), [94,175] both enabling superior LiPSs adsorption compared to the undoped structure in a preconceived manner. Notably, now that a different degree of localization for s, p, d, and f orbitals is acknowledged by the public, engineering gradient orbital coupling is shining in the near future. [188][189][190] It is worth noting that although the d-band center descriptor has been successfully applied in metal/alloy systems, its inadaptability for some electrocatalysts in which p orbitals/nonbonding interaction dominate should attract sufficient attention, such as metal oxides or metal-free catalysts. [103,188,[191][192][193][194][195][196] 2.6.2 | Amorphization-induced geometrical configuration Accompanied by in-plane strain located at the amorphous-crystalline phase boundaries, amorphous electrocatalysts with long-range disorder are flexible in defect manipulation, chemical homogeneity, and electronic structure modulation. [197] To fully consider the role of ligands in metal oxides, structural descriptors can relate the geometry of electrocatalytic active sites to their properties. [103,117,175,193] Originating from the geometrical-site-dependent reactivity, Wang and coworkers offer an intriguing concept for surface electronic structure modulation by transforming the crystalline counterpart (c-CoO nanosheets) with CoO 6 octahedrons into amorphous a-CoO nanosheets with partial distorted or truncated CoO 4 tetrahedrons ( Figure 7G). [111] In conjunction with theoretical calculations and X-ray absorption spectroscopy analysis, it is elucidated that the amorphization-induced reformed crystal fields makes more electrons occupy a high energy level and results in more electrons transferring to Li 2 S 4 , in response to the high binding energy with LiPSs for favorable adsorption. Along this line, there still exists plenty of room at the bottom waiting for digging in depth to decipher the interplay between LiPSs and electrocatalysts, in conjunction with the electrostatic interactions in crystal field theory and the covalent interactions in molecular orbitals. [93,103]

| Piezoelectric electrocatalysts
It is well known that the large volume expansion from S 8 to Li 2 S should be suppressed as much as possible. [198] Nevertheless, the concomitant stress evolution might allow us to turn waste into treasure without additional costs if coupled with piezoelectric electrocatalysts. The merits of piezoelectric coating layers on Li + diffusion at the cathode-electrolyte interface have been verified by Bai et al., [66,[199][200][201][202] which is appealing to dig in-depth insight into the interface coupling strategy between mechanical and EF to purposefully regulate the interfacial kinetics of negatively charged polysulfides and positively charged Li + . Several representatives of piezoelectric semiconductors, such as ZnO, MoS 2 , and MoSe 2 , occupy the top position of potential piezoelectric electrocatalysts.
SF is a localized effect that manipulates the electronic structure to optimize the geometrical-site-dependent reactivity. Once extends to general mechanical fields such as ultrasound, stirring, and microwave fields, homogeneous and fast charge transfer caused by reinforced convective flow stimulates rapid sulfur redox kinetics, of which importance is also self-evident. [203][204][205] Taking the ineluctable energy power input into consideration, strategies to reduce energy consumption are extremely urgent. Meanwhile, now that additional weight and volume are involved in integrated batteries, how to balance the relationship between energy density enhancement and building cost is another key issue to be addressed. [1]

| CONCLUSIONS AND PERSPECTIVES
As an integrated strategy, field-assisted electrocatalysts provide an additional degree of freedom to purposively regulate the reactant concentration or flexibly modulate the electronic structure of electrocatalysts, thereby attaining enhanced sulfur redox kinetics. For each field effect, the effect-dependent mechanisms behind their enhanced catalytic performance toward LiPSs are systematically collected, classified, and summarized, as shown in Table 1. According to the simplified kinetic equation mentioned above, they are divided into two main factors. It is well known that higher charge transfer kinetics or LiPSs concentration inevitably accelerates sulfur redox kinetics. In sharp contrast, there are still controversies in determining the origin of decreased E a from the aspect of interplay between electrocatalysts and LiPSs, where various types of thermodynamic, electronic, and geometric descriptors have been used to predict the catalytic performance. [191] To this end, once the following criteria are fulfilled, a simplified descriptor of metal-S covalency is applied to describe the positive relation with E a , which paints a full picture of the ambiguous correlations among electronic structure, surface adsorption properties, and catalytic performance.
1. LiPSs adsorption competition on multiactive sites is excluded. [103] 2. The interplay between electrocatalysts and LiPSs relies on metal-S bond, namely, d-p hybridization. Li bond chemistry or dipole-dipole interaction is excluded. [206,207] T A B L E 1 The basic mechanisms for enhanced sulfur redox kinetics in response to different field effects. 3. Although sulfur redox is a complex conversion, the targeted model is limited to an elementary reaction. [35] Meanwhile, the desorption of products and backward reactions are excluded. [177] 4. The phase, morphology, and structure changes during the conversion are excluded. [208,209] Enlightened by the Brønsted-Evans-Polanyi relation, [210] the stronger d-p hybridization between electrocatalysts and LiPSs triggers a stronger covalence of metal-S bond, consequently weakening the S-S bond in LiPSs, followed by a lower E a . Correspondingly, the antibonding state occupation, orbital energy level, and orientation of the hybrid orbitals determine the strength of hybridization. [120,178,[211][212][213][214][215][216][217][218][219] When we are constantly looking at a bright future with field-assisted electrocatalysts, we cannot help but worry about whether the field effects can be used as a commercial boosting method or only as a supplementary means to experimentally elucidate the mechanisms behind enhanced sulfur redox kinetics in the conceptual stage. Admittedly, a highly interdisciplinary field-assisted electrochemistry, referring to a considerable amount of uncertainty constrained by multi-interferences in physical parameter variability, weakens the universality and inevitability of the corresponding conclusion. Here, some prospects are proposed, which could serve as guidance for further explorations.
1. It is imperative to establish the quantitative correlation between the physical parameter variation of field effects and the electronic structure modulation of electrocatalysts to comprehensively understand the origin of enhanced electrocatalytic activity. By means of accurately measuring the intensity and orientation changes of physical fields and subsequently quantizing the actual driving effect, controllable and flexible parameter optimization contributes to ascertaining universal guidelines for the rational design of electrocatalysts. Meanwhile, from the holistic standpoint of electronic structure modulation of electrocatalysts, the development of characterization technologies to precisely investigate the intrinsic charge-spin-orbital coupling between electrocatalysis and LiPSs is helpful in decohering the regulation mechanisms of fieldassisted electrochemistry. Established on the strongly correlated model, further efforts can be leveraged to exploit the potential manipulation applicability of field effects to newly emerged systems, such as microfluidic flow cells. 2. To realize real-time manipulation by monitoring the variability of different parameters, customized experimental devices are the conventional strategy.
Although a visualized description for the sophisticated functions is drawn from the particular conditions, whether the equivalent vector values and resultant functions can be transplantable in universal devices such as coin cells is still controversial. As expected, those noncontact field effects such as MF seem promising for adaptation to conventional coin cells, while the feasibility of LF-assisted electrocatalysts remains at the conception stage. Of note, the current laboratory-scale technologies for field effect deployment are still in their infancy, it is considerable to give full play to creative thinking, and adopting engineering strategies associated with research efforts to implant field effects matching the optimal battery arrangement might trigger great innovations for the large-scale commercialization of Li-S batteries. 3. Electrochemistry coupled with a single physical field certainly fails to satisfy the appetite for further performance improvements. Photo-and-magnetic field-assisted Li-O 2 batteries have proven to be a good combination. In an anticipated system driven by the coupling of multiple fields, demands for the compatibility and optimal integration of different fields acting at different regions or parts of the batteries rely on the complementary development of experimental characterizations and simulation tools to acknowledge an in-depth understanding of the working mechanism. On the one hand, because of the complicated intermediate evolution, dynamic interplay changes, and possible reconstruction of the electrocatalysts during the charge-discharge process, various in-situ technologies are indispensable for investigating the electronic structure variation of electrocatalysts in real-time. Although prevailing tactics of in-situ/operando characterization technologies are capable of probing the reaction mechanisms of conventional Li-S batteries, breakthroughs in innovating and designing characterization methods in scenarios of coupling field effects might be more imminent than ever. On the other hand, in parallel with experimental characterizations, theoretical tools such as multiple field simulation require an upgrade, which functions better in resolving the mechanistic research of field-assisted electrocatalysis. 4. There are limited regulatory strategies for precisely catalyzing the multistep sulfur redox in most conventional electrocatalysis. In sharp contrast, the flexible parameter manipulation of field effects can dynamically regulate the electronic structure of electrocatalysts in real-time, thereby maintaining the optimal "adsorption-diffusion-conversion" process of LiPSs when crossing the hugely different redox energy barrier in multistep conversion. Besides, the judiciously coupling of multiple fields might reconcile the variant demands with different conversion steps and integrate them with the optional combination. For instance, the spin regulation derived from the MF contribution is sensitive to strengthening long-chain LiPSs adsorption via the spin pinning effect, while TF significantly accelerates the liquid-solid redox kinetics. In addition, field effects might shine in the construction of complicated electrocatalysts, where photoinduced strategies have been successfully used in materials synthesis, such as preintroducing S 3 •− .
5. The sword concerning the stability of electrocatalysts and intermediates with respect to the durable electrocatalytic activity hangs over our heads. For example, photo-corrosion of metal sulfide-based semiconductor and consequent photocatalytic performance degradation have been well documented. More attention should be paid to improving the robustness of electrocatalysts in response to different application scenarios. Specifically, to circumvent the issue of deactivation and passivation of electrocatalysts resulting from the contamination of the electrocatalyst surface by sulfur species, conventional strategies of doping with heteroatoms and intriguing concepts for self-cleaning surface are highly desirable and thus merit further exploration.
In summary, we anticipate that this review could serve as an opening remark to attain adequate attention for field-assisted electrocatalysts emerging in Li-S batteries. We firmly believe that flexible manipulation of field effects will help advance the fundamental understanding of the complex electrochemistry of Li-S batteries and could break through some of the current bottlenecks that are difficult to regulate in conventional electrochemical processes.