Review on Recent Progress of Nanostructured Anode Materials for Li-Ion Batteries

Lithium ion battery (LIB) is one of the promising power storage devices in today’s world. Lithium ion battery like other types of electrochemical cell has anodic and cathodic electrode in which lithium ion is intercalated and dein-terclated during charging and discharging process respectively. The capacity of lithium ion battery is improved by the development of innovative kinds of electrode. Carbon, metal/semiconductor, metal oxides and metal phosphides/ nitrides/sulfides based nanomaterials improve the capability of LIBs due to their high surface area, low diffusion distance, high electrical and ionic conductivity. Nanostructured materials represent a rapidly growing area in the field of Li-ion batteries because of their substantial advantages in terms of mass transport. In this review anode nanomaterials classified based on type of transition metal/semiconductor such as carbon, silicon, titanium and tin based nanomaterials are discussed. Additionally, different electrochemical reactions, comparative influence of anode materials on LIBs and their applications are widely explained.

. Overview of the most commonly commercialised Li-ion battery concepts.
The cathode half-reaction: The anode half reaction: 6 6 Li In addition to graphite, there are many alternative materials used as anodes in lithium ion batteries, such as carbon based materials (CNT, graphene and porous carbon), metal (Sn, In, etc.), metalloids (Si, Sb and Ge, etc.) and transition metal oxides (Fe, Mn, Ni, Cu and Co) [7].

Nanostructured Material for Rapidly Growing Lithium Ion Battery Technology
Now a day's nanostructured materials are widely used in the field of Li-ion batteries because of their great advantages in terms of mass transport, higher electrode-electrolyte contact area, and better movement of lithium ion during intercalation and deinterclation. Generally, nanocomposites show higher reversible capacity and better cycling behavior than their large structural materials. In addition, nanocomposites have large surface/volume ratio and the geometry which make it efficient electrode materials for powerful electrochemical energy storage devices with both high energy and power density. C, Si, TiO 2 and SnO 2 based nanomaterials have received a great attention, and considered as alternative anode materials for lithium ion batteries [8].
Graphite anodes have a theoretical lithium storage capacity of 372 mAh/g to form LiC 6 intercalation compound. Recently, many efforts in research are currently made to increase this limited capacity. Nanostructured anodes can be used to increase this limited capacity due to three reasons: 1) Decreasing the size of the active particles to the nano range; result in larger surface/volume ratio which implies a larger specific capacity and contact area with the electrolyte leading to high lithium ion movement across the electrode/ electrolyte interface [9].
2) The reduction of the size of the particles result in decreasing the path length for lithium ion implies a reduction of the length of the electron and Li + inside the particles. Therefore the conductivity is going to be increased, because after the electron has reached the surface of a particle it is driven to the current collector in a conductive medium immediately [10].
3) The intercalation of lithium ion is accompanied with a dilatation of the particles, which may result in the formation of cracks, or even pulverization when the particle size is in the micron range [9] [10].
During Charging/discharging process in Li-ion batteries mainly three types of reactions occurs:  Diffusion of Li + and electron within solid electrode materials (anode and cathode).  Charge transfer at electrode electrolyte interface.  Movement of Li + ion in the electrolyte.
Diffusion of Li ions across Solid state is known as the rate-controlling step for Li-ion batteries which determine the rate of charge/discharge. The time for Li ion diffusion can be expressed as a function of diffusion coefficient of Li ions and the diffusion length as following: where L ion is the diffusion length of solid materials and DLi is the diffusion coefficient of Li ions. As shown from Equation that a faster diffusion can be achieved in a system with higher diffusion coefficient and lower diffusion length. Diffusion coefficient of Li ions in most electrode materials is very low (10 −7 -10 −11 cm 2 /s) and has large diffusion length, which result in more diffusion time. Nanostructured materials are capable to improve rate performance of Li ion batteries because of its low diffusion length and high diffusion coefficient [11]. Here the authors compared the data with different anode materials and are listed in Table 1.

Classification of Anode Nanomaterials
Different materials can be used as anode in lithium ion batteries, which have different capacity. The large size of nanomaterials offers low capacity as compared to its nanostructure. In order to address this large capacity nanomaterials different semiconductor, metals and metalloids can be synthesized currently.
Some of nanomaterials that used most commonly in lithium ion batteries can be discussed in this review.

Carbon Nanotubes (CNTs)
Carbon nanotubes consist of carbon atoms that are structured in layers of graphene into the shape of a cylinder. One carbon atom of a graphene is symmetrically bound to the other three carbon atom [12].
Carbon nanotubes (CNTS) are one dimensional nanomaterial and classified in to:  Single-walled carbon nanotube (SWCNT) rolled up by one-layer graphene.
CNTs have light weight and prefect connection in their hexagonal structure which offers it excellent mechanical, electrical and chemical properties. Currently, CNTs has many emerging application due to its excellent electrochemical properties, low density, high rigidity, and high tensile strength. CNTs are widely used in lithium-ion batteries, both as the anode material and the conductive additive in the composite electrodes [13]. Li-induced embrittlement. In contrast the promising results reported, the carbon nanotubes have not found a place in the industry of the lithium-ion batteries, because of their cost and the difficulty to prepare them free of any large structural defects and high voltage hysteresis [14].

Graphene
Graphene is a monolayer of graphite, which contain sp 2 hybridized carbon atoms arranged in a honeycomb crystal lattice. Graphene forms the basic structure for other carbon materials like graphite, carbon nanotubes and fullerenes.
Also, it is two-dimensional carbon material with a specific surface area of 2600 m 2 /G resulting in higher lithium storage capacity. Furthermore, its high electron mobility (15,000 cm 2 /(V·s)), outstanding thermal conductivity (3000 W/(m·K)), good chemical stability and excellent mechanical properties make unique for forming composite materials used as negative electrode. Composite electrodes also allow for the storage of more lithium ions and increase the battery's capacity. As a result, the life of batteries containing graphene can last significantly longer than conventional batteries [1].
In conventional lithium ion batteries, no storing of lithium ions (lithium ions are inserted and removed immediately) in the electrode materials, as a result the materials will swell and shrink, leading to a quicker breakdown. This can be avoided through the addition of graphene, which has efficient conductivity and less heat resistant within the electrode, so batteries can operate at lower temperatures, which ultimately improves the battery's safety. Graphene has many additional characteristics such as the quantum hall effect, bipolar field-effect, ferromagnetism, superconductivity and high electron mobility. These properties make graphene suited for use in many fields [15].
The introduction of heteroatoms such as nitrogen in to graphene can further enhance its electrochemical performance. Nitrogen is more electronegative than carbon but has a similar atomic diameter, which lead to enhanced interaction between a nitrogen-doped carbon material and lithium ions. In addition the presence of nitrogen introduces more defects, providing more active sites for lithium storage [16] [17]. Graphene is building blocks for carbonaceous materials of other dimensionalities (that is, graphite, fullerenes and carbon nanotubes) and listed in Table 2.

Hard Carbon Nanomaterial for Lithium Ion Batteries
Hard carbons have small graphitic grains with disordered orientation, and are much less relevant to exfoliation. These grains also have nanovoids between

Silicon Based Anode Nanomaterials for Lithium Ion Batteries
Silicon has attracted attention of many researchers due to its excellent feature such as high theoretical specific capacity, higher safety and stability than graphite (lithiated silicon is more stable in typical electrolyte than lithiated graphite). Si anode materials suffer from some drawbacks such as: drastic volume change while the alloying/de-alloying reactions of Li ion, the intrinsic low electrical conductivity, and the unstable solid electrolyte interphase (SEI) formed in the common electrolyte of LiPF 6 [19]. American Journal of Analytical Chemistry

Pure Si Nanostructures
Si-based bulk materials exhibited a marked loss of capacity in a few cycles.  [19].
The mechanism of electrochemical lithiation of Si is critical to improve the performance of Si anode. In order to find out the lithiation mechanism, X-ray diffraction (XRD) analysis was performed to investigate the phase transition, and the reaction mechanism is explained as follows [20]: During discharge:   aggregating. Thus, the material of 65% Si nanoparticles/35% graphene composite has the highest initial capacity [24].

Si/Graphene Composite
This indicates that the 65% Si nanoparticles 35% graphene composite has good cycling performance, and the capacity retention after 20 cycles is 96.72%.
The initial capacity of the sample 65% Si nanoparticles is 1354 mA·g −1 , and this value falls to 1271 mA·g −1 at the second cycle. Then, it keeps above 850 mA·g −1 at the 100th cycle. This is because parts of the active materials are not completely utilized during the first cycles, and they are activated along with the charge/ discharge process, which makes the diffusion path of Li + wider and results in the capacity enhancement of the material. The structure of composites results in excellent cycle performances [25].

Tin Based Anode Nanomaterials for Lithium Ion Batteries
Sn anode research is different from Si and Ge anodes in that much focus has been placed on oxides, which showed significantly improved cycle stability. In the fully lithiated state, Sn forms Li 22 Sn 5 , which places its theoretical capacity limit at 990 mAh·g −1 . While Sn specific capacity is significantly less than that of Si, its volumetric capacity is comparable. However, the theoretical capacity of pure Sn has not been achieved with stable cycling, possibly due to the brittleness of the fully lithiated phase [26]. Many tin-based intermetallics and their composites have been examined as lithium storage materials and have shown increased long-term cycling stability compared to metallic Sn. These intermetallics include an alternative anode material for rechargeable lithium ion batteries [27].
The specific capacities of the as-deposited and post heat-treated composite samples were 534 and 737 mAh·g −1 , respectively at the 70th cycle and the corresponding energy densities of the as-deposited and heat-treated composites were 1240 and 1760 Wh/kg, respectively. This enhancement in electrochemical behavior of tin oxide and graphene composites relative to pure tin oxide films is attributed from the addition of graphene [28].

Pure Tin Anode
The theoretical capacity of pure Tin is is three times that of the graphite anode (372 mAh/g), based on the end lithiated phase Li 4 ·4Sn. Later, crystallographic studies suggested that the realistic form of this end phase could be Li 17 Sn 4 (thus, 4.25 Li per Sn). Therefore, its maximum gravimetric capacity could be 959.5 mAh·g −1 , which is still much higher than most common graphite anodes. Also, the potential of the tin-based anodes is slightly higher than that of graphite, which reduces the potential safety problems with dendrite formation on the host anode, during rapid charging and discharging. Unfortunately, they are affected by the large volume change of these metals during lithium insertion/extraction [1].
Whittingham group demonstrate, pure tin foil (bulk) can be cycled as 600 mAh·g −1 for 10 to 15 deep cycles. However, the expansion and contraction of the electrode crystalline lattice cause some breaking-up of the material. Consequently, the loss of electronic contact between active materials and the current collector dramatically decreases the reversible capacity after 15 cycles. In order to overcome these effects, nanostructures or nanocomposite have been extensively applied [3]. purity Sn-C composite delivers a reversible and stable capacity on the order of

Tin-(M)-Carbon (M = Co, Fe, Ti)
Sn-Co exists as 5-nm particles encapsulated in carbon, which most likely used to prevent much contact between the electrolyte and the metal. Thus, few side reactions happened during electrochemical cycling. The electrochemical behavior of the crystalline tin anode was found to be inferior to that of amorphous tin-based materials. Heating the sample in H 2 /He atmosphere can crystallize the Sn-Co amorphous material and also maintain carbon content. The crystallization primarily happened at 300˚C. When the amorphous Sn-Co carbon composite were heated to 250˚C, the capacity of the material was about 300 mAh·g −1 for the initial cycle and rapidly dropped to about 200 mAh·g −1 after 10 cycles. After heating to 450˚C, the capacity is only about 50 mAh·g −1 . Despite its remarkable performance, cobalt is expensive and toxic [1].
Recently, the Whittingham group reported Sn/Fe/C composite prepared by mechanical milling using Ti, Al and Mg as the reducing agent and different grinding media. The specific capacity of 600 mAh·g −1 , close to the theoretical capacity, was obtained on titanium reduced Sn-Fe carbon composite with good capacity retention on cycling 200 cycles. Also, Sn-Fe-carbon composite has a comparable rate capability with Sn-Co-C materials at a current density of 5 mA/cm 2 . It still can achieve a specific capacity of 250 mAh·g −1 when charged and discharged between the 0.01 -1.5 V voltage [10].

Metal Oxide Based Anode Nanomaterials for Lithium Ion Batteries
Transition metal oxides are found to be potential substitutes for graphite as anode materials in LIBs due to their wide ranging electronic, chemical and mechanical properties. Nowadays much interest has been aroused in nanostructured TiO 2 for obtaining fast lithium insertion/removal due to its excellent safety, stability and cost effectiveness [21].
There are two types of reaction mechanisms between transition metal oxides and Unlike the uniform SEM layer of carbon based materials, the SEI layer on transition metal oxides is easily destroyed by the large volume changes during cycling.
Therefore, to achieve reliable cycling performance and high Coulombic efficiency, there are several approaches, such as decreasing the particle size, forming the deliberated designed porous structure, loading on conductive and flexible host (graphene, CNT or porous carbon based materials) [3].
Nanostructured metal oxides exhibit good electrochemical properties, and they are regarded as promising anode materials for high-performance LIBs. Three different categories of metal oxides nanomaterials with distinct lithium storage mechanisms classified as follows:  Tin dioxide (SnO 2 ), which utilizes alloying/dealloying processes to reversibly store/release lithium ions during charge/discharge.  Titanium dioxide (TiO 2 ), where lithium ions are inserted/deinserted into/out of the TiO 2 crystal framework.
Transition metal oxides including iron oxide and cobalt oxide, which react with lithium ions via an unusual conversion reaction.

SnO2 Nanomaterials for High-Capacity LIBs
Sn-based materials have drawn much attention as alternative LIB anodes in both the scientific community and industry. SnO 2 has been demonstrated to be one of the most promising anode materials for high performance LIB due to its high theoretical specific storage capacity (782 mAh·g −1 ), compared with the commercially used graphite (372 mAh·g −1 ) [29]. As a representative material for this category, SnO 2 shows attractive lithium storage properties based on the following two reactions:  [30].

TiO2 Nanomaterials for High-Power LIBs
Titanium dioxide (titania) anode is a very promising material for applications in the LIBs field, and suitable for mass production, cost effective, and TiO 2 has admirable advantages such as high electro-activity, strong oxidation capability, good chemical stability, high abundance and structural diversity. Also, TiO 2 shows excellent safety and stability characteristics at the working potential of 1.5 V vs. Li/Li + . These characteristics make TiO 2 a good anode material in LIBs, especially for HEV applications. Titania can host 1 mol of lithium per 1 mol of American Journal of Analytical Chemistry TiO 2 with a theoretical maximum capacity of 330 mAh·g −1 . The lithium intercalation/de-intercalation process depends on its crystallinity, particle size, structure and surface area. Titania has many allotropic forms; the most well-known are rutile and brookite. Even though anatase titania has been considered the most electroactive form, other allotropes are also widely studied for anode purposes [31].
Decreasing the particle size of rutile form of TiO 2 to 15 nm allows a larger capacity of 378 mAh·g −1 at first discharge and subsequent stable capacity of 200 mAh·g −1 (0. 6 Li per one molecule of rutile TiO 2 ) over 20 cycles, at the current density of 0.05 A·g −1 . With particle size of 300 nm, the initial and the 20th cycle capacities of 110 and 50 mAh·g −1 were observed, respectively. The improvement of the capacity and the Li-ion uptake are related to the nanosize characteristic and to the high surface area. Similarly, it was reported that 6 nm particle size of TiO 2 anatase maintains high capacity at more than 200 mAh·g −1 over 20 cycles at the current density of 0.1 A·g −1 . Furthermore, at the current rate of 10 A·g −1 , the capacity of 125 mAh·g −1 was attained. Moving to higher particle sizes of TiO 2 , such as 15 nm and 30 nm, make the lithium ion battery to have lower capacities of 80 and 71 mAh·g −1 , respectively [32].
Lee's research group synthesized anatase TiO 2 microspheres by solvothermal process. These TiO 2 microspheres are formed by the combination of ultra-fine 6 -8 nmTiO 2 nanocrystals with 4 -6 nm pore size microstructures. The performances of TiO 2 microspheres are increased and it has high lithium storage capability, high number of charging discharging cycles and high tap density. This characteristic improve the capacity, the rate capability as well as the cycling life of titania based anodes. For instance, Gentili et al. synthesized nanotube of anatase TiO 2 with wall thickness of 2 -3 nm, an external diameter of 8 -10 nm and length in the range 100 -300 nm. The synthesized nanotubes exhibited maximum capacity at around 300 mAh·g −1 with lithium uptake roughly 0.98 Li for unit formula of titania. Along with high capacity, these titania nanotubes showed high rate capability and good cycling life with a capacity around 250 mAh·g −1 over 100 charge-discharge cycles. Similarly, Brown et al. have synthesized mesoporous TiO 2 -B microspheres, with 12 nm pore size. This porous titania has proved to be a good anode material at different current densities. Interestingly, a lithium storage capacity of 120 mAh·g −1 was attained at the high current rate of 60˚C. This improved rate performance was related to the fast kinetics from the pseudo capacitive electrochemical behavior of microspheres of TiO 2 -B [31].
Further development of TiO 2 in terms of power density and cycling life can be achieved by combining titania nanostructures with with other nanostructure materials such as carbon, CNTs and graphene. The obtained specific capacity of this composite was more than 300 mAh·g −1 in the potential range from 1.0 to 3.0 V vs. Li/Li + , and stability of these composites was proved over few thousands of charge discharge cycles, from low to high currents, namely from 10 mA·g −1 -8000 mA·g −1 , along with very good columbic efficiency. These promising results were possible due to the nanotubes morphology and to the electronic interac-American Journal of Analytical Chemistry tions between the hybrid components [33].

Conclusion
Li-ion batteries have become a prominent technology in the global battery market and are currently manufactured in large scale production to be used in small electronic devices, such as cellular phones, portable computers and mobile electro-optic equipment. Due to their high power and energy density they are also good candidates for automotive applications. There are different types of Li-ion batteries based on different anode materials, cathode materials, electrolytes and separators. Graphite and LiCoO 2 are the well known anode and cathode materials respectively, which used lithium ion batteries. In order to improve the capacity of batteries different researchers can synthesize many materials which used as electrodes. Recently, in addition to graphite, there are many alternative materials used as anodes, such as CNT, graphene, porous carbon, metal (Sn, In, etc.), metalloids (Si, Sb and Ge, etc.) and transition metal oxides (Fe, Mn, Ni, Cu and Co, etc.). In order to further improve the theoretical capacity of anodes in lithium ion batteries, different nanomaterials and their composite can be used. Nanomaterials can improve the capacity of lithium ion batteries because it decreases the diffusion distance of electrodes, and increase surface volume ratio of electrodes. The most commonly used anode materials in lithium ion batteries are: carbon silicon, tin, and metal oxide based nanomaterials. Composite of these nanomaterials can further improve the capacity of lithium ion batteries than single nanomaterial. Lithium ion batteries still now have limitation applying to large electric vehicles, in order to improve these problems or to increase the capacity of anode materials much research is going on.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Funding Statement
There is no funding contribution from the agency. The authors are grateful to Department of Chemistry, Mizan tepi University for doing this review work.