Porous carbon materials, such as microporous (pore size < 2 nm), mesoporous (2 nm < pore size < 50 nm), macroporous (pore size > 50 nm) materials, can provide extra capacities for lithium ion batteries, which are much higher than that of graphite. Porous structures, offer large surface area and providing more active positions to combine Li-ions, which provide extra capacities for lithium ion batteries. Many researchers have demonstrated that micro/mesopores can store large amounts of Li-ions. Therefore, porous carbon materials often result in enhanced capacities and good cycling performances, owing to large surface areas and small mechanical stresses of volume expansion/contraction during cycling [7].

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.

➢ Multiwalled carbon nanotube (MWCNT) rolled up by multilayer 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].

Single wall carbon nanotubes (SWCNTs) are promising nanomaterials which have reversible capacity 1116 mAh·g⁻¹. A purified single-wall carbon nanotube (SWCNT) delivered a capacity larger than 1050 mAh·g⁻¹ [3]. Multi-wall carbon nanotubes (MWCNT) which have high capacity can be formed introducing 10 nm thick layer of Al₂O₃ by atomic layer deposition (ALD) on to electrodes. The resultant MWCNT anode has a stable capacity of 1100 mAh·g⁻¹ in 50 charge-discharge cycles at the current rate of 372 mA·g⁻¹. Al₂O₃ adsorbed to EC (Ethylene Carbonate) molecules of the electrolyte decreases the decomposition of the electrolyte. However, the results are obtained at the laboratory scale and different from result gained with commercial products. Commercial MWCNTs have a typical capacity close to 250 mAh·g⁻¹, and after purification, the capacity rise to 400 mAh·g⁻¹. Li insertion induce large stresses during cycling in lithium ion batteries because MWCNTs close structure does not allow expansion of graphene sheets as in graphite, therefore MWCNTs show a tendency to suffer from 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 is a monolayer of graphite, which contain sp² 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 m²/G resulting in higher lithium storage capacity. Furthermore, its high electron mobility (15,000 cm²/(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 carbons have small graphitic grains with disordered orientation, and are much less relevant to exfoliation. These grains also have nanovoids between

them, resulting in reduced isotropic volume expansion. Nanovoids and defects in hard carbons result in theoretical capacity greater than 372 mAh·g⁻¹ and excess gravimetric capacity. These properties make hard carbons a high capacity high cycle life material. However, hard carbon has high fraction of exposed edge planes which increases absolute quantity of SEI formed and reducing the coulombic efficiency in the first few cycles. Additionally, the void spaces in the grains of hard carbon significantly reduce the density of the particles, further decreasing volumetric capacity [2].

Hard carbon can form composite with Si to increase the capacity of Li⁺ batteries. Kim et al. successfully dispersed nanocrystalline Si (c-Si) in amorphous Si (a-Si) encapsulating hard carbon (HC) as an anode material for fast chargeable lithium-ion batteries. The HC-(c-Si)-(a-Si) anode shows excellent cycle retention 97.8% even after 200 cycles at a 1˚C discharge/charge rate. Furthermore, the LiCoO₂/HC-(c-Si)-(a-Si) full-cell showed excellent rate capability and very stable long-term cycle. Even at a rate of 10˚C discharge/charge, the capacity retention of the full-cell was 50.8% of its capacity and showed superior cycle retention of 80% after 160 cycles at a rate of 1˚C discharge/charge, which indicates the possibility of its practical application [18].

Si-based bulk materials exhibited a marked loss of capacity in a few cycles. Diverse Si nanostructures have been developed to improve the absolute volume change during charging/discharging process. The improvement can be result in shortening of lithium ion diffusion distance and enhancing the electro activity toward Li uptake/release. Controlling the morphology and size of nanowires makes the structure to have good cycling stability. The limited nanowire diameter allows for better accommodation of the large volume change and provides 1D electronic pathways allowing for efficient charge transport. The Si nanowires display high capacities at higher current density. Even at 1˚C rate, the capacities remains 2100 mAh·g⁻¹, and a reversible capacity of over 3000 mAh·g⁻¹ is maintained after 10 cycles [20].

Two-dimensional (2D) Si thin film is another promising nanostructure with improved cycle stability and rate capabilities. Basically, nanosized thin film can minimize the volume variation and retain structural integrity. The cycling stability and the Li accommodation capacity mainly depend on the film thickness as thinner films offer larger accommodation capacity. A 50 nm-thick Si film was found to give a charge capacity over 3500 mA h·g⁻¹ and maintained during 200 cycles at a rate of 2˚C, while a 150 nm-thick film showed around 2200 mAh·g⁻¹ during 200 cycles at 1˚C in the electrolyte of 1 mol/L LiClO₄ in polyethylene carbonate. Although Si thin films offer high specific capacity, good capacity retention and fast charge/discharge rate, the practical application is limited because of their prohibitively high synthesis costs for industry and low active material content [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:

Si ( crystalline ) + x Li + + x e − → Li x Si ( amorphous ) + ( 3.75 − x ) Li + + ( 3.75 − x ) e − (1)

Si ( crystalline ) + x Li + + x e − → Li 15 Si 4 ( crystalline ) (2)

During charge:

Li 15 Si 4 ( crystalline ) → Si ( amorphous ) + y Li + + y e − + Li 15 Si 4 ( residual ) (3)

McCormac et al. encapsulated Si nanoparticles within a highly porous TiO₂ nanofiber matrix using a unique sulphur-templating method combined with electrospinning. After electrochemical characterization, it was determined that the Si/TiO₂ composite nanofibers have specific surface area as large as 387 m²·g⁻¹ and the outside porous TiO₂ nanofiber matrix is able to compensate for 300% volume expansion during the Si lithiation/delithiation process. Therefore, Si/TiO₂ porous nanofibers have high electrochemical performance, which gives a stable specific capacity of above 1586 mA h·g⁻¹ even after 180 cycles at 0.135 A·g⁻¹ [21].

Molybdenum oxide (MoO₃) is also a potential durable material which forms composite with Si because of little chemical and mechanical degradation during repeated lithium ion insertion and de-insertion. Martinez-Garcia et al. designed a silicon decorated MoO₃ nanoplatelet composite material using a hot wire chemical vapor deposition and ultrasonic spray method, which is desirable to realize a high rate and durable, binder free anode in lithium-ion batteries. The specific capacity of the synthesized silicon decorated MoO₃ started at 1475 mA h·g⁻¹ and dropped to about 1000 mA h·g⁻¹ at a cycling rate of 100 mA·g⁻¹ in the second cycle, and then it retained this capacity for over 50 cycles. Even at a high current rate of 10 mA·g⁻¹, the charge capacity in the silicon sprayed MoO₃ composite remained above 1037 mA h·g⁻¹ after 50 cycles, which means there is almost no steady state capacity fading during cycling [18].

The combination of Si and carbon materials can highly improve the specific capacity, cycle stability and rate capability for LIBs. Kim et al. prepared silicon/carbon composite (Si-C) by thermal decomposition and subsequently formed three-dimensional (3D) electrode structures by the laser ablation process. This 3D Si-C electrode with a conformal carbon coating and empty voids is effective in mitigating large volume expansion and achieving a stable cycling performance. The increased performance exhibited a high specific capacity (>1200 mA h·g⁻¹ over 300 cycles) and good rate capability (1170 mA h·g⁻¹ at 8 A·g⁻¹) [22].

The specific capacity and cyclability of nanostructured Si-C composite electrodes are measured by constant discharge/charge testing. The Si-C electrode delivered a discharge capacity of about 2000 mAh·g⁻¹ in the first cycle, but with an irreversible capacity of about 550 mAh·g⁻¹. Part of this irreversible capacity is used to form a SEI layer on the surface of the negative electrode. The reversible capacity was improved from the second cycle [23].

Silicon nanoparticles coated with graphene have become a promising candidate as LIB anode, due to the large surface area, mechanical flexibility, chemical stability and excellent conductivity of graphene. The reversible specific capacity of Si composites can reach 1500 mAh·g⁻¹ after 200 cycles. A very stable cycling performance of the composite has been observed. The capacity remains 1205 mAh·g⁻¹ even after 150 cycles [24].

The cycle performances of anode of different composites of Si nanoparticles and graphene ratios are recorded and visibly at a rate of 100 mA·g⁻¹, all composites have better capacity retention than the Si nanoparticles or graphene, because of better dispersion of Si nanoparticles in the graphene sheet. The synthesis process leads the Si nanoparticles dispersing well on the graphenes, which guarantees that there is abundant interspace between the Si nanoparticles. The abundant interspace can accommodate the volume change of Si nanoparticles in the charge/discharge processes effectively, which also keeps the Si-based anodes to maintain stable in cycling [10].

The composite of 65% Si nanoparticles exhibits the highest capacity after 40 cycles. The high reserve rate of sample 65% Si nanoparticles after 40 cycles is a significant improved property compared with other Si and graphene composite materials. Only the composite with weight ratios of 65:35 has the highest capacity after 40 cycles. This means that the cycle performances of the Si nanoparticles/graphene composite are visibly influenced by the concentration of the graphenes in the composite, which affects the Si nanoparticles dispersion severely. The materials exhibit a low capacity when there are more graphenes due to the lower content of Si nanoparticles. On the contrary, the materials have a larger initial capacity. But the capacity decreases gradually, because there are no enough graphenes to construct the frame work, resulting in the Si nanoparticles aggregating. Thus, the material of 65% Si nanoparticles/35% graphene composite has the highest initial capacity [24].

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].

The theoretical capacity of pure Tin is is three times that of the graphite anode (372 mAh/g), based on the end lithiated phase Li₄·4Sn. Later, crystallographic studies suggested that the realistic form of this end phase could be Li₁₇Sn₄ (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].

Kim et al. reported Sn/C composite by infiltrating tetraethyltin into mechanically milled polystyrene resin powder. The obtained composite has a capacity of 480 mAh·g⁻¹ with good capacity retention over 30 cycles. Recently, Hassoun et al. were able to synthesize carbon-tin composites by infiltration of a tin precursor into an organic gel, followed by calcination under argon. Uniform dispersed tin particles were found in the carbon matrix. The particle size of tin was around 50 nm at the surface and less than 10 nm in the bulk. Also, a small amount of residual SnO₂ (8.8%) are found in the composite. properly designed nano-composites show excellent cycling performance. At a 0.8˚C rate, the electrode delivers a specific capacity on the order of 500 mAh·g⁻¹ and remains stable over more than 200 cycles. At 5˚C rate, the composite recovered 200 mAh·g⁻¹, which is about 40% of its total capacity. Later, Scrosati et al. further improved the synthesis procedures and obtained a pure tin compound without a trace of tin oxide. The tin component in the composite is highly stable stored in open air without undergoing any decomposition, because of the protective carbon matrix. The high purity Sn-C composite delivers a reversible and stable capacity on the order of 450 mAh·g⁻¹ [10].

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 H₂/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/cm². 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].

Sn-based materials have drawn much attention as alternative LIB anodes in both the scientific community and industry. SnO₂ 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, SnO₂ shows attractive lithium storage properties based on the following two reactions:

SnO 2 + 4Li + 4e − → Sn + 2Li 2 O (1)

Sn + x Li + + x e − → Li x Sn       ( 0 ≤ x ≤ 4.4 ) (2)

Reaction (1) is generally considered as irreversible, although it has been suggested to be partially reversible in some studies. The alloying/dealloying reaction described by reaction (2) endows SnO₂ with the capability to accommodate maximally 4.4 Li per Sn, corresponding to a theoretical specific capacity of 790 mA h·g⁻¹ [30].

Titanium dioxide (titania) anode is a very promising material for applications in the LIBs field, and suitable for mass production, cost effective, and TiO₂ has admirable advantages such as high electro-activity, strong oxidation capability, good chemical stability, high abundance and structural diversity. Also, TiO₂ shows excellent safety and stability characteristics at the working potential of 1.5 V vs. Li/Li⁺. These characteristics make TiO₂ a good anode material in LIBs, especially for HEV applications. Titania can host 1 mol of lithium per 1 mol of TiO₂ 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 TiO₂ 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 TiO₂) 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 TiO₂ 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 TiO₂, 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 TiO₂ microspheres by solvothermal process. These TiO₂ microspheres are formed by the combination of ultra-fine 6 - 8 nmTiO₂ nanocrystals with 4 - 6 nm pore size microstructures. The performances of TiO₂ 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 TiO₂ 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 TiO₂-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 TiO₂-B [31].

Further development of TiO₂ 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 interactions between the hybrid components [33].