Lithium-ion batteries are nowadays playing a pivotal role in our everyday life thanks to their excellent rechargeability, suitable power density, and outstanding energy density. A key component that has paved the way for this success story in the past almost 30 years is graphite, which has served as a lithium-ion host structure for the negative electrode. And despite extensive research efforts to find suitable alternatives with enhanced power and/or energy density, while maintaining the excellent cycling stability, graphite is still used in the great majority of presently available commercial lithium-ion batteries. A comprehensive review article focusing on graphite as lithium-ion intercalation host, however, appeared to be missing so far. Thus, herein, we provide an overview on the relevant fundamental aspects for the de-/lithiation mechanism, the already overcome and remaining challenges (including, for instance, the potential fast charging and the recycling), as well as recent progress in the field such as the trade-off between relatively cheaper natural graphite and comparably purer synthetic graphite and the introduction of relevant amounts of silicon (oxide) to boost the energy and power density.
 The anode is the negative electrode of a primary cell and is always associated with the oxidation or the release of electrons into the external circuit. In a rechargeable cell, the anode is the negative pole during discharge and the positive pole during charge. Lithium Anode The anode in the battery deserves an equal say in the overall performance of a battery. For an effective development of a high energy density battery, the use of high capacity electrode materials (anode & cathode) is an essential factor. For such systems, alkali metals are perhaps the obvious choice. The most promising types of advanced batteries currently under production are based on lithium anodes. The choice of the anode material is very much restricted by the need for a high energy content, which is unavoidably linked, to the use of an alkali metal as the main anode material.Lithium is generally preferred, since it can be more easily handled (though with care) than other alkali metals and more significantly, the lightest and the most electropositive among the alkali metal family. Also, the low density of lithium metal (0.534g/cc) leads to the highest specific capacity value of 3.86Ah/g, which stands exceptional. Therefore, lithium batteries possess the highest voltage and energy density of all other rechargeable batteries and are therefore favored in applications related to portable appliances where low weight and small volume are the major constraints. The advantages of using lithium metal as the anode are as follows: Good reducing agent Highly electropositive(so higher voltage is obtained depending upon the cathode used) High electrochemical equivalence High capacity (3.82Ah/g) and energy density (1470Wh/Kg) Good conducting agent Good mechanical stability Ease of fabrication/compact design The essential reaction of metallic lithium anode is very simple: But, in spite of this simplicity, the practical application of Li metal to a rechargeable anode has been very difficult due to some crucial issue. The most important one is that Li metal usually will tend to deposit as a dendrite or mossy structure during charge, and the disordered metallic deposit gives rise to a poor coulombic efficiency. This happens because such a fine Li metal often acts as an active site inducing reductive decomposition of electrolyte components. Part of the deposit may become electrically isolated and shedding may also occur. Furthermore, the fine metallic lithium may easily penetrate into the separator and eventually cause internal short, this resulting in heat generation and contingent ignition. One of the main reasons for the failure of rechargeable lithium systems lies in the reactivity of lithium with electrolytes]. Hence the hazardous nature of Li has paved way to identify some other safer anode materials, possessing comparably the same electrochemical features as that of lithium. Alternate anodes for lithium batteries Carbonaceous materials, which allow the intercalation of Li within the layers, are clearly the most suitable candidates, leading to the popularly known lithium-ion or shuttlecock or Lithium Rocking Chair Batteries (RCB). Most carbon varieties including graphite are gaining importance as attractive candidates of anode materials for rechargeable lithium batteries, because they can accommodate lithium reversibly and offer high capacity, good electronic conductivity and low electrochemical potential (with respect to Li metal). The maximum amount of lithium that can be intercalated within the graphite structure is 1 per 6 carbon atoms, yielding a specific capacity of 372mAh/g. The cost, availability, performance and potential (vs. Li metal) of carbon-based materials are all acceptable and even preferable when compared to lithium metal anode for practical cells. An important evidence for this is the commercial availability of LiCoO2/carbon cells manufactured by Sony Inc. There is no significant swelling of, or stack pressure generation by the carbon electrode on prolonged cycling and therefore Li-ion cells can be constructed as flat or prismatic cells with thin-walled cases or in any other cell configurations. The shortcomings on the deployment of different types of anode materials are displayed in Table :
Graphene, as a fabulously new-emerging carbonaceous material with an ideal two-dimensional rigid honeycomb structure, has drawn extensive attention in the field of material science due to extraordinary properties, including mechanical robustness, large specific surface area, desirable flexibility, and high electronic conductivity. In particular, as an auxiliary material of electrode materials, it has the potential to improve the performance of lithium-ion batteries. However, wide utilization of graphene in lithium-ion batteries is not implemented since tremendous challenges and issues, such as quality, quantity, and cost concerns, hinder its commercialization. There remains a debate whether graphene can act as an impetus in the evolution of lithium-ion batteries. In this review, we summarize the desirable properties, several common synthesis methods as well as applications of graphene as the anode in lithium-ion batteries, seeking to provide insightful guidelines for further development of graphene-based lithium-ion batteries. With high energy density and favorable cyclic stability, lithium-ion batteries (LIBs) have been widely applied as the main power supplies of portable electrical devices, such as mobile phones, laptops, and other digital products. However, the limitations of traditional anode/cathode materials including the poor energy capacity, short cyclic life, and low power density, have impeded the development of high-performance LIBs for electrical vehicles and energy storage systems. During cycling, the graphite-based anode is likely to undergo layers exfoliation and mechanical fracture due to constant volumetric changes and the formation of lithium dendrite, detrimental to the cyclic performance of the batteries. Constant decomposition and recovery of solid-electrolyte-interface (SEI) films are major consumption of recyclable lithium and electrolyte, leading to capacity fading, especially at high temperature. Under decades of development, the new anode materials are required to meet the demand for further application or marketization, in following aspects: 1) high specific capacity for the development of high cruising batteries for electrical vehicles; 2) high safety performance with high thermal stability and structural stability; 3) high rate performance with moderate electrochemical reactivity from kinetics perspective. Graphene with attractive behaviors is promising to make revolutionary breakthroughs in the field of electrochemical energy storage. Graphene formed by a flat monolayer of carbon atoms with the structure of two-dimensional (2D) honeycomb lattice, has drawn extensive attention. Plentiful efforts have been devoted to exploring its properties and applications. As a basic building block for graphitic material, graphene can be processed into the 0-dimensional fullerenes, 1-dimensional nanotubes or even 3-dimensional graphite[5]-[9]. In comparison with graphite, graphene has larger specific surface area, higher electronic and thermal conductivity. The performance of graphene-based LIBs is promoted significantly due to its inherent excellent properties. For instance, the exceptional high electronic conductivity enables the electron to move freely and thus decreases electrical impedance and polarization during the electrochemical process, accelerating the electrochemical process. Substantial heat release can be significantly reduced during cycling on account of the relatively low resistance, guaranteeing the moderate working temperature for the battery pack. In addition, during lithium insertion and exertion upon electrode, the inevitable swell and flex may possibly lead to destruction and pulverization of morphological topology and microstructure, degrading the cyclic stability and service life. The desirable mechanical behaviors of graphene , can maintain the structural stability of functional materials and address the problems mentioned above, In addition, the large specific area of graphene provides mass channels for ionic transport and intercalation, promoting the rate performance of LIBs in theoretical dynamics. Hence, graphene is promising to assist in developing significantly enhanced LIBs from all-round dimensions. The rise of graphene contributes to the increasing investigations of other 2D inorganic materials, including transition metal oxides, transition metal sulfides, and other graphene analogues (silicene, phosphene, borophene)[16]. The success in the exfoliation of these 2D functional materials leads to the applications in high-efficiency energy storage devices. Layered silicon-nanosheets with high theoretical capacity are thermally unstable upon active interaction, which is the major disadvantage in compared with graphene. Unlike graphene, the empty d-orbits make silicon-based nanosheets perturbed under exterior disturbance, such as light radiation. Layered black phosphorus can be mechanically removed from the bulk volume. Its anisotropy presents desirable properties in optical, electronic, chemical application. Layered black phosphorus shows wide bandgap of black phosphorus (0.76 eV to 2.1 eV). However, black phosphorus suffers from environmental instability and poor conductivity[17], 2D transition metal oxide and sulfide have attractive applications in LIBs, supercapacitors, solar cell and other devices, due to their highly-active catalytic property. However, in comparison with graphene, they are inferior in the charge mobility and specific surface area and cost-efficiency. The term graphite is derived from the Greek word “graphein,” which means to write. The material is typically grayish-black in color, opaque, and has a radiant black sheen. Graphite is a distinct material as it displays the properties of both a metal and a non-metal. Although graphite is flexible, it is not elastic and has high electrical and thermal conductivity. It is also chemically inert and highly refractory. Since graphite displays low adsorption of X-rays and neutrons, it is very valuable in nuclear applications. This uncommon combination of properties is due to graphite’s crystalline structure. The carbon atoms are set hexagonally in a planar condensed ring system. The layers are stacked parallel to each other. The atoms within the rings are bonded covalently, while the layers are loosely linked together by van der Waals forces. Graphite has a high degree of anisotropy, which is caused by two types of bonding acting in different crystallographic directions. For example, graphite’s ability to develop a solid film lubricant is the outcome of these two contrasting chemical bonds. As weak Van der Waals forces control the bonding between each layer, they can slide against one another, making graphite an ideal lubricant. In 2000, worldwide graphite production was estimated to be about 602,000 tons, with China as the largest producer followed by India, Mexico, Brazil, and the Czech Republic. Graphite Classifications Graphite can be divided into two main types—natural and synthetic. Natural Graphite Natural graphite is a mineral composed of graphitic carbon. It varies considerably in crystallinity. Most of the commercial (natural) graphites are mined, and typically contain other minerals. After graphite is mined, it usually requires a considerable amount of mineral processing like froth flotation to concentrate the graphite. Natural graphite is an excellent conductor of heat and electricity, stable over a broad range of temperatures, and a highly refractory material with a high melting point of 3650 °C. There are three types of natural graphite: High crystalline Amorphous Flake Crystalline Graphite It is said that crystalline vein graphite came from crude oil deposits that have transformed into graphite through time, temperature, and pressure. Vein graphite fissures typically measure between 1 cm and 1 m in thickness and usually have a purity of more than 90%. Although this type of graphite can be found globally, only Sri Lanka commercially mines it, using conventional shaft or surface mining techniques. Amorphous Graphite Amorphous graphite is the least graphitic among the natural graphites. However, the term “amorphous” is incorrect as the material is still crystalline. Amorphous graphite can be found as minute particles in beds of mesomorphic rocks such as coal, slate, or shale deposits. The graphite content varies from 25% to 85% according to the geological environment. Conventional mining techniques are used to extract amorphous graphite, which occurs mainly in Mexico, North Korea, South Korea, and Austria. Flake Graphite Flake graphite can be found in metamorphic rocks evenly spread through the body of the ore or in concentrated lens-shaped pockets. The range of carbon concentrations varies from 5% to 40%. Graphite flake can be found as a lamella or scaly form in specific metamorphic rocks such as limestone, gneisses, and schists. Froth flotation is used to extract flake graphite. “Floated” graphite has 80%–90% graphite content. Over 98% of flake graphite is made using chemical beneficiation processes. Flake graphite can be found in numerous places worldwide. Synthetic Graphite Synthetic graphite can be produced from coke and pitch. Although this graphite is not as crystalline as natural graphite, it is likely to have higher purity. There are basically two types of synthetic graphite. One is electrographite, pure carbon produced from coal tar pitch and calcined petroleum coke in an electric furnace. The second is synthetic graphite, produced by heating calcined petroleum pitch to 2800 °C. Essentially, synthetic graphite has higher electrical resistance and porosity, and lower density. Its enhanced porosity makes it unsuitable for refractory applications. Synthetic graphite contains mainly graphitic carbon that has been attained by graphitization, heat treatment of non-graphitic carbon, or chemical vapor deposition from hydrocarbons at temperatures over 2100 K. Recent demand for electric and hybrid vehicles, coupled with a reduction in prices, has caused lithium-ion batteries (LIBs) to become an increasingly popular form of rechargeable battery technology. According to a new IHS Isuppli Rechargeable Batteries Special Report 2011, global lithium-ion battery revenue is expected to expand to $53.7 billion in 2020, up from $11.8 billion in 2010.1 However, graphite (Prod. Nos. 496596, 636398, and 698830), the traditional anode material in lithium-ion batteries, does not meet the high energy demands of the advanced electric and hybrid automobile market due to its limited theoretical specific capacity of ~370 mAh g−1.2 This has led to the proposal of a large number of anode materials with enhanced storage capacity, high energy density, and improved cycle characteristics for lithium-ion batteries over the last decade.3-7 Table 1 summarizes the properties of several different anode materials. Among these advanced anode materials, Si has attracted substantial attention as an alternative for Li-ion batteries, primarily due to 1) its specific capacity of 4,200 mAhg-1 and volume capacity of 9,786 mAh cm-3, the highest known for a LIB anode; 2) relatively low working potential (0.5 V vs. Li/Li+); and 3) the natural abundance of element Si and its environmental benignity. However, the practical implementation of Si anodes is still blocked due to three major problems. First, poor cycle-life of silicon materials results from pulverization during the huge volumetric fluctuations (>300 %) which accompany lithium ion intercalation and deintercalation. Second, drastic irreversible capacity loss and low coulombic efficiency is caused by mechanical fracture of Si anodes during the alloying/dealloying process. Finally, the solid electrolyte interphase (SEI) breaks as the nanostructure shrinks during delithiation. This results in the exposure of the fresh silicon surface to the electrolyte and the reformation of the SEI, resulting in the SEI growing thicker with each charge/discharge cycle, as shown in Nanostructured Silicon Anode Materials To address these issues, several strategies have been developed to accommodate the huge volumetric changes. One effective strategy is to reduce the active particle size to the nanometer range, at which point, nanosized particles can accommodate significant stress without cracking, as well as decreasing the electronic and ionic transport distance. Moreover, the high density of grain boundaries in nanomaterials also provides a fast diffusion path for Li ions and acts as additional Li-storage sites.13-16 Huang et al. have shown the effect of Si nanoparticle size on the release of structural stress by in situ transmission electron microscopy (TEM) and suggested the stored strain energy from electrochemical reactions was insufficient to drive crack propagation in Si nanoparticles if the particle diameter is <150 nm .Recently, Kim et al. reported 5, 10, and 20 nm-sized Si nanoparticles can be synthesized under high pressure at 380 °C by using various surfactants.18 Cycling these materials between 0 and 1.5 V at a rate of 0.2 C, a capacity of 2,500 mAh g-1 can be achieved for over 40 charge/discharge cycles with capacity retentions of 71, 81, and 67%, respectively. Si-based Carbon Composite Anode Materials Another approach to overcome the volume change during cycling is to form a composite material.23 The matrix does not experience significant volumetric change, which may buffer the expansion of silicon, maintain the structural integrity of the electrode, and enhance stability by reducing silicon aggregation or electrochemical sintering.10 One promising research area is silicon-based carbon composites, the benefits of which are attributed to the improved electric conductivity and the expansion buffering effect of a carbon matrix.24-27 In addition, the carbon additives have the advantages of exceptional ionic conductivity and Li- storage ability.28,29 However, a conformal carbon coating on Si active material would rupture during cycling, resulting in Si exposure to electrolytes and additional SEI deposition. Therefore, a form of carbon coating that can accommodate the large volume fluctuation of Si is necessary.

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