Carbon anode materials widely used in commercial lithium-ion batteries can be roughly divided into three categories: hard carbon, natural graphite, and artificial graphite. The advantages and disadvantages of these three carbons are illustrated in the following examples.
- Natural graphite
Currently, natural graphite is the most promising material for lithium-ion battery anode materials due to its low price, low potential and smooth curve, high Coulombic efficiency in suitable electrolytes, and relatively high reversible capacity (330~350 mA h/g). one. On the other hand, it suffers from two major disadvantages: low rate capacity and incompatibility with PC-based electrolytes.
The low rate capacity of natural graphite actually stems from its high anisotropy. As shown in Fig. 1, the graphite flakes exhibit a typical disk-like pattern with greatly shortened dimensions in the direction of the c-axis and greatly widened dimensions in the direction perpendicular to the c-axis. After coating on the current collector (copper foil) and rolling, these particles are perpendicular to the current collector along the C axis [39]. The above inference of the directionality of the graphite fine particles means that the intercalation of lithium ions occurs in the vertical direction of the current. In addition, the resistivity of graphite varies with the direction of the graphite crystals. In the c-axis direction, the resistivity is 1O-²Ωcm, while in the a-axis direction the resistivity is 4X10-5Ωcm[165].
Therefore, the unfavorable orientation of the graphite particles can lead to delayed lithium ion intercalation and insufficient electrical contact between the graphite particles and the copper foil. These factors lead to the low rate capacity of natural graphite, especially at low temperature. To solve this problem, the natural graphite flakes can be ground into small pieces by mechanical grinding method [166]. In this way, the crystallographic orientation in the natural graphite flake particles is distorted to some extent by the individual graphite fragments, as shown in Fig. 1. Many research groups have also attempted to combine small particles of different orientations into larger graphite particles.
The incompatibility of graphite with PC-based electrolytes has been extensively studied. There are usually two ways to solve this problem: one is to modify the electrolyte with additives; the other is to modify the graphite by coating. Saga University and Mitsui Mining have applied superheated steam decomposition (TVD) technology to completely and uniformly coat the surface of natural graphite particles with carbon.
High-quality anode materials have been developed and commercialized. Before TVD carbon coating, small natural graphite fragments have been stacked into large fusiform particles, while the edge layer surface is mostly exposed, as shown in Fig. 2.
This morphology is easy for Li+ intercalation, but easily decomposes the PC electrolyte. Electrolyte decomposition can be substantially suppressed after TVD carbon coating. In addition, the fusiform structure disperses the graphite particles on the copper foil into various directions, which can improve the rate performance of natural graphite.
TVD-coated carbon and fusiform natural graphite have many advantages, but there is still a lot of room to improve their electrochemical performance. For example, the density of TVD-coated carbon (1.86 g/cm3) is smaller than that of natural graphite (2.27 g/cm3), which reduces the energy density of the negative electrode material. In addition, the TVD process increases the preparation cost of the negative electrode material, so carbon-coated natural graphite not only needs to suppress the decomposition of the electrolyte, but also how to reduce the amount of carbon coating. In order to meet the above requirements, it is necessary to reduce the surface activity of the core part (natural graphite). Before TVD carbon coating, the graphite flakes were packed into spherical shape, as shown in Fig. 2.
Since most of the surface of the end faces is hidden in the sphere, the surface of the inert basal plane occupies the outer surface before TVD carbon coating, so only a small amount of carbon is sufficient to coat the remaining end faces. The electrochemical performance of TVD carbon-coated spherical natural graphite is better than that of TVD carbon-coated fusiform natural graphite [172]. In addition, in each spherical natural graphite particle, the high orientation of the graphite layer is largely destroyed, and this morphology is very beneficial to improve the rate capability.
- Artificial graphite
Artificial graphite has many of the same properties as natural graphite. In addition, artificial graphite has many remarkable advantages, such as high purity, and its structure is suitable for smooth intercalation and deintercalation of Li+. However, due to the high temperature (>2800 °C) required to process the soft carbon precursor, its cost is higher, and its reversible capacity is slightly lower than that of natural graphite. Graphitized MCMB, mesophase pitch carbon fiber (MCF), and vapor grown carbon fiber (VGCF) are typical representatives of synthetic graphite anode materials for lithium-ion battery applications in the market today.
MCMB precursors are usually isolated from hot pitches containing mesophase microspheres before graphitization. MCMB is available in different types of fibers such as Brooks-Taylor type, Honda type, Kovac-Lewis type and Huttinger type. In Japan, two major companies, Osaka Gas and Kawasaki Steel Co., Ltd., manufacture MCMB in large quantities, and the product is of the Brooks-Taylor type.
Graphitized MCMB has many advantages:
①High bulk density ensures high energy density;
② Small surface area reduces the irreversible capacity generated by electrolyte decomposition;
③ Most MCMB spherical surfaces are composed of end faces, so it is easier to intercalate lithium ions and improve rate performance;
④MCMB is easy to coat on copper foil.
MCF supplied by Petoca Co., Ltd. is produced from naphthalene spherical mesophase pitch by melt blast method. Figure 3 shows an SEM image of a typical cross-section of a graphitized MCF. MCF has a radial structure on the surface and a layered structure inside. The surface radial structure can smoothly intercalate lithium ions and improve the rate performance. On the other hand, the layered structure in the core seems to keep the carbon fiber stable and avoid the volume change during Li ion intercalation and deintercalation. In addition, the carbon fibers are easily disintegrated into fragments after several charge-discharge cycles, and the cycle performance becomes poor.
VGCF is formed by the decomposition of smoke compounds at 1000~3000 °C using transition metals as catalysts. The characteristic of this type of carbon fiber is that the graphite layers are arranged along the axis of the fiber. Figure 4 shows an SEM image of VGCF (Grasker™) produced by Nikkiso. Since the outer surface of the long fibers is usually composed of basal planes, they are usually cut to about 10 μm long so that the cross-section exposes more of the end face. Graphitized short VGCFs can be prepared by two methods: truncation followed by graphitization and graphitization followed by truncation. The preparation method and the length and diameter of the fibers are the key factors affecting the electrochemical performance.
So far, MAG (mass artificial graphite) produced by Japan’s Hitachi Chemical Co., Ltd. has occupied 70% of the Japanese mobile phone market and has become one of the most commonly used artificial graphites for lithium-ion batteries. The rich pores (thin channels) inside the MAG particles can be filled with electrolyte, thus facilitating the migration of lithium ions in the electrode. If the electrolyte contains functional additives, the same stable and safe interfacial layer is formed inside the MAG particles as elsewhere.
- Hard carbon
If hard carbons can adequately intercalate lithium, they can release high capacities in the low potential range (<0.2V, vs. Li/Li+). In general, it is very difficult to achieve this under normal circumstances. The random arrangement of the graphitic layers of hard carbon provides many spaces to accommodate lithium, however, the diffusion of lithium ions within hard carbon is like a labyrinth, so the diffusion of lithium ions becomes very slow, and the rate capability of hard carbon is usually poor. The space inside the hard carbon also occupies a certain volume. Although the mass specific capacity of hard carbon appears to be much higher than that of graphite, the volumetric specific capacity is indeed much lower than expected. Of course, hard carbon still has many advantages over graphite. For example, at the end of the discharge, the remaining capacity can be displayed by a sloped voltage and capacity graph, which is very valuable.
Kureha Chemical prepared hard carbon (Carbotron® P) from phenol resin [189,190]. This hard carbon has a charge capacity of up to 600mA h/g and discharge capacity of up to 500mA h/go The average distance (d002) between two adjacent graphite layers in Carbotron ® P is up to 0.38nm (compared to graphite d002 = 0.3354nm). After full lithium intercalation, the d002 value of Carbotron® P increased by only 1% (about 10% of the volume expansion of graphite). This shows that the crystal structure of Carbotron® P is very stable during the lithium ion deintercalation process, so its cycle performance is very good. In addition, Carbotron® P is relatively stable in PC electrolyte and is difficult to fall off.
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