①The electrochemical reaction of graphite
During the charging process, the carbon material participates in the reduction reaction, and lithium ions are inserted into it to form LixC compounds. During the discharge process, an oxidation reaction occurs, and lithium ions are released from the carbon material. In the charge-discharge reaction, with the difference in crystallinity, microstructure and particle shape of carbon materials, its electrochemical characteristics such as reaction potential and lithium storage capacity will be different.
In graphite, lithium ions are embedded in the material through defects in the end surface or base surface. Most of the intercalation reactions proceed when the voltage is lower than 0.25 V. In the initial intercalation stage, if the concentration of lithium ions is low, a single lithium ion layer will be formed, and lithium ions will not be intercalated in adjacent graphite layers. The graphite layers that do not contain rational ions are arranged periodically, and the concentration of lithium ions in the graphite will increase. As more lithium ions are inserted into it, the number of graphite layers without rational ions between the lithium ion layers will decrease. In the LiC6 composition, the number of intercalation of slang ions is the largest, and the lithium ion layer and the graphite layer are arranged at intervals in sequence. This gradual intercalation of lithium ions between the graphite layers is called a staged phenomenon, as shown in the description in Figure 1.
As shown in Figure 1, in the charge potential curve measured under constant charge, we can observe the stage of lithium ion intercalation in the form of continuous flat potential. The platform potential means the coexistence of the two phases. As the lithium concentration increases, the high-order is transformed into the low-order. When lithium ions are extracted (discharge), the opposite process occurs. When lithium ions are intercalated, the graphite layer is converted from ABAB to AAAA. In the fully charged stage 1 of LC6, two adjacent graphite layers are arranged in a highly ordered sequence, and their interplanar spacing is 0.370 nm, as shown in Figure 2a. In LiC6, the lithium ions embedded in the graphite layer are not arranged closely, and the distance between them is 0.430 nm, as shown in Figure 2b. Corresponding to the theoretical capacity of graphite, the specific capacity per unit mass is 372 mAh/g.
In the design of lithium secondary batteries, the volume expansion caused by the change of the graphite structure during the charging process should be considered. If this factor is not taken into consideration, the expanded negative electrode will cause deformation of the electrode, which is not conducive to the cycle life of the battery and other battery performance indicators.
②Design of graphite particles
An example of artificial graphite is MCMB (Mesophase Carbon Microspheres). When raw materials such as coal tar, pitch, and petroleum coke are heated to 400°C, anisotropic pellets are formed. Through the thermal decomposition and condensation reaction, the planar molecules of the polycyclic aromatic hydrocarbon compound will stack in one direction, and finally form the layered structure shown in Figure . 3. Due to the difference in viscosity and the surrounding environment, the small ball will exist in the form of liquid crystal. When the pitch is in the form of semi-coke, the small spheres are transformed into a mesophase with optical anisotropy and fluidity. This mesophase sphere is called MCMB, which is a typical artificial graphite.
Figure 4 shows the SEM image of the commercial MCMB-25-28 (average particle size of 25 um, graphitization temperature of 2800 °C) graphite negative electrode. Figure 4b is the enlarged image of Figure 4a. Observe the enlarged image. It can be found that spherical particles have been formed in the liquid crystal.
MCMB grows up in a liquid medium. Due to the amorphous phase on the particle surface, the graphite layer is not directly exposed to the electrolyte. The capacity of MCMB is generally around 320 mAh/g, but by reducing the number of amorphous phases, its capacity can be increased to 340 mAh/g.
Figure 5 shows the charge curve and discharge curve of artificial graphite MCMB. When charging starts, the voltage drops sharply, and then it is at a plateau voltage, which indicates that the crystallinity is good. Also note that its capacity is about 325 mAh/g. Before the use of natural graphite, MCMB was used as a negative electrode material for lithium primary batteries due to its high capacity. The shape of the particles in MCMB is also suitable for the manufacture of electrodes.
Figure 6 shows a schematic diagram of the particle shape and graphite layer arrangement in mesophase pitch-based carbon fiber (MPCF), which is a kind of artificial graphite. We can see that the synthesis of various types of MPCF depends on the arrangement of carbon regions. A radial or linear shape is more conducive to the improvement of rate performance, but excessive exposure of the end face leads to an increase in irreversible capacity. On the other hand, the onion skin-like morphology greatly reduces the irreversible capacity by exposing the end face as little as possible, but it also limits the diffusion of lithium into the graphite layer. The fibrous negative electrode material has a lower compaction density, and the density will rebound after compaction, which leads to the expansion of the electrode. The lower energy density will cause the material as a negative electrode to have many limitations in the battery application of mobile phones and notebook computers. Recently, these materials have attracted attention because of their excellent durability in hybrid electric vehicles.
MPCF-3000, prepared by heat treatment at 3000°C, is a commercialized fibrous artificial graphite. Figure 7a shows the shape of the synthesized MPCF-3000, and Figure 7b shows the material ground through spherical zirconia. The grinding density can be increased by changing the size of the fiber. Too much fiber is not conducive to the uniformity of slurry viscosity during slurry manufacturing or electrode coating. Other examples of graphitized materials are graphitized carbon black, graphitized nanofibers, and multi-walled carbon nanotubes.
Figure 8 is an SEM image of natural graphite. From Figure 8a, we can see that the unprocessed natural graphite exists in the form of flat flake particles. The capacity of natural graphite is close to its theoretical value. The high specific surface area of irregular flakes and the electrolyte decomposition caused by the direct exposure of the end face may cause irreversible reactions. In the electrode manufacturing process, the flaky particles are not conducive to the coating process of the active material slurry. In addition, the difficulty of pressing after the coating process can lead to undesirable electrode density. Therefore, in order to reduce irreversible reactions and improve processing performance, natural graphite needs to be ground and reorganized to obtain a smooth surface, as shown in Figure 8b. By coating the asphalt layer to prevent direct exposure of the end face, irreversible reactions that damage the end face will be reduced. The electrochemical performance of natural graphite has also been improved. The scaly natural graphite can also be reconstituted into the spherical structure shown in Fig. 8c by grinding. This process minimizes the specific surface area, reduces the decomposition of the electrolyte on the surface of the active material, increases the compaction density of the electrode, and improves the uniformity of electrode coating.
The natural graphite extracted by the above method may be commercialized, and the capacity of graphite of 365 mAh/g is conducive to increasing the capacity of the battery. However, the large capacity caused by the increase in the concentration of lithium ions per unit surface area will cause a decrease in rate performance.