Reaction Caused by Decomposition of Electrolyte in Amorphous Carbon

Figure 1 shows the charge and discharge curves of graphitic carbon materials under constant charge. Among them, charging is lithium ion insertion, and discharging is lithium ion deintercalation. Theoretically, the insertion and extraction of lithium in carbon materials is completely reversible. From the actual charge and discharge curve, we can see that more lithium is consumed, which is more than the theoretical capacity of graphite of 372 mAh/g, and only 80%-95% is recovered during the discharge process. In the second cycle, fewer lithium ions are inserted during charging, and most of the lithium will be released during discharge.

The difference in capacity (C_irr) in the first charge/discharge reaction is called irreversible capacity loss, and the reversible insertion/extraction capacity of carbon materials (C_rev) is called reversible capacity. In commercial lithium batteries, lithium ions are provided by a lithium metal oxide positive electrode, and the carbon negative electrode does not contain lithium. Because of this, it is very important to minimize the loss of irreversible capacity in the initial stage of charging and discharging. It is well known that the loss of irreversible capacity is due to the decomposition of the electrolyte on the surface of the carbon material. Within the potential range of lithium ion insertion, the electrolyte is in a thermodynamically unstable state on the surface of the negative electrode. In the first charging reaction, the electrochemical reaction caused by the decomposition of the electrolyte generates an SEI film on the carbon surface. For crystalline graphite, the reaction occurs at about 0.8 V (Li⁺/Li), and it appears as a plateau when charged with a constant current. The length of the plateau depends on the degree of electrolyte decomposition. The formation of the protective film requires a large amount of lithium ions and also causes a loss of reversible capacity. However, by selectively migrating lithium ions without electron transfer, further decomposition of the electrolyte is also suppressed, and the cycle characteristics are also improved. The formation of SEI film is largely affected by the composition and characteristics of the electrolyte. The different characteristics of the SEI film depend on the structural characteristics of the material surface. Generally speaking, when the specific surface area of ​​the material is large, the absolute number of SEI membranes will increase, resulting in a greater loss of irreversible capacity. In graphitized carbon materials, the decomposition of the electrolyte occurs more at the end surface, because the end surface has higher electrochemical activity than the base surface.

In addition to the formation of the SEI film, other factors that contribute to the irreversible reaction include the reduction of impurities (such as H₂O and O₂), the reduction of surface functional groups containing oxygen on the surface of the material, and the reduction of the reversibility of lithium that has been reduced without being oxidized. .

For crystalline graphite, lithium ions and polar molecules in the electrolyte can be intercalated between the graphite layers, and the intercalation of lithium ions and polar solvents proceeds at the same time. This reaction enlarges the spacing between the graphite layers and destroys them. Graphite structure. Figure 2 shows the embedding of the solvent, which can also be understood as the exfoliation of the graphite layer.

In the early stage of lithium ion insertion, that is, when the concentration of lithium ions in crystalline graphite is low (x≤0.33 in Li_xC₆), due to thermodynamic reasons, lithium ions tend to be intercalated together with the electrolyte solvent. In order to avoid this situation, additives are added to the electrolyte to form an SEI film on the graphite surface before the lithium ions and the electrolyte solvent are co-intercalated.

Because the simultaneous insertion of lithium ions and electrolyte solvent occurs between the graphite layers on the end surface,therefore, this is difficult to achieve for low crystallinity soft carbon and amorphous hard carbon with structural defects and disordered arrangement characteristics.By coating graphite particles on the surface of low crystallinity soft carbon and amorphous hard carbon, the occurrence of co-intercalation can be suppressed, and the irreversible reaction caused by the formation of the SEI film is also greatly reduced.

The structure and surface area of ​​the carbon material are important factors that determine the characteristics of the electrode-electrolyte reaction. Since the insertion/extraction of lithium ions occurs on the surface of the carbon particles during charging and discharging, the surface structure and surface area are key factors that determine the power characteristics of a battery.

Figure 3 shows the trend of the BET specific surface area of ​​hard carbon as a function of heat treatment temperature. It can be observed from the figure that the BET specific surface area decreases with increasing temperature. This is because the exposed surface or the micropores inside the particles are closed at high temperatures. The reduction of the specific surface area not only inhibits the decomposition of the electrolyte on the surface, but also limits the movement of lithium ions and weakens the rate performance of the battery. These factors should be carefully considered in the material design during the battery production process to ensure that the battery obtains excellent performance.

As summarized in Table 1, the surface of carbon materials is composed of various chemical functional groups. These functional groups affect the chemical and electrochemical properties of carbon materials. Figure 4 shows the results of X-ray photoelectron spectroscopy (XPS) analysis of various functional groups on the surface of carbon materials.

When the degree of oxidation is relatively low, the C-OH bond has a stronger peak intensity, while the peak intensities of the C=O bond and the HO-C=OOH bond are relatively weaker; when the degree of oxidation is relatively high, C-OH The peak intensity of the bond is relatively weak, while the C=O bond and the HO-C=OOH bond exhibit stronger peak intensity. From this, we can know that during oxidation, most of the oxygen reacts with the carbon on the surface of the carbon particles.