① The electrochemical reaction of low crystallinity carbon
Graphite carbon is obtained by heat treatment of soft carbon, but if the heat treatment temperature is lower than 2000°C, it will result in lower crystallinity and greater structural disorder of the stone wish carbon. The details of the reaction of low-crystallinity carbon at different heat treatment temperatures are as follows.
When soft carbon is heated at a temperature lower than 900°C, the resulting carbon has a higher lithium storage capacity than crystalline graphite. And there is a hysteresis phenomenon, that is, the potential of lithium ion deintercalation is higher than that of intercalation. As shown in Figure 1, the lower the heat treatment temperature, the greater the lithium storage capacity of the obtained carbon, and the more obvious the hysteresis.
The high capacity and hysteresis characteristics of soft carbon prepared by heat treatment at low temperature are related to the amount of hydrogen in the carbon material. Figure 2 shows the relationship between the amount of hydrogen and the heat treatment temperature expressed in the H/C atomic ratio. The amount of hydrogen in carbon (H/C atomic ratio) is inversely proportional to the heat treatment temperature. By observing the low crystallinity carbon obtained by heat treatment at a temperature lower than 1200°C, we can assume that the reaction of hydrogen and lithium ions in the soft carbon is beneficial to the large capacity and hysteresis characteristics. From Figure 3 we can see that the voltage plateau of 1 V increases with the increase of the H/C atomic ratio. The increased capacity due to hydrogen gradually decreases with repeated charge and discharge, and the 1V voltage plateau also decreases with the increase of the heat treatment temperature, which will cause insignificant hysteresis characteristics.
The reaction between hydrogen and lithium ions caused by the hysteresis characteristic is related to the hydrogen-lithium bond. Due to the bonding between lithium atoms and hydrogen at the end faces of the hexagonal grid structure, carbon is transferred from sp2 hybrid to sp3 hybrid orbital, so a large activation energy is required during the lithium insertion/extraction process. This is manifested in the hysteresis performance on the charge and discharge curve. At the same time, as the carbon material is repeatedly charged and discharged, the capacity associated with hydrogen gradually decreases, and the cycle performance is also weakened. The high specific capacity characteristics of carbon materials obtained at low heat treatment temperatures can be explained by different reaction mechanisms. Some typical examples are shown in Figure 4.
When the soft carbon is heat-treated at a temperature higher than 1000°C, most of the hydrogen will be released, and the graphite layers will be stacked in parallel. However, they are cross-linked and arranged in disorder on the c-axis. When lithium is intercalated between the graphite layers of crystalline graphite, the surface of the graphite layer is transferred, and the accumulation changes from ABAB to AAAA. However, it is very difficult to realize the intercalation of lithium between the random layers of graphite sheets. Due to the small crystals of the carbon material obtained under low-temperature heat treatment, compared with graphite, there is almost no space between the graphite layers of this material for lithium insertion. The reversible lithium storage capacity changes with the disordered arrangement in the e-axis direction and the size of the graphite layer. The chaotic structure provides multiple active sites for lithium intercalation. Compared with crystalline graphite, because lithium is distributed into different positions, the charge-discharge curve of this material cannot show any platform. Figure 5 shows the charge and discharge characteristics of the soft carbon in the first cycle. We can see that its reversible capacity is 220 mAh/g.
Compared with graphite, soft carbon has a low capacity but has a high specific surface area and a stable crystal structure. Recently, it has been considered as a negative electrode material for lithium secondary batteries for hybrid vehicles. First, the slope of its charge-discharge curve is very large, even under high current, metal lithium electrodeposition is difficult to achieve; secondly, as the specific surface area involved in the lithium reaction increases, its rate performance is also enhanced; finally , It is easy to adjust the charging depth, so the voltage change can be controlled. In order to realize the commercialization of soft carbon, it is necessary to reduce the irreversible capacity caused by surface defects.
②The electrochemical reaction of amorphous carbon
The carbon material prepared from hard carbon is not a stack of graphite layers but an amorphous structure with many micropores. Like soft carbon, when the temperature is lower than 800 °C, hard carbon contains a lot of hydrogen after heat treatment, and the charge-discharge curve is similar to that of soft carbon. Hard carbon treated at a temperature of 1000 °C has a higher reversible capacity. As a large amount of hydrogen is removed, the charge-discharge curve does not show hysteresis characteristics. In Figure 6, a plateau at a low potential of 0.05 V can be observed. The high reversible capacity of hard carbon can be explained by the adsorption of lithium on the surface of the carbon layer or the formation of lithium clusters in the microporous structure. When the heat treatment temperature exceeds 10,000 °C, with the reduction of micropores and the reduction of lithium adsorption space, the capacity of hard carbon is greatly reduced.
As shown in Figure 7, the micropores formed in the carbon material obtained by the high-temperature heat treatment allow the electrolyte to be immersed, and the reversible storage capacity of the carbon material is reduced due to the inability to store lithium.
In the process of lithium insertion/extraction, graphite and soft carbon will expand or shrink by 10% in volume, while hard carbon has no change in volume due to the existence of large-sized micropores. For hard carbon electrodes, since there is no core deformation caused by volume expansion, it can have very stable life characteristics. In hard carbon, carbon migrates quickly through surface pores, so its rate performance is better than soft carbon. Hard carbon is very suitable for batteries that require excellent rate performance, but the manufacturing cost is also relatively high.
Figure 8 is a graph showing the relationship between the size of the reversible capacity per unit mass of soft carbon/hard carbon and the change in heat treatment temperature. When soft carbon is heat-treated at a temperature lower than 1000 °C, it will obtain a fairly high capacity value. This value will decrease to the minimum value around 1800~2000°C. Then as the heat treatment temperature increases, it will reach a theoretical value of 372 mAh/g. At the same time, when the heat treatment temperature of hard carbon is 1000°C, its reversible capacity is as high as 600mAh/go. When the temperature is higher than 2000°C, the micropores in the structure and the space for lithium adsorption are reduced, so the hard carbon The reversible capacity is lower than that of soft carbon. In Figure 8, area 1 shows the graphitic carbon obtained by treating soft carbon when the heat treatment temperature is higher than 2400°C, and area 2 is equivalent to soft carbon or hard carbon treated at 500 ~-700°C. The product contains a lot of Hydrogen, zone 3 represents that hard carbon contains many micropores and there is almost no accumulation of graphite layers.