Lithium metal has a body-centered cubic structure (bcc), has a strong tendency to ionize, and has an atomic radius of 0.76Å. It has small atomic mass (6.941) and low density (0.534g/cc, standard electrode potential is very low (-3.04VSHE). The specific capacity of lithium is as high as 3860 mAh/g. However, the reason why batteries using lithium metal electrodes have not been commercialized It is the metal’s low melting point of 180.54°C and safety issues due to lithium dendrite growth. Recent studies are trying to stabilize the lithium metal surface by coating polymers or inorganic substances. Despite many efforts, However, metal lithium is still accompanied by many difficulties, such as the danger of explosion when it is exposed to water (moisture) and the complexity of the electrode manufacturing process.
If these difficulties of potassium metal electrodes are overcome, metal lithium may become the negative electrode of lithium secondary batteries. Recently, some companies have tried to use metallic lithium as an auxiliary material. After the metal is added, the initial irreversibility of the negative electrode is compensated by the oxidation of lithium. Therefore, in order to increase the energy density of the battery, it is necessary to prevent excessive consumption of lithium resources in the positive electrode.
There are many types of carbon materials, for example, sp2-hybrid orbital graphite, sp3-hybridized orbital diamond, and sp–hybridized orbital carbon block. These carbon allotropes have different structures and physical and chemical properties. Some of them allow lithium to be inserted and extracted. These carbon materials can be used as negative electrode materials for lithium-ion batteries and can be divided into graphite-based materials and non-graphite-based materials.
Since the commercialization of Cheng secondary batteries in 1999, Shi Yu and other carbon materials have been used as anode materials for lithium batteries. From the early development stage, the performance of the battery has been greatly improved, including a double increase in energy density. As the charging voltage of the positive electrode increases, the specific capacity of the negative electrode material is allowed to increase from 170 mAh/g of amorphous hard carbon to 360 mAh/g of high-density graphite.
As mobile devices become more lightweight, compact and multifunctional, it is required that the energy density of lithium secondary batteries should be increased to meet the requirements of long-term operation of the device. For commercially available graphite, its lithium storage capacity (LiC6) is limited to 372 mAh/g (or 820 mAh/cm3). This problem can be solved by using a negative electrode material with a larger lithium storage capacity. In addition to graphite, Si and Sn are excellent high-capacity materials, and they can react with lithium to form alloys. Research on various alloys related to these metals is actively underway.
Figure 1 shows the relationship between the voltage and energy density of a typical negative electrode material. The working voltage of Sn and Sn alloy is 0.6 V, which is 0.2 V higher than that of Si and Si alloy. The metals Sn and Si both show high specific capacity similar to lithium metal. Unalloyed metals will cause volume expansion during charging. Therefore, more research is needed for commercial applications. Other elements such as Al, Ge, and Pb have low reversible reaction efficiency and high average working voltage, so they are not suitable for negative electrode materials.
Because metals and alloys containing Si have a higher voltage than graphite, when they are applied to actual batteries, the potential difference between the electrodes will be lower, that is, the voltage of the battery will decrease. Figure 2 shows the discharge curves of LiCoO2 for the positive electrode and graphite and Si-C for the negative electrode. Compared with LiCoO2/Si-C, the discharge curve of LiCoO2/graphite has a relatively high average voltage, but the high capacity of LiCoO2/Si-C varies greatly with the cut-off voltage. When the cut-off voltage is set at 3.0V, the capacity is increased by 10%: when the cut-off voltage is reduced to 2.5V, the capacity is increased by 15%. Metal Si and other electrode materials with high voltage ratio may be used as the most negative electrode, but the actual energy density is much lower than expected. These characteristics must be carefully considered when designing the battery.
Carbon-based materials are mainly used as anode materials for lithium secondary batteries. Artificial graphite was generally used in the past, but it is now being replaced by natural graphite. New negative electrode materials represented by silicon and tin are being considered to overcome the shortcomings of low theoretical capacity of graphite and improve battery performance. However, due to the excessive volume expansion and short cycle life caused by the separate use of silicon and tin, they are moving towards the direction of preparing composite materials by compounding with carbon.
Thermal stability is a very important issue for lithium secondary batteries, and research on it is now very active. The focus of research is especially on the three aspects of thermal reaction mechanism, heat generation and heat dissipation rate to prevent thermal runaway in the battery.
High-energy and high-power lithium secondary batteries will be widely used in energy storage and hybrid electric vehicles. Carbon materials with high stability and excellent charge-discharge characteristics are being studied for use as negative electrode materials. At the same time, non-carbon materials will be applied to small batteries, and after further development, they will be applied to high-capacity, high-power batteries.