The development history of anode materials
In the early stages of the development of negative electrode materials, metal Li was used as a negative electrode material for lithium secondary batteries. In the repeated charge and discharge process, Li metal can reach a higher specific capacity, but the formation of dendritic structure on the surface of metal Li will cause certain safety problems in lithium batteries, such as short circuits. For this reason, the quantitative production of lithium batteries is restricted, and special care must be taken during production. At the same time, it is also necessary to prevent a large amount of heat from being in contact with moisture.
In order to solve these problems, many researches were carried out from the 1970s to the 1980s, which used various materials to replace the metal Li as the negative electrode material. These studies are mainly focused on graphite-like carbon materials, metals and metal compounds that can react with lithium ions.
Since lithium ions can be inserted into the negative electrode and maintained in a stable state, the use of carbon-based negative electrode materials can help solve the safety problem of metal lithium electrodes. In this case, the electrochemical reaction potential of the carbon-based material with lithium ions is close to that of metallic lithium. When lithium ions are inserted or extracted from the carbon-based negative electrode, the product structure of the carbon-based negative electrode material does not change significantly, so the redox reaction can be continued and repeated. These key factors enabled the realization of high energy density and long cycle life of lithium secondary batteries, and they were commercialized in 1991.
The diversified structure of carbon determines the energy storage mechanism of lithium. Therefore, the modification of the carbon-based anode material is to increase its energy storage capacity to obtain a high-performance lithium secondary battery. The large secondary battery market including electric vehicles requires lithium secondary batteries to have high output characteristics. At the same time, in order to achieve a high specific capacity of the negative electrode, research on non-carbon-based negative electrode materials such as silicon and tin is underway. In addition, in order to obtain a high-output negative electrode, it is also necessary to develop carbon-based materials with excellent electronic conductivity and ion conductivity.
Overview of anode materials
During the discharge process of the lithium secondary battery, an oxidation reaction occurs on the negative electrode material, and a reduction reaction occurs on the positive electrode material. For example, in a lithium secondary battery composed of LixC/Li1-xCoO2, the negative electrode material LiXC provides electrons and lithium ions, that is, itself is oxidized. Similarly, the cathode material Li1-xCoO2 receives electrons and lithium ions, that is, Li1-xCoO2 is reduced. During the charging and discharging process, lithium ions are correspondingly stored and released in the negative electrode.
For graphite anode, 1 Li corresponds to 6 C in theory, as shown in the following reaction formula. The voltage range of the graphite negative electrode corresponding to the lithium electrode (Li+/Li) is 0.0-0.25V, and its theoretical specific capacity is 372 mAh/g. The potential of pure graphite is 3.0v, but when lithium is inserted into the graphite, the graphite potential drops rapidly. For the positive and negative electrodes, with the increase of lithium in the electrode active material, the potential of the lithium electrode continues to decrease, eventually reaching 0V.
LixC6→C6+xLi＋＋xe﹣ 0.00 0.25 V vs Li＋/Li
Since the reduction potential of the electrolyte is higher than that of lithium, the electrolyte is decomposed on the surface of the negative electrode during the charging process. The decomposition of the electrolyte not only causes the formation of a solid electrolyte membrane on the electrode surface, but also inhibits the electron transport between the negative electrode and the electrolyte, and further prevents the decomposition of the electrolyte. The performance of lithium batteries is largely affected by the characteristics of the SEI film on the electrode surface. Various methods are being tried to add additives to produce a denser SEI film with excellent electrochemical properties before the electrolyte is decomposed.
The negative electrode material affects the performance of lithium secondary batteries, including energy density, power density, and cycle life. In order to optimize the performance of the lithium secondary battery, the anode material should meet the following conditions
① The negative electrode material should have a low potential, consistent with the standard electrode, and provide a high battery voltage together with the positive electrode. The potential is closely related to the electrochemical reaction, so the potential value of the negative electrode material must be as close as possible to the electrochemistry of metal lithium Electric potential.
②When reacting with lithium ion, the crystal structure of the negative electrode cannot be significantly changed. Structural changes will lead to the accumulation of crystal tension and limit the reversibility of the electrochemical reaction, ultimately leading to a shortened cycle life
③ The negative electrode material should have a high degree of reversibility for lithium ions to participate in the reaction. The ideal reversible reaction has a charge-discharge efficiency of 100%, which means that the reaction efficiency does not change as the cycle progresses.
④The active electrode material in the negative electrode must have a high lithium ion diffusion coefficient, because this is of great importance to the performance of the battery.
⑤The negative electrode material must have high electronic conductivity, so that it can more effectively promote the movement of electrons in the electrochemical reaction.
⑥The negative electrode active material should be relatively dense, so that it has a high electrode density. It is generally considered that this is an important factor for increasing battery energy. For example, the theoretical density of graphite material is 2.2g/ml, and its theoretical capacity density is 818 mAh/ml,
⑦The negative electrode material should store a large amount of charge (coulomb) per unit mass.
Other important factors that determine energy density and power are specific surface area, tap density, particle size and distribution. Since the negative electrode has a high mass specific capacity, it is more difficult to insert and extract lithium ions when compared with the positive electrode. Therefore, when designing the negative electrode, full consideration should be given to promoting the rapid movement of lithium ions to improve the performance of the lithium battery.