Anode materials for lithium-ion batteries

Since the birth of lithium-ion batteries, related anode materials researched mainly include carbon-based materials (graphitized carbon materials and amorphous carbon materials), alloys, oxides, silicon-based materials, etc. As a negative anode material for lithium-ion batteries, the following basic requirements must be met:

①The oxidation-reduction potential of lithium ion to the negative anode material is as low as possible, close to the potential of metal lithium, so as to ensure the high output voltage of the battery;

②The reversible degree of lithium ion desorption/intercalation in the negative anode material should be as large as possible, the first irreversible capacity is small, and the Coulomb efficiency is high;

③In the whole process of extraction/intercalation, the negative anode material has a small change in unit cell volume, and has high structural stability, chemical stability and thermal stability, which helps to maintain the stability of the electrode and maintain the cycle capacity;

④The diffusion coefficient of the main material for lithium removal/intercalation is relatively large, in order to improve the charge and discharge efficiency of the battery and the lithium ion insertion/extraction speed, which is conducive to rapid charge and discharge;

⑤The electronic conductivity and ionic conductivity of the negative anode material should be as large as possible, so as to reduce polarization and facilitate charging and discharging;

⑥The host material has a good surface structure and can form a good SEI (solid-electrolyte interface) membrane with the liquid electrolyte;

⑦The insertion compound has good chemical stability in the entire voltage range, and will not react with electrolytes after forming the SEI film;

⑧From a practical point of view, negative anode materials should be selected as far as possible from materials with abundant resources, cheap prices, wide sources, and simple preparation processes to reduce battery costs, etc., and the negative anode materials should be stable in the air, non-toxic, and environmentally friendly.

At present, the commercial anode materials for lithium-ion batteries are mainly carbon-based materials, including graphite, hard carbon, and soft carbon. Carbon materials have the advantages of low electrode potential (<1.0Vvs.Li/Li+), high cycle efficiency (>95%), long cycle life, good safety performance, abundant sources, cheap and easy to obtain, non-toxic and harmless, etc. The research on carbon materials used in lithium-ion battery anodes began in the 1980s, and LixC6, an intercalation matrix anode, was initially used to solve safety problems. Replace metal lithium. The first commercially available anode material for lithium-ion batteries is petroleum coke. Coke-based materials have a high specific capacity (180mA•h/g), and have good compatibility with the electrolyte, and can stably exist in the electrolyte of propylene carbonate. However, its large specific surface area increases the area of ​​the SEI membrane, and the irreversible capacity is larger for the first time. In addition, the disorder of coke materials is considered to hinder the improvement of specific capacity, so graphite electrodes are gradually gaining attention. Spherical graphite electrodes, especially mesophase microspheres (MCMB), have a higher specific capacity (350mA•h/g) and a low specific surface area. However, the theoretical capacity of carbon materials is relatively low (the specific capacity is 200~400mA•h/g) and has reached the limit of the theoretical capacity. It has been unable to meet the high-capacity requirements of the increasingly developed electric vehicles for lithium-ion batteries. Therefore, research The author began to research new materials.

The anode materials under study are: a. Alloy materials; b. Metal oxide series; c. Other anode materials. Among the many anode materials for lithium-ion batteries, the specific capacity of transition metal oxides is generally higher than that of traditional carbon materials, which is conducive to the development of a new generation of large-capacity lithium-ion batteries. At the same time, the discharge platform of metal oxides is generally higher than that of graphite, which can avoid the generation of lithium dendrites to a certain extent, which is conducive to improving battery safety performance. The application of transition metal oxide MxOy (M=Mn, Fe, Co, Ni, Cu) to lithium-ion batteries began in 2000 with the pioneering work of J.M. Tarascon and others. Different from the traditional intercalation reaction (carbon material) and alloy reaction (Si and Sn) mechanisms, transition metal oxides store lithium through a reversible “conversion reaction” with lithium. Generally speaking, Li2O is very stable and will not react with metal M under normal circumstances, but the size of the transition metal particles M produced is only a few nanometers, so it is highly electrochemically active and can decompose Li2O. Transition metal oxides usually have a large first irreversible capacity loss, and the first coulombic efficiency is generally between 40% and 70%. This is because the electrolyte will undergo a side decomposition reaction on the surface of the transition metal to form an SEI film, which needs to consume lithium ions, and the SEI film will not be completely decomposed in the subsequent charging reaction process. However, transition metal oxides are not as good as carbon materials in terms of cycle stability, and the first-time coulombic efficiency is relatively low. Therefore, how to solve the cycle stability of transition metal oxide anode materials and improve the first coulombic efficiency of anode materials is a crucial issue for lithium-ion batteries.

Although we have introduced the basic requirements that must be met as anode materials for lithium-ion batteries, it is difficult for existing anode materials to meet the above requirements at the same time. Therefore, research and development of new anode materials with better electrochemical performance has become a hot topic in the field of lithium ion battery research.

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