Lithium-ion battery anode materials have developed rapidly in recent years. For example, conventional crystalline silicon anodes and oxidized silicon anodes have a specific capacity of more than 1,000 mAh/g. In contrast, cathode materials have been developing relatively slowly, and currently have relatively mature NCAs. The NCM ternary material has a capacity of about 180 mAh/g, although the capacity of some high-nickel NCM materials can reach 200 mAh/g or more, but the cycle performance is often not stable, and the requirements for the production process of high nickel materials are much higher than The traditional LiCoO2 material, so the ternary material to further increase the capacity of the space is not very large, but there is a class of materials, its capacity can easily achieve more than 200mAh / g, and can even do 300mAh / g, can be a lithium-ion battery belt To a large increase in energy density, this material is a lithium-rich material.
Lithium-rich materials have high capacity and low-cost advantages. They can be said to be ideal lithium-ion battery cathode materials. Of course, nothing can be perfect. In order to play a high-li Capacity, to use high-voltage activation, in addition to the formation of Li2MnO3 phase we need, this process will also generate Li2O, but also release active oxygen, which will not only destroy the structure of lithium-rich materials, but also lead to electrolyte Oxidative decomposition causes a high irreversible capacity. In addition, the lithium-rich materials still have a tendency to change from a layered structure to a spinel structure during the cycle, which also leads to a continuous decline in the voltage platform of the lithium-rich material during the cycle, and the capacity is continuously attenuated, making the lithium-rich material to circulate. The performance is poor and the reaction mechanism is shown in the figure below .
Lithium-rich material activation system is an important factor affecting its cycle performance. Israeli scientists Prasant Kumar Nayak et al.  showed that activation voltage and cycle voltage have very significant effects on the cycle performance of lithium-rich materials. For example, they found Li1. .17Ni0.25Mn0.58O2 in a battery that is activated at 4.8V and cycles between 2.3-4.6V. Although the capacity is as high as 242mAh/g, the cycle performance is very poor, and the voltage platform is reduced from 3.62V to 100 cycles. The 3.55V battery that was activated by 4.6V and cycled from 2.3V to 4.3V had a lower specific capacity of only 160mAh/g, but the cycle performance was excellent, and there was no plateau voltage drop for 100 cycles. As shown in the figure, without the high-voltage-activated material, the capacity is only about 100 mAh/g, and it can be seen that the activation system and the circulation system of the lithium-rich material have a huge influence on the circulation of the lithium-rich material.
Lithium-rich materials have the problem of poor interface stability at higher operating voltages. Therefore, element doping and surface coating of materials are the main methods for overcoming the poor cycling performance and voltage drop of lithium-rich materials. Dai Changsong, Harbin Institute of Technology  developed a Se-doped lithium-rich material Li1.2[Mn0.7Ni0.2Co0.1]0.8-XSeXO2, which has a more regular crystal structure than undoped lithium-rich materials. Cationic mixing is also less. Electrochemical tests revealed that the material had an efficiency of 77% for the first time, and it can still exert a capacity of 178mAh/g at a large rate of 10C. Meanwhile, the doped Se element can well suppress the voltage drop of the lithium-rich material. Cycle 100 The sub-capacity decay is only 5%, and the mechanism study shows that Se element inhibits O2- from being oxidized to O2, thereby reducing the material transition from layered structure to spinel structure, thereby increasing the material's rate performance and cycle performance.
Lithium-rich materials have poor interfacial stability, which can easily lead to side reactions and affect the cycle life of the battery. An effective solution is “surface coating”. Materials such as AlF3, Al2O3, and Li3PO4 can be used on the surface of lithium-rich materials. Coating improves the surface stability of lithium-rich materials. HIT University’s Du Chunyu et al.  proposed a SnO2 coating scheme. They used oxygen vacancies in SnO2 to promote the formation of Li2MnO3 structures, which not only improved the cycle performance and rate performance of lithium-rich materials, but also improved The lithium-rich material has a capacity of 264.6 mAh/g, which is 38.2 mAh/g higher than that of the lithium-free material without a coating treatment. This also provides a new idea for surface modification of lithium-rich materials.
Dongrui Chen et al.  of South China Normal University applied Li3PO4 to cover the lithium-rich material by means of a polydopamine stencil method. The thickness of the Li3PO4 cladding layer was only about 5 nm. Li3PO4 coating greatly improved the cycle performance of lithium-rich materials, 0.2C, 2.0-4.8V cycle 100 times, the capacity retention rate was 78%, and the capacity retention rate of lithium-free materials without coating treatment was only 58%, while Li3PO4 coating also significantly improved the rate performance of lithium-rich materials, as shown in the figure below.
The low-temperature performance of lithium-rich materials is also an important factor hindering the application of lithium-rich materials. Guobiao Liu et al.  of the Chinese Academy of Engineering Physics have conducted detailed studies on the mechanism of lithium-rich materials at low temperature, and generally believe that lithium-enriched materials Because of the low amount of Li2MnO3 material produced at low temperature due to activation, the capacity of the material is low, but Guobiao Liu's research found that even if the content of Li2MnO3 in the material is high, the capacity is low at low temperatures. The content of Li2MnO3 is not The decisive factor influencing the capacity of the material, Guobiao Liu believes that the poor electrode dynamics at low temperatures will inhibit the Mn4+/Mn3+ reaction, resulting in a lower capacity of the material. Cycling performance studies have shown that although low temperature results in a lower capacity of lithium-rich materials, it significantly improves the cycle performance of lithium-rich materials, as shown in the figure below (Battery A circulates at 25°C and contains a large amount of Li2MnO3, Battery B, which contains a relatively small amount of Li2MnO3, circulates at a low temperature of -20[deg.] C. Battery C is first activated at 25[deg.] C. and then cycled at -20[deg.] C. and contains a relatively large amount of Li2MnO3). After three cycles of batteries A, B, and C, the capacity retention rates were 68.3%, 80.9%, and 88.1%, respectively. By studying the structure of the lithium-rich materials in the three types of batteries, it was found that the low temperature suppresses the richness very well. Lithium materials transform from a layered structure to a spinel structure, thereby significantly improving the cycle performance of lithium-rich materials.
The main problems faced by lithium-rich materials are poor crystal structure stability and many surface side reactions. Currently, the main solutions are: doping, surface coating, and new activation processes. The main purpose of doping is to stabilize the Ni and Mn elements, and then improve the structural stability of lithium-rich materials. Co doping is a common doping method. Compared with element doping methods, surface coating is a more effective method for improving the performance of lithium-rich materials. The coating materials are divided into electrochemically active materials and inactive materials. Commonly used active materials are spinel materials, spinels. The material has good stability and can significantly improve the performance of lithium-rich materials. However, when the spinel material is below 3V, irreversible phase change occurs. This is also a point we need to pay special attention to when using spinel cladding. . Inactive material coating materials mainly include metal oxides, carbon, and metal fluorides. These materials can significantly improve the interfacial stability of lithium-rich materials and improve the cycle performance of materials. Common coating materials mainly include AlF3, Li3PO4, and ZrO2 and other materials. The activation process of lithium-rich materials has a crucial influence on the structural stability of lithium-rich materials, which leads to a decrease in the surface stability of the lithium-rich material particles during the activation process and an increase in interfacial side reactions, for this reason for the activation system. Research is particularly important (the picture below is a lithium-rich material development diagram ).
After many years of research on lithium-rich materials, people have gradually deepened their understanding of the electrochemical reaction mechanism. Through the adjustment of material structure, element doping, and surface coating, the structure and surface stability of lithium-rich materials have been significantly improved, and the activation system has been adopted. The research shows that the cycle stability and rate performance of lithium-rich materials have been greatly improved. Although it is still difficult to shake up the status of ternary materials in a short time, it is believed that lithium-rich materials will continue to mature as lithium-rich material technologies mature. With the advantage of high capacity, it can become a strong contender for the next generation of cathode materials for high specific energy lithium ion batteries.