Peking University New Energy Materials and Devices Group: Major Progress in the Research Area of Lithium Battery Negative Pole

Advanced lithium-ion batteries with outstanding energy storage advantages in terms of capacity, safety and stability have become an indispensable part of people's daily work and life. They have been widely used in portable electronic products, electric vehicles, and even power grid peaking and other secondary power systems. However, since it was widely used in the 1990s, the specific capacity of lithium-ion batteries has not significantly increased, so it is increasingly unable to meet the requirements of smart-phones for long standby time, electric vehicles for longer runs, and the grid for peak power storage. The fundamental reason for this dilemma is that the electrode material capacity of lithium batteries is difficult to break through. For example, commercial negative electrode materials can only use low-specific capacity carbon-based materials with a theoretical capacity of 372 mAh/g. Although studies have shown that elements such as Si, Ge, and Sn have a high specific capacity as negative poles, they are limited by the rapid decay of capacity after multiple uses and are difficult to apply in practice. In recent years, SnO2 negative electrode materials have received great attention for their superior cyclic properties. Their theoretical capacity(783 mAh/g) has reached twice the negative electrode of graphite. However, the existing SnO2 and the elemental negative electrode materials can not overcome the bottleneck of volumetric expansion during the electrochemical process of lithium ion batteries, and the cyclic stability is difficult to meet the application needs. Therefore, how to develop a new high cyclic stability/high capacity SnO2 lithium electrode material is of great significance.

Recently, the New Energy Materials and Devices Group of the School of Chemistry and Molecular Engineering of Peking University and the Institute of Silicate of the Chinese Academy of Sciences, the University of Pennsylvania, and the Beijing University of Technology have jointly researched a black tin dioxide nanometer based on original preparation technology. The material, which has a reversible capacity of 1340 mAh/g as a lithium negative electrode, is far superior to the theoretical capacity limit of SnO2 (783 mAh/g). The material is more excellent in cycle stability and rate performance after being compounded with graphene. The capacity is not attenuated after circulating 100 cycles at a current density of 0.2 A/g, maintaining a capacity of 950 mAh/g; a large current at 2 A/g. It is maintained at a capacity of 700 mAh/g.

Through in-depth and detailed research, the researchers realized that the unique new black tin dioxide material is different from the existing tin dioxide, has the characteristics of excellent electron conductivity and rich oxygen vacancies, and induces an isotropic reduction reaction of Nano active materials. Thus, a highly thermodynamic and highly stable Sn and Li2O uniformly dispersed microscopic composite Nanostructures were formed, which finally solved the scientific problem of metal Sn agglomeration during the cycle process. The researchers were surprised to find that this special microscopic composite nanostructure can ensure that metal tin is completely reversible oxidized to tin dioxide in the electrochemical reaction of energy storage. This phenomenon and mechanism have not been reported in the literature. Based on the new storage mechanism, the theoretical capacity of tin dioxide negative electrode materials has been increased from the original 783 mAh/g to 1494 mAh/g of the new mechanism. The black tin dioxide invented by the researchers provides a new idea for the design and synthesis of other new types of electronegative materials, and it also has the industrial application value of high-capacity lithium electronegative materials.

The results of the study were published on April 21, 2017 as "ArbustandCondutiveBlackTinOxideNanostructure-IonBatteriesPossible", published on April 21, 2017. The first graduate student of the International Top Science Journal, Dongwujie, and the first graduate student of the Chinese Academy of Chemical Sciences, Xujijian(DOI: 1002/2017), and the Chinese Academy of Chemical Sciences, were the first graduate students of the University of Peking University. Professor Huangfuqiang is the communication author. The project is supported by the National key basic research and development plan, the National Natural Science Foundation Committee, the Shanghai Municipal Science and Technology Committee and the main research projects of the Chinese Academy of Sciences.

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