NiMH No.7 batteries

　　New breakthroughs made in research on cathode materials for nanometer super-capacity NiMH No.7 batteries

　　Electric vehicles will become an important green means of transportation in the future, and there is an urgent need to develop new high-capacity, high-stability and high-safety NiMH No.7 batteries. Scientists are also constantly trying various methods to improve the performance of NiMH No.7 batteries. Nanonization is a common method to improve the electrochemical properties of materials, especially for materials with low conductivity such as lithium iron phosphate, which has a significant improvement effect. The advantage of nanometerization is that the transmission path of lithium ions is shortened and better rate performance can be obtained. Compared with bulk materials, the disadvantages of nanometerization include the reduction of the binding energy of surface lithium and interface atoms, which will lead to a loss of capacity and a decrease in voltage; the large specific surface area after nanometerization will bring more active sites, a large number of The contact between the active sites and the electrolyte will also become an important factor affecting the charge and discharge stability of NiMH No.7 batteries; the reduction in tap density and energy density caused by nanotechnology is a problem that cannot be ignored in industrial production.

　　(A) Charge and discharge curves of 40nm LiFePO4 with ordinary carbon and super-capacity carbon; (B) Charge and discharge curves of 83nmLiFePO4 with ordinary carbon and super-capacity carbon; (C) Exposed LiFePO4 interface; (D) After reconstruction LiFePO4 interface (N represents the ordinary carbon coating method, E represents the super capacity carbon coating method)

　　In order to carry forward the advantages of nanotechnology and overcome its shortcomings, after three years of hard work, Professor Pan Feng's research group from the School of New Materials, Peking University Shenzhen Graduate School finally made an important breakthrough. They cleverly covered the surface of nano-lithium iron phosphate with a shell that can coordinate with the interface (C-O-Fe chemical bonds generated on the surface of lithium iron phosphate), which not only increased the binding energy of lithium ions on the surface, but also appeared additional lithium ion storage sites. Using the reconstructed nano-lithium iron phosphate material, when the average particle size is 42nm, the material capacity can reach 207mAhg-1, exceeding 21% of its theoretical capacity (170mAhg-1), so it is a new type of super-capacity nanocathode. Material. The material has good stability and can maintain 99% of its capacity after 1,000 cycles at 10°C.

　　(A) The effect of LiFePO4 particle size on capacity after reconstruction (black dots and blue dots are theoretical values, red dots are experimental values); (B-D) Ordinary carbon-coated and super-coated lithium iron manganese phosphate, lithium manganese phosphate and lithium cobalt phosphate. Charge and discharge curve of capacity package carbon

　　Through a combination of quantum chemical theoretical calculations and experiments, the team revealed the mechanism of nanometer super-capacity energy storage. This discovery is of great significance to the development of new nanometer energy storage materials. People can reconstruct the interface shell by designing coordination groups, not only It can achieve ultra-capacity energy storage and also improve the stability of nanomaterial applications. This work was recently published in NanoLetters (DOI: 10.1021/acs.nanolett.7b02315, impact factor 12.7, one of the natural publishing index journals), an excellent international journal in the field of materials. This work was completed under the guidance of Professor Pan Feng and in collaboration with Dr. Duan Yandong, Dr. Zhang Bingkai, Dr. Zheng Jiaxin and 2015 doctoral student Hu Jiangtao. Important collaborators in this work include Professor Khalil Amine and Professor Yang Ren of Argonne National Laboratory, Professors Lin-WangWang and Wanli Yang of Berkeley National Laboratory, and Professor Chong-MinWang of Pacific Northwest National Laboratory. This work was supported by the National New Energy Vehicle (Power Lithium Battery) Technology Innovation Project, the Guangdong Provincial Science and Technology Innovation Team Introduction Project, the Guangdong Provincial Natural Science Foundation, the Shenzhen Science and Technology Innovation Commission Fund, the U.S. Department of Energy, and the Shenzhen National Supercomputing Center .

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