CR2320 battery.Research on cathode materials for high-voltage lithium cobalt oxide lithium-ion batteries has made progress

　　Lithium cobalt oxide (LiCoO2) is the earliest commercialized cathode material for lithium-ion batteries. Due to its high material density and electrode compaction density, lithium-ion batteries using lithium cobalt oxide cathode have the highest volume energy density, so lithium cobalt oxide is the most widely used cathode material in the consumer electronics market. As consumer electronics products, especially 5G mobile phones, increasingly require lithium-ion battery life and size, there is an urgent need to further improve battery volume energy density. Increasing the charging voltage of lithium cobalt oxide batteries can increase the volumetric energy density of the battery. Its charging cut-off voltage has gradually increased from 4.20V when it was first commercialized in 1991 to 4.45V (vsLi+/Li), and the volumetric energy density has exceeded 700Wh/L. . At present, developing the next generation of higher-voltage lithium cobalt oxide materials has become a hot spot of common concern among the scientific research community and enterprises. As the charging voltage increases, lithium cobalt oxide materials will gradually suffer from irreversible structural phase changes, decreased surface and interface stability, and decreased safety performance, which limits its practical application. Researchers usually modify lithium cobalt oxide materials by using trace doping with various elements to improve their stability during high-voltage charge and discharge processes. Understanding the mechanism of action of different doping elements is crucial to designing lithium cobalt oxide materials with better performance. However, experimentally determining the mechanism of action of each trace doping element is challenging.

　　Doctors Zhang Jienan and Li Qinghao from the E01 Clean Energy Laboratory of the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics, under the guidance of researchers Li Hong and Yu Xiqian, used trace doping of three elements: Ti, Mg, and Al. (Doping ratio <0.1wt%), the cycle stability and rate characteristics of lithium cobalt oxide materials during the 4.6V high voltage charge and discharge process have been greatly improved. The team further cooperated with relevant research institutions such as Brookhaven National Laboratory, Stanford National Accelerator Laboratory, Lawrence Berkeley National Laboratory, Jiangxi Normal University and Hunan University to use synchrotron radiation X-ray nano-three-dimensional imaging, resonance inelastic X-ray Advanced experimental techniques such as ray scattering systematically studied the mechanism of trace doping of Ti, Mg, and Al on improving the performance of lithium cobalt oxide materials, revealing the unique effects of different doping elements on improving material properties. The research results were recently published in Nature Energy (Nature Energy, 2019, DOI: 10.1038/s41560-019-0409-z).

　　The research team first used high-resolution transmission electron microscopy combined with EDSEELS characterization to explore the distribution patterns of different doping elements on the surface of material particles and within the bulk phase. The results showed that under the same material synthesis conditions, Mg and Al elements are more easily doped into In the crystal structure of the material, the Ti element tends to be enriched on the surface of lithium cobalt oxide particles. Laboratory in-situ X-ray diffraction results show that Mg and Al doped into the lithium cobalt oxide lattice can inhibit the structural phase change that occurs during 4.5V high voltage charge and discharge. This structural phase change is generally considered to be the cause of lithium cobalt oxide materials. One of the main reasons for performance degradation under high voltage charge and discharge. Subsequently, through synchrotron radiation X-ray three-dimensional imaging technology, it was found that Ti is unevenly distributed in lithium cobalt oxide particles. Ti elements are not only enriched on the surface of lithium cobalt oxide particles, but also enriched at the grain boundaries inside the particles, which can provide cobalt. The internal primary particles of lithium acid particles provide good interfacial contact, thereby improving the rate performance of the material (Figure 2). Further use of resonance inelastic X-ray scattering (RIXS) technology found that the Ti element enriched on the surface can effectively inhibit the oxidation activity of oxygen ions on the surface of the material under high voltage, thereby slowing down the side reaction between the material and the organic electrolyte under high voltage and stabilizing The surface of the material. Finally, through first-principles calculations, the research team further theoretically confirmed the doping rules and modification principles of the Ti element. They believed that the Ti element tends to be doped on the surface of the material and can delithiate the surrounding oxygen atoms. Adjust the charge distribution under the material to effectively reduce its oxidation activity.

　　This work reveals the mechanism of Ti, Mg, and Al co-doping to improve the performance of lithium cobalt oxide materials, and illustrates the importance of comprehensive material design from different dimensions such as crystal structure, electronic structure, and material submicron scale microstructure to improve material performance. property, providing a theoretical basis for the design of high-voltage and high-capacity cathode materials. It also demonstrates the importance of multi-scale, high-precision analytical characterization methods in revealing the intrinsic physical and chemical processes of materials. The conclusions obtained from this work also have reference significance for the design of electrode materials for other battery systems. Relevant work is supported by the Key R&D Program of the Ministry of Science and Technology (2016YFB0100100), the Innovation Group Fund of the NSFC (51421002), and the Outstanding Youth Fund of the NSFC (51822211).

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