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 cathodes have high volumetric energy density. Therefore, lithium cobalt oxide is a widely used cathode material in lithium-ion batteries for consumer electronics. As consumer electronic products continue to increase the battery life time requirements of lithium-ion batteries, there is an urgent need to further increase the volumetric energy density of the battery. Increasing the charging voltage of lithium cobalt oxide batteries can increase the volumetric energy density of the battery. Therefore, the development of the next generation of higher voltage lithium cobalt oxide materials has become a hot spot for the scientific research community and enterprises. At present, the charge cut-off voltage of lithium cobalt oxide batteries has gradually increased from 4.20V when it was first commercialized in 1991 to 4.45V (vs Li/Li+), and the volumetric energy density has exceeded 700Wh/L. However, with the increase of charging voltage, lithium cobalt oxide materials will gradually appear irreversible structural phase transition, surface and interface stability decline, safety performance and other problems, which limit its practical application.
Researchers in the E01 group of the Key Laboratory of Clean Energy at the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics have developed a solid electrolyte material Li1.5Al0.5Ti1.5(PO4)3 (LATP) coated with lithium cobalt oxide technology. The lithium cobalt oxide material modified by this technology has the best room temperature and high temperature electrochemical performance reported in the laboratory. The research team further cooperated with the researcher Gu Lin of the Institute of Physics and others. Through careful study of the surface structure of the modified material, it was found that during the material synthesis process, LATP reacts with the lithium cobaltate material, which is transformed into a higher structure and electrochemical stability on the surface. And a uniform interface layer with excellent ionic and electronic conductivity characteristics, thereby effectively solving the problem of surface stability of lithium cobalt oxide materials during high-voltage charging. The research results were recently published on Advanced Energy Materials under the title An In Situ Formed Surface Coating Layer Enabling LiCoO2 with Stable 4.6 V High-Voltage Cycle Performances.
In recent years, the research team has been focusing on high-voltage lithium cobalt oxide material technology development and basic scientific research. Earlier studies have shown that the modification of high-voltage lithium cobalt oxide materials requires a combination of surface and bulk modification techniques. After the research team successfully developed Ti-Mg-Al trace element doping modification technology last year and revealed the mechanism of action of each doping element through a combination of various experimental methods (Nature Energy, 2019, 4, 594), the research team recently In cooperation with Brookhaven National Laboratory in the United States and Stanford Linear Accelerator National Laboratory in the United States, the advanced synchrotron radiation X-ray three-dimensional nano-diffraction imaging technology was further used to study the particle structure and material charging of Ti-Mg-Al co-doped lithium cobalt oxide materials. The relationship between the reversibility of the reaction during the discharge. This experimental technique can observe the crystal structure defects and their spatial distribution at the 50nm spatial scale inside the micron-sized granular material. The research results show that doping elements can control the defects and their distribution in lithium cobalt oxide particles, thereby inhibiting the structural phase transition of lithium cobalt oxide materials that causes the degradation of the electrochemical performance of the material during high-voltage charging and discharging. The result was recently published in the Cell sub-Journal Chem under the title Hierarchical Defect Engineering for LiCoO2 through Low-Solubility Trace Element Doping.
These research results clarify the importance of comprehensive design of materials from different dimensions such as bulk structure, surface structure and material submicron microstructure to improve material performance, and provide a theoretical basis for the design of high-voltage and high-capacity cathode materials. At the same time, it also demonstrates the importance of multi-scale, high-precision analysis and characterization methods for revealing the intrinsic physical and chemical processes of materials. The conclusions obtained from this work also have reference significance for the design of other battery system electrode materials.
Related work has been supported by the Key Research and Development Program of the Ministry of Science and Technology (2016YFB0100100), the Outstanding Youth Fund of the National Natural Science Foundation of China (51822211), the Joint Fund Key Project of the Natural Science Foundation of China (U1932220) and the International Partnership Program of the Chinese Academy of Sciences (GJHZ2068).
Figure 1. Surface structure and modification mechanism of LATP modified LiCoO2 material
Figure 2. Comparison of electrochemical performance of unmodified and LATP modified LiCoO2 materials
Figure 3. Comparison of internal structure defect distribution of undoped and Ti-Mg-Al doped LiCoO2 particles
Figure 4. The internal defects of Ti-Mg-Al doped LiCoO2 particles regulate the phase transition mechanism of charge and discharge structure
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