Lithium cobaltate (LiCoO2) is the earliest commercially available lithium ion battery anode material. Because of its high material density and electrode compaction density, lithium ion batteries using lithium cobaltate cathodes have the highest volume energy density, so lithium cobaltate is the most widely used cathode material in the consumer electronics market.
As consumer electronics products, especially 5G mobile phones, continue to increase the requirements for battery life and volume, there is an urgent need to further increase battery volume energy density. Increasing the charging voltage of a lithium cobaltate battery can increase the volume energy density of the battery. Its charge cut-off voltage has gradually increased from 4.20V at the earliest commercialization in 1991 to 4.45V (vs Li + / Li). The volume energy density has exceeded 700Wh / L. At present, the development of next-generation higher-voltage lithium cobaltate materials has become a hot spot of common interest in the scientific research community and enterprises.
With the increase of the charging voltage, lithium cobaltate materials will gradually appear problems such as irreversible structural phase transition, reduced surface interface stability, and reduced safety performance, which limits their practical applications. Researchers usually modify the lithium cobaltate material by using a variety of elemental trace doping methods to improve its stability during high voltage charging and discharging. Understanding the mechanism of action of different doping elements is critical to designing better cobalt cobalt materials. However, experimentally determining the mechanism of action of each trace doping element presents challenges.
Under the guidance of researchers Li Hong and Yu Xiqian, under the guidance of researchers Li Hong and Yu Xiqian, Dr. Zhang Jienan and Li Qinghao of the E01 team of the Clean Energy Laboratory of the Institute of Physics, Chinese Academy of Sciences / Beijing National Research Center for Condensed Matter Physics (Doping ratio <0.1 wt%), which greatly improves the cycle stability and rate characteristics of lithium cobaltate materials during high-voltage charging and discharging at 4.6 V (Figure 1).
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, etc., using synchronous radiation X-ray nano-three-dimensional imaging, resonance inelastic X Advanced experimental techniques such as ray scattering systematically study the mechanism of the effects of trace doping of Ti, Mg, and Al on the performance improvement of lithium cobaltate materials, and reveal the unique effects of different doping elements on the improvement of material properties. The results of the study were recently published in Nature-Energy, and the article was titled Trace doping of multiple elements enables stable battery cycling of LiCoO2 at 4.6 V.
The research team first used high-resolution transmission electron microscopy combined with EDS EELS characterization to explore the distribution of different doping elements on the surface and bulk phase of the material particles. The results show that under the same material synthesis conditions, Mg and Al elements are more easily doped. The impurities enter the crystal structure of the material, and the Ti element tends to be enriched on the surface of the lithium cobaltate particles. In-situ X-ray diffraction results in the laboratory show that Mg and Al doped into the lithium cobaltate lattice can suppress the structural phase transition that occurs during high-voltage charging and discharging at 4.5 V. This structural phase transition is generally considered to cause lithium cobaltate materials. One of the main reasons for performance degradation under high voltage charge and discharge.
Subsequently, through synchrotron X-ray three-dimensional imaging technology, it was found that Ti was unevenly distributed in the lithium cobaltate particles. The Ti element was not only enriched on the surface of the lithium cobaltate particles, but also on the grain boundaries inside the particles, which could be cobalt. The primary particles within the lithium acid particles provide good interfacial contact, thereby improving the rate performance of the material (Figure 2). Further using the resonance inelastic X-ray scattering (RIXS) technology to discover 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 stable The surface of the material (Figure 3). Finally, through first-principles calculations, the research team further theoretically confirmed the doping rule and modification principle of Ti element, and believed that Ti element tends to be doped on the surface of the material and can deoxidize the oxygen atoms around it. Under the control of the charge distribution, its oxidation activity is effectively reduced.
This work reveals the mechanism of Ti, Mg, Al co-doping on the performance improvement of lithium cobaltate materials, and clarifies the importance of comprehensive design of materials from different dimensions such as crystal structure, electronic structure and material submicron scale microstructure to improve material performance. It provides a theoretical basis for the design of high-voltage, high-capacity cathode materials. At the same time, it also shows the importance of multi-scale and high-precision analytical characterization methods to reveal the intrinsic physical and chemical processes of materials. The conclusions obtained from this work are also of significance for the design of electrode materials for other battery systems.