Lithium cobalt oxide (LiCoO2) is the earliest commercially available cathode material for lithium ion batteries. Because of its high material density and electrode compaction density, lithium-ion batteries using lithium cobalt oxide cathodes 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, continue to increase the requirements for battery life and volume, there is an urgent need to further increase the battery energy density. Increasing the charging voltage of the lithium cobalt oxide battery can increase the volumetric energy density of the battery. The charging cut-off voltage 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. At present, 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. As the charging voltage increases, lithium cobaltate materials will gradually exhibit irreversible structural phase changes, reduced surface interface stability, and decreased safety performance, which limits its practical application. Researchers usually modify lithium cobalt oxide materials by means of trace doping of various elements to improve their stability during high voltage charge and discharge. Understanding the mechanism of action of different doping elements is critical to designing better performance lithium cobaltate materials, but experimentally determining the mechanism of action of each trace doping element presents challenges.
Dr. Zhang Jienan and Li Qinghao, group E01, Clean Energy Laboratory, Institute of Physics, Chinese Academy of Sciences / National Research Center for Condensed Matter Physics, Beijing, under the guidance of researchers Li Hong and Yu Xiqian, doped with trace elements of Ti, Mg and Al (Doping ratio <0.1 wt%), the cycle stability and rate characteristics of lithium cobaltate materials during 4.6 V high-voltage charge and discharge have been greatly improved (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 synchrotron radiation X-ray nano three-dimensional imaging, resonance inelastic X Advanced experimental techniques such as ray scattering systematically studied the mechanism of Ti, Mg, Al trace doping on the performance improvement of lithium cobalt oxide materials, and revealed the unique effect of different doping elements on the performance improvement of materials. The results of this study were published recently in Nature Energy (Nature Energy, 2019, DOI: 10.1038 / s41560-019-0409-z). The article is 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 of the material particles. The results show that under the same material synthesis conditions, Mg and Al elements are easier to dope The impurities enter the crystal structure of the material, and the Ti element tends to be enriched on the surface of the lithium cobalt oxide particles. Laboratory in-situ X-ray diffraction results show that Mg and Al doped into the lithium cobaltate lattice can suppress the structural phase transition that occurs during 4.5 V high-voltage charge and discharge. 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 radiation X-ray three-dimensional imaging technology, it was found that Ti showed an uneven distribution in the lithium cobalt oxide particles. The Ti element was not only enriched on the surface of the lithium cobalt oxide particles, but also enriched at the grain boundaries inside the particles, which could be cobalt The lithium acid particles provide good interface contact between the primary particles, thereby improving the rate performance of the material (Figure 2). Further using resonant inelastic X-ray scattering (RIXS) technology, it is found that the Ti element enriched on the surface can effectively inhibit the oxidation activity of the oxygen ions on the surface of the material at high voltage, thereby slowing down the side reaction of the material and the organic electrolyte at high voltage and stabilizing The surface of the material (Figure 3). Finally, through first-principle calculations, the research team further theoretically confirmed the doping law and modification principle of Ti element, and believed that Ti element tends to dope on the surface of the material and can deoxidize the surrounding oxygen atoms. Under the charge distribution is adjusted to effectively reduce its oxidation activity.
This work reveals the mechanism of Ti, Mg and Al co-doping on the performance improvement of lithium cobalt oxide 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 analysis and characterization methods to reveal the inherent physical and chemical processes of materials. The conclusions drawn from this work are also of reference significance for the design of electrode materials for other battery systems. Related work was supported by the Ministry of Science and Technology's Key R & D Program (2016YFB0100100), the Fund's Innovation Group Fund (51421002), and the Fund's Excellent Youth Fund (51822211).
Figure 1. Comparison of half-cell and full-cell performances of Ti, Mg and Al co-doped LiCoO2 (TMA-LCO) and undoped LiCoO2 (Bare-LCO)
Figure 2. Synchrotron radiation X-ray three-dimensional imaging technique reveals the spatial distribution of Ti, Al and Co elements in LiCoO2 particles
Figure 3. Resonance inelastic X-ray scattering (RIXS) results show that the oxidation activity of oxygen ions is suppressed when Ti, Mg, and Al-doped LiCoO2 materials are charged to a high voltage state, making the material have a more stable surface
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