Spotlight: New achievements in basic research of lithium-ion batteries

The high-capacity cathode material is the hotspot of the current research on the third generation of high-energy density lithium-ion batteries. Among them, the lithium-rich phase nanocomposite positive electrode material formed by the rock salt structure Li2MnO3 and the hexagonal layered LiMO2 structural unit has received extensive attention. The reversible lithium storage capacity of this type of material is twice that of the first-generation lithium-ion battery cathode material LiCoO2, reaching 250-300 mAh / g. It is generally believed that such a high-capacity lithium-rich phase cathode material has such a high capacity and the composition of Li2MnO3 structural unit is high Capacity related (theoretical capacity is 458 mAh / g). The actual application of this material currently needs to solve the bottleneck technology such as voltage attenuation, improved rate characteristics and cycle. Due to the complex microstructure of lithium-rich phase materials, there is a lack of accurate knowledge and experimental evidence on its structural evolution, charge transfer mechanism and its relationship with material properties. In response to this phenomenon, the Key Laboratory of Clean Energy and Advanced Materials and Structure Analysis Laboratory, Beijing Spallation Neutron Source Spectrometer Engineering Center, and Brookhaven National Laboratory in the United States have cooperated with Li2MnO3 The structural evolution and charge transfer in lithium-rich phase materials have been studied in depth.

Dr. Wang Rui from the Clean Energy Laboratory, Dr. He Lunhua and Researcher Wang Fangwei from the Beijing Spallation Neutron Source Spectrometer Engineering Center first used neutron diffraction technology to determine the initial structure of Li2MnO3 and the structural changes after chemical delithiation. After delithiation, the c-axis shrinks and Mn atoms may shift. Researcher Gu Lin of the Advanced Materials and Structure Analysis Laboratory further used spherical aberration corrected transmission electron microscopy technology to successfully obtain atomic-level images of each atomic occupation and stacking method including light atoms Li in the structure. On this basis, further accurate observations revealed that after electrochemical delithiation, lithium ions can be simultaneously and unevenly extracted from the Li layer and the transition metal LiMn2 layer in the material structure, and at the same time, Mn can be between the transition metal layer and the Li layer mobile. This corrects the traditional understanding that the skeleton atoms of the layered material generally do not deviate from the equilibrium lattice position during the intercalation and deintercalation of lithium, that lithium ions are mainly extracted from the Li layer, and the existence of transition metals in the transition metal layer hinders the extraction of lithium ions.

The study of Li2MnO3 material also found that a new nano-scale spinel phase appeared in the material after the first charge delithiation. Previous in-situ XRD studies have shown that a new phase of spinel structure generally appears after multiple charge and discharge, and the results of spherical aberration electron microscopy show that this phase transition has occurred since the first charge, indicating that the Li2MnO3 structure is unstable during the delithiation process , Easy to change, the emergence of new phases should be related to lithium ion extraction leading to Mn easy to move in the crystal lattice. The appearance of a new phase is the essential cause of the potential drop. The characteristics of this structural evolution are also reflected in the reaction kinetics of lithium-rich phase materials. In collaboration with Dr. Yu Xiqian and Dr. Xiaoqing Yang of Brookhaven National Laboratory in the United States and Dr. Yingying Lu from the Clean Energy Laboratory of the Institute of Physics, time-energy-resolved synchrotron radiation absorption spectroscopy was used to compare Ni, Co, and Mn elements in lithium-rich The charge transfer and local structure evolution during the intercalation and deintercalation of materials reveal for the first time that the bottleneck that affects the rate of intercalation and deintercalation of the lithium-rich phase anode material with nanocomposite structure is the Li2MnO3 structural unit. This is consistent with the previous spherical aberration correction electron microscope found that Mn will be displaced with lithium extraction, which is a slow step at room temperature. The above research results indicate that the intercalation and deintercalation behavior of the Li2MnO3 structural unit is the key to the cycle stability, rate characteristics, and voltage decay of the lithium-rich phase cathode material. The design and optimization provided important guidance.

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