In Situ Multitechnical Investigation into Capacity Fading of High-Voltage LiNi_0.5Co_0.2Mn_0.3O₂

LiNi_0.5Co_0.2Mn_0.3O₂ positive electrode materials of lithium ion battery can release a discharge capacity larger than 200 mAh/g at high potential (>4.30 V). However, its inevitable capacity fading, which is greatly related to the structural evolution, reduces the cycling performance. The origin of this capacity fading is investigated by coupled in situ XRD-PITT-EIS. A new phase of NiMn₂O₄ is discovered on the surface of the LiNi_0.5Co_0.2Mn_0.3O₂ upon charging to high voltage, which blocks Li⁺ diffusion pathways. Theoretical calculations predict the formation of cubic NiMn₂O₄. Moreover, corrosion, cracks, and microstress appear to increase the difficulty of Li⁺ transportation, which are attributed to the protection degradation of the interfacial film on the positive electrode material at high voltage. After 50 electrochemical cycles, the increase in degree of crystal defects by low-angle grain boundary, evidenced through HR-TEM, leads to poor Li⁺ kinetics, which in turn causes capacity loss. The in situ XRD-PITT-EIS technique can bring multiperspective insights into fading mechanism of the high-voltage positive electrode materials and provide a solution to control or suppress the problem on the basis of structural, kinetic, and electrochemical interfacial understandings.