Mechanism of Structural Change and the Trigger of Electrochemical Degradation of Li-Rich Layered Oxide Cathodes during Charge–Discharge Cycles

Li-rich layered oxides are promising candidates for next-generation cathode materials for Li-ion batteries because of their high capacity. However, a low cycle durability accompanied by structural changes poses a serious problem in their practical use. In this study, we focused on transition metal (TM) ion migration, which induces structural changes, and investigated the mechanism that triggers the degradation of the Li1.16Ni0.19Co0.19Mn0.46O2 structure, using X-ray diffraction spectroscopy (XDS), soft X-ray absorption spectroscopy (S-XAS), and first-principles calculations. XDS revealed that the Mn and Ni ion contents in the Li layer increase after the charge–discharge cycles. S-XAS showed that Mn, Co, and Ni become divalent after the charge–discharge cycles, suggesting that the divalent TM ions are constituents of the rock-salt-type metal monooxides at the surface. First-principles calculations indicated that the potential energy of the structure in which the TM ions exist in the Li layer is lower than that of the structure in which the TM ions exist in the TM layer with oxygen defects at the charging state. It was concluded that the mechanism of structural degradation involved the Mn and Ni ions that migrated to the Li layer during charging. Even during discharging, the migrated TM ions remain and accumulate at the surface, where oxygen defects could be easily formed. The structures in which the TM ions exist in the Li layer are rock-salt-type metal monooxides, which degrade the electrochemical properties by decreasing the potential of discharging. Thus, the oxygen defects are considered to trigger the structural degradation.