Impact of Surface Layer Formation during Cycling on the Thermal Stability of the LiNi0.8Co0.1Mn0.1O2 Cathode

In this study, the effects of charge/discharge cycling on the thermal stability of LiNi0.8Co0.1Mn0.1O2, a high-Ni cathode material, are systematically investigated. X-ray diffraction measurements show that there is almost no change in the bulk structure of the cathode after cycling. However, X-ray absorption fine structure measurements indicate that Ni in the surface layer is reduced and stable rock-salt structures are formed. Differential scanning calorimetry (DSC) measurements show that the heat generation at the lowest temperature, which can trigger thermal runaway in batteries that use high-Ni cathodes, decreases significantly with the formation of rock-salt structures on the active material surface. This finding indicates that the rock-salt layer on the surface enhances the thermal stability of a high-Ni cathode. The change in the total heat generation with degradation, indicated by DSC measurements, is similar to that in the K-edge of Ni (i.e., the Ni valency), suggesting a strong correlation between the heat generation and crystal structure changes during cycling.