Combination Effect of Bulk Structure Change and Surface Rearrangement on the Electrochemical Kinetics of LiNi_0.80Co_0.15Al_0.05O₂ During Initial Charging Processes

The correlations between bulk/surface structure change and electrochemical kinetics of LiNi0.80Co0.15Al0.05O2 are systematically investigated at atomic level, including the initial charged, half-charged, and over-charged states. In the initial stage of charge, surface rearrangement occurs and an amorphous Li2CO3 layer forms on the surface, which can release stress and provide a stable interface. The Li2CO3 surface layer decomposes upon charging, resulting in decreased interface resistance for charge transfer. Meanwhile, the bulk structure goes through the two-phase reaction region toward the solid solution region, which demonstrates higher electrical conductivity and faster Li-ion mobility. Along with the charging process, more substantial surface rearrangement and the decomposed Li2CO3 layer lead to surface degradation. Together with the anisotropic volume change-induced mechanical stress, microcracks stem from the surface and provide access for electrolyte penetration. All of these cause high kinetic barriers for Li-ion extraction, as demonstrated by the high interface and charge-transfer resistance and slow lithium diffusion in this region.