High-voltage performance of LiCoO^sub 2^ cathode studied by single particle microelectrodes –influence of surface modification with TiO^sub 2

The LiCoO2 particle with TiO2 coating presents improved columbic efficiency, overcharge performance, rate capability and cycling stability at high voltage. To gain deeper insight into the performance improvement mechanism of the TiO2-coated LiCoO2 cathode materials, the first-principles calculation combined with single particle microelectrode technique are employed in this work. The first-principles calculation is firstly used to simulate the bulk structure, electronic properties and lithium ion diffusion of TiO2-coated LiCoO2 at different delithiation states, while the microelectrode technique is used to evaluate the thermodynamic and kinetic behaviors of TiO2-coated LiCoO2 on a single particle level without interference of binder, conductive agent, and porous structure of the electrode. It has been revealed that from thermodynamics point of view, the improved electrochemical performance of TiO2-coated LiCoO2 can be ascribed to the reduced surface oxygen activity and surface oxygen loss, which should suppress the electrolyte decomposition and layered structure degradation of LiCoO2 at high voltage. From kinetics point of view, the increased Li-slab space and decreased Li ion transfer activation energy barrier facilitate Li ion diffusion in the solid state of LiCoO2 and Li ion transfer across the solid/electrolyte interphase.