Enhanced Interfacial Kinetics and High-Voltage/High-Rate Performance of LiCoO2 Cathode by Controlled Sputter-Coating with a Nanoscale Li4Ti5O12 Ionic Conductor.

The selection and optimization of coating material/approach for electrode materials have been under intensive pursuit to address the high-voltage induced degradation of lithium ion batteries. Herein, we demonstrate an efficient way to enhance the high-voltage electrochemical performance of LiCoO2 cathode by postcoating of its composite electrode with Li4Ti5O12 (LTO) via magnetron sputtering. With a nanoscale (∼25 nm) LTO coating, the reversible capacity of LiCoO2 after 60 cycles is significantly increased by 40% (to 170 mAh g-1) at room temperature and by 118% (to 139 mAh g-1) at 55 °C. Meanwhile, the electrode's rate capability is also greatly improved, which should be associated with the high Li+ diffusivity of the LTO surface layer, while the bulk electronic conductivity of the electrode is unaffected. At 12 C, the capacity of the coated electrode reaches 113 mAh g-1, being 70% larger than that of the uncoated one. The surface interaction between LTO and LiCoO2 is supposed to reduce the space-charge layer at the LiCoO2-electrolyte interface, which makes the Li+ diffusion much easier as evidenced by the largely enhanced diffusion coefficient of the coated electrode (an order of magnitude improvement). In addition, the LTO coating layer, which is electrochemically and structurally stable in the applied potential range, plays the role of a passivation layer or an artificial and friendly solid electrolyte interface (SEI) layer on the electrode surface. Such protection is able to impede propagation of the in situ formed irreversible SEI and thus guarantee a high initial columbic efficiency and superior cycling stability at high voltage.