In situ impedance analysis on BaTiO₃-LiCoO₂ composite cathodes for lithium ion batteries

In situ electrochemical impedance spectroscopy (EIS) was undertaken to investigate the contribution of a ferroelectric artificial solid electrolyte interface (SEI) to the enhancement of the rate capability of lithium ion batteries. Resistance elements, consisting of the cell reactions, the resistance of the electrolyte, Rsol, that of the Li metal anode reaction, RLi, and the charge transfer resistance, Rct, were measured. A small ferroelectric BaTiO₃ (BT) load, ∼1 mol %, notably reduced Rct and Rsol compared with bare LiCoO₂ (LC), indicating that loaded ferroelectric BT SEIs effectively promote Li inter/deintercalation into and from the active material, LC, and restrict cobalt ion dissolution into the electrolyte liquid. Lower Rct and Rsol resulted in a significantly higher capacity retention ratio at a 10C rate compared with the initial cycle for small BT load, ∼1 mol %. The capacity retention dropped rapidly, accompanied by a slight increase in Rct for larger BT loads, 5 and 15 mol %, which may be attributed to the thicker BT layer and the existence of the impurity phase, BaCO₃. These results imply that the ferroelectric SEI affected the kinetics of mobile Li ions at the cathode-electrolyte interface, significantly enhancing the rate capability.