Surface Design with Cation and Anion Dual Gradient Stabilizes High-Voltage LiCoO2

LiCoO2 (LCO) is the most successful cathode material for commercial lithium-ion batteries. Cycling LCO to high potentials up to 4.5 V or even 4.6 V can significantly elevate the capacity but cause structural degradation due to the serious surface side reaction between the highly oxidized Co4+ and O- species with organic electrolytes. To tackle this concern, a new strategy, constructing cation and anion dual gradients at the surface of LCO (DG-LCO), is proposed. Specifically, the electrochemically inactive cation and anion are selected to substitute Co3+ and O2- at the surface in a gradated manner, thus minimizing the highly oxidized Co4+ and O- species at high potentials and suppressing the induced surface side reactions. Unexpectedly, this dual gradient design leads to a spinel-like surface structure coherently with bulk layered structure, which facilitates Li+ diffusion kinetics. Thus, DG-LCO achieves high capacity and excellent cycling stability at 4.6 V (approximate to 216 mA h g(-1) at 0.1 C, a capacity retention of 88.6% after 100 cycles in 1.8 A h pouch full cell at 1 C), as well as improved rate capability (approximate to 140 mA h g(-1) at 5 C). These studies provide useful guidelines for future design of cathode materials with long lifespan and high rate capability.