A bi-functional strategy involving surface coating and subsurface gradient co-doping for enhanced cycle stability of LiCoO2 at 4.6 V

Layered LiCoO2 (LCO) acts as a dominant cathode material for lithium-ion batteries (LIBs) in 3C products because of its high compacted density and volumetric energy density. Although improving the high cut-off voltage is an effective strategy to increase its capacity, such behavior would trigger rapid capacity decay due to the surface or/and structure degradation. Herein, we propose a bi-functional surface strategy involving constructing a robust spinel-like phase coating layer with great integrity and compatibility to LiCoO2 and modulating crystal lattice by anion and cation gradient co-doping at the subsurface. As a result, the modified LiCoO2 (AFM-LCO) shows a capacity retention of 80.9% after 500 cycles between 3.0 and 4.6 V. The Al, F, Mg enriched spinel-like phase coating layer serves as a robust physical barrier to effectively inhibit the undesired side reactions between the electrolyte and the cathode. Meanwhile, the Al, F, Mg gradient co-doping significantly enhances the surficial structure stability, suppresses Co dissolution and oxygen release, providing a stable path for Li-ions mobility all through the long-term cycles. Thus, the surface bi-functional strategy is an effective method to synergistically improve the electrochemical performances of LCO at a high cut-off voltage of 4.6 V.