LiMgPO₄-Coating-Induced Phosphate Shell and Bulk Mg-Doping Enables Stable Ultra-High-Voltage Cycling of LiCoO₂ Cathode

Stable cycling of LiCoO2 (LCO) cathode at high voltage is extremely challenging due to the notable structural instability in deeply delithiated states. Here, using the sol-gel coating method, LCO materials (LMP-LCO) are obtained with bulk Mg-doping and surface LiMgPO4/Li3PO4 (LMP/LPO) coating. The experimental results suggest that the simultaneous modification in the bulk and at the surface is demonstrated to be highly effective in improving the high-voltage performance of LCO. LMP-LCO cathodes deliver 149.8 mAh g(-1)@4.60 V and 146.1 mAh g(-1)@4.65 V after 200 cycles at 1 C. For higher cut-off voltages, 4.70 and 4.80 V, LMP-LCO cathodes still achieve 144.9 mAh g(-1) after 150 cycles and 136.8 mAh g(-1) after 100 cycles at 1 C, respectively. Bulk Mg-dopants enhance the ionicity of Co-O bond by tailoring the band centers of Co 3d and O 2p, promoting stable redox on O2-, and thus enhancing stable cycling at high cut-off voltages. Meanwhile, LMP/LPO surface coating suppresses detrimental surface side reactions while allowing facile Li-ion diffusion. The mechanism of high-voltage cycling stability is investigated by combining experimental characterizations and theoretical calculations. This study proposes a strategy of surface-to-bulk simultaneous modification to achieve superior structural stability at high voltages.