No abstract

LiCoO2 (LCO) with a high theoretical capacity of 274 mAh g–1 can rarely achieve a high practical capacity even at an upper cutoff voltage of 4.6 V due to severe structural instability and interface side reactions. Herein, an in situ strategy of gas–solid modifications during synthesis is proposed to improve the performance of LCO using sulfocompound‐contained expanded graphite (EG) as templates. In situ generated SO2 gas from EG enables the Co‐coating of coherent spinel LixCo2O4 and Li2SO4 and the trace doping of high‐valence S mainly in the near‐surface regions via its reactions with the precursors. The modified LCO possesses excellent structural reversibility, interfacial stability, slight dissolution of Co2+, high diffusion coefficients of Li+, and low O 2p band top. This endows LCO with remarkably improved high‐voltage performance, 222 and 143 mAh g‐1 at 0.1 and 20 C, respectively, and 88% capacity retention over 100 cycles at 1 C for LCO/Li half cells between 2.8 and 4.6 V, and 202 mAh g–1 at 1 C and 87% capacity retention over 1000 cycles between 2.8 and 4.5 V for LCO/graphite full cells. This study provides a unique, simple, and upscalable strategy for performance improvement of electrode materials.

Elevating the charge voltage of LiCoO 2 increases the energy density of batteries, which is highly enticing in energy storage implementation ranging from portable electronics to e-vehicles. However, hybrid redox reactions at high voltages facilitate oxygen evolution, electrolyte decomposition and irreversible phase change, and accordingly lead to rapid battery capacity decay. Here significantly improved high-voltage cycling stability of Mg-Al-Eu co-doped LiCoO 2 is demonstrated. It is found that element co-doping induces a near-surface high-entropy zone, including an innately thin disordered rock-salt shell and a dopant segregation surface. The high-entropy complex can effectively suppress oxygen evolution and near-surface structure deconstruction. The phase change reversibility between O3 and H1-3 and thermal stability of the cathode are greatly enhanced as well. As a result, the co-doped LiCoO 2 exhibits a remarkable cycling performance, retaining 86.3% and 72.0% of initial capacity over 800 and 2000 cycles, respectively, with a high cut-off voltage of 4.6 V. The feasible co-doping approach broadens the perspective for the development of stable lithium-ion batteries with high operating voltages.