Medium-entropy-induced in-situ surface spinel phase towards stable Co-free Li-rich cathode material

Co-free Li-rich Li_1.2Ni_0.2Mn_0.6O₂ (LR) cathode shows the highest working capacity that can be applied to high-energy density Li-ion batteries (LIBs). However, poor cycle stability and voltage decay caused by phase transition are always hindering its further development. Herein, a novel medium-entropy Li-rich Mn-based cathode material (LRMEF) was synthesized via a simple sol-gel method. The introduction of multivalent ions (Al³⁺/Cu²⁺ doping at Mn sites and F⁻ doping at O sites) effectively mitigates the Jahn-Teller distortion of Mn ions and suppresses oxygen release. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images confirm that this synergistic doping strategy induces the in-situ formation of an approximately 3 nm-thick spinel surface layer, which significantly enhances structural stability and ion diffusion kinetics. Besides, a series of in-situ/ex-situ characterization methods and density functional theory (DFT) calculations have been carried out to fundamentally shed light on the optimized structure-activity relationship and reaction mechanism. As a result, the LR material with entropy regulation and anion doping exhibits excellent cycling stability (189.2 mAh g⁻¹ at 1 C with 84 % capacity retention after 300 cycles), rate performance (164.1 mAh g⁻¹ at 5 C), and voltage retention (82.7 % at 1 C after 300 cycles), demonstrating great application prospects in future high-energy-density LIBs.