Entropy tuning and artificial CEI synergistically enhance the stability and kinetics of P2-type layered oxide cathode for high-voltage sodium-ion batteries

Abstract

P2-type layered oxide Na_2/3Ni_1/3Mn_2/3O₂ (NM) is a promising cathode material for sodium-ion batteries (SIBs). However, the severe irreversible phase transition, sluggish Na⁺ diffusion kinetics, and interfacial side reactions at high-voltage result in grievous capacity degradation and inferior electrochemical performance. Herein, a dual-function strategy of entropy tuning and artificial cathode electrolyte interface (CEI) layer construction is reported to generate a novel P2-type medium-entropy Na_0.75Li_0.1Mg_0.05Ni_0.18Mn_0.66Ta_0.01O₂ with NaTaO₃ surface modification (LMNMT) to address the aforementioned issues. In situ X-ray diffraction reveals that LMNMT exhibits a near zero-strain phase transition with a volume change of only 1.4%, which is significantly lower than that of NM (20.9%), indicating that entropy tuning effectively suppresses irreversible phase transitions and enhances ion diffusion. Kinetic analysis and post-cycling interfacial characterization further confirm that the artificial CEI layer promotes the formation of a stable, thin NaF-rich CEI and reduces interfacial side reactions, thereby further enhancing ion transport kinetics and surface/interface stability. Consequently, the LMNMT electrode exhibits outstanding rate capability (46 mA h g⁻¹ at 20 C) and cycling stability (89.5% capacity retention after 200 cycles at 2 C) within the voltage range of 2–4.35 V. The LMNMT also exhibits superior all-climate performance and air stability. This study provides a novel path for the design of high-voltage cathode materials for SIBs.