Regulation of Lattice Oxygen and Phase Transition in O3-Type High-Entropy Layered Sodium-Ion Cathodes

Abstract Irreversible phase transition and lattice-oxygen loss above 4.0 V cause rapid capacity decay in high-capacity O3-type layered cathodes for sodium-ion batteries (SIBs). Herein, a novel high-entropy layered cathode of Na0.95Li0.05Ni0.3Fe0.1Mn0.3Zn0.05Ti0.2O2 (LNFMZT) with regulated lattice oxygen through a high-entropy configuration and multi-element compositional regulation is successfully synthesized by a high-temperature solid-state method. We show that high-entropy modification stabilizes the single-phase structure, expands Na+ diffusion pathways, and optimizes the transition-metal (TM)–O coordination environment. The LNFMZT delivers a high initial capacity (163.7 mAh g–1 at 0.1 C) and good cycling stability (64.0% retention over 250 cycles at 1 C) at a high-voltage window of 2.0–4.2 V. Combining in situ and ex situ experimental characterizations, we find that a high-entropy layered cathode exhibits reduced charge-transfer resistance, enhanced Na+ diffusion dynamics, good reversibility of lattice oxygen redox, and smoother O3-P3-OP2 evolution, which greatly improve the structural integrity and electrochemical durability. This work underscores the critical role of high-entropy modification in coupling lattice oxygen stability with phase transition reversibility for advanced SIB cathodes.