Mechanical Stress Relief and Na⁺ Diffusion Enhancement via Bond Tuning and Vacancy Disorder in Low-Sodium O3 Cathodes.

Abstract

Layered transition-metal (TM) oxide cathodes have garnered considerable attention for their low cost and abundant sodium reservoir toward sodium-ion batteries (SIBs). However, their practical application is severely hindered by slow ion diffusion kinetics, as well as lattice stress accumulation and structural degradation induced by complex phase transitions. Herein, a low-sodium O3-type material, Na_0.72Li_0.1Mn_0.35Cu_0.08Ni_0.25Fe_0.1Ti_0.12O₂, is proposed through a lattice engineering strategy based on low-cation-potential high-entropy doping. The low Na content is inferred to introduce disordered vacancies, which help to improve the apparent Na⁺ diffusion kinetics. Meanwhile, theoretical calculations suggest that Ti incorporation can enhance Ti-O covalency, constructing a rigid lattice framework that helps to suppress interlayer sliding and mitigate lattice stress accumulation. The synergistic "bond-tuning" and "vacancy-disorder" effects mitigate the unfavorable O3-O'3 phase transition, with a lattice volume change of merely 1.68%, alleviating internal stress accumulation. This material demonstrates excellent energy density (427.36 Wh kg^-1 based on the cathode), long-term cycling stability (80.40% capacity retention after 1000 cycles at 1 A g^-1), with an average discharge voltage decay of only 0.0002 V per cycle, and broad temperature adaptability (-30 to 60 °C). This work provides valuable guidance for designing layered cathodes with long-cycle life and wide temperature adaptability at high rates.