Engineering hydrolytic stability and reversible O3/P3/OP2 transitions in O3-type sodium-ion battery cathodes through Cu2+ doping

Abstract

O3-type layered oxides have emerged as promising cathode materials for sodium-ion batteries (SIBs) owing to their competitive energy density and structural robustness. However, their inherent sensitivity to moisture remains a critical obstacle for practical application. In this study, we introduced Cu²⁺ doping into NaNi₀.₃Fe₀.₂Mn₀.₅O₂ (NFM325) to synthesize NaNi₀.₃Fe₀.₂Cu₀.₁Mn₀.₄O₂ (NFCM3214). Comprehensive analyses (XRD, FTIR, TGA, XPS, SEM, TEM) demonstrate that Cu incorporation prevents bulk degradation and suppresses surface reactions such as Na⁺/H⁺ exchange and the formation of NaOH and Mn hydroxides upon water exposure. These local structural modifications enhance both bulk and surface stability against moisture. Electrochemical evaluations confirm that NFCM3214 retains 89% of its initial capacity after direct water contact, with significantly improved Na⁺ diffusivity, rate capability (7.2 → 88.8 mAh g⁻¹ at 5C), and cycling retention (71.4% → 84.5% after 100 cycles). Mechanistic insights further reveal that Cu²⁺ doping mitigates Jahn–Teller distortion, promotes the formation of an OP2 intermediate phase during deep desodiation, and facilitates reversible OP2–O/P transitions during cycling. Finally, full-cell tests using an N-doped MoS₂ anode demonstrate high capacity and stable cycling performance, underscoring the practical potential of Cu-doped O3-type layered oxides for advanced sodium-ion batteries.