Realizing long-term cycling stability of O3-type layered oxide cathodes for sodium-ion batteries

O3-type layered oxide cathodes are promising for practical sodium-ion batteries (SIBs) owing to their high theoretical capacity, facile synthesis, and sufficient Na⁺ storage. However, they face challenges such as rapid capacity loss and poor cycling stability, mainly attributed to irreversible phase transitions. To address these challenges, a novel cathode material, Li/Sn co-substituted O3-Na_0.95Li_0.07Sn_0.01Ni_0.22Fe_0.2Mn_0.5O₂ (LSNFM), has been designed by regulating the electronic structure, in which Li⁺ activates more redox reactions of Ni^2+/3+ and Fe^3+/4+ above 2.5 V and suppresses the redox reactivity of Mn^3+/4+ below 2.5 V, while Sn⁴⁺ can prevent the charge delocalization in the transition metal layer, contributing to structural stability. Due to this synergistic effect, the as-prepared LSNFM electrode with high structural reversibility displays a 27.2% capacity increase contributed by the high-voltage transition metal ion redox activity and exhibits excellent long-term cycling stability, an 84.0% capacity retention after 500 cycles at 1 C and an 84.7% capacity retention after 2000 cycles at 5 C. The fundamental mechanism is fully investigated using systematic in situ/ex situ characterization techniques and density functional theory computations. This work provides a paradigm for designing long-term cycle life cathode materials by synergistically regulating the electronic structure in practical SIBs.