Promoting high energy density Mn-based layered oxide cathodes via surface structural regulation for sodium-ion batteries

Construction of a structurally compatible Na 0.44 MnO 2 -derived interphase for enhanced interfacial stability and Na + transport kinetics. • In situ SXRD and XAS analyses reveal a reversible P2-OP4-P2 phase evolution and a stabilized Mn-centered local coordination environment. • The optimized material suppresses moisture/CO 2 -induced surface carbonation, Na + /H + exchange, and hydrated-phase formation, thereby markedly enhancing its air stability. • Elucidating the multiscale mechanisms of surface structural regulation for modulating structural evolution, local coordination environments, and air stability. Mn-based layered oxides are attractive cathode candidates for sodium-ion batteries (SIBs) because of their high capacity, cost effectiveness, elemental abundance, and scalable synthesis. However, their practical implementation is hindered by moisture/CO 2 -induced surface degradation, surface residual alkali accumulation, sluggish Na + transport kinetics, and unstable interfacial chemistry. Herein, Na 0.55 Ni 0.1 Fe 0.1 Mn 0.8 O 2 (NNFM) is selected as a model Mn-based layered oxide cathode to establish a residual-alkali-conversion-driven surface structural regulation strategy. Owing to the formation of a structurally compatible and chemically stable interphase, the optimized Na 0.55 Ni 0.1 Fe 0.1 Mn 0.8 O 2 @5%Na 0.44 MnO 2 (NNFM-T5) delivers a specific capacity of 197.14 mAh g −1 at 0.1C and an energy density of 556.9 Wh kg −1 based on the cathode active-material mass, demonstrating excellent electrochemical performance. NNFM-T5 also exhibits a reversible P2-OP4-P2 structural evolution, reduced interfacial polarization, and accelerated Na + transport kinetics, as evidenced by operando in situ synchrotron X-ray diffraction and distribution of relaxation times analysis. In addition, ex situ X-ray absorption spectroscopy indicates reversible Ni-centered redox, limited Fe valence variation, and stabilized Mn-centered coordination. Air-aging experiments and dynamic contact angle measurements further verify the improved air tolerance of NNFM-T5. This work shifts the focus of surface modification from empirical coating toward residual-alkali-conversion-driven surface structural regulation, providing a design principle for Mn-based layered oxide cathodes that combine high capacity, air stability, and favorable interfacial kinetics. A surface structural regulation strategy based on residual alkali conversion is developed for Mn-based layered oxide cathodes, enabling the construction of a Na 0.44 MnO 2 -derived interphase. This strategy significantly enhances the structural stability, air stability, and Na + transport kinetics of the material, providing an effective approach for the design of high-energy-density layered oxide cathodes for sodium-ion batteries. Download: Download high-res image (176KB) Download: Download full-size image