Bulk Ca-doping-induced surface modification enabling high-performance O3-type layered cathodes for sodium-ion batteries

Abstract

Layered O3–NaNi_1/3Fe_1/3Mn_1/3O₂ (NMF) is a promising cathode material for commercialization of sodium-ion batteries (SIBs), but it still faces challenges due to its poor cycling stability and air instability. Herein, we demonstrate that through optimized synthesis conditions (sintering and composition), a single Ca-dopant additive can synergistically achieve bulk-doping and surface-modification effects. Microscopic characterizations reveal that a robust rock-salt structure is constructed on the non-(003) surface facets due to Ca surface enrichment, which is developed through the Ca dopant exsolution process during high-temperature sintering. Combining bulk-doping and surface-modification effects, the 3% Ca-doped NFM cathode demonstrates superb cycling stability and improved rate capability, boosting the capacity retention from 61.4% to 90.5% after 200 cycles at 2.0–4.0 V. Further microanalysis reveals that the surface Ca-enriched modification layer not only effectively suppresses the cycling-induced surface degradation but also significantly improves the air storage stability by acting as a physical barrier layer to prevent moisture degradation. This work offers a new pathway to achieve bulk and surface modifications by a simple doping strategy and reveals the dual effects of Ca-doping for enhanced performance of O3-type layered oxide cathodes.