Enhanced air stability by calcium doping in Na2/3[Fe1/2Mn1/2]O2 cathode material for Na-ion batteries

Abstract

Surface instability under ambient conditions in layered sodium transition metal oxides, such as P2 type Na_2/3[Fe_1/2Mn_1/2]O₂ (NFM), leads to initial capacity loss, poor cyclability, and gelation of the polyvinylidene fluoride slurry through defluorination. Although NFM is a high-performance cathode material, primarily due to its high capacity and stable electrochemical behavior, this issue presents a significant challenge to its practical application in low-cost sodium ion batteries. In this study, we investigated Ca²⁺ doping into the Na⁺ layer of NFM to enhance structural and environmental stability. We found that Ca²⁺ doping at 1 wt.% improved crystallinity and increased interlayer spacing which led to an improved rate performance while maintaining high discharge capacities of 190 mAh g⁻¹. Furthermore, the air and water stability of the Ca²⁺-doped NFM samples was significantly improved, as demonstrated via X-ray diffraction, transmission electron microscopy, and other analyses. While the as-synthesized undoped samples exhibited a rapid Na⁺/H⁺ exchange process, we found that this reaction was suppressed in Ca-doped samples. Electrochemical testing after 2 days of air exposure showed that NFM lost ∼35% of its discharge capacity, but Ca-doped samples showed no losses. Based on surface analyses, the improved stability appears to stem from spontaneous Ca migration during air exposure which leads to a Ca-enriched surface layer that suppresses decomposition processes such as Na⁺/H⁺ exchange and Na deintercalation. This previously undescribed mechanism appears to be quite effective in mitigating surface degradation reactions in layered oxides.