소듐이온전지용 Ni-Fe-Mn 기반 층상형 양극재의 전압 조건에따른 전기화학적 특성 연구

Abstract

Sodium-ion batteries are considered as a promising alternative to lithium-ion batteries in large-scaleapplications. We propose a new P2-type layered oxide as an excellent intercalation cathode material forsodium-ion batteries. However, Fe-Mn-based layered oxides suffer from structural changes such as the directphase transition of P2-O2, which causes power performance degradation. In this work, we substitute redoxactive elements to increase reversible capacity, and various experiments demonstrated reversible capacity canbe enhanced. We compare P2-type Na0.7Fe0.4Mn0.6O2 synthesized via a solid state synthesis method with P2-type Na0.7Fe0.4-xNixMn0.6O2 (x=0, 0.1, 0.2) with partial substitution of Ni in the Fe site. Phase changes duringelectrochemical reactions are important for understanding the relationship between layered structure and electrochemicalproperties. In-situ x-ray diffraction reveals the phase transition mechanism of Fe-Mn-based layeredoxides with P2 stacks. We also find that partial substitution of Ni, an active element, can significantly improvethe battery performance by smoothing the electrochemical charge/discharge profile. The P2-type Na0.7Ni0.2Fe0.2Mn0.6O2cathode exhibits a high specific discharge capacity of ~162.8 mAh g−1at 20 mA g−1. The smooth and continuousstructural changes resulted in improved cycling performance, including reduced voltage attenuationduring cycling. Partial substitution of Ni can improve cycling performance, including reduced voltage attenuationduring cycling. These results suggest that structural stability is one of the most efficient ways to improvecapacity and charge/discharge performance in P2 Fe-Mn-based layered oxides.