Revisit of layered sodium manganese oxides: achievement of high energy by Ni incorporation

Abstract

Sodium-ion batteries (SIBs) have been intensively investigated as a potential alternative to lithium-ion batteries. Among the studied cathodes, the cost-effective P2-type Na_2/3MnO₂ cathode is particularly attractive because it can deliver high capacity and high energy density. However, its cyclability during prolonged use remains an issue because of the Jahn–Teller distortion associated with the presence of Mn³⁺. In this study, the effect of Ni doping on the electrochemical properties of Na_2/3MnO₂ was investigated by varying the Ni content in the range of x = 0–0.2 in Na_2/3[Mn_1−xNi_x]O₂. Of these materials, Na_2/3[Mn_0.8Ni_0.2]O₂ exhibited the best electrochemical performance in terms of capacity and retention as well as improved thermal properties. Although in situ operando synchrotron X-ray diffraction analysis of the structural stability indicated that Na_2/3[Mn_0.8Ni_0.2]O₂ underwent a bi-phasic reaction (a P2–O2 transformation when charged to 4.3 V), the resulting volume change from P2 to O2 was only approximately 10%. This low volume change was possible because of the Ni²⁺ substitution of partial Mn³⁺ in the crystal structure, which is thought to have suppressed the cooperative Jahn–Teller distortion, as demonstrated by extended X-ray absorption fine structure analysis. As a result, the post-cycled Na_2/3[Mn_0.8Ni_0.2]O₂ was able to maintain its original structure, whereas structural disintegration was observed for Na_2/3MnO₂. Our findings provide a potential new path to utilize cost-effective Mn-rich high-capacity cathode materials for SIBs.