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

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.