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)]O-2. Of these materials, Na-2 3[Mn0.8Ni0.2]O-2 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[Mn0.8Ni0.2]O-2 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[Mn0.8Ni0.2]O-2 was able to maintain its original structure, whereas structural disintegration was observed for Na2/3MnO2. Our findings provide a potential new path to utilize cost-effective Mn-rich high-capacity cathode materials for SIBs.