New insights into the critical role of inactive element substitution in improving the rate performance of sodium oxide cathode material

In layered oxide cathodes of sodium-ion batteries, electrochemically inactive element substitution, especially into the sodium layer, has emerged to be an effective strategy for structural stabilization and performance improvement. However, the underlying mechanism for how these inactive elements function remains elusive and controversial. Herein, based on the systematic study of a series of P2-Na0.84Mn0.67Ni0.3-xMgx□0.03O2 cathodes with different Mg doping concentrations, we achieve a superior rate and cycling performance and more importantly unravel the underlying mechanism. Specifically, two opposite sides of the roles of inactive Mg dopants in the sodium layer are unambiguously clarified, which include the beneficial structural stabilizing effect, and the detrimental sodium-ion diffusion blocking effect, as confirmed by combining advanced electron microscopy, in-situ X-ray diffraction with detailed electrochemical measurements. Wherein, an optimal balance of these two kinds of effects reaches superior comprehensive performances in the Na0.84Mn0.67Ni0.2Mg0.1□0.03O2 cathode, including surpassing rate (156.5 and 71.0 mAh g−1 at 0.1 and 20 C, respectively) and cycling performances (98.3% capacity retention after 50 cycles at 0.1 C). This study gives new insight into the roles of inactive element substitution in general, which could provide new perspectives on the material design and new opportunities for the property improvement of sodium oxide cathodes.