On the synthesis and potential benefit of Na-rich P-type layered oxides for high power Na-ion batteries

Layered oxides with the general composition NaxMO2 (M = transition metal) have attracted a lot of attention for their composition diversity and promising electrochemical performances as positive electrode for sodium-ion batteries. P-type compositions offer power capabilities owing to the fast diffusion of sodium cations but are plagued by limited specific capacities because of the nominal low amount of sodium in the structure. Therefore, sodium enriching of P-type structure is consequently of interest to propose electrode candidates combining high energy density and high power capabilities. Herein co-precipitation syntheses of sodium-rich P-type NaxMn2/3Ni1/3O2 and NaxMn1/2Ni1/2O2 have been attempted and the synthesis mechanism carefully monitored by X-rays diffraction follow up in temperature. The optimization of the synthesis protocol led to stabilize only P3-type single-phases. The combination of several characterization techniques shows that the actual Na content is limited to x = 0.8 far from targeted stoichiometry but still higher than conventionally reported for most of P3-type layered oxides. The investigation of the electrochemical behavior of Na-rich P3-type compounds shows that they behave similarly to stable P2-type and O3-type homologue used as reference. Consequently, efficiency of highly sodiated P3-type layered oxides as a way to combine high energy density and power capabilities is questioned while the interest in stabilizing Na-rich P2-type remains an open question.