Effects of Particle Size and Polytype on the Redox Reversibility of the Layered Na0.76Ni0.38Mn0.62O2 Electrode

Layered sodium nickel manganese oxides (NaxNiyMn1–yO2) have gained great interest as one of the positive electrode materials in sodium-ion batteries toward sustainable energy storage systems. However, the significant capacity fade of those materials necessities to be solved for practical application. Partial substitutions for Ni and Mn at the expense of delivered capacity have been widely suggested to address the poor cycle life, whereas relatively little investigation on particle morphology and/or surface engineering has been carried out. Herein, P3- and P2-type Na0.76Ni0.38Mn0.62O2 powders were prepared using 4 and 10 μm (Ni0.38Mn0.62)(OH)2 precursors to understand the influence of primary and secondary particle sizes and polytype on their electrochemical performance in Na cells. P3-type Na0.76Ni0.38Mn0.62O2 with smaller primary particles and the absence of impurity have superior cyclability and rate capability compared to the P2-type one. In addition, larger secondary particles improve cycling performance in both polytypes. Formation of microcracks is inevitable over 100 cycles, especially with the upper cutoff voltage of 4.4 V; however, more severe pulverization and microcracks are shown in 4 μm P3-type Na0.76Ni0.38Mn0.62O2 compared to 10 μm P3-type Na0.76Ni0.38Mn0.62O2. The severity in prevalent microcracks, rapid growth of resistance over cycling as well as aging, and CO2 gas release upon charge to 4.4 V supports the degradation of 4 μm P3-type Na0.76Ni0.38Mn0.62O2 driven by parasitic surface reactions causing capacity fade.