Effects of Particle Size and Polytype on the Redox Reversibility of the Layered Na_0.76Ni_0.38Mn_0.62O₂ Electrode

Abstract

Layered sodium nickel manganese oxides (Na x Ni y Mn 1−y O 2 ) 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 Na 0.76 Ni 0.38 Mn 0.62 O 2 powders were prepared using 4 and 10 μm (Ni 0.38 Mn 0.62 )(OH) 2 precursors to understand the influence of primary and secondary particle sizes and polytype on their electrochemical performance in Na cells. P3-type Na 0.76 Ni 0.38 Mn 0.62 O 2 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 Na 0.76 Ni 0.38 Mn 0.62 O 2 compared to 10 μm P3-type Na 0.76 Ni 0.38 Mn 0.62 O 2 . The severity in prevalent microcracks, rapid growth of resistance over cycling as well as aging, and CO 2 gas release upon charge to 4.4 V supports the degradation of 4 μm P3-type Na 0.76 Ni 0.38 Mn 0.62 O 2 driven by parasitic surface reactions causing capacity fade.

Understanding the NaxMnO2 System: A Thermodynamics and XPS Approach

“…The decrease in discharge capacity from ∼119.5 mAh g –1 for Na 0.39 MnO 2 to ∼107.2 mAh g –1 for Na 0.44 MnO 2 could be due to decrease in the particle size diminishes from 0.6 to 0.3 μm. The effect of the particle size on the electrochemical behavior of the sodium-ion batteries has been studied by Komaba group for the Na 0.76 Ni 0.38 Mn 0.62 O 2 system. The authors have shown that an increase in the particle size increased the discharge capacity.…”