The Role of Divalent (Zn2+/Mg2+/Cu2+) Substituents in Achieving Full Capacity of Sodium Layered Oxides for Na-Ion Battery Applications

Abstract

O3-type layered sodium transition metal oxides, for example, NaNi0.5Mn0.5-zTizO2, having one sodium per transition metal ion could be attractive positive electrode materials for achieving high energy density sodium-ion batteries, provided that we can reversibly utilize their full Na content. However, the layered structure on cycling undergoes a series of phase transitions in which the fully desodiated O1 phase shows a huge reduction in cell volume together with cation migration, both of which are detrimental for long-term cycling performance. Hence, the practical capacity of layered oxides is restricted to solely ∼0.5-0.6 Na (oxidation up to ∼4 V vs Na+/Na0), avoiding the complete removal of sodium. Herein, we show that the partial substitution of a redox-active Ni2+ cation by an inactive one (e.g., Zn2+ to form NaNi0.45Zn0.05Mn0.4Ti0.1O2) suppresses the phase transitions at high voltage (>4 V vs Na+/Na0) and helps in utilizing the maximum capacity of the material (170 mAh g-1 with ∼0.8 Na) without much degradations upon long cycling. The fully charged phase (Na0.2Ni0.45Zn0.05Mn0.4Ti0.1O2), as determined by high-resolution electron transmission microscopy, shows a P3-O1 intergrowth structure in which the O1 phase is present only locally as nanoscale domains. We believe that the formation of P3-O1 intergrowths in the Zn-substituted material, in contrast to the distinct O1 phase for unsubstituted NaNi0.5Mn0.4Ti0.1O2, restricts structural degradations during cycling and improves the long-term cycling stability. Similar substitution chemistry can be extended to Cu2+ and Mg2+ ions as well. The NaNi0.45Zn0.05Mn0.4Ti0.1O2 positive electrode material on implementation in 18650 Na-ion cells show electrochemical performances comparable to that of polyanionic Na3V2(PO4)2F3/C cells.