The role of divalent (Zn2+/ Mg2+/ Cu2+) substituents in achieving full capacity of sodium layered oxides for Na-ion battery applications

O3 type layered sodium transition metal oxides, e.g. 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 we can reversibly utilize their full Na content. However, the layered structure on cycling undergoes series of phase transitions in which the fully de-sodiated O1 phase shows 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 l...