O3-type NaNi0·33Li0·11Ti0·56O2-based electrode for symmetric sodium ion cell

Layered materials Na0.67+3xNi0.33LixTi0.67-xO2 with Li-substitution as x = 0, 0.05 and 0.11 have been synthesized and investigated as “bi-functional” electrodes for symmetric sodium ion cells. The samples with lithium substitution up to 0.11 are confirmed to be a single phase without impurities and introducing lithium into the transition metal layer increases the interlayer space of the layered materials. It is found that O3-type NaNi0·33Li0·11Ti0·56O2 exhibits two electrochemical working windows, 0.4-0.8 V and 3.1-3.75 V, for sodium ion storage. The as-proposed material thus can be employed as both positive and negative electrodes. As positive electrode, it shows a high working voltage of ca. 3.75 V versus Na+/Na and an initial capacity of 91 mAh g−1 with 19% capacity loss after 100 cycles. When utilized as negative electrode, it delivers a low average voltage of ca. 0.65 V versus Na+/Na, along with a reversible capacity of 125 mAh g−1 and 76% capacity retention after 200 cycles. A symmetric full cell based on the O3-type NaNi0·33Li0·11Ti0·56O2 “bi-functional” electrode has been developed. The cell exhibits a high voltage of 3.1 V and an energy density of 100 W h kg−1 based on the total mass of active electrode materials.