Novel high-capacity hybrid layered oxides NaxLi1.5-xNi0.167Co0.167Mn0.67O2 as promising cathode materials for rechargeable sodium ion batteries

Relatively low capacity is a technological bottleneck of the development of sodium ion batteries. Herein, we present a series of hybrid layered cathode materials NaxLi1.5-xNi0.167Co0.167Mn0.67O2 (x = 0.5, 0.6, 0.7, 0.8, 0.9, 1) with composite crystalline structures, which are prepared by co-precipitation method. The combined analysis of XRD, SEM and TEM reveals that the materials are composed of P2 structure, alpha-NaFeO2 structure and small amount of Li2MnO3. Among the as-prepared materials, Na0.6Li0.9Ni0.167Co0.167Mn0.67O2 delivers an initial reversible capacity of 222 mA h g(-1) at 20 mA g(-1). Even at 100 mA g(-1) it shows a remarkable discharge capacity of 125 mA h g(-1) in the first cycle and remains 60 mA h g(-1) after 300 cycles. Such high capacity is achieved by the specific composite structure and sodium ions are proved to be able to intercalate/deintercalate in Li1.5Ni0.167Co0.167Mn0.67O2 with alpha-NaFeO2 structure. The Ex-situ XRD results of Na0.6Li0.9Ni0.167Co0.167Mn0.67O2 in the first cycle show that the P2 structure is well maintained along with irreversible phase transition of alpha-NaFeO2 structure, which is responsible for the long-term capacity fading. Owing to the high discharge capacity, the novel hybrid layered oxides NaxLi1.5-x,Ni0.167Co0.167Mn0.67O2 with composite structures can be considered as promising cathode materials to promote progress toward sodium-ion batteries.