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 Na_xLi_1.5-xNi_0.167Co_0.167Mn_0.67O₂ (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, α-NaFeO₂ structure and small amount of Li₂MnO₃. Among the as-prepared materials, Na_0.6Li_0.9Ni_0.167Co_0.167Mn_0.67O₂ 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 Li_1.5Ni_0.167Co_0.167Mn_0.67O₂ with α-NaFeO₂ structure. The Ex-situ XRD results of Na_0.6Li_0.9Ni_0.167Co_0.167Mn_0.67O₂ in the first cycle show that the P2 structure is well maintained along with irreversible phase transition of α-NaFeO₂ structure, which is responsible for the long-term capacity fading. Owing to the high discharge capacity, the novel hybrid layered oxides Na_xLi_1.5-xNi_0.167Co_0.167Mn_0.67O₂ with composite structures can be considered as promising cathode materials to promote progress toward sodium-ion batteries.