Cr-doped Fe2F5·H2O with open framework structure as a high performance cathode material of sodium-ion batteries

The sphere-like Fe(2-x)CrxF5·H2O (x = 0, 0.03, 0.05, 0.07) composites with open framework structure are prepared via an IL based assisted approach, and served as the cathode material for Na-ion batteries (NIBs). The physicochemical and electrochemical properties of the Cr-doped Fe2F5·H2O cathode materials are systematically characterized. The results indicate that the Cr-doped materials not only reduce the crystalline size, but also remarkably enhance electronic conductivity. Meanwhile, the electrochemical tests further show that Fe1.95Cr0.05F5·H2O as the cathode active material of NIBs exhibits a high initial discharge capacity of 357 mAh g−1 and retains a discharge capacity of 171 mAh g−1 after 100 cycles at 0.1 C (1 C = 200 mAh g−1). Moreover, even at high rate of 1 C, it can still deliver a high discharge capacity of 147 mAh g−1. Compared with the Fe2F5·H2O, the Fe1.95Cr0.05F5·H2O shows higher discharge capacity, excellent cycle stability and better rate capability, which can be attributed to the improvement of structure stability and electronic conductivity due to the appropriate amount (5%) of Cr3+ doping. Therefore, the preparation of Cr-doped Fe2F5·H2O sample provides a unique perspective to enhance the electrochemical performance of Fe2F5·H2O, which is an essential step for the development of high specific energy sodium-ion batteries.