Copper-substituted, lithium rich iron phosphate as cathode material for lithium secondary batteries

Carbon-free, copper-doped, lithium rich iron phosphates, Li1+xFe1−yCuyPO4 (0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.005), have been synthesized by a solid-state reaction method. From the optimization, the Li1.05Fe0.997Cu0.003PO4 phase showed superior performances in terms of phase purity and high discharge capacity. The structural, morphological, and electrochemical properties were studied and compared to LiFePO4, Li1.05FePO4, LiFe0.997Cu0.003PO4, and materials. X-ray photoelectron spectroscopy (XPS) was conducted to ensure copper doping. Only smooth surface morphologies were observed for lithium rich iron phosphates, namely Li1.05FePO4 and Li1.05Fe0.997Cu0.003PO4. The Li/Li1.05Fe0.997Cu0.003PO4 cell delivered an initial discharge capacity of 145 mAh/g and was 18 mAh/g higher than the Li/LiFePO4 cell without any carbon coating effect. Cyclic voltammetry revealed excellent reversibility of the Li1.05Fe0.997Cu0.003PO4 material. High rate capability studies were also performed and showed a capacity retention over 95% during the cycling. We concluded that substituted Li and Cu ions play an important role in enhancing battery performance of the LiFePO4 material through improving the kinetics of the lithium insertion/extraction reaction on the electrode.