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

Abstract

Carbon-free, copper-doped, lithium rich iron phosphates, Li 1 +x Fe 1 −y Cu y PO 4 (0 ≤ x ≤ 0.15, 0 ≤ y ≤ 0.005), have been synthesized by a solid-state reaction method. From the optimization, the Li 1.05Fe 0.997Cu 0.003PO 4 phase showed superior performances in terms of phase purity and high discharge capacity. The structural, morphological, and electrochemical properties were studied and compared to LiFePO 4, Li 1.05FePO 4, LiFe 0.997Cu 0.003PO 4, 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 Li 1.05FePO 4 and Li 1.05Fe 0.997Cu 0.003PO 4. The Li/Li 1.05Fe 0.997Cu 0.003PO 4 cell delivered an initial discharge capacity of 145 mAh/g and was 18 mAh/g higher than the Li/LiFePO 4 cell without any carbon coating effect. Cyclic voltammetry revealed excellent reversibility of the Li 1.05Fe 0.997Cu 0.003PO 4 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 LiFePO 4 material through improving the kinetics of the lithium insertion/extraction reaction on the electrode.