Mechanism of the First Lithiation/Delithiation Process in the Anode Material CoFeOPO₄@C for Li-Ion Batteries

Abstract

A cobalt iron oxyphosphate CoFeOPO4@C (abbreviated as CFP@C) anode was prepared via a solid-state route, and its electrochemical performance was investigated vs Li+/Li over a wide voltage range of 0.01–3.0 V at different current rates C/n (n = 20, 10, 5, 2, and 1). This anode material is able to intercalate more than six lithium ions into the structure at the C/10 current rate, delivering a specific capacity of 748.23 mAh g–1, which is much higher than the theoretical capacity (593.7 mAh g–1) calculated when the insertion of a five lithium ions is considered. A reversible capacity of 200 mAhg–1 was maintained after 30 cycles. Raman spectroscopy confirmed the incorporation of carbon layers into the CoFeOPO4@C composite. Scanning electronic microscopy revealed that CFP@C particles have an angular-flake shape with particle sizes ranging between 1 and 5 μm. In situ X-ray absorption spectroscopy of Fe and Co at the K-edge showed that both transition metals are active during the whole discharge and charge. In operando high-energy X-ray diffraction revealed that this material undergoes a gradual evolution of the structure with lower crystallinity after the first discharge. Correlating electrochemical performance to the structural and electronic features indicated that the cycling mechanism of the CFP@C anode material exhibits a combination of intercalation and conversion processes.