Copper Phosphate as a Cathode Material for Rechargeable Li Batteries and Its Electrochemical Reaction Mechanism

In the search for new cathode materials for rechargeable lithium batteries, conversion-type materials have great potential because of their ability to achieve high specific capacities via the full utilization of transition metal oxidation states. Here, we report for the first time that copper phosphate can be used as a novel high-capacity cathode for rechargeable Li batteries, capable of delivering a reversible capacity of 360 mAh/g with two discharge plateaus of 2.7 and 2.1 V at 400 mA/g. The underlying reaction involves the formation as well as the oxidation of metallic Cu. The solid-state NMR, in situ XAFS, HR-TEM, and XRD results clearly indicate that Cu can react with Li3PO4 to form copper phosphate and LixCuyPO4 during the charging process, largely determining the reversibility of Cu3(PO4)2. This new reaction scheme provides a new venue to explore polyanion-type compounds as high-capacity cathode materials with conversion reaction processes.