Electrochemical reactions of lithium with CuP₂ and Li_1.75Cu_1.25P₂ synthesized by ballmilling

Copper phosphide (CuP₂) and lithium copper phosphide (Li_1.75Cu_1.25P₂) were synthesized by high-energy ballmilling at room temperature. The electrochemical reactions between lithium and these samples have been studied. The first lithium insertion into the CuP₂ phase till 0.0 V vs. Li leads to copper reduction and the formation of lithium phosphide (Li₃P), corresponding to a long voltage plateau. The subsequent lithium extraction until 1.3 V vs. Li presents three voltage plateaus related to the formation of new phases such as Li₂CuP, giving a reversible capacity about 810 mAh/g and faradic yield about 61%. It means that both copper and phosphorus face a change of the oxidation state for the electrochemical insertion and extraction. Lithium copper phosphide exhibits a similar reaction process. However, it provides a reversible capacity of 750 mAh/g and faradic yield of 100% at the first cycle.