Mechanochemically synthesized Cu3P/C composites as a conversion electrode for Li-ion and Na-ion batteries in different electrolytes

Copper phosphide (Cu₃P) is a potentially high volumetric capacity conversion electrode for the use in Li-ion as well as in Na-ion batteries. Here, we study the lithium and sodium storage properties of Cu₃P/Carbon (Cu₃P/C) composites containing 70 wt% Cu₃P and 30 wt% carbon black. Cu₃P is prepared by reactive ball milling from the elements while in a second step Cu₃P is mixed with carbon black by non-reactive ball milling. Structure and morphology are characterized by X-ray diffraction (XRD) as well as scanning and transmission electron microscopy (SEM, TEM). The electrochemical properties are studied in Li and Na half cells with different types of electrolytes based on carbonates (EC:DMC mixture) or diglyme, with the latter clearly leading to better results such as higher capacity, better cycle life and smaller polarization. After 120 cycles, the Li-cell showed a capacity of 210 mAh g⁻¹ while around 120 mAh g⁻¹ were found for the Na cell. The contribution of the carbon black is negligible in case of the Li cell while it becomes an important factor in the Na cell. Electrode expansion/shrinkage of the electrode during cycling (“breathing”) as determined by in situ dilatometry is fairly constant in diglyme electrolytes while rapid fading is observed in carbonate electrolytes.