Low-Temperature Tailoring of Copper-Deficient Cu_3–xP—Electric Properties, Phase Transitions, and Performance in Lithium-Ion Batteries

A convenient approach for a controlled and high-yield synthesis of copper-deficient Cu3–xP (0.1 < x < 0.7) is reported that makes use of ionic liquids with highly nucleophilic “naked” halide anions. Halide anions drastically enhance the reactivity of the white phosphorus precursor and kinetically disfavor the formation of phosphorus-rich side products. Cu3–xP shows a high degree of tolerance for cation vacancies without major structural reorganization, as evidenced by X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy. Measurements of the electric properties reveal that Cu3–xP is a bad metallic p-type conductor. The resistivity is composition-dependent and displays a distinct anomaly from a phase transition, leading to the discovery and structural characterization of two hitherto unknown low temperature polymorphs. Electrochemical evaluation of copper-deficient Cu3–xP as anode material for lithium ion batteries reveals a drastic change in the cycling mechanism leading to an increase of the initial capacities by about 70%. This work gives a comprehensive insight into the chemical and structural features of copper-deficient Cu3–xP and should lead to an improved understanding of its properties, not just for battery applications.