Computational Investigation of Copper Phosphides as Conversion Anodes for Lithium-Ion Batteries.

Using first-principles structure searching with density-functional theory (DFT), we identify a novel 3̅ phase of CuP and two low-lying metastable structures, an 4̅3-CuP phase and a -CuP phase. The computed pair distribution function of the novel -CuP phase shows its structural similarity to the experimentally identified -CuP phase. The relative stability of all Cu-P phases at finite temperatures is determined by calculating the Gibbs free energy using vibrational effects from phonon modes at 0 K. From this, a finite-temperature convex hull is created, on which 3̅-CuP is dynamically stable and the Cu P ( < 1) defect phase 2-CuP remains metastable (within 20 meV/atom of the convex hull) across a temperature range from 0 to 600 K. Both CuP and CuP exhibit theoretical gravimetric capacities higher than contemporary graphite anodes for Li-ion batteries; the predicted CuP phase has a theoretical gravimetric capacity of 508 mAh/g as a Li-ion battery electrode, greater than both CuP (363 mAh/g) and graphite (372 mAh/g). CuP is also predicted to be both nonmagnetic and metallic, which should promote efficient electron transfer in the anode. CuP's favorable properties as a metallic, high-capacity material suggest its use as a future conversion anode for Li-ion batteries; with a volume expansion of 99% during complete cycling, CuP anodes could be more durable than other conversion anodes in the Cu-P system, with volume expansions greater than 150%. The structures and figures presented in this paper, and the code used to generate them, can be interactively explored online using Binder.