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 $Fm\bar{3}m$ phase of Cu${}_2$P and two low-lying metastable structures, an $I\bar{4}3d$–Cu${}_3$P phase, and a $Cm$–Cu${}_3$P${}_{11}$ phase. The computed pair distribution function of the novel $Cm$–Cu${}_3$P${}_{11}$ phase shows its structural similarity to the experimentally identified $Cm$–Cu${}_2$P${}_7$ 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 $Fm\bar{3}m$–Cu${}_2$P is dynamically stable and the Cu${}_{3-x}$P ($x<1$) defect phase $Cmc{2}_1$–Cu${}_8$P${}_3$ remains metastable (within 20 meV/atom of the convex hull) across a temperature range from 0 K to 600 K. Both CuP${}_2$ and Cu${}_3$P exhibit theoretical gravimetric capacities higher than contemporary graphite anodes for Li-ion batteries; the predicted Cu${}_2$P phase has a theoretical gravimetric capacity of 508 mAh/g as a Li-ion battery electrode, greater than both Cu${}_3$P (363 mAh/g) and graphite (372 mAh/g). Cu${}_2$P is also predicted to be both non-magnetic and metallic, which should promote efficient electron transfer in the anode. Cu${}_2$P’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, Cu${}_2$P anodes could be more durable than other conversion anodes in the Cu–P system with volume expansions greater than 150%.