Abstract
Using first-principles structure searching with density-functional theory (DFT), we identify a novel Fm3̅m phase of Cu2P and two low-lying metastable structures, an I4̅3d–Cu3P phase and a Cm–Cu3P11 phase. The computed pair distribution function of the novel Cm–Cu3P11 phase shows its structural similarity to the experimentally identified Cm–Cu2P7 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 Fm3̅m–Cu2P is dynamically stable and the Cu3–xP (x < 1) defect phase Cmc21–Cu8P3 remains metastable (within 20 meV/atom of the convex hull) across a temperature range from 0 to 600 K. Both CuP2 and Cu3P exhibit theoretical gravimetric capacities higher than contemporary graphite anodes for Li-ion batteries; the predicted Cu2P phase has a theoretical gravimetric capacity of 508 mAh/g as a Li-ion battery electrode, greater than both Cu3P (363 mAh/g) and graphite (372 mAh/g). Cu2P is also predicted to be both nonmagnetic and metallic, which should promote efficient electron transfer in the anode. Cu2P’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, Cu2P 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.
Highlights
Graphite is the most commonly employed lithium-ion battery (LIB) anode but is inherently limited by a maximum theoretical capacity of 372 mAh/g upon the formation of LiC6
In addition to bulk or powdered TMPs being used as LIB conversion anodes,[6] nanostructured TMPs can often display improved electrochemical cycling performance.[7]
TMPs have yet to be widely adopted as conversion anodes, given the large volume expansion
Summary
Graphite is the most commonly employed lithium-ion battery (LIB) anode but is inherently limited by a maximum theoretical capacity of 372 mAh/g upon the formation of LiC6. By performing crystal structure prediction, combining both ab initio random structure searching (AIRSS) and a genetic algorithm (GA), in addition to structural prototyping with known crystal structures of related chemistries,[18−20] we produced the compositional phase diagram of the copper phosphide system We describe this approach to structure prediction and the application of open-source Python packages matador (v0.9),[21] for high-throughput first-principles calculations, and ilustrado (v0.3),[22] for computational structure prediction with GAs. Crystal structure prediction for battery anodes is a well-tested method,[23] used for identifying both novel anode materials[4] and unknown phases, which form during battery cycling.[24,25] AIRSS has been used previously to search for additional phases of Li−P and Na−P, which form during battery cycling.[26] The GA was employed to search for new phases of Na−P, which were confirmed experimentally by solidstate NMR spectroscopy.[27] As applied here to Cu−P, these methods predict a novel metallic Fm3̅m−Cu2P phase at 0 K, within the target composition range of Cu1
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