Ab Initio Prediction of Metal Phosphide Anode Materials for Lithium and Beyond Lithium Batteries

Abstract

Identifying high capacity battery materials is critical for creating better energy storage to lower our reliance on non-renewable energy resources. While Li-ion batteries are the state-of-the-art, their graphite anodes are limited by a theoretical capacity of 372 mAh/g. Phosphorus is one alternative which has a high capacity of 2596 mAh/g and can alloy with both Li$^+$ and Na$^+$ ions, but suffers from large volume changes upon cycling. To mitigate this destructive effect, transition metals act as stabilising agents, limiting volume change and retaining high capacities. In this dissertation, I investigate two classes of transition metal phosphides (TMPs) as candidates for high capacity Li and Na-ion battery anodes. Herein, I employ a computational approach which combines density-functional theory (DFT) with structure searching methods including $Ab$ $Initio$ Random Structure Searching (AIRSS) and Genetic Algorithms (GA). I conduct an AIRSS and GA search of the Li-Cu-P system, as well as an AIRSS search of the Na-Fe-P system, and study their ground state electrochemical properties with DFT. I investigate the lithiation pathway in Cu-P, and find that LiCu may form during cycling, increasing the overall capacity of all Cu-P anodes. Additionally, I calculate the capacity of CuP$_{10}$, to be 2225 mAh/g, while the highest capacity Cu-P to date is CuP$_2$ at 1495 mAh/g. This suggests that it should be tested in future experimental work. Using AIRSS, I identify a ground state $I$mm2 Cu$_2$P structure, which has not been identified experimentally, and find it is a stable semimetal at high temperature and pressures up to 10 GPa. I also find an AIRSS identified structure of Cu$_3$P with Cu vacancies (Cu$_8$P$_3$) which has different vacancy orderings to previously identified Cu$_{3-x}$P, suggesting this structure has several possible ground state orderings. Finally, I assess the effects of pressure on Cu-P, and find that several GA-identified $P$1 structures are low in energy at high pressure, suggesting they may form during extreme conditions on the battery anode. To conduct an AIRSS search on the Fe-P system, I investigate the possible ways to introduce spin polarisation into the search, and determine that breaking the spin state on each atom can be included as a post-processing step of high-throughput searching. Furthermore, the experimental sodiation pathway for FeP$_4$ has not yet been identified, though it was considered to be a conversion anode. From the results of the ternary AIRSS search on Na-Fe-P, I propose a theoretical sodiation pathway via an insertion process for FeP$_4$ which includes an as-yet unidentified $P$m ternary compound, NaFeP which may limit the overall battery capacity by 298 mAh/g.Funded by the Churchill Scholarship of America, with computing resources from HPC Midlands