Tailoring the Electrochemical Performance of Olivine Phosphate Cathode Materials for Li-Ion Batteries by Strain Engineering: Computational Experiments

Abstract

The open-circuit voltage (OCV), Li-ion diffusion, and electronic properties of a cathode material can significantly affect the performance of lithium-ion batteries (LIBs). In this work, DFT + U was carried out to investigate the effect of strain engineering in terms of biaxial compressive and tensile strains (BCS and BTS) on the electrochemical properties of olivine-based LiMPO4 (M = Co and Ni). The results show that biaxial compressive strain decreased the average bond lengths of O–Co/Ni and O–P, which means a good improvement in the structural stability of LiCoPO4 (LCP) and LiNiPO4 (LNP). The deployment of BCS and BTS reduced the open-circuit voltage of the LCP and LNP systems. Precisely, imposing a biaxial strain of about ε = ±3.5% for LCP and ε = ±5.5% for LNP reduced the high OCV to values below 4.5 V vs Li/Li+, which is recommended for most commercial liquid electrolyte operation. In addition, it was found that biaxial strain reduces the energy barrier for Li+ diffusion, which promotes Li+ diffusion and increases its mobility in these olivine crystal structures. Further analysis of the electronic structures shows that BTS reduces the material’s band gap and improves electronic conductivity. These results are promising for the commercial utilization of LCP and LNP as potential cathode material in next-generation LIBs like their famous big brother LFP.