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

Theopen-circuit voltage (OCV), Li-ion diffusion, and electronicproperties of a cathode material can significantly affect the performanceof lithium-ion batteries (LIBs). In this work, DFT + U was carried out to investigate the effect of strain engineeringin 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 straindecreased 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 BCSand BTS reduced the open-circuit voltage of the LCP and LNP systems.Precisely, imposing a biaxial strain of about & epsilon; = & PLUSMN;3.5%for LCP and & epsilon; = & PLUSMN;5.5% for LNP reduced the high OCV to valuesbelow 4.5 V vs Li/Li+, which is recommended for most commercialliquid electrolyte operation. In addition, it was found that biaxialstrain reduces the energy barrier for Li+ diffusion, whichpromotes Li+ diffusion and increases its mobility in theseolivine crystal structures. Further analysis of the electronic structuresshows that BTS reduces the material's band gap and improveselectronic conductivity. These results are promising for the commercialutilization of LCP and LNP as potential cathode material in next-generationLIBs like their famous big brother LFP.