Enhancing 1D ionic conductivity in lithium manganese iron phosphate with low-energy optical phonons

Lithium manganese iron phosphate (LMFP) is a promising cathode material for lithium-ion batteries due to its enhanced safety and structural stability. However, its ionic conductivity is limited by the 1D channels within its olivine crystal structure. In this study, we investigate the influence of Mn/Fe atomic configurations on LMFP's ionic conductivity by integrating ab initio density-functional theory (DFT) calculations with molecular dynamics (MD) simulations using graph neural network-based interatomic potentials. Our results show that variations in the Mn/Fe environment around Li ions significantly affect the Li-ion migration barrier and transport mechanisms. Specifically, asymmetric Mn/Fe configurations break Li-site degeneracy and induce a low-energy optical (LEO) phonon mode below 50 cm^{- 1}, enhancing ionic conductivity. This study demonstrates that LMFP's ionic conductivity can be improved by more than 60% when this LEO phonon mode is present. These findings highlight the potential to optimize LMFP cathode performance by employing controlled synthesis methods that exploit favorable atomic configurations, laying the groundwork for accelerating the development of safer and more efficient lithium-ion batteries.