Strain engineering of Li+ ion migration in olivine phosphate cathode materials LiMPO4 (M = Mn, Fe, Co) and (LiFePO4)n(LiMnPO4)m superlattices

Abstract

The olivine phosphate family has been widely utilized as cathode materials for high-performance lithium-ion batteries. However, limited energy density and poor rate performance caused by low electronic and ionic conductivities are the main obstacles that need to be overcome for their widespread application. In this work, atomic simulations have been performed to study the effects of lattice strains on the Li⁺ ion migration energy barrier in olivine phosphates LiMPO₄ (M = Mn, Fe, Co) and (LiFePO₄)_n(LiMnPO₄)_m superlattices (SLs). The (LiFePO₄)_n(LiMnPO₄)_m superlattices include three ratios of LFP/LMP, namely SL3 + 1, SL1 + 1 and SL1 + 3, each of which is along three typical (100), (010) and (001) orientations. We mainly discuss two migration paths of Li⁺ ions: the low-energy path A channel parallel to the b-axis and the medium-energy path B channel parallel to the c-axis. It is found that the biaxial tensile strain perpendicular to the migration path is most beneficial to reduce the migration energy barrier of Li⁺ ions, and the strain on the b-axis has a dominant effect on the energy barrier of Li⁺ ion migration. For path A, SL3 + 1 alternating periodically along the (010) orientation can obtain the lowest Li ion migration energy barrier. For path B, SL1 + 3 is the most favorable for Li⁺ ion migration, and there is no significant difference among the three orientations. Our work provides reference values for cathode materials and battery design.