First principles investigation of voltage, structure, ionic and electronic conduction of olivine and maricite NaMnPO4

Mn2+/Mn3+ redox couple is expected to provide high operating voltage, but the Mn-based orthophosphate is difficult to achieve stable and reversible (de)intercalation of alkali-metal ions. Using first principles calculations, we investigated the structure, voltage, ionic and electronic transport properties of olivine and maricite NaMnPO4, trying to unveil the factors affecting the electrochemical performance. Our calculations show that the energetically favored structure transforms from the electrochemically active olivine to the inactive maricite with the decrease of sodium. This is likely to cause Mn to occupy Na site, blocking the diffusion channel along b axis. Lattice strain was found to have significantly influence on the relative structural stability. Both compressive and tensile ac biaxial strains induce the olivine MnPO4 to be in an energetically stable state. The enhanced structural stability of desodiated phase may prevent the irreversible structural transformation, contributing to the stable and reversible (de)intercalation of sodium ions.