Phase transformation, charge transfer, and ionic diffusion of Na4MnV(PO4)3 in sodium-ion batteries: a combined first-principles and experimental study

NASICON-structured Na4MnV(PO4)3 has been recognized as a potential positive electrode material for sodium-ion batteries, but its electrochemical mechanism during de(sodiation) has not been well understood. In this work, the structural transformation, charge transfer, and ionic diffusion properties of Na4MnV(PO4)3 were comprehensively studied by first-principles calculations combined with experimental studies. The results revealed two independent Na sites, Na(1) and Na(2), in the structure of Na4MnV(PO4)3, but only Na(2) can be extracted between 2.5 and 3.8 V. Extraction of the first Na+ caused charge transfer on V3+ and was associated with a solid-solution reaction. In addition, Na+ migrated along the 3D channels in the NASICON structure with low energy barriers of