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 <0.4 eV. With extraction of the second Na+, the charge transfer process occurred on Mn2+. The material underwent a biphasic transition with the Na(1) atom auto-migrating to a nearby interstitial site, Na(3). During this process, Na+ migrated in a 1D channel with a relatively higher diffusion barrier of 0.52 eV. In line with the structural evolution, the Na+ diffusion coefficients remained at 2.0 × 10−12 cm2 s−1 during the first Na+ extraction, and then decreased to 9.7 × 10−14 cm2 s−1 with the extraction of the second Na+.
- This article is part of the themed collection: Journal of Materials Chemistry A HOT Papers