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

Abstract

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⁺.