NMR Evidence for the Multielectron Reaction Mechanism of Na₃V₂(PO₄)₃Cathode and the Impact of Polyanion Site Substitution

Although being regarded as a promising cathode candidate for Na-ion batteries, Na3V2(PO4)3 is still plagued with a congenital drawback that only a limited theoretical specific capacity of 400 Wh kg-1 can be achieved by employing two-electron reaction. This study focuses on enhancing the energy density by enabling a fourth Na+ intercalation upon discharge, which increases the theoretical specific capacity to around 494 Wh kg-1. The reaction mechanism of Na3V2(PO4)3 in the whole potential range of charge/discharge (1.0-3.8 V) is elaborately investigated by the combination of 23Na/31P solid-state nuclear magnetic resonance (NMR) and cryogenic-temperature electron paramagnetic resonance (EPR) for the first time. EPR measurement under 1.8 K manifests the generation of V2+ with rhombohedral distortion upon the fourth Na+ intercalation process of Na3V2(PO4)3. Besides, this study pinpoints the profound impact of polyanion site substitution to the local structural transformation of Na3V2(PO4)3 upon Na+ (de)intercalation, which corroborates that the boron substitution into phosphorus site can broaden the range of solid-solution reaction, accelerate the structural transition toward V2+-containing phase, and refrain the short scale heterogeneity of P and Na nuclei.