NMR Evidence for the Multielectron Reaction Mechanism of Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> Cathode and the Impact of Polyanion Site Substitution
Qing Qiu, Chao Li, Hui Liu, Yuxin Liao, Chong Zhao, Fushan Geng, Ming Shen, Jingxin Li, Wei Tong, Bingwen Hu
Abstract
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.