Probing the Aqueous Electrochemical Degradation in Na2.5V1.5Ti0.5(PO4)3 by Nuclear Magnetic Resonance through the Interplay between Paramagnetic and Diamagnetic Vanadium Ions

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Abstract Although nuclear magnetic resonance (NMR) spectroscopy is a very powerful method for investigating battery materials, its application is often complicated by the inherent presence of paramagnetic transition-metal ions (e.g., V3+/4+, Mn2+/3+, Fe2+/3+, Ni2+/3+, etc.). These ions induce extremely large chemical shift ranges, severe line broadening, and rapid spin relaxation, which often prevents NMR signal detection. In this work, we demonstrate that the interplay between the oxidation states and magnetic properties of vanadium, such as the transition from paramagnetic V3+/4+ to diamagnetic V5+, lead to the formation of local environments suitable for the NMR signal detection of multiple nuclei (e.g., 51V, 31P) which serve as unique and direct probes into the structure and degradation mechanism of vanadium-based battery materials. By systematically analyzing the NASICON-structured Na2.5V1.5Ti0.5(PO4)3 samples, ranging from pristine NVTP powder to electrodes subjected to electrochemical cycling, we develop a comprehensive mechanistic model of NVTP electrochemical degradation in aqueous media. Our findings reveal that vanadium dissolution, together with phosphate, are among the primary drivers of charge capacity loss. The formation of residual Ti-containing phases, which also become NMR-visible due to the formation of diamagnetic V5+, results in blocking layer growth and additional capacity loss. These NMR observations establish a definitive framework for probing the structure and stability of vanadium-based battery materials.

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