Elucidating the Impact of Mg Substitution on the Properties of NASICON‐Na3+yV2−yMgy(PO4)3 Cathodes.

Vanadium multiredox‐based NASICON‐NazV2−yMy(PO4)3 (3 ≤ z ≤ 4; M = Al3+, Cr3+, and Mn2+) cathodes are particularly attractive for Na‐ion battery applications due to their high Na insertion voltage (>3.5 V vs Na+/Na0), reversible storage capacity (≈150 mA h g−1), and rate performance. However, their practical application is hindered by rapid capacity fade due to bulk structural rearrangements at high potentials involving complex redox and local structural changes. To decouple these factors, a series of Mg2+‐substituted Na3+yV2−yMgy(PO4)3 (0 ≤ y ≤ 1) cathodes is studied for which the only redox‐active species is vanadium. While X‐ray diffraction (XRD) confirms the formation of solid solutions between the y = 0 and 1 end members, X‐ray absorption spectroscopy and solid‐state nuclear magnetic resonance reveal a complex evolution of the local structure upon progressive Mg2+ substitution for V3+. Concurrently, the intercalation voltage rises from 3.35 to 3.45 V, due to increasingly more ionic VO bonds, and the sodium (de)intercalation mechanism transitions from a two‐phase for y ≤ 0.5 to a solid solution process for y ≥ 0.5, as confirmed by in operando XRD, while Na‐ion diffusion kinetics follow a nonlinear trend across the compositional series. [ABSTRACT FROM AUTHOR]