Solid-State Nuclear Magnetic Resonance Insights into the Precursor-Dependent Structure and Na-Ion Storage Behavior of Na-Preintercalated Bilayered Vanadium Oxides

Chemically preintercalated bilayered vanadium oxide (BVO) electrodes derived from V2CTx MXene exhibit superior Na-ion storage performance compared to compositionally similar BVO counterparts synthesized from α-V2O5 powder. Here, we report for the first time the precursor-dependent structural differences in Na-preintercalated BVO electrodes (δ-NaxV2O5·nH2O) synthesized from α-V2O5 powder (AD-NVO) and V2CTx MXene nanoflakes (MD-NVO) and show how these differences govern their electrochemical behavior in a nonaqueous Na-ion energy storage system. Our analyses show that AD-NVO and MD-NVO exhibit distinct compositions of δ-Na0.37V2O5·0.46H2O and δ-Na0.33V2O5·0.21H2O, respectively, along with pronounced differences in morphology, electronic structure, and interlayer chemistry. Scanning electron microscopy reveals the formation of 1D nanobelts for AD-NVO, whereas MD-NVO consists of 2D nanoflakes assembled into nanoflower-like agglomerates. X-ray photoelectron spectroscopy indicates that Na preintercalation led to different extents of V5+ to V4+ reduction in AD-NVO and MD-NVO, attributed to differences in the structural water content, which was further supported by the V4+ content quantification via electron paramagnetic resonance. Electrochemical measurements show fundamentally different charge storage behaviors: AD-NVO exhibits largely capacitive responses, while MD-NVO displays pronounced Na+ redox activity and delivers a higher specific capacity, improved rate capability, and superior cycling stability. Magic-angle spinning 23Na solid-state NMR identifies two distinct interlayer Na environments in MD-NVO, in contrast to a single Na site in AD-NVO. These sites play complementary roles, with one facilitating Na+ transport and the other acting as stabilizing pillars, as confirmed by ex situ X-ray diffraction. This study reveals how precursor-dependent structural evolution in chemically preintercalated layered oxides governs interlayer chemistry and electrochemical function, providing design principles for engineering layered metal oxides for advanced energy storage.