Slab-Gliding–Induced Structural Evolution in β-V ₂ O ₅ Enables Reversible High Na-Ion Storage: A Combined <i>Operando</i> Synchrotron Diffraction and <i>Operando</i> XAS Study

High-pressure β-V₂O₅ is a positive electrode material for sodium-ion batteries offering a remarkable high capacity of ∼150 mAh g^-1. Despite its attractive electrochemical properties and interesting crystal chemistry due to the existence of several sodiated phases, the sodium intercalation mechanism that provides reversible insertion is still largely unclear. In this work, we conducted a comprehensive investigation of the structural evolution, oxidation state and local structural changes of high-pressure β-V₂O₅ during sodium intercalation. <i>Operando</i> synchrotron diffraction and <i>operando</i> X-ray absorption spectroscopy together with X-ray photoelectron spectroscopy, reveal the reversibility of sodium (de)-intercalation and allowed us to gain a complete picture of the crystal structure evolution and oxidation state changes during cycling. A full crystal structure determination of the sodiated phases Na _<i>x</i> V₂O₅ (0 ≤ <i>x</i> ≤ 1) was performed for the first time directly from <i>operando</i> synchrotron diffraction and <i>ex situ</i> transmission electron microscopy. Our findings reveal a fully reversible phase transition sequence, <i>P</i>2₁/<i>m</i> → <i>C</i>2/<i>m</i> → <i>P</i>2₁/<i>m</i>, during sodium intercalation, driven by the facile slab-gliding of V₂O₅ layers along the crystallographic <i>b</i> direction to accommodate varying amounts of sodium ions. This storage mechanism was further supported with first-principles density functional theory (DFT) calculations.