Slab-Gliding–Induced Structural Evolution in β-V$_2$O$_5$ Enables Reversible High Na-Ion Storage: A Combined Operando Synchrotron Diffraction and Operando XAS Study

High-pressure β-V$_2$O$_5$ 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-V$_2$O$_5$ during sodium intercalation. Operando synchrotron diffraction and operando 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$_x$V$_2$O$_5$ (0 ≤ x ≤ 1) was performed for the first time directly from operando synchrotron diffraction and ex situ transmission electron microscopy. Our findings reveal a fully reversible phase transition sequence, P2$_1$/m → C2/m → P2$_1$/m, during sodium intercalation, driven by the facile slab-gliding of V$_2$O$_5$ layers along the crystallographic b direction to accommodate varying amounts of sodium ions. ... mehrThis storage mechanism was further supported with first-principles density functional theory (DFT) calculations.