Electrochemical Phase Engineering of γ'‑V₂O₅ Thin Films for Sodium-Ion Storage Electrodes

V₂O₅ is a promising sodium-ion cathode material due to its high theoretical capacity (147 mAh/g) and working voltage (3.3 V vs Na/Na⁺). Among its various crystal phases, γ'-V₂O₅ has a large interlayer spacing, ensuring the reversible insertion-extraction of sodium ions. However, current synthesis methods for γ'-V₂O₅ require high temperatures (>600 °C) and toxic chemicals (NO₂BF₄), which make the preparation demanding. Herein, we put forward an electrochemical phase engineering method combining thermal annealing and electrochemistry to easily prepare thin-film γ'-V₂O₅. Electrochemical characterization shows near-ideal performance as a thin-film cathode material for sodium-ion batteries. It shows a measured initial capacity of 152 mAh/g, a high working voltage (3.3 V vs Na⁺/Na), and an exceptional Coulombic efficiency of 98%, significantly surpassing previously reported values (∼50% CE). Cyclic voltammogram and galvanostatic capacity curves confirm the sodium insertion-deinsertion, which remains stable at 2 C. The γ'-V₂O₅ thin film has electrochemical performance similar to γ'-V₂O₅ powder, indicating another workable morphology of γ'-V₂O₅ for sodium-ion batteries.