γ′-V₂O₅ Polymorph as a Promising Host Structure for Potassium Storage: an Electrochemical and Structural Study

K-ion batteries (KIBs) are receiving increasing interest because of their low K+/K redox potential and reduced cost. This emerging alternative is, however, strongly dependent on the development of cathode materials with a suitable structure for accommodating K+ ions. We show here the promising properties of the puckered layered γ′-V2O5 polymorph that inserts up to 0.9 K+ mol–1 at 3.3 V versus K+/K at C/60. An initial depotassiation capacity of 72 mA h g–1 corresponding to the exchange of 0.5 K+ ions is still delivered at C/10, and a reversible capacity of 48 mA h g–1, stable over 100 cycles, is achieved in the 4.4–2.4 V voltage window. The reaction mechanism, investigated by X-ray diffraction and Raman spectroscopy, involves the formation on the first discharge of a new layered KxV2O5 host structure. The K0.78V2O5 bronze obtained at 2.4 V exhibits unfolded V2O5 sheets and an unexpected moderate expansion of the interlayer spacing compared to γ′-V2O5. Depotassiation–potassiation reversibly occurs within KxV2O5 (0.3 ≤ x ≤ 0.78) with less than 2% breathing. Such findings demonstrate the remarkable structural flexibility of γ′-V2O5 to accommodate the large-sized K+ ions and illustrate the richness of V2O5 polymorphs as positive electrode materials for KIBs.