Unraveling the structural mechanism of Li insertion in γ′-V2O5 and its effect on cycling properties

We report the first electrochemical and structural study of the γ′-V₂O₅ polymorph toward Li insertion, with cycling properties evaluated in the 4 V-2.4 V, 3.6 V-2.4 V and 4 V-2.15 V potential ranges. This cathode material is synthesized through topotactic lithium removal from the ternary γ-LiV₂O₅ bronze, using a strong oxidizing agent. It exhibits two pairs of well-defined reversible steps at 3.58/3.47 V and 2.42/2.36 V separated by a sharp potential drop of about 1 V. A high specific capacity of 285 mAh g⁻¹ is involved in the 4.00 V-2.15 V voltage window corresponding to the insertion of nearly 2 Li/mole of oxide. The γ′-V₂O₅ material can deliver stable specific capacities of 120–185 mAh g⁻¹ over 45 cycles at C/10 when the lower cut-off voltage is limited to 2.4 V. Cycling experiments in the widest 4.00 V-2.15 V potential range induce however a significant capacity decline. A detailed XRD and Raman spectroscopy study delivers a detailed picture of the structural changes occurring in γ′-V₂O₅ as a function of Li content. A complete phase diagram of the γ′-V₂O₅/Li system is provided during the first discharge-charge cycle in the extended 4.00 V–2.15 V voltage range, i.e. for 0 ≤ x ≤ 1.94 in Li_xV₂O₅. It is demonstrated the existence of a wide solid solution domain in the 0.4 ≤ x < 1.4 composition range (3.6 V-2.4 V), leading to a remarkable enhancement of the cycle performance in the corresponding controlled voltage window. The Raman fingerprint of the fully lithiated ζ-Li_1.94V₂O₅ phase is newly identified, indicating that deep structural rearrangements at the atomic scale take place during the γ-Li_1.4V₂O₅ → ζ-Li_1.94V₂O₅ transition. The resulting phase diagram accounts for the potential-composition profile and sheds light on the nature of the cycling properties in the different voltage windows.