Bilayered vanadium oxides by chemical pre-intercalation of alkali and alkali-earth ions as battery electrodes

We report the use of the chemical pre-intercalation synthesis technique to insert alkali (Li+, Na+, K+) and, for the first time, alkali-earth (Mg2+ and Ca2+) ions into the structure of vanadium oxide leading to the formation of the bilayered δ-MxV2O5 (M = Li, Na, K, Mg, Ca) phase with expanded interlayer spacing, enabling a large number of insertion sites for and faster diffusion of charge-carrying ions. By altering the nature of the chemically preintercalated ion, interlayer spacing of the synthesized δ-MxV2O5 materials was varied between 9.62 and 13.40 Å. We for the first time show that the interlayer spacing increases with the increase of the hydrated ion radius. The ion (Na+, K+, Mg2+, Ca2+) stabilization effect was investigated in Li-ion cells, with Li-preintercalated phase, δ-LixV2O5, serving as a reference material. Our analyses indicate that cyclability and rate performance of the δ-MxV2O5 improves with increasing interlayer spacing. The highest initial capacity (198 mAh g−1), greatest capacity retention (81.8% after 50 cycles at 20 mA g−1), and highest capacity retention at higher current rates (74.5% when current rate was changed from C/15 to 1 C) were exhibited by Mg-stabilized δ-V2O5 with the largest interlayer spacing (13.40 Å). This research demonstrates the efficacy of a facile chemical pre-intercalation strategy to synthesize ion-stabilized layered electrode materials with improved electrochemical stability. Ion-stabilized layered materials with large interlayer spacing are attractive for applications that involve electrochemically driven movement of ions through two-dimensional diffusion channels, ranging from beyond Li-ion energy storage and electrochromics to actuation and water treatment.