Sol–gel preparation of V2O5 sheets and their lithium storage behaviors studied by electrochemical and in-situ X-ray diffraction techniques

In this paper, V2O5 sheets and bulks are prepared by a low temperature sol–gel method with and without tartaric acid as chelating agent, respectively. Owing to short diffusion pathways for lithium ions, V2O5 sheets display the reversible lithium storage capacities of 243.2, 167.5, 124.3 mAh g−1 at 0.2C, 1C and 2C in 1.5–4.1 V, respectively. Electroanalytical results reveal that the chemical diffusion coefficient (3.4×10−12 cm2 s−1) in Li-deficient LixV2O5 sheets (x=0.681) is almost one order of magnitude higher than that (3.7×10−13 cm2 s−1) in Li-rich phase (x=2.824). As a result, V2O5 sheets show an outstanding capacity retention of 92.7% after 22 cycles due to 0.912 Li per formula storage in Li-deficient LixV2O5 at 2C. In-situ structural analysis shows that the electrochemical reactions of V2O5 sheets with Li are associated with the formation of α-LixV2O5, ε-LixV2O5, δ-LixV2O5 and γ-LixV2O5 in 1.5–4.1 V. In the reverse delithiation process, γ-LixV2O5 can transforms back into δ-LixV2O5, ε-LixV2O5 and α-LixV2O5, but cannot return to the pristine V2O5. Upon an over-lithiation process to 0.0 V, the crystal structure of LixV2O5 (x>3) sheets will collapse and cannot be rebuilt in 0.0–4.1 V.