Structural and transport evolution in the Li_xAg₂V₄O₁₁ system

We investigated the effect of inserting lithium into Ag₂V₄O₁₁ (ε-SVO) on the structure, electronic properties and redox committed by combining in situ XRD measurements, ESR spectroscopy and 4 probes DC conductivity coupled with thermopower measurements. The electrochemical discharge occurs in three consecutive steps above 2 V (vs. Li⁺/Li). The first one, between 0 < x < ∼0.7 in Li_x-SVO, has been ascribed to the V⁵⁺ reduction through a solid solution mechanism. This reduction competes with a Li⁺/Ag⁺ displacement reaction which leads to a structural collapse owing to the ionic radii mismatch between the withdrawn Ag⁺ and the inserted Li⁺. The silver reduction progresses continuously with two different slopes along two composition-potential plateaus at 2.81 V and 2.55 V. Finally, the reduction continues until we obtain an amorphous structure with V⁴⁺ and a ε of V³⁺. Although, the silver re-enters the structure during the subsequent recharge, the original structure is not recovered. The reduction of silver forming silver metal nano-clusters acts to increase the electronic conductivity from 3.8 × 10^-5 S cm^-1 to 1.4 × 10^-3 S cm^-1. In complement to this study, we also report on a low temperature hydro-(solvo)-thermal approach using HF_(aq) as a mineralizer, which enables the synthesis of nano-sized ε-SVO particles that exhibit superior electrochemical performances compared to conventional particles synthesized by solid-state reaction.