Structural and transport evolution in the Li xAg 2V 4O 11 system

Abstract

We investigated the effect of inserting lithium into Ag 2V 4O 11 ( ɛ-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 V 5+ 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a Li +/Ag + displacement reaction which leads to a structural collapse owing to the ionic radii mismatch between the withdrawn Ag + and the inserted Li +.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 V 4+ and a ɛ of V 3+. 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 ɛ-SVO particles that exhibit superior electrochemical performances compared to conventional particles synthesized by solid-state reaction.