Vanadium and Chromium Molecular Cluster Batteries (MCB): Li- Storage in Transition Metal Complexes

Abstract

Increasing energy demands and the requirement to reduce carbon dioxide emissions at the same time accelerated the world-wide research on renewable energy sources and energy storage. However, for future applications in personal electronics, electro-mobility and decentralised energy storage, new LIB electrode materials based on light-weight and earth abundant components with superior redox activity, long-term stability and high energy density are urgently required. The concept of using molecular transition metal clusters as active electrode materials for reversible Li ion shuttling opens up a new direction for energy storage.[1] These metal complexes are interesting electrode materials due to the ability of the transition metal ions to exist in several oxidation states and reversibly reacting with Li.[2,3] In this study we introduce LIBs based on molecular cluster systems of the early transition metal ions vanadium and chromium as the cathode active material, in a so-called molecular cluster battery (MCB). Transition metal complexes of different ligand systems were characterised by IR, XRD and SEM techniques. Their Li cycling behaviour was investigated by galvanostatic cycling and cyclic voltammetry (CV). High reversible specific capacities with low capacity loss were found and multi-step redox processes were observed reflecting the various oxidation states of these metal ions. The 2D channels of the crystalline compounds, which are revealed in their crystal packing diagram, are suitable for an easy Li ion transportation trough the structure. The Li ion diffusion coefficients were determined by various electrochemical techniques, such as GITT, CV and EIS to understand the Li ion kinetics of these molecular metal clusters. The various oxidation steps of molecular cluster compounds allow multi-step redox changes making these materials interesting compounds with potentially high specific capacities and allow a study of the kinetics of the lithium ion diffusion pathway.