Novel Vanadium Based Cation-Disordered Lithium Transition Metal Oxides for Li-Ion Batteries

Abstract

Secondary lithium batteries are considered as a fundamental device for green and sustainable energy development and are now used as power sources for EV’s. However, the demand for higher energy density LIB’s necessitates the search for new materials and new systems. Compounds with the ability to reversibly exchange more than 1 Li+ per formula unit are of great importance in order to achieve high specific capacities without undergoing severe chemical and structural changes. Prominent examples for multiple-electron transfer cathodes are Li2MSiO4 silicates, Li2M(PO4)F fluorophosphates[1] and Li2MTiO4 titanates.[2] The latter belongs to the class of cation-disordered rocksalt structure (spacegroup Fm-3m) with all cations disordered in the Wyckoff 4a site in the cationic sublattice. Up to now the practical capacities of disordered rocksalt titanates were restricted to 1e- transfer to about 150 mAhg-1. Vanadium-based positive materials are well known to possess a high capacity due to the ability of multiple redox-electron transfer. By the introduction of vanadium into the Li2MTiO4 Dominko et al [3] achieved reversible lithium intercalation-deintercalation beyond 1 Li+. Thus it appears, introducing vanadium into Li2M2O4 rock salt compounds is a promising approach in the search for new materials with high reversible capacity. In this respect we synthesized a novel positive cathode material with disordered rock salt structure. The overall formula is Li2M2O4 with vanadium as one of the redox active transition metals within the structure. To the best of knowledge this compound has not been reported so far. The XRD pattern of the as-prepared compound is shown in figure a) and can be indexed to cation-disordered cubic rock salt structure (space group Fm-3m) with the lattice parameter of a = 4.129 Å. Figure b) is showing the charge-discharge profile for two different voltage windows. A narrow window between 3.5-1.0 V showing a reversible capacity of 170 mAhg-1. For an extended cycling window between 4.5-1.3 V a first discharge of around 245 mAhg-1is achieved. Figure c) shows the respective cycling performance. The OCV is around ~ 2.5 V vs Li/Li+. Various characterization techniques were applied to verify the redox process and reversible structural change upon electrochemical cycling.