Electrochemical and Local Structural Analysis of the Li3V(2-2x/3)Mgx(PO4)3/C (x=0, 0.15, 0.3, and 0.45) Cathode

Electrochemically stable phosphates promise high reversible capacity, high operating voltage, and good ion mobility and have the potential to provide electrochemically superior, environmentally friendly (Co-free) and affordable battery materials.  Li₃V_(2−2x/3)Mg_x(PO₄)₃/C (x=0, 0.15, 0.3, and 0.45) composite phosphate cathode materials show high reversible capacity, high operating voltage and good cycling stability. In order to understand the functioning of this cathode at the molecular level, we have determined the local vanadium structure as a function of charge state using in-situ x-ray absorption spectroscopy (XAS). The XAS results verify for the charge compensation involves V cycling between V³⁺ and V⁵⁺ during charge and discharge and shows that these changes in oxidation/reduction are accompanied by modest changes in V-O bond length but little apparent change the overall local structure of the V and its neighboring atoms. In contrast, there is a significant change in local structure if the cut-off potential is increased from 4.5V to 4.8 V. Once the electrode has been charged to 4.8V it undergoes an irreversible conversion to a vanadyl-like structure.  While the vanadium is still redox active, it is not converted back to the starting form, even at potentials as low as 2.0 V cut off region. Although the details of the behavior depend on the level of Mg doping, similar results are found for all samples.  These data provide for the first time a molecular level explanation for the observation that vanadium phosphate batteries suffer irreversible capacity loss if charges above 4.5 V.