Differentiating Molecular and Solid-State Vanadium Oxides as Active Materials in Battery Electrodes

Molecular vanadium oxides - polyoxovanadates (POVs) - have recently received widespread interest as new, high performance active materials in lithium ion and sodium ion battery electrodes. This is due to their low molecular weight and their high redox activity. However, little attention has been paid to the structural and chemical stability of the POVs under typical electrode fabrication processes. Here, we report how molecular polyoxovanadates can be differentiated from solid-state vanadium oxides as active materials in battery electrodes using combined spectroscopic, X-ray diffraction and element-analytical data. Our study highlights that novel electrode fabrication processes are required as prototype POVs are not stable under classical fabrication conditions. We explore POV degradation pathways and show how thermal POV conversion leads to the formation of layered solid-state lithium vanadium oxide phases. A facile protocol is presented to detect and prevent POV degradation. In future, this will enable energy materials science to unambiguously identify and use molecular or solid-state vanadium oxides as next-generation active materials in lithium or post-lithium battery electrodes.