The mechanism of NaTi2(PO4)3 aqueous electrochemical degradation revisited

NASICON structure type materials are attracting increasing attention as electrode active materials for ion insertion batteries and deionization devices. NaTi2(PO4)3 due to its favorable potential is among the most suitable negative electrodes for various applications in aqueous electrolytes. However, various parasitic processes limit its Coulombic efficiency, self-discharge characteristics, and capacity retention. Understanding and mitigation of these processes are essential in order to fully utilize the potential of such and similar materials. In this study, the degradation mechanisms of NaTi2(PO4)3 in aqueous electrolytes is studied in great detail. The results indicate that, to a different extent, both hydrogen evolution and oxygen reduction reactions are taking place during NaTi2(PO4)3 electrochemical cycling and contribute to pH increase. The latter reaction due to the presence of Ti(III) species is found to be prevalent in aerated electrolytes. There is virtually no degradation and capacity loss observed at pH 7, but it becomes significant at pH 10 even in oxygen-free electrolytes. Contrary to previous studies, this work shows that capacity loss is actually slower at lower C-rates. The post mortem X-ray diffraction analysis shows that only a fraction of capacity loss could be directly attributed to NaTi2(PO4)3 decomposition and even less to its dissolution into electrolyte. Therefore, most of the observed capacity fade during cycling is related to contact loss in the electrode structure which most likely comes from the formation of electron blocking aqueous interphasial layer.