Capacity Fade Mechanisms of NaTi2(PO4)3 When Used As an Aqueous Anode

The capacity fade of NaTi₂(PO₄)₃ when used as an anode material in aqueous electrolytes was investigated using electrochemical methods including electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT), as well as transmission electron microscopy (TEM) and x-ray diffraction (XRD). Both EIS and GITT indicated a continual increase in resistance of the synthesized material with little change in the diffusion rate of the sodium ion into the structure. XRD showed no change in crystallinity of the material after cycling. Morphological characterization using TEM showed an amorphous sodium phosphate layer formed between the in-situ carbon coating and the NaTi₂(PO₄)₃ particles during synthesis when titania particles >100nm were used in the precursor powder. This layer was not present when smaller titania particles were used. Additionally, TEM showed the in-situ carbon coating became detached from the active material upon cycling. We believe this detachment of the conductive coating is the primary cause of capacity fade during cycling when the small titania particles are used in the synthesis. A better understanding of the detachment mechanism is needed to improve the cycle stability of NaTi₂(PO₄)₃ materials in aqueous solutions.