NaTi₂(PO₄)₃ nanoparticles embedded in double carbon networks as a negative electrode for an aqueous sodium-polyiodide flow battery

To demonstrate an aqueous sodium-polyiodide flow battery (SIFB) for the first time, sodium titanium phosphate (NaTi₂(PO₄)₃) was utilized as the negative electrode and I⁻/I₃⁻ couple was adopted as the positive redox active species. SIFB not only shows advantages of cost-effectiveness and environmental friendliness based on earth-abundant elements of sodium and iodine, but also exhibits a high energy density due to the high solubility of sodium iodide (~12.3 M at room temperature). Although NaTi₂(PO₄)₃ has several advantages such as high Na-ion conductivity and good structural stability, its use as a negative electrode is limited because of intrinsically low electrical conductivity. In this study, NaTi₂(PO₄)₃ nanoparticles were embedded in double carbon networks comprising internal and external carbon layers. Individual effects by the internal and external carbon networks on the electrochemical performance of NaTi₂(PO₄)₃ were clearly distinguished. With the simultaneous formation of the two carbon networks, a SIFB exhibited outstanding electrochemical performance (86.0 mAh•g⁻¹ at a rate of 7 C) and a long cycle life (93.5% capacity retention after 100 cycles).