New construction of polypyrrole interphase layers to improve performance stability of NaTi2(PO4)3 anode for aqueous Na-ion batteries

NaTi2(PO4)3 (NTP) as a promising anode material for aqueous sodium-ion batteries has attracted widespread attention. However, the NTP material commonly exhibits significant capacity fade, possibly mainly attributing to the change of the chemical stability of NTP in the aqueous electrolytes. Coating NTP with polypyrrole (PPY) conductive polymers is a promising strategy to solve this problem, but in situ synthesis of the PPY polymer in the presence of the NTP active materials seems not to be easy, reported by the previous work, owing to the inferior adhesion of the synthesized PPY to the NTP surface. Thereby, this work has attempted ex situ synthesis of PPY coatings on the carbon-coated NTP (NTP/C) anode surface in the presence of small amounts of carbon conductive additives and organic binders, and then it shows that synthesized conductive PPY interphase layer can readily adhere to the NTP surface with no visible phase separation through the electrochemical oxidation polymerization under certain potentials in the pyrrole (PY)-containing electrolytes. The corresponding optimized oxidation modification conditions of the anode can be obtained, and the evolution law of the anode interface with the content of the electrolyte additives, the oxidation potential and the oxidation time can be established. Thus, the as-synthesized optimal modified anodes, with the thickness of about 24 μm for the PPY coating, are observed to have fairly large charge/discharge capacity of 228.6 mAh·g−1, high coulombic efficiency (> 95%) and enhanced cycling stability. Furthermore, a schematic for the charge/discharge processes involving possible reaction pathways at the modified anodes with the PPY layers in Na2SO4 aqueous electrolytes has been proposed.