A PVDF/g-C₃N₄-Based Composite Polymer Electrolytes for Sodium-Ion Battery

As one of the most promising candidates for all-solid-state sodium-ion batteries and sodium-metal batteries, polyvinylidene difluoride (PVDF) and amorphous hexafluoropropylene (HFP) copolymerized polymer solid electrolytes still suffer from a relatively low room temperature ionic conductivity. To modify the properties of PVDF-HEP copolymer electrolytes, we introduce the graphitic C₃N₄ (g-C₃N₄) nanosheets as a novel nanofiller to form g-C₃N₄ composite solid polymer electrolytes (CSPEs). The analysis shows that the g-C₃N₄ filler can not only modify the structure in g-C₃N₄CSPEs by reducing the crystallinity, compared to the PVDF-HFP solid polymer electrolytes (SPEs), but also promote a further dissociation with the sodium salt through interaction between the surface atoms of the g-C₃N₄ and the sodium salt. As a result, enhanced electrical properties such as ionic conductivity, Na⁺ transference number, mechanical properties and thermal stability of the composite electrolyte can be observed. In particular, a low Na deposition/dissolution overpotential of about 100 mV at a current density of 1 mA cm^-2 was found after 160 cycles with the incorporation of g-C₃N₄. By applying the g-C₃N₄ CSPEs in the sodium-metal battery with Na₃V₂(PO₄)₃ cathode, the coin cell battery exhibits a lower polarization voltage at 90 mV, and a stable reversible capacity of 93 mAh g^-1 after 200 cycles at 1 C.