A PVDF/g-C 3 N 4 -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 C 3 N 4 (g-C 3 N 4 ) nanosheets as a novel nanofiller to form g-C 3 N 4 composite solid polymer electrolytes (CSPEs). The analysis shows that the g-C 3 N 4 filler can not only modify the structure in g-C 3 N 4 CSPEs 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-C 3 N 4 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-C 3 N 4 . By applying the g-C 3 N 4 CSPEs in the sodium-metal battery with Na 3 V 2 (PO 4 ) 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.