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−C3N4composite solid polymer electrolytes (CSPEs). The analysis shows that the g−C3N4filler 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−C3N4and 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−2was found after 160 cycles with the incorporation of g−C3N4. By applying the g−C3N4CSPEs in the sodium-metal battery with Na3V2(PO4)3cathode, the coin cell battery exhibits a lower polarization voltage at 90 mV, and a stable reversible capacity of 93 mAh g−1after 200 cycles at 1 C.