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.


Introduction
The low reserve, uneven distribution and rising costs of lithium resources are becoming vexing problems for the currently dominant lithium-ion batteries (LIBs) [1]. Therefore, as a promising substitution for LIBs, sodium-ion batteries (SIBs) are attracting more and more attention because they have almost the same electrochemical working principles as LIBs and the abundant reserve and wide distribution of sodium element in the earth's crust [2,3]. In the existing commercial sodium-ion batteries, the highly volatile and flammable organic electrolyte still has safety issues, as to cause a fire or even an explosion, and those disadvantages become more and more serious as the energy density increases. Additionally, for a purpose of controlling costs and improving the performance of SIBs, a common method is to replace carbon with a sodium-metal anode to form sodium-metal batteries (SMBs). However, even though the cost is controlled and performance is enhanced in SMBs, it arouses an even higher safety concern regarding the formation of sodium dendrites [4]. Therefore, all-solid-state batteries is developed using solid-state electrolytes and are considered to be the next-generation batteries due to an excellent balance between high performance and safety [5,6]. Among the various types of solid electrolytes, polymer solid electrolytes play a

Preparation of g−C 3 N 4 CSPE and Assembly of Coin Cell Battery
The solid polymer electrolytes and composite solid polymer electrolytes were prepared by the solution casting method. Specifically, PVDF−HFP (M W = 6 × 10 5 , Aladdin), NaClO 4 (≥ 98.0%, anhydrous, Aladdin) and g−C 3 N 4 were dried under vacuum, stored in an argonfilled glove box (H 2 O and O 2 content below 0.1 ppm, Mikrouna) before used [17]. To synthesize CSPEs, 1 g PVDF−HFP, 0.05 g g−C 3 N 4 , and 0.3 g NaClO 4 were dissolved in 20 mL N-methyl-2-pyrrolidone (NMP, 99.9%, Aladdin). After ultrasonication treatment for 30 min, magnetic stirring was performed for 18 h to obtain a uniform mixed dispersion. Then, the dispersion was cast on a polytetrafluoroethylene (PTFE) mold and vacuumdried at 60 • C for 48 h to obtain a film with a thickness of 0.056-0.114 mm. Initially, the PVDF−HFP/NaClO 4 electrolyte films were transparent and colorless. After combined with g−C 3 N 4 , the color gradually shifted to pale yellow ( Figure S1). This is an implication of the formation of the g−C 3 N 4 CSPEs. For comparison, SPE without g−C 3 N 4 and neat PVDF−HFP film was prepared by the same procedure. All obtained membranes were stored in an argon-filled glove box.
The feasibility of PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs in solid-state sodium metal batteries was investigated by fabricating the coin cell battery assembled with Na metal anodes and a NVP cathode (Na|CSPEs|NVP) [22][23][24]. The performance of coin cells, including charge-discharge cycling performance and galvanostatic cycling performance, was tested at room temperature using the LAND battery testing system (Wuhan Landian LAND-CT3001A).

Characterization
The structures of g−C 3 N 4 , PVDF−HFP, PVDF−HFP/NaClO 4 SPEs and PVDF−HFPg−C 3 N 4 -NaClO 4 CSPEs were characterized by X-ray powder diffractometer (XRD, Rigaku MiniFlex 600) and Fourier transform infrared spectra (FTIR, Bruker VECTOR-22). For XRD, the diffraction angle ranged from 2θ = 10 •~7 0 • , and the scan rate was 5 • min −1 . For FTIR, the samples were recorded in the range of 400~4000 cm −1 to analyze the interactions between groups in the polymer. The tensile samples of PVDF−HFP SPEs and g−C 3 N 4 CSPE were stretched in a gauge length of 35 mm × 2 mm on a universal testing machine (Shandong Wanchen, UTM-4000) with a 50 N sensor to evaluate their mechanical properties with a stretching rate of 10 mm min −1 in the thickness of 80-100 µm. The thermal stability of PVDF−HFP SPEs and g−C 3 N 4 CSPE samples were determined by differential scanning calorimeter (DSC, TA Q2000) and Thermogravimetric Analysis (TGA, TGA-55) in sealed aluminum crucibles and heated at a rate of 10 • C min −1 . The surface morphology analysis of polymer electrolytes was observed with a scanning electron microscope (SEM, FEI Verios 460) and atomic force microscopy (AFM, Rigaku SPI3800N/SPA400).

Electrochemical Test
The ionic conductivity of solid electrolytes was determined by the electrochemical impedance spectroscopy (EIS) test on an electrochemical workstation (Ametek PARSTAT3000A-DX). The electrolyte was sandwiched between two stainless steel plates. The frequency range was between 10 −1 and 10 6 Hz and the temperature range was from 25 to 80 • C. The ionic conductivity of polymer electrolyte was calculated by the following equation: where L is the thickness of the polymer electrolyte, R is the bulk resistance of the polymer electrode which can be obtained from the EIS test, and S is the area of the electrolyte. Linear sweep voltammetry of g−C 3 N 4 CSPE and SPE were tested on an electrochemical workstation (Ametek PARSTAT3000A-DX) at a scan rate of 5 mV s −1 on Na/SPE/ stainless steel equipment prepared using CR 2032 coin cell. The sodium ion transfer number of g−C 3 N 4 CSPE was calculated from the Bruce-Vincent-Evans equation: ∆V is the polarization voltage (∆V = 10 mV), I 0 and R 0 are the initial current and interfacial resistance before polarization, respectively. I ss and R ss are the steady-state current and interfacial resistance after polarization, respectively.

XRD Analysis
To characterize and investigate the composition of CSPEs, the crystallinity of g−C 3 N 4 powder, PVDF−HFP copolymerized electrolytes, PVDF−HFP/NaClO 4 SPEs and PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs was analyzed by XRD in Figure 1. The thickness of SPEs and CSPEs are prepared in the range of 70 to 100 µm ( Figure S2), since it's the common value to lower the battery impedance and ensure flexibility of the membrane [25]. Since PVDF is a semicrystalline polymer and has two main crystal phases, α and β, two partially overlapped diffraction peaks are observed in PVDF−HFP/NaClO 4 at 18.5 • and 20.3 • , which are reflected from the crystal plane of α (100) and β (110) (200) phase, respectively [26,27]. In a comparison of the g−C 3 N 4 powder and PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs, a tiny g−C 3 N 4 characteristic peak at 27.6 • appeared in CEPEs and did not have a significant change before and after hybridization (Figure 1 inset), indicating that the presence of g−C 3 N 4 and good compatibility in the preparation process [12,20]. Furthermore, by calculation, the crystallite size is reduced from 37 nm in PVDF−HFP/NaClO 4 SPEs to 23 nm in PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs according to the Williamson-Hall Methods [28,29]. The Segel crystallinity index (CI) for the polymer electrolyte was calculated from the ratio of the total intensity of the α (100) and β (110) (200) phase and the intensity of the amorphous α phase peak, which is CI= (I total − I amorphous )/I total [30,31]. The CI for PVDF−HFP/NaClO 4 SPEs and PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs is 0.62 and 0.55. Therefore, it is obvious that the addition of g−C 3 N 4 in PVDF−HFP/NaClO 4 decreased the crystallization, as revealed by a weakened amorphous peak intensity. The high degree of crystallinity of PVDF−HFP results in low ionic conductivity of its corresponding solid electrolyte at room temperature. Meanwhile, the broader PVDF−HFP signal in CSPEs (inset) indicates lowered degree of crystallinity, which would lead to higher RT ionic conductivity. The XRD confirmed that the addition of g−C 3 N 4 nanofiller can reduce the crystallinity of PVDF−HFP and increase the amorphous region, thereby enhancing the mobility of sodium ions [18].

FTIR Spectra Analysis
In Figure 2, the FTIR spectra of PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs in the range of 500-4000 cm −1 are shown. After adding nanofillers, the broad peak emerges at 3450 cm −1 in PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs, which is associated with the residual N-H bonded structure on the surface of g−C 3 N 4 [32]. Characteristics peaks of the C-N(-C)-C and bridging C-NH-C bonded structures appear between 1800-900cm −1 [33]. The peak at 744-875 cm −1 and 838 cm −1 can be considered as the α and amorphous phase of crystalline PVDF−HFP, respectively [34]. The two characteristic peaks weakened or even disappeared in PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs, indicating the interaction between g−C 3 N 4 nanosheets and polymer matrix [35]. The rich N atoms on the surface of g−C 3 N 4 can be considered a Lewis base [18]. Thus, the interaction between N-Na leads to an increase in the dissociation of the sodium salt, which has the ability to enhance ionic conductivity.

FTIR Spectra Analysis
In Figure 2, the FTIR spectra of PVDF−HFP/g−C3N4/NaClO4 CSPEs in the range of 500-4000 cm −1 are shown. After adding nanofillers, the broad peak emerges at 3450 cm −1 in PVDF−HFP/g−C3N4/NaClO4 CSPEs, which is associated with the residual N-H bonded structure on the surface of g−C3N4 [32]. Characteristics peaks of the C-N(-C)-C and bridging C-NH-C bonded structures appear between 1800-900cm −1 [33]. The peak at 744-875 cm −1 and 838 cm −1 can be considered as the α and amorphous phase of crystalline PVDF−HFP, respectively [34]. The two characteristic peaks weakened or even disappeared in PVDF−HFP/g−C3N4/NaClO4 CSPEs, indicating the interaction between g−C3N4 nanosheets and polymer matrix [35]. The rich N atoms on the surface of g−C3N4 can be considered a Lewis base [18]. Thus, the interaction between N-Na leads to an increase in the dissociation of the sodium salt, which has the ability to enhance ionic conductivity.

FTIR Spectra Analysis
In Figure 2, the FTIR spectra of PVDF−HFP/g−C3N4/NaClO4 CSPEs in the range of 500-4000 cm −1 are shown. After adding nanofillers, the broad peak emerges at 3450 cm −1 in PVDF−HFP/g−C3N4/NaClO4 CSPEs, which is associated with the residual N-H bonded structure on the surface of g−C3N4 [32]. Characteristics peaks of the C-N(-C)-C and bridging C-NH-C bonded structures appear between 1800-900cm −1 [33]. The peak at 744-875 cm −1 and 838 cm −1 can be considered as the α and amorphous phase of crystalline PVDF−HFP, respectively [34]. The two characteristic peaks weakened or even disappeared in PVDF−HFP/g−C3N4/NaClO4 CSPEs, indicating the interaction between g−C3N4 nanosheets and polymer matrix [35]. The rich N atoms on the surface of g−C3N4 can be considered a Lewis base [18]. Thus, the interaction between N-Na leads to an increase in the dissociation of the sodium salt, which has the ability to enhance ionic conductivity.

Stress-Strain Tests
Mechanical properties of PVDF−HFP/NaClO 4 SPEs and PVDF−HFP/g−C 3 N 4 / NaClO 4 CSPEs are shown in Figure 3, which is a critical parameter to evaluate the performance of polymer solid electrolytes by inhibiting the formation of Na dendrites [36]. The tensile strength of the CSPEs and SPEs are 16.2 MPa and 8.89 MPa at the maximum of strain, separately. It can be seen that the tensile strength gradually increases by 7.38 MPa, and the corresponding Young's modulus increased from 40.9 MPa to 66.8 MPa. Therefore, it was expected that the PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs prevented the sodium dendrites because of the larger tensile modulus which can delay the nucleation of sodium dendrites. The mechanic properties of our PVDF−HFP/g−C 3 N 4 /NaClO 4 com- posite is also very consistent with that of composite solid electrolytes for sodium batteries reported elsewhere [37].
CSPEs are shown in Figure 3, which is a critical parameter to evaluate the performance of polymer solid electrolytes by inhibiting the formation of Na dendrites [36]. The tensile strength of the CSPEs and SPEs are 16.2 MPa and 8.89 MPa at the maximum of strain, separately. It can be seen that the tensile strength gradually increases by 7.38 MPa, and the corresponding Young's modulus increased from 40.9 MPa to 66.8 MPa. Therefore, it was expected that the PVDF−HFP/g−C3N4/NaClO4 CSPEs prevented the sodium dendrites because of the larger tensile modulus which can delay the nucleation of sodium dendrites. The mechanic properties of our PVDF−HFP/g−C3N4/NaClO4 composite is also very consistent with that of composite solid electrolytes for sodium batteries reported elsewhere [37].  Figure 4a presents the TGA curves of the polymer electrolytes. The weight loss of the electrolyte between 120-180 °C was caused by the dissociation of NaClO4; the rapid weight loss between 400-450 °C can be attributed to the complete decomposition of the PVDF−HFP polymer. Compared with the process of weight loss of the PVDF−HFP/g−C3N4/NaClO4 CSPEs and PVDF−HFP/NaClO4 SPEs, a slower dissociation of NaClO4 in the CSPEs can be found, which indicates that the CSPEs exhibited higher thermal stability. Thereby, the safety of battery with incorporated g−C3N4 can be improved. The solvent residue in the PVDF−HFP CSPEs and SPEs was approximately 11%, as revealed by comparing TGA curves of PVDF−HFP powder and PVDF−HFP membrane prepared without Na salt and nanofiller ( Figure S4). In Figure 4b, the Tg of PVDF−HFP/NaClO4/g−C3N4 was −41 °C, which was decreased compared to that of PVDF−HFP/NaClO4 (−34 °C). As also revealed by DSC, the melting point slightly lowered from 146 °C to 143 °C after g−C3N4 introduced ( Figure S5). This indicates that the introduction of g−C3N4 reduces the crystallinity of PVDF−HFP, resulting in an improvement of the segment mobility in the electrolyte [6,34].  Figure 4a presents the TGA curves of the polymer electrolytes. The weight loss of the electrolyte between 120-180 • C was caused by the dissociation of NaClO 4 ; the rapid weight loss between 400-450 • C can be attributed to the complete decomposition of the PVDF−HFP polymer. Compared with the process of weight loss of the PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs and PVDF−HFP/NaClO 4 SPEs, a slower dissociation of NaClO 4 in the CSPEs can be found, which indicates that the CSPEs exhibited higher thermal stability. Thereby, the safety of battery with incorporated g−C 3 N 4 can be improved. The solvent residue in the PVDF−HFP CSPEs and SPEs was approximately 11%, as revealed by comparing TGA curves of PVDF−HFP powder and PVDF−HFP membrane prepared without Na salt and nanofiller ( Figure S4). In Figure 4b, the T g of PVDF−HFP/NaClO 4 /g−C 3 N 4 was −41 • C, which was decreased compared to that of PVDF−HFP/NaClO 4 (−34 • C). As also revealed by DSC, the melting point slightly lowered from 146 • C to 143 • C after g−C 3 N 4 introduced ( Figure S5). This indicates that the introduction of g−C 3 N 4 reduces the crystallinity of PVDF−HFP, resulting in an improvement of the segment mobility in the electrolyte [6,34].

Morphology Analysis
The morphology of the as-prepared PVDF−HFP based CSPEs was firstly characterized by SEM. As shown in Figure 5a,b, bare PVDF SPE presents a dense and smooth sur-

Morphology Analysis
The morphology of the as-prepared PVDF−HFP based CSPEs was firstly characterized by SEM. As shown in Figure 5a,b, bare PVDF SPE presents a dense and smooth surface with micrometer-sized pores. Meanwhile, the surface of CSPE with g−C 3 N 4 has a nodularlike structure with increased micropores. The evolution in morphology suggests that the addition of g−C 3 N 4 weakened the consistency of the polymer matrix, which greatly lowered the crystallinity of the electrolyte as previously revealed by XRD analysis. The addition of g−C 3 N 4 was beneficial to improve the ionic conductivity of the composite solid polymer electrolyte. The corresponding EDS mapping shows that the C, F elements are uniformly distributed in SPE and CSPE film along with PVDF−HFP aggregation (Figure 5c,d). The N element signal appeared in the CSPEs, confirming the successful combination of g−C 3 N 4 and polymer matrix.

Morphology Analysis
The morphology of the as-prepared PVDF−HFP based CSPEs was firstly charac ized by SEM. As shown in Figure 5a,b, bare PVDF SPE presents a dense and smooth s face with micrometer-sized pores. Meanwhile, the surface of CSPE with g−C3N4 has a n ular-like structure with increased micropores. The evolution in morphology suggests the addition of g−C3N4 weakened the consistency of the polymer matrix, which gre lowered the crystallinity of the electrolyte as previously revealed by XRD analysis. addition of g−C3N4 was beneficial to improve the ionic conductivity of the composite s polymer electrolyte. The corresponding EDS mapping shows that the C, F elements uniformly distributed in SPE and CSPE film along with PVDF−HFP aggregation (Fig  5c,d). The N element signal appeared in the CSPEs, confirming the successful combina of g−C3N4 and polymer matrix.  Figure 6. Comparing the 3D AFM images of the SPE (Figure 6a) and CSPE (Figure 6b), a clear mountainvalley pattern in the CSPE was observed, and this indicates a rough surface was formed in CSPEs. Furthermore, in 2D AFM images of the SPE and CSPE (Figure S3), the root- By further investigation, the topography images of PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPE obtained by AFM show a clear mountain-valley pattern, as shown in Figure 6. Comparing the 3D AFM images of the SPE (Figure 6a) and CSPE (Figure 6b), a clear mountain-valley pattern in the CSPE was observed, and this indicates a rough surface was formed in CSPEs. Furthermore, in 2D AFM images of the SPE and CSPE (Figure S3), the root-mean-roughness (RMS) in CSPE was calculated as 134.9 nm, which is a huge increase from 63.9 nm SPE without the nanofiller. This indicates the presence of g−C 3 N 4 nanosheets modifies the construction in CSPEs and shows a good complexation between the polymer and the g−C 3 N 4 . No small particles are observed on the surface, indicating the g−C 3 N 4 nanosheets have been uniformly dispersed into the host polymer. Figure 5. SEM images of (a) PVDF/NaClO4 and (b) PVDF/g−C3N4/NaClO4; EDS images of (c PVDF/NaClO4 and (d) PVDF/g−C3N4/NaClO4. By further investigation, the topography images of PVDF−HFP/g−C3N4/NaClO CSPE obtained by AFM show a clear mountain-valley pattern, as shown in Figure 6. Com paring the 3D AFM images of the SPE (Figure 6a) and CSPE (Figure 6b), a clear mountain valley pattern in the CSPE was observed, and this indicates a rough surface was formed in CSPEs. Furthermore, in 2D AFM images of the SPE and CSPE (Figure S3), the root mean-roughness (RMS) in CSPE was calculated as 134.9 nm, which is a huge increase from 63.9 nm SPE without the nanofiller. This indicates the presence of g−C3N4 nanosheet modifies the construction in CSPEs and shows a good complexation between the polyme and the g−C3N4. No small particles are observed on the surface, indicating the g−C3N nanosheets have been uniformly dispersed into the host polymer.

The Electrochemical Characterization
To understand the effect of g−C3N4 filler on the ionic transport capability o PVDF−HFP polymer solid electrolyte, the ionic conductivity of the CSPE was calculated based on electrochemical impedance spectroscopy (EIS) results in the frequency range o 0.01~10 6 Hz. The Arrhenius plot of ionic conductivity in the temperature range from 25 °C to 80 °C is presented in Figure 7a. After the addition of the g−C3N4 nanofiller, the RT ionic conductivity experienced a significant improvement, from 5.17 × 10 −5 to 1.67 × 10 −4 S cm −1 The increase in ionic conductivity resulted from the enhanced segment motion of the pol ymer, which was owing to the decrease in crystallinity of the polymer induced by nano filler incorporation, as demonstrated by previous characterization [13,17]. Besides, the RT

The Electrochemical Characterization
To understand the effect of g−C 3 N 4 filler on the ionic transport capability of PVDF−HFP polymer solid electrolyte, the ionic conductivity of the CSPE was calculated based on electrochemical impedance spectroscopy (EIS) results in the frequency range of 0.01~10 6 Hz. The Arrhenius plot of ionic conductivity in the temperature range from 25 • C to 80 • C is presented in Figure 7a. After the addition of the g−C 3 N 4 nanofiller, the RT ionic conductivity experienced a significant improvement, from 5.17 × 10 −5 to 1.67 × 10 −4 S cm −1 . The increase in ionic conductivity resulted from the enhanced segment motion of the polymer, which was owing to the decrease in crystallinity of the polymer induced by nanofiller incorporation, as demonstrated by previous characterization [13,17]. Besides, the RT ionic conductivity of our work was much higher than that of the reported PEO/g−C 3 N 4 /LiClO 4 (1.76 × 10 −5 S cm −1 at 25 • C), PEO/g−C 3 N 4 /LiTFSI solid electrolytes (1.70 × 10 −5 S cm −1 at 30 • C) [17,18]. The electrochemical stability window of SPE/CSPE was determined by linear sweep voltammetry (LSV) at room temperature using SS|CSPE|Na structured coin cells. As shown in Figure S8, the electrochemical window of the PVDF−HFP/g−C 3 N 4 /NaClO 4 composite electrolyte is 4.8 V, while the electrochemical window of the PVDF−HFP/NaClO 4 composite electrolyte is only 3.6 V. The addition of g−C 3 N 4 improved the stability of electrolyte due to the interaction between PVDF−HFP and the nanofiller, and thus preventing the reaction between the end groups and Na metal [18,35]. In the polymer solid electrolyte system, positive and negative ions can move at the same time, and the number of negative ion migrations is usually more significant than the number of positive ion migrations [18]. It is crucial to determine the sodium ion transport number (t Na+ ) of polymer electrolytes, since the polarization potential during recycling can effectively be reduced with a higher sodium ion mobility number [17]. Figure S6 shows the chronoamperometry and AC impedance spectra (inset) before and after polarization of the Na|SPE|Na cell. The t Na+ of the polymer electrolyte was then measured and calculated. CSPE has a sodium ion migration number of 0.78, while that of PVDF−HFP/NaClO 4 composite solid electrolyte was 0.61. The interaction between nitrogen atoms and Na + promotes the dissociation of sodium salts in the polymer, thus leading to higher t Na+ . For sodium metal batteries, a higher sodium ion migration number also increases the ionic conductivity of sodium ions, thereby making the sodium metal anode more stable [12,18]. To evaluate the compatibility and the cycling stability of the interface between PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPE and sodium metal, the galvanostatic cycling performance of Na|CSPE|Na symmetric cells was performed at 1 mA cm −2 . The behavior of Na metal deposition/dissolution can be reflected by polarization voltage during charge/discharge, as displayed in Figure 7b [21]. The cell with g−C 3 N 4 CSPE presented low deposition/dissolution overpotential of 100 mV, and excellent cycling stability over 180 h without short circuit and polarization voltage increase. By contrast, the cell using SPE without filler has a high initial overpotential (300 mV) and exhibits instability after 40-60 h cycling due to the internal short circuit caused by sodium dendrites formation. The g−C 3 N 4 composite promoted uniform Na deposition/dissolution, leading to improved interfacial compatibility and long-term stability and finally resulting in enhancement of battery life. the chronoamperometry and AC impedance spectra (inset) before and after polarization of the Na|SPE|Na cell. The tNa+ of the polymer electrolyte was then measured and calculated. CSPE has a sodium ion migration number of 0.78, while that of PVDF−HFP/NaClO4 composite solid electrolyte was 0.61. The interaction between nitrogen atoms and Na + promotes the dissociation of sodium salts in the polymer, thus leading to higher tNa+. For sodium metal batteries, a higher sodium ion migration number also increases the ionic conductivity of sodium ions, thereby making the sodium metal anode more stable [12,18]. To evaluate the compatibility and the cycling stability of the interface between PVDF−HFP/g−C3N4/NaClO4 CSPE and sodium metal, the galvanostatic cycling performance of Na|CSPE|Na symmetric cells was performed at 1 mA cm −2 . The behavior of Na metal deposition/dissolution can be reflected by polarization voltage during charge/discharge, as displayed in Figure 7b [21]. The cell with g−C3N4 CSPE presented low deposition/dissolution overpotential of 100 mV, and excellent cycling stability over 180 h without short circuit and polarization voltage increase. By contrast, the cell using SPE without filler has a high initial overpotential (300 mV) and exhibits instability after 40-60 h cycling due to the internal short circuit caused by sodium dendrites formation. The g−C3N4 composite promoted uniform Na deposition/dissolution, leading to improved interfacial compatibility and long-term stability and finally resulting in enhancement of battery life.

Battery Performance Based on PVDF−HFP/g−C3N4/NaClO4 CSPE
The performance of coin cell type battery consists of PVDF−HFP/NaClO4 SPE or the PVDF−HFP/g−C3N4/NaClO4 CSPE, Na metal anode and NVP cathode was evaluated. At the same current density of 0.1 C, the Na|PVDF−HFP/g−C3N4/NaClO4|NVP cell exhibits

Battery Performance Based on PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPE
The performance of coin cell type battery consists of PVDF−HFP/NaClO 4 SPE or the PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPE, Na metal anode and NVP cathode was evaluated. At the same current density of 0.1 C, the Na|PVDF−HFP/g−C 3 N 4 /NaClO 4 |NVP cell exhibits a higher specific capacity of 100.3 mAh g −1 than that of Na|PVDF−HFP/ NaClO 4 |NVP cell (81.5 mAh g −1 ) (Figure 8b) [38,39]. Meanwhile, the Na|PVDF−HFP/ g−C 3 N 4 /NaClO 4 |NVP cell exhibits a flatter charging/discharging voltage plateau and lower polarization voltage (0.09 V) than that of Na|PVDF−HFP/NaClO 4 |NVP cell (0.21 V) ( Figure S7). The specific discharge capacity of Na|PVDF−HFP/g−C 3 N 4 /NaClO 4 |NVP is 100.3, 95.3, 92.5, 86.7 and 79.2 mAh g −1 at 0.1, 0.2, 0.5, 1 and 2 C, respectively (Figure 8a). The PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPE exhibited excellent rate capability, which was also improved compared with SPE without g−C 3 N 4 . The enhancement was ascribed to the minimized Na + concentration gradient and the accelerated transport ability of Na + in the PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPE. It can be seen from Figure 8c that the diameter of the semicircle with the g−C 3 N 4 CSPE is smaller than the SPE without the g−C 3 N 4 . It indicates that the intercalation and deintercalation of sodium ions are easier. mAh g −1 after 200 cycles. Meanwhile, the coulombic efficiency of the cells with g−C3N4 CSPE is as high as 99.5%. By contrast, the cells assembled using SPE without g−C3N4 decayed severely after 200 cycles with only 69.3% capacity retention. Therefore, the lifetime of solid-state sodium metal batteries is prolonged due to the enhanced interfacial stability between the PVDF−HFP/g−C3N4/NaClO4 CSE and Na metal anodes, as well as the fast Na + transport.

Conclusions
In summary, a PVDF−HFP/g−C3N4/NaClO4 composite polymer electrolyte with excellent comprehensive properties was prepared for the first time. The process of incorporation of g−C3N4 nanofiller in PVDF−HFP to form CSPEs are characterized by The cycling performance at the current density of 1C is shown in Figure 8d. The cell with the g−C 3 N 4 CSPE had an initial capacity of up to 95 mAh g −1 , which retained at 93 mAh g −1 after 200 cycles. Meanwhile, the coulombic efficiency of the cells with g−C 3 N 4 CSPE is as high as 99.5%. By contrast, the cells assembled using SPE without g−C 3 N 4 decayed severely after 200 cycles with only 69.3% capacity retention. Therefore, the lifetime of solid-state sodium metal batteries is prolonged due to the enhanced interfacial stability between the PVDF−HFP/g−C 3 N 4 /NaClO 4 CSE and Na metal anodes, as well as the fast Na + transport.

Conclusions
In summary, a PVDF−HFP/g−C 3 N 4 /NaClO 4 composite polymer electrolyte with excellent comprehensive properties was prepared for the first time. The process of incorporation of g−C 3 N 4 nanofiller in PVDF−HFP to form CSPEs are characterized by measurements in the XRD and FTIR. A significant crystallinity reduction, an even distribution of g−C 3 N 4 nanofiller in CSPEs are also found through morphological measurements in the SEM and AFM. The presence of g−C 3 N 4 nanosheet in CSPEs provides an efficient interface with polymers, and promotes Na + dissociation via the interaction of the surface N atoms with the sodium salt. Consequently, the mechanical properties and ionic conductivity are simultaneously enhanced through the thermal stability and electrochemical characterization. Regarding to the good mechanical and electrochemical properties, the PVDF−HFP/g−C 3 N 4 /NaClO 4 CSPEs are used in the all solid-state sodium coin cell battery, Resulting a superior cycle performance and high safety performance in comparison of the coin cell battery assembled with the PVDF−HFP/NaClO 4 SPEs. The addition of this novel filler opens up new ideas for further research on composite electrolytes, making solid-state batteries practical in large-scale energy storage, making them the development direction of next-generation rechargeable solid-state batteries.