The synthetic procedure of g-C3N4 nanosheets is shown in Fig. 1 g-C3N4 was directly prepared by a simple calcination process, and a pale-yellow powder was obtained [8]. The PVDF-HFP/NaClO4 electrolyte membrane are transparent and colorless. After combined with g-C3N4, the color of the solid electrolyte changes and gradually becomes pale yellow(Figure S1). It is necessary to keep the thickness of the film in the range of 70 to 100 µm to lower the battery impedance and ensure flexibility of the membrane [15]. For PVDF-HFP, the high degree of crystallinity results in low ionic conductivity of corresponding solid electrolyte at room temperature [16]. It is confirmed that the addition of g-C3N4 nanofiller can reduce the crystallinity of PVDF-HFP and increase the amorphous region, thereby enhancing the mobility of sodium ions [17]. The crystallinity change in the polymer solid electrolytes was analyzed by X-ray diffraction (XRD). PVDF is a semicrystalline polymer and has two main crystal phases, α and β. In Fig. 2a, two partially overlapped diffraction peaks are observed in PVDF-HFP/NaClO4 at 18.5° and 20.3°, which reflected from the crystal plane of α (100) and β (100/200) phase of PVDF, respectively [18, 19]. For PVDF-HFP/g-C3N4/NaClO4, the tiny g-C3N4 characteristic peak at 27.6° appeared [20]. With the addition of g-C3N4, the crystallinity of PVDF-HFP decreased and the intensity of characteristic amorphous peak weakened. The diffraction peak of g-C3N4 powder did not change significantly before and after hybridization, indicating that g-C3N4 and PVDF-HFP have good compatibility in the preparation process [9].
The Fourier transform infrared (FTIR) spectra of PVDF-HFP/g-C3N4/NaClO4 CSPE in the range of 500–4000 cm− 1 are shown in Fig. 2b. After adding nanofillers, characteristic peaks at 800, 900–1800 and 3000–3600 cm− 1 emerges, confirms the existence of C-N heterocycles in g-C3N4. The peak at 3450 cm− 1 is associated with residual N-H group on the surface of g-C3N4 [21]. Characteristics peaks of the triangular C-N(-C)-C and bridging C-NH-C units group appears between 1800-900cm− 1 [22]. 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 [23]. The two characteristic peaks weakened or even disappeared in PVDF-HFP/g-C3N4/NaClO4 composite, indicating the interaction between g-C3N4 nanosheets and polymer matrix [24]. The surface of g-C3N4 is rich in nitrogen atoms, which can be considered as a Lewis base [14]. The resulted interaction between N-Na leads to an increase in dissociation of the sodium salt, and thus enhancing the ionic conductivity.
Mechanical properties are a critical index of the performance of polymer solid electrolytes, which have positive effect on Na dendrite formation inhibition. As shown in Fig. 2c, the tensile strength of the CSPE is 16.2 MPa, in comparison with 8.89 MPa of bare PVDF-HFP SPE. It can be seen that the tensile strength gradually increases by 7.38 MPa, and corresponding Young’s modulus increased from 40.9 MPa to 66.8 MPa. Therefore, it was expected that the PVDF-HFP/g-C3N4/NaClO4 composite solid electrolyte improves the sodium dendrite inhibition ability in sodium batteries, because the larger tensile modulus 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 [25].
The morphology of the as-prepared PVDF-HFP-based CSPE was characterized by SEM. As shown in Fig. 3a, 3b, bare PVDF SPE presents dense and smooth surface with micrometer sized pores. Meanwhile, the surface of CSPE with g-C3N4 has a nodular-like structure with increased micropores. The evolution in morphology suggests that the addition of g-C3N4 weakened the consistency of the polymer matrix, which greatly lowed the crystallinity of the electrolyte as previously revealed by XRD analysis. The addition of g-C3N4 was beneficial to improve the ionic conductivity of the composite solid polymer electrolyte. The corresponding EDS mapping shows that the C, F element uniformly distributed in SPE and CSPE film along with PVDF-HFP aggregation (Fig. 3c, d). The N element signal appeared in CSPE film, confirmed the successful combination of g-C3N4 and polymer matrix.
The topography images of PVDF-HFP/g-C3N4/NaClO4 CSPE obtained by AFM show a clear mountain-valley pattern, as is shown in Fig. 4a, b. The increase in the roughness of the polymer electrolyte was related to the nanofiller addition, indicated a good complexation between the polymer and the g-C3N4 [16, 26]. No small particles are observed on the surface, indicating that the g-C3N4 nanosheets have been uniformly dispersed into the host polymer. Figure 4c presents the TGA curves of the polymer electrolytes. The weight loss of the electrolyte between 120–180°C in Fig. 4c was due to the decomposition of NaClO4; the rapid weight loss between 400–450°C can be attributed to the complete decomposition of the PVDF-HFP polymer. The PVDF-HFP/g-C3N4/NaClO4 electrolyte exhibited higher thermal stability, thereby improving the safety of the battery. The DSC curve shows that the melting point of g-C3N4 CSPE decreased from 96℃ to 74℃ compared with the solid polymer electrolyte without filler addition (Fig. 4d). The introduction of g-C3N4 reduced the crystallinity of PVDF-HFP, so adding g-C3N4 in the polymer matrix g-C3N4 can improve the segment mobility of polymers by reducing the crystallinity of polymers [4, 21].
To understand the effect of g-C3N4 filler on the ionic transport capability of PVDF-HFP polymer solid electrolyte [11], the ionic conductivity of the CSPE was calculated based on electrochemical impedance spectroscopy (EIS) results in the frequency range of 0.01 ~ 106 Hz. The Arrhenius plot of ionic conductivity in the temperature range from 25 ℃ to 80 ℃ is presented in Fig. 5a. 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 enhanced segment motion of polymer, which was owing to the decrease in crystallinity of the polymer induced by nanofiller incorporation as demonstrated by physicochemical characterization previously [10, 17]. The RT ionic conductivity of our work was much higher than that of the reported PEO/g-C3N4/LiClO4 (1.76×10− 5 S cm− 1 at 25℃), PEO/g-C3N4/LiTFSI solid electrolytes (1.70×10− 5 S cm− 1 at 30°C) [13, 17]. The electrochemical stability window of SPE/CSPE was determined by linear sweep voltammetry (LSV) at room temperature using a SS|CSPE|Na structured coin cells. As shown in Fig. 5b, the electrochemical window of the PVDF-HFP/g-C3N4/NaClO4 composite electrolyte is 4.8 V, while the electrochemical window of PVDF-HFP/NaClO4 composite electrolyte is only 3.6 V, indicating that the addition of g-C3N4 improved the stability of polymer by preventing the reaction between the end groups and Na metal [27]. In the polymer solid electrolyte system, positive and negative ions can move at the same time, and the number of negative ion migration is usually more significant than the number of positive ion migration [13]. It is crucial to determine the sodium ion transport number (tNa+) of polymer electrolytes, since the polarization potential during recycling can effectively be reduced with higher sodium ion mobility number [17]. Figure 5c shows 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+ that 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 [9, 14]. 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 Fig. 5d [2, 28]. 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 [9, 25]. The g-C3N4 composite promoted the uniform Na deposition/dissolution, leading to improved interfacial compatibility and long-term stability and finally resulting in enhancement of battery life.
The feasibility of PVDF-HFP/g-C3N4/NaClO4 CSE in solid-state sodium metal batteries was investigated using coin cells assembled with Na metal anodes and Na3V2(PO4)3 (NVP) cathode (Na|CSPE|NVP) [28–30]. At the same current density of 0.1 C, the Na|PVDF-HFP/g-C3N4/NaClO4|NVP cell exhibits higher specific capacity of 100.3 mAh g-1 than that of Na|PVDF-HFP/NaClO4|NVP cell (81.5 mAh g-1) (Fig. 6b) [31, 32]. Meanwhile, the Na|PVDF-HFP/g-C3N4/NaClO4|NVP cell exhibits flatter charging/discharging voltage plateau and lower polarization voltage (0.09 V) than that of Na|PVDF-HFP/NaClO4|NVP cell (0.21 V) (Figure S2). The specific discharge capacity of Na|PVDF-HFP/g-C3N4/NaClO4|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 (Fig. 6a). The PVDF-HFP/g-C3N4/NaClO4 CSPE exhibited excellent rate capability, which was also improved compared with SPE without g-C3N4. The enhancement was ascribed to the minimized Na+ concentration gradient and the accelerated transport ability of Na+ in PVDF-HFP/g-C3N4/NaClO4 CSPE. It can be seen from Fig. 6c that the diameter of the semicircle with g-C3N4 CSPE added is smaller than that without, indicating that the intercalation and deintercalation of sodium ions are easier.
The cycling performance at current density of 1C is shown in Fig. 6d. The cell with the g-C3N4 CSPE had an initial capacity 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-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 life time 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 [6, 34–35].