The synthesis process of the sample involves only two steps: (1) uniformly mixing commercial FeS, S, and Ge in an agate mortar, followed by heating, and (2) ball milling the yield powder with commercial graphite, Fig. 1a is the schematic of the experimental processes. The crystal structure and purity of the sample were analyzed using XRD and XPS. Figure 1b shows the XRD patterns of bulk Fe2GeS4 and Fe2GeS4/G-15%. The diffraction peaks observed in bulk Fe2GeS4 correspond to the standard peaks of orthorhombic Fe2GeS4. In the case of Fe2GeS4/G-15%, some peaks correspond to Fe2GeS4, but there is also a strong diffraction peak around 26° that matches with graphite (PDF#25–0396). This suggests that the graphite has not been fully exfoliated. Figure 1c shows the XPS spectra of Fe2GeS4/G-15%, the Fe, Ge, S, C, and O were detected. The presence of O elements is believed to be caused by small amounts of H2O and CO2 that have been absorbed. The Fe 2p spectrum of Fe2GeS4/G-15% shows Fe-S 2p3/2 and Fe-S 2p1/2 peaks at 711.06 and 724.54 eV, respectively as seen in Fig. 1d [33]. In Fig. 1e, the Ge 3d spectrum for Fe2GeS4/G-15% shows a Ge-S fitting located at 32.12 eV [34]. The S 2p spectrum of Fe2GeS4/G-15% can be fitted as Ge-S 2p3/2, Ge-S 2p1/2, Fe-S 2p3/2, and Fe-S 2p1/2, located at 161.85, 163.11, 163.52, and 164.72 eV, respectively (Fig. 1f). The C 1s spectrum can be fitted as C-C, C = C, C-S, located at 283.52, 284.83, and 285.66 eV, respectively (Fig. 1g)[35–38]. Besides the characteristic peaks of Fe, Ge, S, and C, no impurity peaks were detected, indicating that Fe2GeS4 was successfully synthesized using FeS, Ge, and S powders as reactants, further confirming the high purity of the synthesized material.
The morphology and structure of the sample were studied using SEM and TEM. As shown in Fig. 2a, bulk Fe2GeS4 exhibited an irregular rock-like morphology with a relatively smooth surface, and the dimensions measured up to 10 µm. Figure 2b shows the SEM image of Fe2GeS4/G-15%, where the primary particles had a size smaller than 1 µm. To study the distribution of Fe2GeS4/G-15%, we conducted a TEM analysis which revealed that Fe2GeS4/G-15% is attached to the surface of the exfoliated graphite flakes. This can be observed in Fig. 2c. Figure 2d show the lattice spacing of 3.08 Å, which is the (3 1 1) crystal plane of Fe2GeS4/G-15%, and the lattice spacing of 4.29 Å is the (1 1 1) crystal plane of Fe2GeS4/G-15%. Selected area electron diffraction (SAED) patterns show scattered electron diffraction spots (Fig. 2e), indicating the polycrystalline nature of Fe2GeS4/G-15%. Furthermore, the elemental distribution mapping showed a uniform distribution of S, C, Fe, and Ge elements at the nanoscale (Fig. 2f).
The sodium storage mechanism was explored through CV measurements, and the findings are depicted in Figs. 3a, b. Fe2GeS4/G-15% exhibited similar redox peaks to Fe2GeS4, indicating that the electrochemical reactions involved in the charge/discharge processes were the same for both samples. Based on recent literature reports, the process of sodiation/dissociation of Fe2GeS4 can be expressed through the following equations [39].
1.45/0.73 V: 2Fe + Ge + 4Na2S ⇔ 2Fe + NaGe + 4Na2S (8)
Total reaction: 2Fe + NaGe + 4Na2S ⇔ 2Fe + Ge + 4Na2S ⇔ Fe2GeS4 (9)
Figure 3c illustrates the cycling performance and coulombic efficiency of the materials at a current density of 0.1 A g− 1. The initial discharge capacities of bulk Fe2GeS4 and Fe2GeS4/G-15% are 755.2 and 775.5 mAh g− 1, respectively, with corresponding coulombic efficiencies of 81.61% and 100%. In the second cycle, bulk Fe2GeS4 exhibits a discharge capacity of 620.6 mAh g− 1, whereas Fe2GeS4/G-15% demonstrates a higher discharge capacity of 665.5 mAh g− 1. After the 60th cycle, the reversible capacity of the Fe2GeS4 electrode rapidly decreases to 132.8 mAh g− 1 due to poor conductivity and significant volume change during cycling. In contrast, the capacity of Fe2GeS4/G-15% remains stable at 526.1 mAh g− 1. The results illustrate that crushing Fe2GeS4 and anchoring it onto the surface of exfoliated graphite significantly enhances the rechargeable capacity of the material. Figure 3d, e shows the discharge/charge curves of Fe2GeS4 and Fe2GeS4/G-15%. The Fe2GeS4/G-15% sample reproduces the profile from the 1st to 60th cycle, while the Fe2GeS4 shows a significant polarization phenomenon from the 10th cycle, further proving the enhances the rechargeable capacity of the Fe2GeS4/G-15%. Figure 3f shows the rate cycling performances of Fe2GeS4 and Fe2GeS4/G-15%, the Fe2GeS4/G-15% delivered 615.8, 588.2, 538.0, 525.6, 490.4, 416.3, 341.5, 272.3, and 227.0 mAh g− 1 at current densities of 0.1, 0.2, 0.5, 1, 2, 4, 6, 8, and 10 A g− 1, respectively. Furthermore, even if the current density is reverted to 0.1 A g− 1, a reversible capacity of 635.9 mAh g− 1 is achieved, demonstrating excellent capacity retention. The Fe2GeS4 delivered 491.5, 342.5, 234.2, 157.7, 102.1, 65.8, 45.2, 31.5, and 23.5 mAh g− 1 at 0.1, 0.2, 0.5, 1, 2, 4, 6, 8, and 10 A g− 1, lower than that of Fe2GeS4/G-15%. Figure 3g illustrates the discharge/charge cycling performances of Fe2GeS4 and Fe2GeS4/G-15% under 1.0 A g− 1. The results indicate that Fe2GeS4/G-15% exhibits excellent cycling stability even under 1.0 A g− 1. Conversely, Fe2GeS4 demonstrates significantly inferior cycling stability, with its reversible capacity falling below 200 mA g− 1 after the 20th cycle. The rated capacity of Fe2GeS4/G-15% was compared with recently reported transited metal-contained binary metal sulfide anodes (Fig. 3h) and the Fe2GeS4/G anode delivered higher reversible capacity than those anodes [40–49].
An analysis of surface pseudocapacitance was conducted as part of the investigation into the Na+ diffusion mechanism. Figure 4a, b shows the rate CV curves of Fe2GeS4 and Fe2GeS4/G-15%. In the range of 0.2 mV s− 1 to 1.0 mV s− 1, the Fe2GeS4/G-15% exhibits an increasing peak current and a slight right shift in peak voltage. For the Fe2GeS4, the peak current is relatively lower as the scan rate increases, indicating lower Na+ adsorption/diffuse activity. The peak current (i) and voltage (v) in the CV curves was fitted by the following equation [50]:
i = avb (10)
When b is close to 1 indicates that the process is predominantly capacitance-controlled and b value equal to 0.5 suggests that the charge/discharge process is entirely diffusion-controlled. Figure 4c, d show that the b value for Fe2GeS4 is less than 0.5, while the b values for Fe2GeS4/G-15% are all greater than 0.5, indicating that the capacitance-controlled process plays a dominant role in the nanostructured Fe2GeS4/G-15%. The capacitance ratio can be calculated according to the following equation[51, 52]:
i = k1v + k2v1/2 (11)
Where k1 and k2 are specific values for the electrochemical reaction at a fixed voltage. As shown in Fig. 4e, at a scanning rate of 1.0 mV s− 1, the capacitance contribution of Fe2GeS4/G-15% reached 78.70%. Figure 4f shows the pseudocapacitive values of Fe2GeS4/G-15% and Fe2GeS4, the Fe2GeS4/G-15% exhibits higher pseudocapacitive than Fe2GeS4 at each scan rate, indicating the graphite exfoliation improved the Na+ adsorption/diffusion and further enhances the battery performance [53, 54].
To further investigate the positive effect of graphite exfoliation on the performance of SIBs, Fig. 5a presents the results of EIS tests conducted on the batteries after 100 cycles of charge and discharge. Fe2GeS4/G-15% showed a smaller semicircular arc than Fe2GeS4 in the high-frequency region, indicating a higher Na+ conductivity in Fe2GeS4/G-15%. After two charge-discharge cycles at 0.1 A g− 1, the batteries underwent GITT tests, and the results are shown in Fig. 5b. The DNa+ was calculated and presented in Fig. 5c. Fe2GeS4/G-15% exhibited a higher DNa+ of 10− 11 to 10− 10 cm2 s− 1 compared to Fe2GeS4, which had a DNa+ of 10− 11 to 10− 12 cm2 s− 1. This indicates that exfoliated graphite significantly enhances the sodium ion transport rate in the battery. Figure 5d illustrates how Na+ diffuses boosted by exfoliated graphite, and recent literature suggests that the negatively charged surface accelerates Na+ diffusion.
DFT calculations were conducted to unveil the molecular-level mechanism of Na+ storage. Considering the Fe2GeS4 generates a series of intermediate products during the charging/discharging process, including FeS, Na2S, NaGe, Ge, and Fe, the density of states (DOS) of Fe2GeS4, FeS, NaGe, Na2S, Ge, and Fe were calculated and shown in Fig. S1a-f, and the band gap values calculated from DOS were depicted in Fig. S2a-f. The band gaps measured for Fe2GeS4, FeS, NaGe, Na2S, Ge, and Fe are 0.394, 0, 1.11, 2.39, 0, and 0 eV, respectively. A lower band gap indicates a smaller energy difference between the energy bands in the material, making it easier for electrons to transition. Next, the work functions of Fe, Fe2GeS4, FeS, Ge, S, Fe/G-15%, Fe2GeS4/G-15%, FeS/G-15%, Ge/G-15%, and S/G-15% was calculated and shown in Figure S3a-j. The electron emission energies of Fe2GeS4-G, FeS-G, NaGe-G, Na2S-G, and Ge-G are lower than that of bulk Fe2GeS4, indicating that exfoliated graphite enhances electronic conductivity (Fig. 6a). Figure 6b illustrates the charge density difference of Fe2GeS4, Ge, FeS, S, NaGe, and Na2S anchored on exfoliated graphite. The overlapping of electron clouds with exfoliated graphite indicates that these materials are forming chemical bonds with exfoliated graphite.
Figure S4a-f show the electronic fluctuations of Fe2GeS4/G-15%, Ge/G-15%, FeS/G-15%, S/G-15%, NaGe/G-15%, and Na2S/G-15%. The electronic fluctuations in the Z-direction are stronger than in the X or Y-direction, further confirming the formation of chemical bonds between Fe2GeS4, Ge, FeS, S, NaGe, and Na2S exfoliated graphite. The Na+ diffusion coefficients of Fe/G-15%, Fe2GeS4/G-15%, FeS/G-15%, Ge/G-15%, S/G-15%, Fe, Fe2GeS4, FeS, Ge, and S were calculated based on MD simulation, and results show in Fig. 6c and Fig. S5a-e. The calculated DNa+ for Fe/G-15%, Fe2GeS4/G-15%, FeS/G-15%, Ge/G-15%, S/G-15%, and S/G-15% are 5.03× 10− 6, 1.13 × 10− 5, 5.08 × 10− 6, 2.46 × 10− 5 and 5.93×10− 6 cm2 s− 1, the corresponding DNa+ values for Fe, Fe2GeS4, FeS, Ge, and S are 1.10 × 10− 7, 7.60 × 10− 7, 1.128 × 10− 7, 3.41 × 10− 7, and 4.78 × 10− 7 cm2 s− 1, respectively. The MD simulations demonstrate that the exfoliated graphite accelerates Na+ diffusion.