The nickel (Ni) foam was cleaned with acetone, anhydrous ethanol, and deionized water. First, 0.701 g of SnCl₄.5H₂O and 0.225 g of thioacetamide (TAA) were added to 60 ml of glycol and stirred for 20 min to form a uniform solution. Then, the prepared solution and a piece of cleaned Ni foam were transferred to a 100 ml lined Teflon stainless steel autoclave and maintained at 130°C for 8 h. Afterward, the sample was taken out, cleaned with anhydrous ethanol and deionized water, and then placed in the oven at 60°C for 12 h to obtain Ni@SnS₂ samples.

The morphology was characterized by cold field emission scanning electron microscopy (SEM, Hitachi 4,800) and transmission electron microscopy (TEM, JEM 2100F). The phase structure of the sample was analyzed by X-ray Powder Diffractometer (XRD, Rigaku D/Max-2400 diffractometer) and micro-Raman spectroscopy (Jobin-Yvon LabRAM HR800 UV, YAG 532 nm). The surface composition and chemical state of the sample are analyzed by X-ray photoelectron spectroscopy (XPS, K-ALPHA 0.5 eV). The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. The pore size distribution (PSD) was derived from the adsorption branches of isotherms by the Barrett-Joyner-Halenda (BJH) method.

The electrochemical performance was measured using an electrochemical workstation (CHI660e). Ni@SnS₂ was directly used to test the electrochemical performance of three electrodes. Saturated calomel was used as the reference electrode, a platinum electrode was used as the counter electrode, Ni@SnS₂ (1 × 1 cm²) was used as the working electrode, and 2 M KOH was used as the electrolyte. Ni@SnS₂ was tested by cyclic voltammetry (CV, sweep speed 5–100 mV s⁻¹) and constant current charge-discharge curves (GCD, current density 1–100 mA cm⁻²). Electrochemical impedance spectroscopy (EIS) was performed at AC frequency of 0.01 Hz–100 kHz and an amplitude of 5 mV.

Figure 1Ashows the XRD patterns of the Ni@SnS₂ nanosheet arrays. As is seen, the strong diffraction peaks centered at 44.5°, 51.7°, and 76.3°, correspond to the (111), (200), and (220) crystalline facets of Ni foam (PDF#04-0850), respectively. Due to the weak peak intensity, the characteristic reflection at 28.1° can be assigned to the (100) plane of SnS₂ (PDF#23-0677) (Li et al., 2018). In compared to the SnS₂ samples left from the Ni foam (Supplementary Figure S1), the pure SnS₂ samples (Supplementary Figure S2) exhibit the same diffraction peaks in the XRD patterns, confirming the existence of SnS₂ used as real active materials. Raman spectrum (Figure 1B) proves the existence of SnS₂ on the surface of Ni foam with a high band at ∼ 305 cm⁻¹, which is assigned to the A_1g mode of SnS₂ (Jiang et al., 2013; Qu et al., 2014). Furthermore, Supplementary Figure S3 shows the XPS full survey spectrum, confirming the existence of Sn, Ni, and S elements. In the high-resolution narrow spectrum of S 2p (Figure 1C), the two peaks located at 161.9 and 163.1 eV respectively correspond to S 2p_3/2 and S 2p_1/2, accompanying the satellite peak. The Sn 3d spectrum (Figure 1D) contains two-orbit peaks at 487.1 and 495.4 eV, corresponding to Sn 3d_3/2 and Sn 2d_5/2. The BET surface area and pore size distribution of Ni@SnS₂ were conducted by nitrogen isothermal adsorption/desorption measurement. The typical type IV isotherm curves (Supplementary Figure S4A) exhibit the evident hysteresis loop and the according surface area is about 1.14 m²⋅g⁻¹. The pore diameter (Supplementary Figure S4B) is located at 2.17 and 10.97 nm, indicating the existence of mesoporous structure.

Figures 2A–D display the morphological features of the Ni@SnS₂ nanosheet arrays by SEM. In the low-magnification SEM image (Figure 2A), a homogeneous and complete SnS₂ nanosheet arrays coated the skeleton of the Ni foam. Figures 2B–D show the high-magnification SEM images, revealing that the vertically aligned SnS₂ nanosheets are highly packed on the Ni foam and are interconnected to form numerous porous structures. A single Ni@SnS₂ nanosheet array was further observed by TEM. Figures 2E,I show images of the intersected SnS₂ nanosheet structures, which are consistent with the SEM analysis. From the high-resolution TEM (HRTEM) image, Figures 2F,G shows a lattice fringe of 0.21 nm, corresponding to the (102) plane of SnS₂. The selected area electron diffraction (SAED) pattern is shown in Figure 2H, implying that the polycrystalline rings can be indexed to the (101), (100) and (102) planes of SnS₂ (Liu et al., 2021). Furthermore, elemental mappings (Figures 2I–M) were measured to demonstrate the porous structure and homogeneous distribution of Sn, and S supported on Ni foam.

Figure 3A shows a typical Ni@SnS₂ electrode with a pair of redox peaks at a scan rate of 5 mV s⁻¹. With increasing scan rates, the shapes of these CV curves nearly retain their initial state when the scan rate is as high as 100 mV s−¹. As shown in Figure 3B, the GCD curves of the samples display almost symmetric shapes at all current densities from 1 to 100 mA cm−², suggesting ideal pseudocapacitance properties (Li et al., 2021).

When current density was increased from 1 to 100 mA cm−² (Figure 3C), the specific capacitance decreased from 1965.56 to 733.78 mF cm−². Furthermore, 75% of the initial specific capacitance was maintained even at a high current density of 20 mA cm−². Here, the specific capacitances of bare Ni foam measured in Supplementary Figure S5 show much less than to the values of Ni@SnS₂ electrode, suggesting the ignorable influence. The following relationship (Bian et al., 2022) is established between the peak current i) and scan rate (v): i = a v b , where a and b are constants. As shown in Supplementary Figure S6, the b-values of the anodic and cathodic peaks are 0.548 and 0.531, respectively, reflecting the diffusion-controlled behavior in the charge storage process. The cycle stability of the Ni@SnS₂ electrode shown in Figure 3D was measured by the GCD method at a high current density of 20 mA cm−². After 10,000 cycles, the capacitance retains 78.3% of its initial value, displaying its excellent application potential. Compared with the SEM images of the Ni@SnS₂ electrode after the long cycle (Supplementary Figure S7), the nanosheet arrays were partly retained while the morphologies of Ni foam were dilapidated, decreasing the active site of the electrode material and causing the capacity decay after 10,000 cycling tests. According to the EIS Nyquist plots of the Ni@SnS₂ electrode (Supplementary Figure S8), the real axis intercepts in the high-frequency region show a solution resistance (Rs) of about 1.0 Ω. The quasi-semicircle arc in the high-to-medium-frequency region corresponds to the charge transfer resistance (R_ct). Notably, the samples possessed a higher R_ct (3.968 Ω) compared with the initial R_ct (0.85 Ω) after a long cycle, indicating the increased electrochemical system resistance of the electrode material.