1 Introduction

Exploiting environmentally safe and high-performance electrical energy storage devices is vital, given the continuous consumption of traditional fossil fuels and severe environmental pollution. The energy storage industry has seen growth in the utilization of lithium-ion batteries (LIBs) mainly owing to their exceptional cycle stability and high energy density [1]. Nevertheless, the use of LIBs in large-scale storage systems has been limited by the poor specific capacity of graphite electrodes as well as the scarcity and high cost of lithium [2]. Because of the abundant natural resources and the low redox potential of K and Na (− 2.92 V (vs. K/K+) and − 2.71 V (vs. Na/Na+)), electrochemical potassium-ion batteries (PIBs) and sodium-ion batteries (SIBs) are attracting considerable attention and demonstrating a vast commercialization potential for low-cost and large-scale energy storage systems [3,4,5,6]. Notably, the large radius and weight of K+/Na+ result in a massive diffusion barrier, slow reaction kinetics, and poor power density, which adversely affect rate capability and cycle life [7]. Therefore, designing effective electrode materials with high comprehensive performance for PIBs/SIBs is imperative.

It is recognized that a heterostructure that combines the advanced functions and strengths of multiple materials has the synergistic effect of employing the benefits of each individual component while avoiding their drawbacks to accomplish the impact of 1 + 1 > 2 [8]. Charge transfer and adsorption are facilitated by built-in electric fields generated at the interface owing to the potential difference between the two materials in the heterostructure [9]. Meanwhile, a remarkable number of heterointerface defects and lattice mismatches offer an essential effect on maximizing electrochemical performance [10]. In particular, the heterojunction interface provides a large number of active sites and cause increased ionic diffusion and a maximum charge storage capability to accelerate the storage performance of K+/Na+ with a large radius [11, 12]. Additionally, heterostructures exhibit structural stability, which is beneficial for alleviating volume expansion and enhancing cycle life. In the past several years, heterostructure engineering has been reported as a novel and beneficial technique, and studies have confirmed that using the same metal or different metal elements can allow for the construction of heterojunction interfaces that improve the energy storage performance, such as SnS/SnO2 [13], Co3O4/TiO2 [14], CuS/FeS2 [15], and WSe2/NiSe [16]. Notably, studies have tended to neglect the issue of the effective contact area at the heterojunction interface, and only the effective contact region can afford a built-in electric field to facilitate the rapid flow of electrons or ions, which is the most important factor affecting electrochemical performance enhancement. Therefore, optimization of the heterostructure with a better-matched heterointerface is essential for enlarging the interfacial contact area and expanding the range of built-in electric fields.

Transition-metal chalcogenides (TMCs) have garnered considerable attention as potential electrode materials for PIBs/SIBs because of their large interlamellar spacing, high theoretical capacity, and excellent redox reversibility [17, 18]. Among multitudinous TMCs materials, metal selenides, having appropriate working potentials, narrow bandgaps, and high electrical conductivities, are conducive to potassium/sodium storage. Moreover, they have weaker metal–selenium bond energies, which is kinetically favorable for K+/Na+ intercalation and deintercalation [19, 20]. As a characteristic layered metal selenide, MoSe2 has been widely employed as PIB/SIB electrode material by right of an intrinsically higher theoretical capacity (422 mAh·g−1) in a four-electron reaction [21]. The transition metals, such as W and Mo, are homologous elements. When building a heterostructure using WSe2 and MoSe2, they can be well matched because of their similar space groups (P63/mmc) and almost equal lattice spacings [22, 23], offering a better option for constructing heterojunction systems with more stable structures and larger effective contact areas. Furthermore, the large interlamellar space (0.65 nm) in WSe2 and MoSe2 allows for an abundance of large K+ or Na+, and the weak van der Waals interactions enhance the reversibility of the electrochemical reaction [24, 25]. Of note, utilization of WSe2/MoSe2 heterojunctions for sodium or potassium storage has not been reported.

In this study, WSe2/MoSe2 nanosheets with heterostructures were uniformly embedded in carbon nanofiber frameworks (WSe2/MoSe2/CNFs) via a straightforward electrospinning technique combined with an in situ selenization procedure. The one-dimensional (1D) nanofibers could shorten the diffusion pathways of ions and enhance the electrical conductivity of anodes. WSe2/MoSe2 nanosheets with large specific surface areas alleviated the notable volume expansion that occurred during cycling and ensured that K+/Na+ was rapidly stored. More importantly, the constructed heterointerface possessed a large charge transport driving force due to the formation of built-in electric fields owing to WSe2/MoSe2 with a better-matched and stable heterojunction interface, and it offered abundant active sites to accelerate K+/Na+ migration. Benefiting from these advantages, the WSe2/MoSe2/CNF composite displayed excellent cycling behavior and high-rate ability in PIBs and SIBs. After 500 cycles at 5 A·g−1 for PIBs, it delivered a high reversible capacity of 125.6 mAh·g−1. For PIBs and SIBs, it was sustained at 133.4 mAh·g−1 and 243.5 mAh·g−1, respectively, at a high current density of 20 A·g−1.

2 Experimental

Phosphotungstic acid (PW12; 0.23 mmol) and phosphomolybdic acid (PMo12; 0.23 mmol) were dissolved in 10 mL of N,N-dimethyl formamide (DMF). Thereafter, 1.5 g of polyvinylpyrrolidone (PVP) was added to the solution, which was then stirred for 12 h. The obtained solution was transferred to a 5-mL syringe, and electrospinning was performed at 50 °C. The distance between the collection device and the needle was 30 cm. A voltage of 19 kV was applied, and the solution supply speed was controlled at 0.03 mm·min−1. After pre-oxidation at 280 °C for 2 h, the obtained fibers and Se powders (1:3 by mass) were heated at 900 °C under N2 to synthesize WSe2/MoSe2/CNFs. The WSe2/CNFs and MoSe2/CNFs were prepared using similar methods without PMo12 and PW12, respectively. Detailed material characterizations, density functional theory (DFT) calculations, and electrochemical measurements are described in Supporting Information.

3 Results and discussion

Figure 1 shows the synthesis pathway used for the WSe2/MoSe2/CNF composites. First, homogeneous precursor solutions were prepared by mixing PW12, PMo12, and PVP with DMF. Electrospinning was performed to obtain PW12/PMo12/PVP. The WSe2/MoSe2/CNF composites were then formed by carbonization and selenization of the resulting PW12/PMo12/PVP fibers in an N2 atmosphere. When exposed to high temperatures, PVP transformed into a fibrous carbon substrate, and WSe2/MoSe2 nanosheets with heterointerfaces were generated on the surface of the CNFs. Scanning electron microscopy and transmission electron microscopy (TEM) were used to characterize the internal structure and surface morphology of the materials. The smooth surfaces of PW12/PMo12/PVP fibers with diameters of 200–300 nm are shown in Fig. S1. Luxuriant WSe2/MoSe2 nanosheets formed on the surface of the CNFs, forming a unique hierarchical heterostructure (Fig. 2a, b). Similarly, the TEM image shows that the surface of the fibers exhibited a distinct wrinkled and sheet-like structure (Fig. 2c). Figure S2 shows the (002) crystal planes of WSe2 and MoSe2 according to the high-resolution TEM (HRTEM) images [26, 27]. By contrast, Fig. 2d presents the enlarged interlayer spacing of 0.667 nm, which can be attributed to the (002) heterogeneous interface composed of WSe2 and MoSe2. Furthermore, as shown in Fig. 2e, the magnified images in Regions I and II reveal the significant alternating distribution of WSe2 and MoSe2 crystal planes, and the atomic arrangement of the (002) crystal plane illustrates the interfacial atom configuration [28]. These results suggest the distinct two-phase heterointerface of WSe2 and MoSe2 in the bulk material. Moreover, WSe2 and MoSe2 exhibit the same hexagonal symmetry with a (002) lattice mismatch of approximately 0.46% (Table S1), which provides an ideal heterostructure for energy storage. C, W, Mo, and Se in the WSe2/MoSe2/CNF composites were evenly distributed, according to energy-dispersive spectroscopy (EDS) mapping images (Fig. 2f). Figure S3 shows the morphologies of WSe2/CNFs and MoSe2/CNFs.

Fig. 1
figure 1

Schematic synthesis of WSe2/MoSe2/CNFs composites

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Fig. 2
figure 2

a, b SEM images of WSe2/MoSe2/CNFs; c–e TEM and HRTEM images of WSe2/MoSe2/CNFs; f SEM image and corresponding EDS mapping of WSe2/MoSe2/CNFs; g XRD patterns of WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs; h W 4f, i Mo 3d, and j Se 3d XPS spectra for as-prepared samples; k Raman spectra and l TG analysis curves of three samples

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X-ray diffraction (XRD) patterns were used to identify the phase components of the as-prepared samples, as shown in Fig. 2g. Without any impurity phases, all characteristic peaks matched well with those of hexagonal MoSe2 (PDF No. 29–0914) and WSe2 (PDF No. 38–1388) [29, 30]. X-ray photoelectron spectroscopy (XPS) measurements were used to determine the electronic states and surface chemical environments of all three samples. In Fig. 2h, for the W 4f spectrum of the WSe2/MoSe2/CNFs, W 4f7/2 and W 4f5/2 are the two characteristic peaks at 32.4 and 34.5 eV, respectively [31]. Notably, the binding energies shifted slightly to values lower than those of the WSe2/CNFs. For the Mo 3d spectrum of the WSe2/MoSe2/CNFs (Fig. 2i), the peaks at 229.0 and 232.1 eV represented Mo 3d5/2 and Mo 3d3/2, respectively [32]. The positions of the peaks shifted toward higher binding energies compared with those of the MoSe2/CNFs. These binding energy shifts were attributed to heterophase boundaries, indicating the successful preparation of the WSe2/MoSe2 heterostructure [33]. Meanwhile, the electron shift from MoSe2 to WSe2 led to the redistribution of interfacial electrons. The existence of a heterointerface contributed to enhancing K+/Na+ mobility and electrochemical reaction kinetics. The peaks at 54.6 and 55.5 eV were well matched to Se 3d5/2 and Se 3d3/2 (Fig. 2j), respectively, suggesting a successful selenization reaction [34]. In the Raman spectra (Fig. 2k), the D peak at 1358 cm−1 and the G peak at 1586 cm−1 corresponded to amorphous carbon and graphitic carbon, respectively. The graphitization degree was evaluated using the ID/IG ratio, which reflects the conductivity of composites [35]. The WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs displayed ID/IG values of 0.92, 0.98, and 0.96, respectively, indicating their superior electrical conductivity. Notably, the WSe2/MoSe2/CNFs included specific quantities of carbon defects, which provided appropriate active sites for moving ions and electrons. The thermal properties of the three samples were characterized using thermogravimetric analysis performed in air (Fig. 2l). The early stage of the thermogravimetric analysis was prone to a weight increase, which may have been related to the solid WO3, MoO3, and SeO2 formation [36, 37]. The weight percentages of carbon in MoSe2/CNFs, WSe2/CNFs, and WSe2/MoSe2/CNFs were predicted to be 53.8%, 56.2%, and 59.3%, respectively. Combined with an inductively coupled plasma test (Table S2), the weight percentages of WSe2 and MoSe2 in the WSe2/MoSe2/CNF composite were 23.4% and 17.3%, respectively. From N2 adsorption/desorption isotherms (Fig. S4a), the WSe2/MoSe2/CNFs showed a larger specific surface area of 33.4 m2·g−1 than those of the WSe2/CNFs (9.4 m2·g−1) and MoSe2/CNFs (15.8 m2·g−1). The pore diameters of all composites were centered in the range of approximately 5–15 nm, indicating a mesopore structure (Fig. S4b). The high specific surface area and abundant mesopores provided more active sites and promoted ion diffusion, thereby improving the electrochemical performance of the WSe2/MoSe2/CNF electrode.

Cyclic voltammetry (CV) curves for WSe2/MoSe2/CNFs at a scan speed of 0.1 mV·s−1 and a potential range of 0.005–3.0 V are depicted in Fig. 3a (Fig. S5 for WSe2/CNFs and MoSe2/CNFs). During the initial cathodic scanning, the reduction peak at 1.07 V may have been due to the generation of KxWSe2/KxMoSe2 phases via K+ intercalation, the peak at 0.74 V was ascribed to irreversible solid electrolyte interphase (SEI) layer formation resulting from the decomposition of the electrolyte, and the peak at 0.30 V was associated with KxWSe2/KxMoSe2 conversion to metallic W/Mo and K2Se [38]. The peak at 1.65 V in the anodic scan was ascribed to oxidation that produced WSe2 and MoSe2 [27]. The CV curves for the succeeding cycles nearly overlap, demonstrating the better reversibility of WSe2/MoSe2/CNFs for potassium-ion batteries. The discharge and charge curves of the WSe2/MoSe2/CNFs with the potential range of 0.005–3.0 V and a current density of 0.1 A·g−1 are presented in Fig. 3b. The initial discharge and charge capacities were 1088.6 and 463.4 mAh·g−1, respectively, and the generation of the SEI was primarily responsible for the enormous capacity loss. The subsequent cycle processes exhibited overlaps in the voltage profiles, and the steady voltage platforms agreed with the CV test. The cyclic stabilities of the electrodes for the WSe2/MoSe2/CNFs, WSe2/CNFs, MoSe2/CNFs, and CNFs at 0.1 A·g−1 are shown in Fig. S6. In the 50th cycle, the WSe2/MoSe2/CNFs maintained a high discharge capacity of 483.8 mAh·g−1. In comparison, after 50 cycles, the discharge capacities of the WSe2/CNFs and MoSe2/CNFs drop rapidly to 331.5 mAh·g−1 and 409.6 mAh·g−1, respectively. The CNFs exhibited a capacity of 99.2 mAh·g−1 after 50 cycles. Therefore, it can be deduced that WSe2 and MoSe2 account for the majority of the capacity contribution in WSe2/MoSe2/CNFs. Figure 3c displays the cycle performance of the WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNF electrodes at 1 A·g−1. The specific capacity of WSe2/MoSe2/CNFs stabilized at 426.1 mAh·g−1 over 100 cycles after pre-stabilization with three cycles at a low current density of 0.1 A·g−1. This was considerably greater than that of WSe2/CNFs (195.2 mAh·g−1) and MoSe2/CNFs (326.3 mAh·g−1). In addition, the WSe2/MoSe2/CNF anode also delivered a relatively high Coulombic efficiency of nearly 99.0%. As shown in Fig. 3d, long-term cycling of the WSe2/MoSe2/CNFs was carried out at an elevated current density of 5 A·g−1. A higher reversible capacity of 125.6 mAh·g−1 with 99.4% Coulombic efficiency was achieved up to 500 cycles. Notably, a gradual increase in the specific capacity was observed at the beginning of the galvanostatic charge/discharge tests (Fig. 3c, d). In view of this phenomenon, the electrochemical impedance spectroscopy (EIS) tests were conducted for different cycles at 5 A·g−1 (Fig. S7). The fresh cell showed a larger K+ diffusion resistance of 539 Ω, which was rapidly reduced to 272 Ω after only 10 cycles. As the number of cycles increased, the K+ diffusion resistance gradually reduced to 215, 147, and 107 Ω at the 20th, 40th, and 80th cycles, respectively, which was in line with the gradual capacity increase. These results can be attributed to the KFSI electrolyte, which electrochemically activates the electrode materials and extends the potassification duration [39]. HRTEM was used to characterize the WSe2/MoSe2/CNF electrode in the initial fully discharged/charged states at 5 A·g−1. In the fully discharged state at 0.005 V (Fig. S8a), the lattice fringe was indexed to the (220) plane of K2Se. In the charged state at 3.0 V, the lattice fringe was indexed to the (004) plane of WSe2 or MoSe2 (Fig. S8b). This result indicates the structural stability and reversibility of WSe2/MoSe2/CNFs during the initial charge and discharge processes. Subsequently, the evolution process of the WSe2/MoSe2/CNF electrode in the 5th, 25th, 50th, 100th, 200th, 300th, 400th and 500th cycles was characterized (Fig. S8c–j). With increase in cycling, the visualized crystalline regions of the material gradually decrease, while clear lattice fringes can still be observed, which corresponds to the cycle performance, as shown in Fig. 3d. The cyclic performance of WSe2/MoSe2/CNFs was further tested from the 500th to the 700th cycle at 5 A·g−1 (Fig. S9a). It should be noted that the degradation of the capacity with a longer cyclic test was possibly due to the deformation of the crystal structure (Fig. S9b, S9c).

Fig. 3
figure 3

Electrochemical performance for PIBs: a CV curves of WSe2/MoSe2/CNFs at 0.1 mV·s−1 in a voltage range of 0.005–3.0 V; b discharge/charge curves of WSe2/MoSe2/CNFs at 0.1 A·g−1; c cycling performance of WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs at 1 A·g−1 (after initial 3 cycles at 0.1 A·g−1); d long-term cycling performance of WSe2/MoSe2/CNFs at a current density of 5 A·g−1 (after the first three cycles at 0.1 A·g−1); e comparison of rate performance of WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs anodes; f comparison of rate capacity retention of three samples based on e; g comparison of rate performance with other reported similar anode materials; h Nyquist plots and equivalent circuit model of WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs; i the corresponding plots of the real part of impedance (Z′) as a function of inverse square root of angular frequency (ω−1/2) in the Warburg region

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Figure 3e shows a comparison of the rate capabilities of the three samples. The WSe2/MoSe2/CNF electrode exhibited specific capacities of 580.6, 576.5, 540.5, 413.7, 386.2, 281.3, 179.5 and 133.4 mAh·g−1 at 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20 A·g−1, respectively. As the current density turned to 0.1 A·g−1, the reversible capacity quickly recovered to 592.1 mAh·g−1, indicating that it can withstand the variations of various current densities. As shown in Fig. 3f, the WSe2/MoSe2/CNF electrode sustains excellent capacity retention at various current densities. Furthermore, a rate behavior comparison of the as-prepared materials is shown in Fig. 3g, and the observations suggest that WSe2/MoSe2/CNFs have obvious advantages over other anode materials [23, 25, 27, 30, 40,41,42,43,44,45]. The prominent potassium storage ability of WSe2/MoSe2/CNFs can be ascribed to the fact that WSe2/MoSe2, with a better-matched and stable heterojunction interface, enhances the useful area of the built-in electric field, which leads to a higher charge flux rate and faster potassium-ion diffusion kinetics, guaranteeing the realization of outstanding electrochemical properties. Figure 3h shows the EIS of the three-electrode materials. The results revealed that WSe2/MoSe2/CNFs with a smaller charge transfer resistance (Rct) are indicative of a lower resistivity and faster electron/ion transfer rate [39]. In order to corroborate this, Fig. 3i illustrates the relation plot between the real part of impedance (Z′) and the angular frequency (ω−1/2) in the low-frequency Warburg region. The linear slope value presents Warburg coefficient (σ), and it is related to the K+ diffusion coefficient (D). The small σ indicates the large K+ diffusion coefficient [46]. The relevant values were calculated based on the following formulas [47]:

$$Z^{\prime}={R}_{\text{SEI}}+{R}_{\text{ct}}+\sigma {\omega }^{-1/2}$$
(1)
$${D}_{{\text{K}}^{+}}={R}^{2}{T}^{2}/2{A}^{2}{n}^{4}{F}^{4}{C}^{2}{\sigma }^{2}$$
(2)

where RSEI, R, T, A, n, F, and C denote the resistance of the SEI, gas constant, absolute temperature, zone area at the electrode interface, electron concentration involved in the electrochemical process, Faraday’s constant, and K+ concentration in the electrode material, respectively. The slope value of WSe2/MoSe2/CNFs (506.37) was evidently less than those of WSe2/CNFs (681.25) and MoSe2/CNFs (627.91), representing a higher K+ diffusion coefficient, which indicates enhanced K+ transfer kinetics during the electrochemical reaction process of the WSe2/MoSe2/CNF electrode. Figure S10 shows the EIS curves of the WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNF electrodes after 500 cycles at 5 A·g−1. WSe2/MoSe2/CNFs still exhibited the lowest impedance compared to WSe2/CNFs and MoSe2/CNFs, which is mainly attributed to the coupling of the different components to form a heterostructure that can accelerate charge transfer and enhance structural integrity. This results in excellent cycling performance.

To investigate the kinetic behavior of K+ storage in the WSe2/MoSe2/CNF anode, CV tests were performed at various scan speeds (Fig. 4a). The connection between the scan rate (ν) and peak current (i) can be presented as follows [48]:

Fig. 4
figure 4

Reaction kinetics analyses for PIBs: a CV curves of WSe2/MoSe2/CNFs in potential range of 0.005–3.0 V with different scan rates; b b value analysis using relationship between peak currents and scan rates for WSe2/MoSe2/CNFs; c capacitive contribution (pink region) of WSe2/MoSe2/CNFs for K+ storage at 0.2 mV·s−1; d ratios of capacitive contribution at different scan rates for WSe2/MoSe2/CNFs; e, f plots of peak current versus square root of scan rate; g–i GITT curves and K+ ionic diffusion coefficients as a function of time for WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs

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$$i=a{v}^{b}$$
(3)

The b value was determined using the slope of a lgν versus lgi plot. When b value was close to 1, capacitive behavior was predominant in the electrochemical process. Moreover, the electrochemical reaction was mainly governed by the ion diffusion behavior when b value was near 0.5. As shown in Fig. 4b, the acquired b values of peak 1 and peak 2 are 0.92 and 0.99, respectively, which reveals that the capacitive behavior dominates the discharge/charge processes of WSe2/MoSe2/CNFs. Equation (4) was used to quantify the capacitive contribution at different scan speeds [49].

$$i={k}_{1}v+{k}_{2}{v}^{1/2}$$
(4)

The contributions of the pseudocapacitance and diffusion behavior to the reactions are represented by k1v and k2v1/2, respectively. At the low scanning rate of 0.2 mV·s−1, as demonstrated in Fig. 4c, the contribution of pseudocapacitive behavior was 85.1%. The percentage contribution of capacitance increased as the scan rate increased (Fig. 4d). The high pseudocapacitive behavior is attributed to the constructed heterogeneous structure, which can enhance the K+ transport kinetics. WSe2/MoSe2 nanosheets with large specific surface areas are also beneficial for the fast storage of K+. Therefore, the remarkable cycling performance and rate behavior of the WSe2/MoSe2/CNF anode can be explained by its high pseudocapacitive contribution. The Randles–Sevcik equation was used to determine the K+ diffusion coefficient using a set of CV curves (Figs. 4a, S11) [30]:

$${I}_{\text{p}}=2.69\times {10}^{5}{D}^{1/2}{n}^{3/2}{v}^{1/2}A{C}_{0}$$
(5)

A positive correlation was observed between the K+ diffusion coefficient and slopes of the curves (Ip/v1/2). Clearly, WSe2/MoSe2/CNFs displayed the highest slopes among the three samples (Fig. 4e, f), indicating a faster diffusion rate of K+. The galvanostatic intermittent titration technique (GITT) was used to further analyze the potassium storage kinetics for as-prepared electrodes (Fig. 4g). The K+ diffusion coefficients of the WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs were determined using the following equation [26]:

$${D}_{{\text{K}}^{+}}=\frac{4{L}^{2}}{\uppi \tau }{\left(\frac{\Delta {E}_{\text{s}}}{\Delta {E}_{\text{t}}}\right)}^{2}$$
(6)

where τ is the current pulse time, ΔEs is the deviation of each equilibrium voltage, ΔEt is the deviation voltage during the current pulse, and L is the average thickness of the electrode. The single-step GITT measurement yielded the parameters for computing the K+ diffusion coefficient, as illustrated in Fig. S12. Figure 4h, i demonstrates the relevant K+ diffusion coefficients for the discharge and charge cycles. The K+ diffusion coefficients of WSe2/MoSe2/CNFs were significantly larger than those of WSe2/CNFs and MoSe2/CNFs during the entire charge/discharge process, suggesting faster electrochemical reaction kinetics, which is advantageous for supporting the outstanding rate property and cycling durability of the WSe2/MoSe2/CNF anode.

In situ XRD measurements were performed to investigate the potassium storage mechanism of WSe2/MoSe2/CNFs (Fig. 5a). The initial discharge/charge process was performed in an in situ cell with a potential range of 0.005 to 3.0 V. The increase in intensity is indicated by a color change from blue to red in the diffraction peaks. Throughout the cycling process, the diffraction peaks of Be (41.3°) and BeO (38.7° and 44.0°) did not change [50]. Strong characteristic peaks at 13.8°, 28.6° and 41.9° were observed for the WSe2/MoSe2/CNFs in their pristine state, which matched the (002), (004), and (006) planes of WSe2/MoSe2, respectively. With further discharging, the strength of these feature peaks progressively weakened, and three broad peaks related to the K2Se planes appeared at 10.4°, 20.7° and 33.5°, indicating that the conversion reaction involved an intermediate product of K2Se [23]. However, for W and Mo, no distinctive reflections were observed at this stage, which may be due to their amorphous nature or small crystallite sizes. The peak intensity of K2Se gradually decreased and eventually disappeared throughout the charging process, indicating the occurrence of a reverse conversion reaction between K2Se and W/Mo selenides. In the fully charged state, the peak of WSe2/MoSe2 reappeared at approximately 13.8°, whereas the other peaks did not resurface, which might be partially explained by the poor crystallinity or small particle size of WSe2/MoSe2 after undergoing potassium removal [51]. A similar phenomenon is shown in Fig. 5b. As seen from Fig. S13, the (002) characteristic peak of WSe2/MoSe2 moved slightly to a lower angle during potassiation, suggesting that K+ intercalates into WSe2/MoSe2 to form KxWSe2/KxMoSe2. Subsequently, to further study the changes in the chemical components, ex situ XPS of the WSe2/MoSe2/CNF anode was performed at various reaction stages. As observed in Fig. 5c, the peaks at approximately 228.3 and 233.1 eV were identified as being for Mo4+ after being discharged to 0.005 V. The peaks at 226.5 and 230.6 eV belonged to elemental Mo0, representing the reduction of MoSe2 to Mo metal [26]. Only the peaks of Mo4+ were visible when the cell was recharged to 3.0 V, demonstrating the reversibility of MoSe2 during potassification and depotassification. For W 4f spectra (Fig. 5d), when discharging to 0.005 V, it displayed the presence of metallic W0 at 35.4 eV and 37.6 eV, accompanied by the potassiation process [31]. However, W0 was not fully oxidized after being fully charged, indicating the partial reversibility of the conversion process of WSe2. This phenomenon can also explain why WSe2/CNFs display poor electrochemical performance compared to MoSe2/CNFs. Importantly, according to the above discussion, the potassium storage mechanism of the WSe2/MoSe2/CNF anode is summarized as follows:

Fig. 5
figure 5

Reaction mechanism exploration of WSe2/MoSe2/CNFs anode for PIBs: a contour plots of in situ XRD analysis at a cutoff voltage of 0.005–3.0 V during initial cycle; b fine XRD diffraction spectra under different discharge and charge states; ex situ high-resolution c Mo 3d and d W 4f XPS spectra of WSe2/MoSe2/CNFs at fully discharged (0.005 V) and charged (3.0 V) states; e side views of potassium adsorption energies in MoSe2, WSe2, and WSe2/MoSe2 heterojunction interface, respectively; electrostatic potentials of f MoSe2 and g WSe2; h DOS of WSe2/MoSe2 heterostructure, MoSe2, and WSe2

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Discharged process:

$${\text{WSe}}_{2}+x{\text{K}}^{+}+x{\text{e}}^{-}\to {\text{K}}_{x}{\text{WSe}}_{2}$$
(7)
$${\text{MoSe}}_{2}+x{\text{K}}^{+}+x{\text{e}}^{-}\to {\text{K}}_{x}{\text{MoSe}}_{2}$$
(8)
$${\text{K}}_{x}{\text{WSe}}_{2}+\left(4-x\right){\text{K}}^{+}+\left(4-x\right){\text{e}}^{-}\to \text{W}+2{\text{K}}_{2}\text{Se}$$
(9)
$${\text{K}}_{x}{\text{MoSe}}_{2}+\left(4-x\right){\text{K}}^{+}+\left(4-x\right){\text{e}}^{-}\to \text{Mo}+2{\text{K}}_{2}\text{Se}$$
(10)

Charged process:

$$\text{W}+2{\text{K}}_{2}\text{Se}\to {\text{WSe}}_{2}+4{\text{K}}^{+}+4{\text{e}}^{-}$$
(11)
$$\text{Mo}+2{\text{K}}_{2}\text{Se}\to {\text{MoSe}}_{2}+4{\text{K}}^{+}+4{\text{e}}^{-}$$
(12)

The influence mechanism of the WSe2/MoSe2 heterojunction interface on K+ storage was further studied using DFT calculations. Figures 5e, S14 show the side and top views of the adsorption models, as well as the adsorption energy (Eads) values of the WSe2, MoSe2, and WSe2/MoSe2 heterostructures for potassium. The adsorption energy of potassium on the WSe2/MoSe2 heterointerface (− 1.76 eV) was remarkably lower compared with those of WSe2 (− 1.02 eV) and MoSe2 (− 1.06 eV), suggesting that the potassium is more handily adsorbed on the WSe2/MoSe2 heterointerface. Figure 5f, g illustrates that MoSe2 displayed a lower work function (4.70 eV) than WSe2 (4.91 eV), demonstrating that the electrons in MoSe2 move to WSe2 to reach uniform Fermi levels once the heterointerface is built. As demonstrated in Fig. S15, an additional electromotive force that promotes electron migration was provided by the built-in electric field pointing from WSe2 to MoSe2; the MoSe2 side was surrounded by a positively charged space, whereas the WSe2 side was covered by a negatively charged space. Figure S16 shows the band structures of the MoSe2, WSe2, and WSe2/MoSe2 heterostructures. Clearly, MoSe2 and WSe2 exhibited semiconducting characteristics. In contrast, the constructed WSe2/MoSe2 heterojunction shows that the electrons cross the Fermi level, and the conductivity is greatly improved. Compared to the band gap values of MoSe2 and WSe2, the WSe2/MoSe2 heterostructure exhibited distinct metallic properties (Fig. 5h) [52]. Moreover, the Mo d and W d orbitals of the WSe2/MoSe2 heterojunction showed a similar distribution trend to MoSe2 and WSe2, which play a major energy-level contributing role at positions near the Fermi level. The new electronic states generated by Mo and W in the conduction band of the WSe2/MoSe2 heterojunction increased the carrier concentration, accelerated electron transfer, and improved the conductivity of the WSe2/MoSe2 heterojunction. These advantages effectively improved the energy storage performance of PIBs.

SIBs were assembled to investigate the sodium storage properties of the as-prepared composites. As shown in Fig. 6a, a CV test of WSe2/MoSe2/CNFs was performed at 0.1 mV·s−1 (Fig. S17 for WSe2/CNFs and MoSe2/CNFs). In the initial cathodic reaction, the reduction peak located at 1.19 V resulted from Na+ intercalation in WSe2/MoSe2 to generate NaxWSe2/NaxMoSe2, the SEI film formation was confirmed in the peak at 0.60 V, and the conversion of NaxWSe2/NaxMoSe2 into metallic W/Mo and Na2Se was offered by another peak at 0.48 V [37]. The subsequent anodic curve exhibits one oxidation peak at 1.78 V, which was linked to the transformation of W/Mo into WSe2/MoSe2 [53]. Clearly, minor changes in the CV curves can be seen in the following two cycles, demonstrating the outstanding electrochemical reversibility of the WSe2/MoSe2/CNF anode. Figure 6b shows the discharge and charge curves of the WSe2/MoSe2/CNF anode in the first five cycles at 0.1 A·g−1. The comparatively low Coulombic efficiency during the first cycle can be attributed to the irreversible capacity loss caused by the formation of the SEI film. In addition, the 3rd, 4th, and 5th charge/discharge cycles almost exactly coincided with the 2nd cycle, revealing the outstanding reversibility of WSe2/MoSe2/CNFs for Na+ storage. Figure 6c compares the cycling stabilities of the three-electrode materials at 0.1 A·g−1. After 50 cycles, the WSe2/MoSe2/CNF composite exhibited a discharge capacity of 484.9 mAh·g−1, outperforming the WSe2/CNFs (273.2 mAh·g−1) and MoSe2/CNFs (411.4 mAh·g−1). Furthermore, the WSe2/MoSe2/CNF electrode exhibited a superior rate performance (Fig. 6d). At current densities of 0.1, 0.2, 0.5, 1, 2, 5, 10 and 20 A·g−1, the discharge specific capacities were 482.5, 452.9, 423.3, 399.6, 367.6, 322.0, 283.6 and 243.5 mAh·g−1, respectively. The discharge capacity increased to 417.1 mAh·g−1 when the current density was brought back to 0.1 A·g−1. The discharge/charge profiles of WSe2/MoSe2/CNFs with current densities from 0.1 to 20 A·g−1 are shown in Fig. 6e. The shapes of the discharge and charge profiles are almost identical at different current densities, proving the reversibility of the WSe2/MoSe2/CNFs. A comparison of the rate performance of WSe2/MoSe2/CNFs with those of other relevant anode materials is shown in Fig. S18. The prepared WSe2/MoSe2/CNFs exhibited excellent electrochemical performance as anodes for SIBs. Additionally, a prolonged cycling test under 10 A·g−1 was performed (Fig. 6f). It is noteworthy that the WSe2/MoSe2/CNF anode could deliver a superior reversible capacity of 144.5 mAh·g−1 and a high Coulombic efficiency of 97.3% after 100 cycles. Although the WSe2/MoSe2/CNF electrode exhibited an increase in the capacity for potassium storage in the first few tens of cycles, this phenomenon was not observed in SIBs. This difference can be attributed to the sluggish diffusion of K+, which has a relatively large ionic radius, and slow penetration of the KFSI electrolyte into the electrode via electrochemical activation [30, 39].

Fig. 6
figure 6

Electrochemical properties and kinetic analysis for SIBs: a CV curves of WSe2/MoSe2/CNFs at 0.1 mV·s−1; b charge/discharge curves at 0.1 A·g−1 for WSe2/MoSe2/CNFs; c cycling performance of WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs at 0.1 A·g−1; d rate performance at various current densities from 0.1 to 20 A·g−1; e charge/discharge curves of WSe2/MoSe2/CNFs anode at different current densities; f long-term cycling performance of WSe2/MoSe2/CNFs at 10 A·g−1 (after initial three cycles at 0.1 A·g−1); g EIS curves of WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs; h ratios of capacitive contribution at different scan rates for WSe2/MoSe2/CNFs; i Na+ ionic diffusion coefficients as a function of time for WSe2/MoSe2/CNFs, WSe2/CNFs, and MoSe2/CNFs

Full size image

The electrochemical resistances of the as-prepared samples were examined by EIS (Fig. 6g). The results reveal that the WSe2/MoSe2/CNFs showed lower charge transfer resistance compared to the WSe2/CNFs and MoSe2/CNFs, indicating superior electrical conductivity. Meanwhile, the lower slope value of the WSe2/MoSe2/CNFs means more rapid Na+ transfer and increased reaction kinetics (Fig. S19). The CV curves at various scan speeds were used to evaluate the electrochemical kinetic behavior of the WSe2/MoSe2/CNFs (Fig. S20a). The two major peaks of the WSe2/MoSe2/CNF anode were observed in the electrochemical process at b values of 0.93 and 0.90 (Fig. S20b). All b values were close to 1, indicating that the capacitance process was dominant and the WSe2/MoSe2/CNF electrode possesses ultrafast Na+ diffusion kinetics. Figure S21 shows the detailed capacitive contribution (83.8%) to the overall sodium storage at 0.2 mV·s−1, based on quantity measurements. The capacitive contribution percentages of the WSe2/MoSe2/CNF electrode at different scan speeds are shown in Fig. 6h. These findings demonstrate that potassium and sodium storage have comparable electrochemical behaviors. The surface pseudocapacitive contributions were, respectively, 83.8%, 87.5%, 90.9%, 93.7% and 96.1% of the entire capacity when scanning speed increases from 0.2 to 1.0 mV·s−1. The surface pseudocapacitive mechanism was dominant in the sodium storage process, particularly at high scan rates. In addition, CV curves were obtained at various scanning speeds to further explore the sodium storage kinetics (Figs. S20a, S22). The highest slope values of the WSe2/MoSe2/CNF anode among all the samples indirectly indicated the rapid diffusion kinetics of sodium ions, which is conducive to the greater rate performance and cycle durability of the WSe2/MoSe2/CNF anode. GITT tests of the three anodes were carried out by utilizing a range of pulse current at 0.1 A·g−1 for 10 min and a relaxation time of 30 min during the initial cycle stages (Fig. S23). The WSe2/MoSe2/CNFs exhibited smaller polarization and higher diffusion coefficients in both the sodiation and desodiation processes (Fig. 6i), which further verifies the accelerated Na+ transport behavior. Based on the investigations of the electrochemical reaction kinetics presented above, the steady cycle lifespan and excellent rate stability of the WSe2/MoSe2/CNFs can be explained by the dominant pseudocapacitive behavior and accelerated sodium-ion migration kinetics.

Based on the above discussion, WSe2/MoSe2/CNFs as anode materials in PIBs and SIBs exhibit remarkable advantages, which are mainly due to the following reasons. (1) The heterogeneous structure constructed by WSe2 and MoSe2 with better matching elevates the effective contact area of the heterointerface, which supplies numerous active sites and enlarges the utilization range of the built-in electric field to hasten the transport of electrons/ions and further accelerates the cycling kinetics. (2) The ultra-high pseudocapacitance contribution originating from the heterostructure and morphological features of the WSe2/MoSe2 nanosheets is conducive to enhancing high-rate energy storage. (3) The incorporation of 1D carbon nanofibers as a sturdy skeleton substrate to support the WSe2/MoSe2 nanosheets not only increases the conductivity of the composite but also mitigates the large volume change of the anode upon electrochemical cycling, achieving a relatively stable cycling performance and long cycle life.

4 Conclusion

In summary, an advanced PIB/SIB anode material based on a WSe2/MoSe2/CNFs heterostructure is reported, fabricated through a facile synthesis process that incorporates the electrospinning of a 1D fiber template and the surface in situ growth of WSe2/MoSe2 nanosheets. This hierarchical structure restrains the volume expansion of the active material during cycling. The WSe2/MoSe2 heterojunction interface, with better matching and stability, displayed a strong potassium adsorption energy, which facilitated the provision of effective active sites and rapid ion storage at the interface. The enlarged effective utilization range of the built-in electric field at the heterointerface provided a powerful driving force for accelerating the transfer of electrons and ions, thus enabling an exceptional cycling capability. In addition, the contribution of ultra-high pseudocapacitance was beneficial for enhancing high-rate energy storage. As expected, the WSe2/MoSe2/CNF composite exhibits excellent rate performance (133.4 mAh·g−1 at 20 A·g−1) and cycling stability (125.6 mAh·g−1 over 500 cycles at 5 A·g−1) when used as the PIB anode. High-rate performance was also realized in sodium-ion storage (243.5 mAh·g−1 at 20 A·g−1). This study incorporates the advantages of heterogeneous interface engineering and structural regulation in the construction of PIB/SIB anodes, offering a potential strategy for high-performance energy storage materials.