Binding SnO₂ Nanoparticles with MoS₂ Nanosheets Toward Highly Reversible and Cycle-Stable Lithium/Sodium Storage

1 Introduction

Lithium-ion batteries (LIBs) have been widely applied in electronic equipments and electric vehicles due to their high energy density and long lifespan.^{[1, 2]} Nevertheless, considering the low theoretical capacity of commercial graphite anode (372 mAh g⁻¹), exploring anode materials owning high capacity/energy density is vital for the further development of LIBs.^{[3, 4]} SnO₂ has been recognized as a promising anode material for LIBs because of its high theoretical capacity (1494 mAh g⁻¹), abundant resources, and environmental friendliness.^[5-8] It has also inspired increasing concern as an anode for sodium-ion batteries (SIBs), showing great prospects to be an alternative to LIBs due to the abundant resources and low cost of sodium.^[9-12]

However, SnO₂ anode faces two major challenges for commercialization. First, the huge volume expansion during lithiation/delithiation processes results in severe internal stress and electrode disintegration.^[13] Second, the coarsening of Sn nanograins incurs incomplete reverse conversion reaction and inferior reversibility.^[14] Fabricating the SnO₂-carbon composites has been considered as the most common and effective way to improve the electrochemical performance of SnO₂, since these carbon matrices can accommodate the volume variation of SnO₂ and promote the electrochemical kinetics of overall electrode.^{[6, 15-18]} However, most of these SnO₂-based anodes exhibit low initial coulombic efficiencies, typically below 60%. Moreover, after long-term cycling, the carbon matrices usually encounter structure deformation in common electrolytes, resulting in unstable structures and severe Sn agglomeration in the long run.^[19-21] Therefore, the reversibility and cyclability of SnO₂ still need to be improved.

Recently, constructing heterostructure with bicomponent hybridization, such as SnO₂/Fe₂O₃, SnO₂@SnS₂, and SnO₂@Nb₂O₅, has been proved to be an effective strategy to achieve high reversible capacities and long-term cyclic life.^{[19, 22, 23]} MoS₂ nanosheet, a typical p-type semiconductor, has also attracted tremendous interest as an excellent anode material owing to its high reversible capacity of 900–1200 mAh g⁻¹.^[24-27] Coupling with MoS₂ nanosheets, the SnO₂ anode (an n-type semiconductor) demonstrates enhanced charge transfer and surface reaction kinetics owing to the built-in electric field between them.^[28] Furthermore, the highly conductive MoS₂ nanosheets with strong mechanical strength can alleviate the volume expansion and particle agglomeration upon cycling.^{[27, 29-31]} Therefore, it is desirable to construct heterostructure of SnO₂ nanoparticles strongly bound with MoS₂ nanosheets to significantly improve the reversibility and cyclability of SnO₂.

In this work, we have developed a SnO₂/MoS₂ heterostructure through a facile one-step method called plasma-assisted milling (P-milling). This process induced strong binding between the SnO₂ nanoparticles and in-situ exfoliated MoS₂ nanosheets predominantly in 1 T phase. The inclusion of conductive MoS₂ nanosheets effectively mitigates the volume expansion of SnO₂ and particle agglomeration of Sn during discharge/charge processes. Additionally, the strong affinity between Mo (formed during the lithiation of MoS₂) and Sn plays a pivotal role in inhibiting the coarsening of Sn nanograins and enhancing the reversibility of Sn to SnO₂ upon cycling. Consequently, the reversibility and cyclability of SnO₂ are significantly enhanced. Remarkably, it also demonstrates excellent electrochemical performance in full LIBs and SIBs.

2 Results and Discussion

Figure 1a illustrates the synthesis of the SnO₂/MoS₂. Through the P-milling procedure, the milling balls exert impact force and the discharge plasma generates rapid heating. These processes effectively refine SnO₂ and exfoliate MoS₂. Consequently, MoS₂ nanosheets encapsulate the SnO₂ nanoparticles, resulting in the formation of core-shell structures. Furthermore, the MoS₂-wrapped SnO₂ nanoparticles agglomerate into secondary microgranules.

The XRD pattern of SnO₂/MoS₂ (Figure 1b) presents the distinctive peaks of SnO₂ (JCPDS No. 41-1445) and MoS₂ (JCPDS No. 37-1492). In comparison to B-MoS₂, the SnO₂/MoS₂ demonstrates a weak and broad peak at (002)MoS₂, which is shifted from 14.4° to 12.4°. This suggests that the exfoliated MoS₂ layers have undergone expansion from 0.61 to 0.71 nm during the P-milling process.^[32-35] Similarly, the Raman spectrum of SnO₂/MoS₂ (Figure 1c) reveals that the E_2g and A_1g bands of MoS₂ shift to lower wavenumbers, and the difference between them decreases. This also implies the exfoliation and interlayer expansion of MoS₂ after P-milling.^{[33, 36]} The content of MoS₂ in the composite can be calculated to be 47 wt% according to the TG results (Figure S3, Supporting Information).

The XPS characterization was employed to evaluate the chemical state of SnO₂/MoS₂. For the Sn 3d spectrum (Figure 1d), two prominent peaks observed at 495.5 and 487.1 eV correspond to the Sn 3d_3/2 and Sn 3d_5/2 peaks of Sn⁴⁺, respectively.^[37] Moving to the Mo 3d spectrum (Figure 1e), a couple of peaks positioned at 229.4 and 232.9 eV are indicative of the 1 T phase of MoS₂, while another pair at 230.1 and 233.5 eV are characteristic of the 2H phase of MoS₂.^[38] Based on the analysis of the peak areas, the SnO₂/MoS₂ composite exhibits a higher fraction of 1 T-MoS₂ (75.1%) compared to B-MoS₂ (28.2%), implying that SnO₂ prevents the restacking of exfoliated MoS₂ nanosheets. Additionally, the peak located at 226.5 eV is due to the S 2 s, and the peak at 236.3 eV represents the Mo⁶⁺ of MoO₃, which may indicate slight oxidation by SnO₂.^[28] This observation is further supported by the presence of additional Sn²⁺ peaks in the Sn 3d spectrum of SnO₂/MoS₂. The S 2p spectrum (Figure 1f) is divided into two distinct sets of doublet peaks, where the peaks located at 162.3/163.4 and 162.9/164.1 eV can be assigned to the 1 T and 2H phases of MoS₂, respectively.^[39]

The microstructure of SnO₂/MoS₂ was thoroughly examined using SEM and TEM techniques. In contrast to the dense bulks of B-MoS₂ (Figure 2a) and the loosely packed nanoparticles of B-SnO₂ (Figure 2b), the SnO₂/MoS₂ exhibits a unique microgranular structure consisting of stacked nanoparticles (Figure 2c,d,g). The SAED pattern (inset of Figure 2d) confirms the presence of SnO₂ and MoS₂ within the microgranule. Element mapping of SnO₂/MoS₂ (Figure 2h–k) reveals that Mo and S are dispersed throughout the microgranuel, whereas Sn appears as concentrated dots, indicating SnO₂ nanoparticles are embedded within the MoS₂ matrix. In the HRTEM images of SnO₂/MoS₂ (Figure 2e,f), the wrapping of few-layered MoS₂ around the SnO₂ nanoparticles is clearly visible. The lattice fringe with interplanar spacing of 0.33 nm corresponds to the (110) plane of SnO₂. At the edge of the SnO₂ nanoparticle, distinct few-layered MoS₂ wrapping (5–10 layers) with spacing of 0.74 nm can be observed, which is in accord with above XRD analysis.

To evaluate the performance of SnO₂/MoS₂ anode for Li⁺ storage, coin-type half-cells were assembled. The CV profiles of SnO₂/MoS₂ (Figure 3a) reveal the combined lithiation/delithiation processes involving both MoS₂ and SnO₂. In the initial cathodic scan, the peaks observed at 1.25 and 0.45 V are attributed to the intercalation of Li⁺ and subsequent conversion reaction of MoS₂ to Mo, respectively.^{[37, 50]} Furthermore, the peak at 1.05 V is assigned to the conversion of SnO₂ to Sn, and the peaks at 0.76 and 0.18 V correspond to the alloying of Sn to Li_xSn.^{[20, 40]} During the anodic scan, the peaks at 0.08 and 0.51 V are related to the dealloying reaction of Li_xSn to Sn. The broad peaks observed between 1.0 and 2.0 V are indicative of the reconversion reactions through which MoS₂ and SnO₂ are restored. However, it is important to note that only a portion of Li₂S is able to react with Mo, while the rest is decomposed into S and Li⁺. This is reflected in the anodic peak at 2.25 V.^{[27, 51, 52]} As a result, starting from the second cycle, a new cathodic peak at 2.0 V occurs, which results from the lithiation of S. The ascription of cathode/anode peaks of SnO₂/MoS₂ has also been affirmed by the CV tests of bare SnO₂ and MoS₂ (Figure S4, Supporting Information).

In the galvanostatic discharge/charge tests, the SnO₂/MoS₂ demonstrates an initial reversible capacity of 1091.1 mAh g⁻¹ at 0.2 A g⁻¹ (Figure 3b). This reversible capacity is closely aligned with the theoretical capacity of 1105.1 mAh g⁻¹, which is calculated based on the weight percentages of SnO₂ (53 wt%) and MoS₂ (47 wt%). Furthermore, the average initial coulombic efficiency (ICE) of five different cells using SnO₂/MoS₂ as the anode is 74.5% (Figure 3c). This confirms the superior reversibility of Li⁺ storage compared to B-SnO₂, B-MoS₂, and SnO₂/GNs (Figure S5, Supporting Information). The capacity remains stable throughout the subsequent cycles (Figure 3d), with the coulombic efficiency increasing to 99.2%–99.9% (Figure 3e). In contrast, the B-SnO₂, B-MoS₂, and SnO₂/GNs suffer serious capacity decay and low CE during cycling. Furthermore, the SnO₂/MoS₂ exhibits higher rate capability than the B-SnO₂, B-MoS₂, and SnO₂/GNs (Figure 3f). The average reversible capacity of SnO₂/MoS₂ only slightly decreases from 1160.5 to 1030.6 mAh g⁻¹ as the rate increases from 0.1 C to 2 C (1 C = 1 A g⁻¹). Even at a rate of 10 C, a reversible capacity of 815.8 mAh g⁻¹ is maintained. When the rate returns to 0.1 C, the capacity rebounds to 1151.8 mAh g⁻¹. Moreover, the SnO₂/MoS₂ exhibits superb long-term cyclic stability (Figure 3g). At 1 C, it demonstrates a reversible capacity of 1044.5 mAh g⁻¹ in the initial cycle and maintains 1005.3 mAh g⁻¹ after 800 cycles, resulting in a capacity retention of 96.2%. At a higher rate of 5 C, the reversible capacity reaches up to 825.8 mAh g⁻¹ and retains 93.1% after 800 cycles. Obviously, the high-rate cycling performance of SnO₂/MoS₂ anode surpasses most reported SnO₂-based anodes (Figure 3h). This highlights the superiority of combining MoS₂ and SnO₂ in the composite structure. To discuss the influence of the contents of SnO₂ and MoS₂ on the performance of composites, we have also prepared composites with MoS₂ additions of 30, 50, and 70 wt% (Figure S6, Supporting Information). As shown in Figure S7, Supporting Information, the SnO₂/MoS₂ composite with MoS₂ addition of 50 wt% exhibits much superior performance than those prepared with MoS₂ additions of 30 and 70 wt%, considering the cycling performance and rate performance.

During the P-milling process, the exfoliation of bulky MoS₂ leads to the formation of metallic 1 T-MoS₂, which wraps the refined SnO₂ nanoparticles and facilitates efficient charge/ion transfer. Thus, the electrical conductivity and ion diffusion of SnO₂/MoS₂ are greatly enhanced compared to those of SnO₂ or MoS₂ alone (Figure S8, Table S1, Supporting Information), which can be directly responsible for the improved rate capability. Moreover, the CV measurements at various scan rates were conducted to investigate the capacitive behavior behind the superior rate capability of SnO₂/MoS₂ (Figure S9a–c, Supporting Information). The current at a given potential can be composed of diffusion-controlled and capacitive currents. Figure S9d, Supporting Information, summarizes the capacitive contributions at various rates, rising form 70.4% (0.2 mV s⁻¹) to 86.5% (1.5 mV s⁻¹), which are higher than those of bare SnO₂ and MoS₂. The superior capacitive storage of SnO₂/MoS₂ results from the effective exfoliation of MoS₂ sheets and refining of SnO₂ particles. Additionally, the wrapping of MoS₂ provides spatial confinement, mitigating the volume change experienced by SnO₂ and contributing to enhanced cyclic performance. Despite the enhanced conductivity achieved through GNs wrapping, the SnO₂/GNs composite exhibits inferior capacity due to the lower capacity of GNs when compared to MoS₂.

To examine the evolution of capacity in SnO₂/MoS₂, the reversible capacity contribution is divided into three parts (Figure 4c), based on the charge profile and differential capacity (dQ/dV) plot (Figure 4a,b). Specifically, the capacity delivered in the range of 0.01–1.0 V represents the dealloying process of Li_xSn. The capacity delivered in the range of 1.0–2.0 V stems from the reconversion reaction of Sn and Mo, represented by Sn + 2Li₂O → SnO₂ + 4Li⁺ + 4e⁻, Mo + 2Li₂S → MoS₂ + 4Li⁺ + 4e⁻. Lastly, the capacity of 2.0–3.0 V originates from the delithiation of Li₂S. Remarkably, the capacity deliveries of SnO₂/MoS₂ remain stable during cycling in the 0.01–2.0 V ranges, as observed by the well-maintained dQ/dV peaks even after 300 discharge/charge cycles. This suggests the superior reversibility of dealloying and reconversion reactions in the composite. Interestingly, within the 2.0–3.0 V range, the corresponding dQ/dV peak becomes more pronounced, and the capacity contribution increases by approximately 130 mA g⁻¹ after 300 cycles, which closely aligns with the increase in total capacity. In contrast, the dQ/dV curves of B-MoS₂ exhibit almost no peak after cycling, implying that the combination with SnO₂ mitigates the dissolution of Li₂S during long-term cycling. Conversely, B-SnO₂ and SnO₂/GNs demonstrate weakened dQ/dV peaks and reduced capacity delivery during cycling in the 0.01–2.0 V region (Figures S10, S11, Supporting Information). These observations can be attributed to the gradually irreversible nature of dealloying and reconversion reactions in these materials.

On the XRD pattern of B-SnO₂ electrode tested at 0.2 A g⁻¹ for 300 cycles, only diffraction peaks corresponding to Sn can be observed (Figure 4d). Similarly, only peaks corresponding to Sn⁰ can be detected on the corresponding Sn 3d XPS spectrum (Figure 4e). The XRD and XPS results indicate the SnO₂ phase cannot be recovered, possibly because the coarsening of conversion product Sn inhibits the reconversion reaction.^[53] In comparison, SnO₂ can be partially recovered in SnO₂/GNs after cycling, indicating the spatial confinement of GNs wrapping partially alleviates the Sn coarsening. However, for the electrode of SnO₂/MoS₂, SnO₂ can be fully recovered without any residue of Sn even after extended cycling, indicating complete reconversion of Sn with Li₂O. Figure 4f shows the Mo 3d XPS of cycled SnO₂/MoS₂ electrode, confirming the recovery of MoS₂ after cycling. The comparisons indicate the combination with MoS₂ is more effective than using GNs to enhance the reconversion of Sn with Li₂O. During the discharge process, the conversion of MoS₂ with Li⁺ gives rise to the Mo cluster, which can interact with Sn and impede its growing. DFT calculations (Figure 4g,h) were conducted to evaluate the affinities of Sn with 1 T-MoS₂, 2H-MoS₂, GNs, Sn cluster, and Mo cluster. The binding energy of Sn with GNs is −0.7 eV, which is lower than the binding energy of Sn with Sn clusters (−3.8 eV). This result suggests that the aggregation of Sn is more favorable than its adsorption on GNs in the SnO₂/GNs system. In contrast, the binding energy of Sn with Mo clusters (−5.5 eV) is higher than the binding energy of Sn with Sn clusters, implying that Sn is preferentially adsorbed on the surface of Mo clusters. The spatial confinement, together with strong interaction, enables the good dispersion of Sn nanograins, which not only promotes the efficiencies of dealloying and reconversion reactions, but also enhances the structural stability of electrode. In contrast to severe cracking observed on the SnO₂ and SnO₂/GNs electrodes after cycling, the SnO₂/MoS₂ electrode remains intact without any cracks (Figure S12, Supporting Information).

Encouraged by the outstanding half-cell performance, we further investigated its efficacy when coupled with LiFePO₄ in full-cells. The LiFePO₄ cathode (Shenzhen Kejing Zhida Technology Co., Ltd) exhibits a high initial reversible capacity of 148.5 mAh g⁻¹, with almost no capacity fading after 150 cycles (Figure S13, Supporting Information). The cathode-to-anode capacity ratio is 1.1:1, and the mass loading of cathode is 5.0–6.0 mg cm⁻². Figure 5a illustrates the discharge/charge profiles of full-cells tested at 0.2 A g⁻¹ and between 0.6 and 3.6 V. These full-cells exhibit noteworthy charge/discharge capacities of 1177.8/860.8 mAh g⁻¹. Moreover, as shown in Figure 5b, they demonstrate remarkable cycling performance, retaining a high charge capacity of 590.5 mAh g⁻¹ after 100 cycles. Even when cycled at 1.0 A g⁻¹ for 100 loops, they still maintain a significant charge capacity of 469.8 mAh g⁻¹. The outstanding performance of these full-cells can be comparable to those of reported SnO₂-based full-cells (Table S2, Supporting Information), indicating the great potential of SnO₂/MoS₂ anode for practical applications.

Additionally, the SnO₂/MoS₂ heterostructure also exhibits excellent performance for Na⁺ storage. The corresponding Na-storage process of the SnO₂/MoS₂ was revealed by the CV analysis and discussed in Figure S14, Supporting Information. In the galvanostatic discharge/charge test, it delivers a reversible capacity of 465.8 mAh g⁻¹ at 0.2 A g⁻¹, with an ICE of 70.6% (Figure 5c). Subsequent cycles show overlapping discharge/charge curves, showing a high capacity retention of 97.5% after 150 cycles. Although the SnO₂/GNs also exhibits improved cyclic stability compared to B-SnO₂, the capacity is only around 200 mAh g⁻¹, much inferior to that of SnO₂/MoS₂ (Figure 5d). Moreover, the SnO₂/MoS₂ demonstrates a higher rate capability than the B-SnO₂ and SnO₂/GNs. It achieves average reversible capacities of 470.5, 441.2, 400.6, 365.8, and 332.2 mAh g⁻¹ at 0.1, 0.2, 0.5, 1.0, and 2.0 A g⁻¹, respectively (Figure 5e). Even cycled at 0.5/1.0 A g⁻¹, the SnO₂/MoS₂ still retains high reversible capacities of 391.2/330.6 mAh g⁻¹ after 200 cycles without any capacity degradation (Figure 5f).

3 Conclusion

In summary, the SnO₂/MoS₂ heterostructure was successfully fabricated via a facile one-step P-milling method, resulting in strong binding between SnO₂ nanoparticles and MoS₂ nanosheets. The MoS₂ nanosheets with high fraction of 1 T phase provide spatial confinement to buffer the volume change of SnO₂ and prevent coarsening of Sn during charge/discharge processes. Furthermore, as confirmed by various ex-situ characterizations and DFT calculations, the strong chemical interaction between Mo clusters and Sn clusters can effectively prevent Sn nanograins from aggregating and ensures the reversibility of Sn to SnO₂, thereby significantly improving the Li⁺/Na⁺ storage. Consequently, the SnO₂/MoS₂ heterostructure exhibited high capacity, high rate, and long life in both LIBs and SIBs. The simplicity of strategy and scalability of fabrication method, combined with the outstanding performance, manifest the potential of the SnO₂/MoS₂ heterostructure for battery applications. Moreover, this viable strategy can be extended to other anode materials for high-performance LIBs.

4 Experimental Section

Conflict of Interest

The authors declare no conflict of interest.