SiO₂-directed surface control of hierarchical MoS₂ microspheres for stable lithium-ion batteries

Abstract

We, for the first time, developed a SiO₂-directed strategy for the surface control of hierarchical MoS₂ microspheres as advanced anode materials for Li-ion batteries. The as-obtained MoS₂ microspheres constructed with nanosheets show excellent battery performance, especially superior stability, due to their unique surface structure and extended interlayer distance of the nanosheets.

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Lithium-ion batteries (LIBs) have become general power sources in portable electronic devices,¹ because of their high energy density, long cycling life, high reversible capacity, are environmentally friendly and so on. In commercialized LIBs, graphite was used as the anode material with the advantages of being earth abundant, a low flat potential platform versus lithium, have high columbic efficiency and structural stability during cycling. However, the low theoretical capacity (372 mA h g⁻¹) of graphite,² in some degrees, cannot fully meet the energy density requirement in EVs. Therefore it is of essential importance to find alternative anode materials for LIBs.

Recently, with the emergence of graphene, researchers began to focus on the special properties of two dimensional materials including h-BN, transition metal chalcogenides (MoS₂, WS₂, FeSe etc.), transition metal oxides (MoO₃, LiCoO₂ etc.), phosphorene etc. Among them, molybdenum disulfide (MoS₂) has attracted widespread attention on its unique properties and has been widely used in lubricant, biosensor, photovoltaic devices, transistors, supercapacitors, batteries etc.^3–10 MoS₂ consists of stacked atom layers (S–Mo–S) held together by week van der Waals force and the layered structure enables facile intercalation of metal ions like Li⁺, Na⁺, Mg²⁺ without obvious volume change.^11–13 With the large lithium storage capacity via a conversion reaction, MoS₂ has become promising alternative anode materials in LIBs to replace graphite. However, like other metal-based electrode materials, the poor cycling stability of MoS₂ cannot meet the requirement for practical applications of LIBs. To solve this problem, various strategies have been developed to enhance the electrochemical performance of MoS₂ in LIBs, including morphology control. Various morphologies of MoS₂ have been reported such as nanosheets,¹⁴ nanotubes,¹⁵ nanorods,¹⁶ flowerlike particles and spheres.^17,18 Hierarchical structures of MoS₂ have also been reported on controllable synthesis and demonstrated to be an effective strategy to improve the electrochemical performance of MoS₂.^19–22 On the other hand, strategies to enlarge interlayer distance of MoS₂ nanosheets are also very popular, which can relax the strain and lower the barrier for Li⁺ intercalation.²³

Herein, we, for the first time, realized the surface control of the hierarchical MoS₂ microspheres composed of nanosheets by employing hydrophilic SiO₂ nanoparticles with the size of 40–70 nm. The hydrophilic SiO₂ nanoparticles were used as directing agents to realize the surface control of the as-prepared MoS₂ microspheres. The presence of SiO₂ nanoparticles could facilitate the formation of dense spheres and suppress the aggregation of the nanosheets. The SiO₂-directed surface control on hierarchical MoS₂ spheres results in excellent electrochemical performance in LIBs, especial the superior stability performance.

Fig. 1 shows the procedure of the SiO₂-directed synthesis of the hierarchical MoS₂ microspheres. In a typical synthesis of MoS₂-x (x = 0, 50, 100), 0.46 g (NH₄)₆Mo₇O₄·4H₂O and 0.6 g NH₂CSNH₂ were dissolved in 60 mL deionized water, and then x mg SiO₂ nanospheres were dispersed in the mixture by ultrasonication followed by the hydrothermal reaction in a stainless autoclave at 200 °C for 16 h (for more details, see ESI†). Abundant hydroxy groups are present on the surface of the commercial hydrophilic SiO₂ nanospheres, with which Mo₇O₄⁶⁻ will be absorbed on the surface through a weak hydrogen bonding when SiO₂ were dispersed in a mixture solution of ((NH₄)₆Mo₇O₄·4H₂O) and thiourea (NH₂CSNH₂), as Mo and S precursors, respectively. It is well-known that small sized particles possess high specific surface energy and tend to reduce the surface energy to a more stable state. In the hydrothermal process, Mo₇O₄⁶⁻ absorbed on the SiO₂ nanospheres reacted with the H₂S decomposed from thiourea to form MoS₂ nucleus and grow to dense spheres, and SiO₂ nanoparticles aggregated on the as-synthesized MoS₂ sphere surface. The presence of SiO₂ could efficiently suppress the re-stack of MoS₂ nanosheets due to the physical blocking effect. In general, surface control of MoS₂ microspheres by directing agents SiO₂ can be due to its complex synergistic effect of hydroxy groups on the surface, the high surface energy of small sized SiO₂ nanoparticles and the physical blocking effect of nanoparticles.

Fig. 1 Schematic diagram for synthesis of hierarchical MoS₂ dense spheres.

After etching the SiO₂, a hierarchical MoS₂ dense sphere was obtained. Before the removal of SiO₂ by etching, the surface of MoS₂ spheres were covered by large amount of SiO₂ nanoparticles, as evidenced by the SEM images given in Fig. S1.† After SiO₂ nanoparticles were etched, no SiO₂ nanoparticles were found on the surface of MoS₂ microspheres. XRD measurements (Fig. S2†) were used to demonstrate our successful synthesis of MoS₂ by SiO₂ assisted method, and no obvious diffraction peak was observed. To further confirm the etching of SiO₂ nanospheres EDX measurements (Fig. S3†) were carried out, a very small amount of silicon was detected. The histogram of the diameter distribution of MoS₂ microspheres was displayed in Fig. S4.† The MoS₂-0 has a size of 0.27–1.82 μm in diameter with a mean size of 0.95 μm. MoS₂-50 has a size of 0.96–2.82 μm in diameter with a mean size of 1.71 μm. The MoS₂-100 has a size of 1.94–3.51 μm in diameter with a mean size of 2.54 μm. A narrower diameter distribution is observed with the increasing addition of the SiO₂ nanospheres. In the absence of SiO₂ during the hydrothermal synthesis, surface of MoS₂-0 (Fig. 2a and b) shows nanosheet structure of aggregation (marked in Fig. 2b). In the presence of SiO₂, with proper amount addition during the hydrothermal synthesis of MoS₂, well-dispersed MoS₂ nanosheet arrays were found on the surface of MoS₂-50 (Fig. 2c and d), indicating that proper amount of SiO₂ present in the hydrothermal reactor could significantly improve the dispersion of surface nanosheets. On the other hand, when 100 mg SiO₂ nanoparticles were added in the MoS₂ synthetic process (MoS₂-100), as shown in Fig. 2e and f, separately dense MoS₂ microspheres were obtained and nanosheets on the surface of MoS₂ microspheres were polished by the excessive SiO₂ with reduced surface area for energy applications. This result demonstrated the presence of SiO₂ as morphology-directing agent could realize the proper surface control on MoS₂ microspheres.

Fig. 2 SEM images of (a and b) MoS₂-0, (c and d) MoS₂-50 and (e and f) MoS₂-100.

In order to observe the effect of SiO₂ on the micro-structure of MoS₂, the TEM images of MoS₂-0 and MoS₂-50 were collected in Fig. 3 and MoS₂-100 was in Fig. S5a.† It is clearly distinguished that MoS₂-0 is constructed fully by nanosheets and MoS₂-50 is the hierarchical dense sphere with nanosheets on the surface (Fig. 3a and c), the MoS₂-100 presented to dense spheres. This comparison demonstrated that the presence of SiO₂ might lead to denser sphere structures probably due to the high surface energy of fine SiO₂ nanoparticles and the confined reaction region. HRTEM images (Fig. 3b and d and S5b†) display the typical interlayer structure of MoS₂ for MoS₂-0, MoS₂-50 and MoS₂-100, the fringe distance of the interlayer is calculated to be 6.2 Å, 6.9 Å and 7.0 Å, respectively. The presence of SiO₂ may extend the interlayer distance of MoS₂, which is helpful for Li⁺ storage. The enlarged interlayer distance of MoS₂ may be attributed to hydroxy on SiO₂ surface, which have the similar blocking effect on (001) direction to form a lamellar structure as before reported.²⁰

Fig. 3 TEM images of (a) MoS₂-0, (c) MoS₂-50; HRTEM images of (b) MoS₂-0, (d) MoS₂-50.

The control on the surface morphology of MoS₂ by SiO₂ nanoparticles must result in effect on the electronic properties probed by Raman spectroscopy. Fig. 4 shows the Raman spectra of above-mentioned three samples excited by 632 nm laser. The characteristic peaks at 385.4 and 403.7 cm⁻¹ correspond to the E¹_2g and A_1g active modes of MoS₂, which result from vibrational modes within the S–Mo–S layer.¹⁴ The intensity ratio of E¹_2g to A_1g increased with addition of SiO₂ nanoparticle in the MoS₂ synthesis solution, which can be attributed to the enlarged interlayer distance,²⁴ which in agreement with the HRTEM results.

Fig. 4 Raman spectra of MoS₂-0, MoS₂-50 and MoS₂-100.

We investigated the electrochemical properties of these three samples as anode materials for LIBs. Fig. 5 shows the representative cyclic voltammetry (CV) and initial three charge and discharge curves at 0.2C rate (1C = 670 mA g⁻¹) of MoS₂-0, MoS₂-50 and MoS₂-100. As shown in Fig. 5a, c and e, during the first CV cycle, the dominant reduction and oxidation peaks are at ∼0.4 V and ∼1.7 V, respectively. Peak at 0.4 V can be attributed to the reduction of Li_xMoS₂ to Mo and LiS₂ via a conversion reaction, accompanied by the formation of the solid-electrolyte interface (SEI) film and electrolyte decomposition.²⁵ The small peak at ∼1.1 V results from insertion of Li⁺ into the interlayer of the MoS₂ (MoS₂ → Li_xMoS₂),²⁶ and the doublet oxidation peaks are related to the delithiation of Li₂S.²⁰ During the second CV cycle, the reduction peak at ∼0.4 V observed during the first cycle disappears, but two new reduction peaks at ∼1.0 and ∼1.7 V are observed. The difference between 1st and the following CV cycles indicated the irreversible reaction in the initial discharge, which agrees with the previous observation in the literature.²⁰ On the other hand, the second and third CV curves for MoS₂-50 overlaps better than that for MoS₂-0 and MoS₂-100 that agrees with the good cycling performance in the following discussion. Fig. 5b, d and f show the initial three charge and discharge curves at 0.2C rate in the voltage range of 0.01–3.0 V (vs. Li/Li⁺), which reveals the plateaus information of the samples. In the first discharge curves, the obvious plateau is at ∼0.6 V, agreeing with the CV curves in the first discharge, which can be attributed to the reduction of Mo⁴⁺ to Mo particle embedded in the Li₂S and then forming a gel-like polymeric layer resulting from electrochemically driven electrolyte degradation.²⁷ In the second and third discharge the potential plateau at ∼0.6 V in the first discharge disappeared and was replaced by two new plateaus of ∼1.9 V and ∼1.4 V. The small plateau at ∼1.1 V on first discharge curve is too small to distinguish and result in a sudden drop in 2.5 V to 0.75 V; and the plateau at ∼1.1 V can be observed in literatures and it is related to irreversible phase change (MoS₂ + Li → Li_xMoS₂).^28–30 In the charge procedure, two potential plateaus at 1.7 V and 2.2 V appeared in the first and subsequent charge curves, which indicated the two steps of lithium extraction process of LiS₂. As shown in the Fig. 5b, d and f, the initial discharge capacities of MoS₂-0, MoS₂-50 and MoS₂-100 are 1221 mA g⁻¹, 1321 mA g⁻¹ and 1053 mA g⁻¹, respectively, where MoS₂-50 shows the highest discharge capacity among these samples; the charge capacity of MoS₂-50 can reach 1065 mA g⁻¹ and the capacity loss can be attributed to the irreversible process of the formation of SEI layer and decomposition of electrolyte.²⁰

Fig. 5 Initial three CV curves of (a) MoS₂-0, (c) MoS₂-50 and (e) MoS₂-100 measured at a scanning rate of 0.2 mV s⁻¹ in the voltage of 0.01–3 V (vs. Li/Li⁺); first three charge–discharge curves of (b) MoS₂-0, (d) MoS₂-50 and (f) MoS₂-100 at 0.2C rate in the voltage of 0.01–3 V (vs. Li/Li⁺).

Cycling performances of the three samples at different rates of 0.2C, 1C and 2C in the range of 0.01 V to 3 V are displayed in Fig. 6a–c. As shown in Fig. 6a, the reversible capacities of MoS₂-0, MoS₂-50 and MoS₂-100 can maintain 244 mA h g⁻¹, 987 mA h g⁻¹ and 356 mA h g⁻¹ at rate of 0.2C after 100 cycles, respectively, and the capacity retention rates are 24.14%, 92.2% and 38.9% corresponding to their second cycle. As shown in Fig. 6b and c, the discharge capacities of MoS₂-0, MoS₂-50 and MoS₂-100 can maintain 136.2 mA h g⁻¹, 675.3 mA h g⁻¹ and 302.7 mA h g⁻¹ at 1C rate after 100 cycles, respectively, and 134.7 mA h g⁻¹, 649 mA h g⁻¹ and 296.4 mA h g⁻¹ at 2C rate. It is clearly observed that the MoS₂-50 manifests excellent cycling ability with little capacity fading during the first 100 cycles compared with other hierarchical MoS₂ or hierarchical MoS₂-based anode materials (Table S1†),^20–22,31 also it shows advanced performance when compared with some other oxides based composite materials as LIB anode (Table S2†).^32–35 In order to evaluate the rate performance, the MoS₂-50 electrode was cycled at various rates from 0.2C to 2C in the voltage range of 0.01–3.0 V. As shown in Fig. 6d, there is a slight capacity fading for MoS₂-50 with increase of the cycle rates. When the cycle rate increased to 2C, high capacity of 649 mA h g⁻¹ can still remained; and also a high discharge capacity of 973 mA h g⁻¹ can be recovered after 20 cycles when decreased the rate to 0.2C. So it is clear that MoS₂-50 demonstrated excellent lithium storage ability. The excellent cycling stability and rate capability of the hierarchical microspheres (MoS₂-50) might be attributed to the unique structure and enlarged interlayer distance mediated by the SiO₂ nanoparticles during the synthesis.

Fig. 6 Cycling performances for electrode MoS₂-0, MoS₂-50 and MoS₂-100 at (a) 0.2C, (b) 1C and (c) 2C rates; (d) rate performance of MoS₂-50.

To further investigate the different electrochemical performance of the MoS₂ anodes, the electrochemical impedance spectroscopy (EIS) measurements were carried out after 5 cycles. As shown in Fig. 7, the MoS₂-50 electrode shows the smallest radius of semi-circle in the Nyquist plots, indicating a low charge-transfer resistance in the battery. On the contrary, MoS₂-100 exhibits the largest charge-transfer resistance due to its dense structure. Hierarchical structure constructed by nanosheets can be effective to improve the electrochemical performance of MoS₂. The large lateral size of the nanosheets in MoS₂-50 also provides a large contact area between MoS₂ and the electrolyte, which offers more active sites for Li⁺ insertion/distraction, resulting in the high specific capacity. Furthermore, the space between the nanosheets can effectively relax the stress to protect the active materials from pulverization during the lithiation/delithiation process. Therefore, the cycling and rate performance of the MoS₂-50 is greatly enhanced.

Fig. 7 Nyquist plots of the electrodes at the frequency of 100 kHz to 0.01 kHz.

Summarily, a simple SiO₂-directed strategy was developed for the successful synthesis of the hierarchical dense spheres constructed by nanosheets with proper surface control and extension of the interlayer distance, leading to excellent Li⁺ storage performance. MoS₂-50 delivers a small capacity fading with a high specific capacity of 987 mA h g⁻¹ at 0.2C and 649 mA h g⁻¹ even at 2C rate after 100 cycles. The excellent electrochemical performance of the hierarchical MoS₂ microspheres (MoS₂-50) can be attributed to the unique physical structure and enlarged interlayer distance. The current method reported in this work provides a novel strategy for the design of anode materials for stable LIBs with advanced performance.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51402100, 50702020, 21573066 and 81171461), the Youth 1000 Talent Program of China, and the Inter-discipline Research Program of Hunan University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra14424h

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