Hierarchically 3D structured milled lamellar MoS₂/nano-silicon@carbon hybrid with medium capacity and long cycling sustainability as anodes for lithium-ion batteries

1. Introduction

In the past few years, the ever-increasing demands for lithium ion batteries (LIBs) with remarkable energy density and long-cycle life for widespread applications such as portable electronic device, mobile equipment and electric vehicle, have become one of the global research hotspots [[1], [2], [3], [4]]. Currently, commercial graphite suffers low power density and potential safety hazard, theoretical specific capacity of 372 mA h g^-1 is hardly to satisfy further large-scale applications, especially for transportation industries [[5], [6], [7], [8]]. Up to now, silicon is well known as one of the most promising candidates due to high theoretical capacity of 4200 mA h g^-1, suitable discharge plateau of $\widetilde{0}$.2 V, low cost and environmental safety [[9], [10], [11]]. However, the large mechanical strain caused by the drastic volume expansion of conventional Si anodes easily leads to serious pulverization of the electrode, rapid capacity decay and deteriorated cycle ability upon continuous lithiation/delithiation reactions, and the practical applications are extremely challenged for next generation high-power batteries [[12], [13], [14]]. To address such dilemma, a variety of scientific methods, including nanowires [15], nanofibers [16], porous and hollow structures [[17], [18], [19]], have been conducted to avoid the intrinsic disadvantage [20,21]. It is well known that the combination with carbonaceous materials (graphite, carbon nanotubes, graphene and polymer sources) can efficiently enhance the electrode performance [22]. Among these, graphene as an attractive two-dimensional (2D) material has been extensively utilized to fabricate various functional hybrids due to some advantages of large surface area, high electrical conductivity, good chemical stability and strong mechanical property. For instance, Zhai et al. prepared porous core-shell P-Si/rGO via in situ reduction followed by a dealloying process, showing specific capacity of more than 2100 mAh g^-1 at a current density of 1000 mA g^-1 [23]. Shi et al. studied the growth of vertical grapheme on silicon monoxide (SiO) microparticles. The results showed 93% capacity retention after 100 cycles even at a high areal mass loading of 1.5 mg cm^-2 [24]. Most recently, transition-metal dichalcogenides (TMDs) as emerging graphene-like materials are of interest to lithium ion batteries, supercapacitors, sensors, transistors and so on [[25], [26], [27], [28]]. Typically, MoS₂ with S-Mo-S layers separated by van der Waals force has been intensively studied [29,30]. Benefiting from adjustable interlayer spacing, layered MoS₂ can accommodate the repetitive insertion/extraction of species and mitigate the volume changes during cycles, and various lamellar MoS₂ have also been reported to enhance electrochemical properties [[31], [32], [33]]. However, there a big gap still exists between scientific researches and commercial applications. Some pending technical dilemmas remain to be solved, including long-term cycling durability, safety issue, application compatibility and so on.

In this work we attempted to yield a medium capacity rather than super high capacity. Since layered MoS₂ is expected to accommodate volumetric expansion upon lithium storage and mitigate strain due to the weak van der Waals interaction between S-S layers. Moreover, the MoS₂ nanosheets tightly immobilized on the carbon can also provide more exposed active sites for Li⁺ adsorption and facilitate electron transfer. So we tried to achieve cyclic stability through designing a smart Si-based architecture combined with MoS₂. Herein, a novel 3D structured milled lamellar MoS₂/nano-silicon@carbon hybrid with medium capacity and long-term lifespan was synthesized successfully. In addition, it is worth mentioning that all raw materials are commercially available in the experiment, and the technological process is relatively simple, facile reliable and eco-friendly without using costly reagents, which is beneficial to the practical applications. Especially, utilizing an efficient ball-milling and scalable spray-drying strategy is different from the stringent technique, in which the high temperature sintering, chemical vapor deposition (CVD) and organic liquid-phase methods are usually used. As illustrated in Scheme 1, commercial nano-silicon and MoS₂ were treated by mechanical milling process at first, then the precursor and appropriate proportion of petroleum pitch were mixed. After that, the compounds were spray-dried and carbonized to obtain the final product. The lamellar MoS₂ sheets played a role in isolating the aggregation of the nano-silicon, while carbon shells pyrolyzed from pitch acted as an elastic matrix to accommodate the volume changes and remained mechanical stability. And the theoretical calculations based on the density functional theory (DFT) also reveal that Si/C and MoS₂/C surfaces could facilitate electrons diffusion due to the improved conductivity. The Si/MoS₂@C hybrid as anodes for LIBs demonstrates a stable reversible capacity and prolonged cycling performance with 99.6% coulombic efficiency over 1200 cycles at 500 mA g^-1. Moreover, the Si/MoS₂@C hybrid delivers an excellent rate capacity of 388.1 mA h g^-1 at a high current of 3000 mA g^-1, implying a promising anode candidate for LIBs.

View original graphic| Download| PPT slide

Scheme 1.   Schematic illustration of milled lamellar MoS₂/nano-silicon@carbon.

2. Experimental

2.1. Theoreotical calculation

The interfacial contact models of Si (111)/C (002) and MoS₂ (002)/C(002) were designed. In order to build up the interfacial contact models, the 2D Si (111), MoS₂ (002) and graphite (002) slabs were directly cut from the optimized 3D Si, MoS₂ and graphic bulk structures along the normal crystal plane. Meanwhile, the 2D slabs were separated by a vacuum region of 20 Å thickness along the c-axis to avoid the interactions from periodic boundary. As a comparison, the crystal models of silicon and MoS₂ were also proposed. All theoretical calculations based on the density functional theory (DFT) were performed using CASTEP code of Material Studio. The generalized gradient approximation (GGA) accompanied by the Perdew-Wang (PW91) scheme was employed to conduct the electronic exchange-correlation potential energy, and the electron-ion interaction was described by ultra-soft pseudopotential (USPP). Pseudo atomic calculations were performed for C (2s²2p²), Si (3s²3p²), Mo (4s²4p⁶4d⁵5s¹) and S (3s²3p⁴). While various Monkhorst-Pack k-points grid was set to 8 × 8×1 and 9 × 9×1 for Si (111)/C (002) and MoS₂ (002)/C (002) interface, respectively. The BroydenFlecher-Goldfarb-Shanno (BFGS) algorithm was used to obtain the optimal structures. The surface layers were fully relaxed and the bottom layers were constrained. Specifically, some convergent parameters were set as 0.01 eV/Å for maximum force, 5 × 10^-4 Å for maximum displacement tolerances, 0.02 GPa for maximum stress, 5 × 10^-6 eV/atom for total energy change in the geometry optimization and 5 × 10^-7 eV/atom for self-consistent field tolerance.

2.1.1. Preparation of Si/MoS₂ precursor (SMP)

2 g nano-silicon ($\widetilde{6}$0 nm) powder (Shanghai Xiang Tian Nano Materials Co., LTD.) was scattered ultrasonically in 30 ml alcohol, then 2 g MoS₂ (Aladdin, AR) powder was added to an ultrasonic bath for 30 min. Next, the mixture was poured into stainless steel jar to achieve Si/MoS₂ composite by ball-milling method with a rotation speed of 300 rpm for 4 h. The precipitates were collected and washed by deionized water and ethanol, followed by drying under vacuum overnight at 60 °C. Additionally, the weight proportions of Si and MoS₂ were designed to be 7:3, 8:2 and 1:1, respectively. The precursors were denoted as SMP (7-3), SMP (8-2) and SMP (1-1).

2.1.2. Preparation of the Si/MoS₂@C composite

The optimized ratio of Si/MoS2 precursor (1:1) was achieved to proceed further experiments. 2.8 g SMP (1-1) precursor was dispersed in 50 mL tetrahydrofuran in an ultrasonic bath for 30 min. 4.2 g pitch was added to the solution, followed by stirring for 30 min. The weight proportion of Si: MoS₂: C was designed to be 2: 2: 6. Then, the well-proportioned suspension was spray-dried by hot air. The inlet and outer temperatures of the spray dryer were maintained at 260 ℃ and 90 ℃, respectively. The solid composite was calcined at 900 ℃ for 3 h under argon atmosphere at a heating rate of 2 ℃ min^-1 to obtain the final product Si/MoS₂@C (named as SMC).

2.2. Structural characterization

XRD measurement was done with PANalytical X’Pert PRO diffractometer at a scan rate of 2°/min with 2θ range from 10° to 90°. The carbon characteristics were determined by micro-Raman spectroscopy (Jobin Yvon LabRam HR800). The morphological and microstructural features were investigated by field emission scanning electron microscopy (FESEM; ZEISS ULTRA 55) and high-resolution transmission electron microscopy (TEM; JEM-2100HR). The pore size distribution was conducted by means of N₂ adsorption/desorption method using an ASAP2460 analyzer.

2.3. Battery assembly and electrochemical measurements

The anodes were composed of the slurry including the as-synthesized active materials (Si, MoS₂, SMP, SMC), carbon black and binder (LA133, Chengdu Indigo Power Sources Co., Ltd) in the weight ratio of 7:2:1. The electrode was obtained by coating the slurries on a copper foil of 10 μm in thickness. The coated copper foil was then dried at 80 ℃ for 12 h. The coin cells (CR2430 cells) were assembled in an argon-filled glove box (MBRAUN LABstar). Li-metal, Celgard 2400 and 1 M LiPF6 (EC: DEC: EMC = 1:1:1, in volume) worked as the counter electrode, separator and electrolyte, respectively. Galvanostatic cycling measurements were performed on a NEWARE battery testing system in the voltage range of 0.01-2.5 V. The cyclic voltammetry (CV) was proceeded on a Solartron 1470E electrochemical workstation.

3. Results and discussion

3.1. Morphology and microstructure

Fig. 1(a) shows the X-ray diffraction (XRD) patterns of the Si/MoS₂@C hybrid. These peaks can be ascribed to two main phases of crystalline Si (JCPDS #27-1402) and hexagonal MoS₂ (JCPDS #37-1492). In detail, several sharp peaks at 28.4°, 47.3°, 56.1°, 69.1° and 79.4° are consistent with (111), (220), (311), (400) and (331) plane of Si. Other strong peaks at 14.4°, 32.7°, 39.6°, 49.8°, 58.3° and 60.1° are in accord with (002), (100), (103), (105), (110) and (008) plane of MoS₂. Specifically, the sharp peak at 26° is corresponding to the pyrolyzed carbon from pitch during the calcination process. Meanwhile, no impurities are observed. Fig. 1(b) shows the Raman spectra of SMC. Two peaks at about 1355 cm^-1 and 1580 cm^-1 can be identified to D band and G band, respectively [34,35]. The D band indicates the defect and disorder in carbon materials. The G band reflects the sp² hybridization of carbon [36]. The integrated intensity ratio (I_D/I_G) is estimated to be 0.42, which manifests partial graphitization and some defects in the Si/MoS₂@C hybrid. Fig. 1(c) shows optical image of SMP precursor and final product SMC from the experiments.

View original graphic| Download| PPT slide

Fig. 1.   (a) XRD pattern and (b) Raman spectra of SMC, (c) optical image of SMP precursor and final product SMC.

Fig. 2 depicts the detailed SEM morphologies of the SMC hybrid. In the ball milling process, nano-silicon and MoS₂ were squeezed with each other owing to the large mechanical momentum. The mixtures were constantly crushed and pressed to gradually form layered-structure compounds. After spray drying and heat treatment, as shown in Fig. 2(a-b), the final products demonstrate a 3D spherical fashion ($\widetilde{2}$0 μm) with porous and rough surface. More information can be observed in Fig. 2(c-d), and it is obvious that nano-silicon (indicated by arrows) and lamellar MoS₂ sheets are covered by the pyrolyzed carbon shells derived from pitch. It is expected that this unique architecture as anodes for LIBs will present more advantages since 3D porous structure provides enough active sites for lithium storage and rapid diffusion channels for ions. In addition, the external carbon shells can avoid the aggregation of nano particles and buffer severe volume changes, thus improving the cycling performance to some extent.

View original graphic| Download| PPT slide

Fig. 2.   SEM images of (a-d) SMC microsphere.

This smart 3D architecture was further observed by TEM and HRTEM. As shown in Fig. 3(a-b), it is clearly shown that nano-silicon particles, MoS₂ sheets and carbon skeleton are interconnected together, implying a high pore volume and ensuring a good buffering effect. In Fig. 3(b-c), silicon and MoS₂ are encapsulated in carbon matrix, which could promote electrical conductivity and prevent active materials from being exposed to electrolyte directly. It can be evidently shown in Fig. 3(d-f) that Si and MoS₂ are coated by carbon shells of $\widetilde{1}$0 nm in thickness generated from the pitch. The marked interplanar spacings of 0.31 nm and 0.61 nm agree well with (111) and (002) planes of Si and MoS₂, respectively. In addition, the pyrolyzation product of the pitch can facilitate the formation of 3D framework to avoid the pulverization and aggregation during the repeated charge/discharge cycling.

View original graphic| Download| PPT slide

Fig. 3.   (a-c) TEM images, (d-f) HRTEM images, (g) Nitrogen absorption-desorption isotherms and (h) BJH pore size distribution.

N₂ adsorption was carried out to investigate the porosity of the SMC. Fig. 3(g-h) exhibits type IV isotherm, which is one of the main characteristics of mesoporous materials [37]. The total adsorption pore volumes at P/P₀ = 0.994 are calculated to be about 0.221 cm³ g^-1. Meanwhile, the SMC also displays a broad pore size distribution ranging from 3.2-30 nm. This unique structure can not only provide more contact areas between the electrode and electrolyte, but also offer sufficient void space and more active sites, thus efficiently facilitating ion diffusion and accommodating the stress relaxation to improve the cyclic stability.

3.2. Electrochemical analysis

To further investigate the lithium storage mechanism, the CV results (Si, MoS₂ and SMC) of the first three cycles are presented in Fig. 4(a-c) with a scan rate of 0.2 mV/s between 0.01 and 2.5 V. Obviously, the 1 st CV curve of SMC electrode (Fig. 4(c)) has broad peaks around 0.7 V and 0.3 V, which is distinctly different from the subsequent cycle, corresponding to the formation of the SEI film as well as lithiation process of MoS₂ [38,39]. The formation of SEI film will result in a distinct capacity loss. At the same time, the cathodic peaks at low potential are related to the formation of various Li-Si phases [40]:

Si + xLi→(1-x/y)Si + x/ya-Li_ySi

View original graphic| Download| PPT slide

Fig. 4.   CV profiles of (a) Nano-silicon, (b) Commercial MoS₂ and (c) SMC in the initial 3 cycles at a scan rate of 0.2 mV/s, (d) Galvanostatic charge-discharge curves of SMC for the 1 st, 2nd and 250th cycles at a current density of 100 mA g^-1.

The relevant anodic peaks at 0.23 V and 2.3 V are in accord with the delithiation of Si and MoS₂ (Li_xMoS₂→Li₂S + Mo) [41]. The metallic Mo can enhance the electrical conductivity and also play a "rake" effect, maintaining the material structure stable. In the 2nd cycle, the subsequent cathodic peak at $\widetilde{1}$.91 V comes from the lithiation reaction of S^2- (MoS₂) and Li⁺ [41,42]. Another peak at 1.04 V corresponds to the formation of Li_xMoS₂ (MoS₂ + xLi⁺ +xe-→ Li_xMoS₂). Meanwhile, the lithiation tendency of Si almost remains unchanged. In the 3rd cycle, the peak profiles of the SMC hybrid gradually overlapped, which is attributed to the good reversibility of the electrode during the charge/discharge process.

Furthermore, as shown in Fig. 4(d), the galvanostatic tests for the 1 st, 2nd and 250th cycles were performed in the voltage of 0.01-2.5 V at the current density of 100 mA g^-1. As can be seen, there is a plateau around 0.7 V at the 1 st cycle, corresponding to the above CV analysis. The following curves almost depict the same tendency, manifesting a high reversible capability of the SMC hybrid. The electrode delivers high discharge specific capacities of 1257.8, 876.4 and 767.5 mA h g^-1 in the 1 st, 2nd and 250th cycle, respectively.

Fig. 5(a-b) depicts the comparable cycling performance of various SMP precursors and final product SMC hybrid at different current densities. Obviously, as shown in Fig. 5(a), SMP precursors with a weight proportion of 7:3, 8:2 and 1:1 can deliver discharge specific capacities of 2812.04, 3058.44 and 2653.3 mA h g^-1, respectively. As the cycling number increased, all precursors displayed capacity attenuation quickly. After 100 cycles, the specific capacities of SMP (7-3), SMP (8-2) and SMP (1-1) are only 378.3, 444.2 and 465.3 mA h g^-1, while the SMC electrode delivers sustainable capacity of 753.3 mA h g^-1. Similar tendency can be also found in Fig. 5(b) at a current density 500 mA g^-1, and the SMP (7-3), SMP (8-2) and SMP (1-1) expressed obvious capacity decay of 325.31, 373.27 and 406.60 mA h g^-1 up to 200 cycle. To achieve ameliorated chemical performance, the optimized ratio of SMP (1-1) abbreviated as SMP in following section was adopted to combine with organic carbon to produce the final product SMC hybrid.

View original graphic| Download| PPT slide

Fig. 5.   Comparable cycling performance of various SMP (7:3, 8:2 and 1:1) precursors and final product SMC hybrid at a constant current density of 100 and 500 mA g^-1.

In order to study practical application of LIBs, rate capability is measured at the current densities ranging from 100 to 3000 mA g^-1, as shown in Fig. 6(a). Surprisingly, the SMC hybrid can present admirable capacities of 1147.2, 723.1, 566.5, 467.3, 415.5, 388.1 and 760.0 mA h g^-1 at the increased current densities of 100, 200, 500, 1000, 2000, 3000 and 100 mA g^-1, respectively. The results implied a good capability and compatibility of high-speed electrons/ions since 3D architecture provided more electrons/ions transport channel, afforded robust mechanical stability and improved electric conductivity.

View original graphic| Download| PPT slide

Fig. 6.   (a) Rate performance of final product SMC at various current densities from 100 to 3000 mA g^-1, (b) Cycling performance of Si, MoS₂, SMP precursor and final product SMC at a constant current density of 100 mA g^-1, (c) Super long service life of final product SMC at a constant current density of 500 mA g^-1.

Fig. 6(b) shows the cycling stability of active materials at a current density of 100 mA g^-1. For the first time, pristine nano-silicon, commercial MoS₂ and SMP precursor have high discharge specific capacities of 4075.1, 1093.7 and 2653.3 mA h g^-1, respectively. At the same time, the SMC electrode attains an initial discharge capacity of 1257.8 mA h g^-1. According to our previous study, the carbon shells pyrolyzed from petroleum pitch only afford a low discharge capacity of about 160 mA h g^-1 at a current density 100 mA g^-1 [35]. Hence, as for Si/MoS₂@C hybrid, the actual capacity mainly depends on the contribution of the Si/MoS₂ hybrid. As the cycling numbers increasing, the capacities of Si, MoS₂ and SMP quickly attenuated. Most obviously, the SMC electrode possesses larger capacity retention and better stability than bare Si and MoS₂. After 250 cycles, the SMC hybrid can exhibit a reversible capacity of 767.52 mA h g^-1 at a current density 100 mA g^-1. Additionally, the SMC hybrid delivers a high initial discharge specific capacity of 1122.8 mAh g^-1 with a coulombic efficiency of 72.7% even at a high current density of 500 mA g^-1, and the reversible capacity can still reach 537.6 mA h g^-1 with a coulombic efficiency of 99.6% up to 1200 cycle (Fig. 6(c)). During the long cycling test, the capacity increases gradually after a slight decrease. The initial large capacity loss and low coulombic efficiency can be ascribed to the formation SEI film and the decomposition of the electrolyte. As the electrode tending to be stable, the SMC presents super long-term lifespan and enhances lithium storage capability, which is supposed to be controllable self-adaption of the construction, good infiltration of the electrolyte, high electroconductivity of carbon skeleton as well as the robust confinement of 3D framework.

To further explore this long-term durability, SEM and TEM images (Fig. 7(a-b)) of the SMC electrode after 1200 cycles at 500 mA g^-1 are used to evaluate the morphological evolution. It is apparent that repeated lithiation/delithiation resulted in partial transformation and pulverization, which is the inevitable and universal phenomenon during long-term conversion process. Meanwhile, moderate cracks could partly work in relaxation stress, contributing to the structural stability. Fig. 7(c-d) shows the images of selected nano-silicon particles after 1200 cycles. The spherical shape still remains unchanged since hierarchically 3D structure alleviates the massive volume variation and promotes electron/ions transfer. As shown in Fig. 7(e-h), energy dispersive spectrometry (EDS) mapping results further indicate the structural integrity and homogenous distribution of Si, C, Mo and S element, respectively. As a result, several morphological changes could ensure good electric contact, enough buffer space and more active sites.

View original graphic| Download| PPT slide

Fig. 7.   (a-b) SEM, (c-d) TEM images and (e-h) EDS mapping of final product SMC electrode after 1200 cycles at a current density of 500 mA g^-1 (the blue, red, purple and green dots stand for Si, C, Mo and S, respectively).

3.3. Theoretical analysis and simulation

It is well known that material performance depends on the electronic structure to a large degree. Based on the experimental researches, we also investigate different interfacial models by using First-principles calculations. According to the preferred orientation of the as-prepared materials shown in XRD and TEM analysis, particular interfacial contact models of Si (111)/C (002) and MoS₂ (002)/C (002) were also designed by using C (002) slab on Si (111) and MoS₂ (002) slab. The simulated geometry structures are displayed in Fig. 8(a-d).

View original graphic| Download| PPT slide

Fig. 8.   The simulated geometry structures of (a) bulk MoS₂, (b) bulk Si, (c) Si/C interface, (d) MoS₂/C interface.

As shown in Fig. 9(a-c), bulk Si and MoS₂ have indirect band gaps of 0.602 and 1.879 eV, respectively. When directly employed as anodes for LIBs, Si and MoS₂ suffer from low electron mobility and poor electronic conductivity, which would limit practical applications to a large extent. In contrast, energy bands of interfacial contact models are crossing Fermi level, as shown in Fig. 9(c-d). It can be concluded that Si (111)/C (002) and MoS₂ (002)/C (002) partially exhibit metallic characteristics near the surface contacting area, which could facilitate electrons and Li-ions diffusion due to the improvement of the conductivity. This phenomenon can be mainly ascribed to the contribution of different work functions.

View original graphic| Download| PPT slide

Fig. 9.   Band structures of (a) bulk MoS₂, (b) bulk Si, (c) MoS₂ (002)/C (002) interface, (d) Si (111)/C (002) interface.

Based on the surface potential profiles, the work functions along c-axis are calculated in Fig. 10(a-e). The corresponding results indicate that C (002) surface owns relatively lower work function (4.251 eV) than those of MoS₂ (002) and Si (111) surface (4.714 and 4.633 eV). The higher work function implies electrons need more energy to escape from the surface. This also means that variant surfaces contacting together would cause electrons transfer from carbon to MoS₂ and Si semiconductors until the Fermi levels are aligned. When the interfaces tend to be stable, the corresponding work functions are 5.429 and 5.159 eV for MoS₂ (002)/C (002) and Si (111)/C (002) interface. The intrinsic change of the electronic structures can also be observed from the total density of states (TDOS) and partial density of states (PDOS).

View original graphic| Download| PPT slide

Fig. 10.   Work functions of (a) MoS₂ (002) plane, (b) Si (111) plane, (c) C (002) plane, (d) MoS₂ (002)/C (002) and (e) Si (111)/C (002) interface.

In Fig. 11(a-d), energy structure at Fermi level indicates a high contribution of Mo-4d, S^-3p and C-2p orbitals. Evidently, this mixed character dominantly derives from the electron orbital hybridizations due to the transfer of some electrons. While Mo-4 s, Mo-4p, Mo-5 s, C-2p and S^-3p orbitals play a little role in MoS₂ (002)/C (002) interface near the Fermi level.

View original graphic| Download| PPT slide

Fig. 11.   Partial density of states of (a) Mo, (b) S and (c) C atoms and (d) Total density of states of MoS₂ (002)/C (002) interface.

Similar phenomenon can be observed in Fig. 12(a-c), the hybridization peaks located at Fermi level mainly come from the interaction between s, p orbitals of C atom and p orbitals of Si atom during the interface optimization process. Besides, other peaks appeared at low energy range also take part in the hybridization. Moreover, porous carbon skeleton coated on active materials could effectively restrain the pulverization due to the repetitive lithium intercalation/extraction during cycling process, thus enhancing the electrochemical performances.

View original graphic| Download| PPT slide

Fig. 12.   Partial density of states of (d) Si, (e) C atoms and (f) Total density of states of MoS₂ (002)/C (002) interface.

According to the above analysis, nano-silicon, MoS₂ and carbon play a cooperative effect on ameliorating the electrochemical performance of the SMC electrode. Especially, the reasons of the high-performance for lithium storage properties of the SMC hybrid can be explained as: (1) Carbon skeletons with enough void space server as an elastic matrix to buffer the huge volume changes and avoid the pulverization, thus accommodating the stress relaxation; (2) The lamellar MoS₂ sheets act as a unique separating layer to isolate the aggregation of the nano-silicon particles; (3) The hierarchical 3D framework provides more contact areas of the electrode-electrolyte interface and enhance the electrical conductivity.

4. Conclusion

In summary, a hierarchically 3D structured milled lamellar MoS₂/nano-silicon@carbon hybrid was successfully synthesized by ball milling process and spray-drying route. The particular structure provides abundant buffering space and enhances electrical conductivity, thus effectively adapting to the intensive volume change during the repeated charge/discharge cycling process. The SMC demonstrates a super long-term lifespan and medium capacity. These consequences manifest that the SMC could be a promising candidate anode for the LIBs to handle the potential energy problems.

Acknowledgements

The authors gratefully acknowledge the financial support of the Outstanding Young Scholar Project (8S0256) from South China Normal University, the Union Project of the National Natural Science Foundation of China and Guangdong Province (U1601214), the Scientific and Technological Plan of Guangdong Province (2017A040405047), the Key Projects of Guangdong Province Nature Science Foundation (2017B030311013) and the Scientific and Technological Plan of Guangzhou City (201607010274).

The authors have declared that no competing interests exist.