Three-dimensional macroporous graphene monoliths with entrapped MoS2 nanoflakes from single-step synthesis for high-performance sodium-ion batteries

Layered metal sulfides (MoS2, WS2, SnS2, and SnS) offer high potential as advanced anode materials in sodium ion batteries upon integration with highly-conductive graphene materials. However, in addition to being costly and time-consuming, existing strategies for synthesizing sulfides/graphene composites often involve complicated procedures. It is therefore essential to develop a simple yet scalable pathway to construct sulfide/graphene composites for practical applications. Here, we highlight a one-step, template-free, high-throughput “self-bubbling” method for producing MoS2/graphene composites, which is suitable for large-scale production of sulfide/graphene composites. The final product featured MoS2 nanoflakes distributed in three-dimensional macroporous monolithic graphene. Moreover, this unique MoS2/graphene composite achieved remarkable electrochemical performance when being applied to Na-ion battery anodes; namely, excellent cycling stability (474 mA h g−1 at 0.1 A g−1 after 100 cycles) and high rate capability (406 mA h g−1 at 0.25 A g−1 and 359 mA h g−1 at 0.5 A g−1). This self-bubbling approach should be applicable to delivering other graphene-based composites for emerging applications such as energy storage, catalysis, and sensing.

scaffolds to create porous composites, so as to simultaneously improve its conductivity as well as buffer the volumetric variation. 12 In this regard, carbon materials, especially graphene, have been repeatedly conrmed to be an efficient conductive additive in electrode materials in resolving the above issues. 28,29 Some examples of such effective treatment on electrode materials include sulfur/graphene cathode in lithium-sulfur batteries, 30 lithium metal phosphates/carbon cathode materials in LIBs, [31][32][33][34] and various metal oxides/graphene anode materials in LIBs. 35 To improve the electrical conductivity and enhance the structural integrity of MoS 2 anode, MoS 2 /graphene composites have been synthesized via several methods and applied in NIBs. 17,18,21,23,26,[36][37][38][39][40][41] For instance, David et al. prepared MoS 2 / graphene composite paper through vacuum ltration of homogeneous dispersions consisting of exfoliated MoS 2 and graphene oxide sheets, followed by thermal reduction at elevated temperatures. 18 Wang et al. and Xie et al. also synthesized MoS 2 /graphene composites via hydrothermal reactions plus thermal annealing, respectively. 23,26 In spite of the signicant synthetic achievements made, the existing strategies for synthesizing MoS 2 /graphene composites present a few shortcomings as these methods oen involve complicated procedures (graphene oxide preparation, MoS 2 preparation, compositing or mixing step, thermal treatments, etc.) in addition to be costly and time-consuming. 28 Another issue with existing MoS 2 /graphene compositing methods is that some of them do not ensure the intimate contact between MoS 2 /graphene interfaces, an unfavorable condition for electrochemical applications (charge-transfer process). 26 Finally, most of the present MoS 2 /graphene compositing methods are faced with the issue of low yield, ranging from several tens to hundreds milligrams of powders under laboratory conditions.
Herein, we report a single-step, template-free, highthroughput "self-bubbling" method for synthesizing MoS 2 / graphene composite. Our method is cost-effective, simple and scalable. The synthesis utilizes the thermal decomposition of solid precursor to generate MoS 2 ; meanwhile, the released gas from the decomposition reaction blows premixed, melted glucose into crowded bubbles, which then evolve into graphene structures during annealing. The nal product is microscopically featured as highly crystalline MoS 2 nano-akes distributed in three-dimensional (3D) macroporous monolithic graphene. With the additional assistance of intimate interfacial contacts between MoS 2 and graphene, our composite demonstrates considerably improved electrochemical performance when compared with those of conventional MoS 2 /graphene composite upon application in NIBs. It is expected that such a unique MoS 2 /graphene composite should hold potential in promoting the development of practical MoS 2 anode in NIBs, while the straightforward self-bubbling method could offer the opportunity in producing MoS 2 /graphene composites in industrial scale as well as synthesizing other advanced graphene-based composites.

Results and discussion
We demonstrate a one-step "self-bubbling" system, for the rst time, to synthesize the graphene/MoS 2 composite in this work. Empirically, thermal decomposition of (NH 4 ) 2 MoS 4 in inert atmosphere leads to MoS 2 while releasing a considerable amount of gases. 42,43 Results from our carefully conducted thermogravimetric and differential scanning calorimetry (TG/ DSC) analysis for (NH 4 ) 2 MoS 4 decomposition in owing Ar (Fig. 1a) suggests the following processes: Inspired by these ndings, we started with a mixture of (NH 4 ) 2 MoS 4 (as MoS 2 source) and glucose (as carbon source) with the setup of a two-stage annealing sequence to produce graphene/MoS 2 hybrid, as shown in Fig. 1b  (NH 4 ) 2 MoS 4 decomposes into MoS 2 and crystallizes while the released gas species (ammonia, hydrogen sulde and sulfur vapor) blow the melted glucose to form crowded bubbles with ultrathin walls, which are then graphitized into 3D graphene networks at high temperature (the released gases actually serve as so templates to direct the growth of graphene structures.). With this simple approach, a rational nanostructure consists of MoS 2 and graphene was obtained (hereaer abbreviated as MoS 2 @G hybrid). Besides, our approach also allows the MoS 2 / graphene ratio in the nal product to be expediently tuned by using different ratios of (NH 4 ) 2 MoS 4 and glucose as precursors for various potential applications (see Fig. S1, † the composites with varied MoS 2 /graphene ratios). The advantages of such novel approach include low cost, high exibility, easy operability and excellent scalability. The complete annealing program is presented in Fig. 1b as well as described in the Experimental methods section in the ESI. † A schematic diagram of the whole process is further shown in Fig. 1c.
The effectiveness of our approach could be fully conrmed by systematic microstructural analysis of the end-product. Firstly, microscopies were involved to reveal the morphological characteristics of the MoS 2 @G hybrid. The single production of the MoS 2 @G hybrid under our laboratory condition is up to ca. 1-2 g when a 1-inch (diameter) tube furnace was used, and the product is foam-like black solid (inset of Fig. 2a). An optical image (Fig. 2a) manifests that the product is composed of largescale crowded bubbles. The walls of these bubbles are so thin that the light can penetrate through them, leading to rainbowlike reections on their surface. The scanning electron microscope (SEM) images ( Fig. 2b and c) suggest the bubbles are mostly polyhedral units, with a broad size distribution from 1 to 50 micrometers in diameter. Enlarged SEM image (Fig. 2d) further reveals that the wall of the bubbles is made up of ultrathin nanosheets, and every three to four bubbles are interconnected by a strut (denoted by red arrow). One can also notice the presence of large areas of wrinkle-like structures on the nanosheets (denoted by black arrows), a typical phenomenon associated with large-sized graphene, which helps to further increase the surface area of the sample. 44 Such structure of monolithic graphene is analogous to the 3D Voronoi structure (which is frequently seen in soap bubbles and styrofoam) 45,46 and provides a number of advantages such as excellent mechanical stability, high surface area, and effective avoidance of the graphene restack.
Our results also show that the graphene walls are decorated with nanosized particulates in the SEM images, most likely the result of the MoS 2 content. Transmission electron microscope (TEM) images in Fig. 2e and f verify the nanoparticles are quasihexagonal nanoakes (in consistent with our previous in situ experiment 22 ), with a lateral size of 50-100 nm and thickness of 5-10 nm. As expected, the selected area electron diffraction (SAED) pattern in Fig. 2g can be readily assigned to hexagonal MoS 2 structure (JCPDS no. 37-1492) while the bright diffraction rings reect high crystallinity (also refer to Fig. S2, † the energy dispersive spectra (EDS) from the nanoakes region). The diffraction rings in the SAED pattern correspond to a polycrystalline character, a result of the cumulative signals from many nanoakes across the selected area aperture (Fig. 2e) although the high-resolution TEM (HRTEM) image in Fig. 2h shows that each MoS 2 nanoake is clearly a single crystal. The intimate contact between MoS 2 nanoakes and graphene nanosheets is also evident from the TEM images ( Fig. 2e-i), a favorable condition for enhancing the electroactivity of the MoS 2 @G hybrid. 26 Interestingly, despite the validation of the inplane d-spacing of MoS 2 (0.26 nm for (100) planes) and interlayer distance of graphene (0.34 nm for (002) planes) as shown Paper in the HRTEM images ( Fig. 2h and i), a slightly expanded interlayer distance of MoS 2 ($0.65 nm, 0.62 nm for natural MoS 2 ) can be identied throughout repeated observations. It should be noted that MoS 2 structure with expanded interlayers is commonly considered to be highly benecial to improve its electrochemical performance (discharge capacity, reaction kinetics, etc.) for battery applications. 20,27 In short, the above results consistently showed that the sample from our one-step self-bubbling approach was MoS 2 nanoakes distributed in macroporous few-layered graphene, in accordance with our original design. For comparison purpose, we also prepared pure MoS 2 samples via the same process without glucose and the nal product is mainly irregular microsized akes (see Fig. S3, † the TEM images of pure MoS 2 sample).
Subsequently, spectroscopic characterizations were employed to further explore the microstructural features of the MoS 2 @G hybrid. Concerning the chemical states of Mo and S in the product, Fig. 3a shows the X-ray photoelectron spectroscopy (XPS) survey scans for MoS 2 and MoS 2 @G hybrid, with their C 1s peak referenced at 284.8 eV. The presence of MoS 2 with Mo and S elemental ratio of $1 : 2 can be identied for both samples, besides the prominent carbon component in the MoS 2 @G hybrid. The insets in Fig. 3a show the high-resolution spectra of MoS 2 @G hybrid, which are the S 2p, Mo 3d and C 1s regions. The Mo 3d possesses two peaks centered at 229.6 and 232.8 eV, in association with the doublet Mo 3d 5/2 and Mo 3d 3/2 for Mo 4+ ions. Another group of peaks, ascribed to the S 2p 3/2 and S 2p 1/2 orbital of divalent sulde ions (S 2À ), are observed at 162.4 and 163.7 eV, respectively. All these results are well consistent with the reported values for MoS 2 . 25,47 The existence of Mo, S and C in the MoS 2 @G hybrid was also veried by the electron energy-loss spectrum (EELS), as shown in Fig. 3b by the S L-edge, Mo Medge and C K-edge. Particularly, the core-loss C K-edge EELS spectrum of the MoS 2 @G hybrid (inset in Fig. 3b) presents a sharp p* peak ($284 eV, due to the excitation from 1s spin level to empty p* orbits of the sp 2 -bonded atoms) as well as a clear s* step ($289 eV, resulting from the transition from the 1s level to empty s* orbits at both sp 2 and sp 3 -bonded atoms), suggesting the high crystalline nature of graphene in the hybrid. 30,48 The crystal structure of the samples was then studied by Xray diffraction (XRD). As shown in Fig. 3c, the XRD patterns for both MoS 2 and MoS 2 @G hybrid match with 2H molybdenite; however, the diffraction peaks of the MoS 2 @G hybrid are much boarder than those of pure MoS 2 , a result of the ne MoS 2 crystalline size. Notably, the peak of (002) planes for the MoS 2 @G hybrid slightly shis towards the direction of low scattering angle, corroborating the expanded interlayer distance as revealed by the above TEM results. The diffraction signal for graphene is not visible due to the intense peaks from MoS 2 crystals, so Raman measurement was applied. As can be seen from the inset of Fig. 3d, the Raman spectrum for MoS 2 @G hybrid exhibits two sharp bands at 1360 (D band, in-plane vibration of sp 3 -bonded carbon) and 1600 cm À1 (G band, vibration mode of sp 2 -bonded carbon), as well as two broad bands at 2690 (2D band) and 2920 cm À1 (D + G band), 49 which directly proved the existence of well-crystallized few-layered graphene structure in the hybrid. The hexagonal layered structure of the MoS 2 in the hybrid was further conrmed by Raman spectrum with two peaks located at 383 and 407 cm À1 (Fig. 3d), which are typical E 1 2g and A 1g modes due to in-plane vibrations within the sulfur-molybdenum-sulfur layers, respectively. 50 Consequently, the carbon content in the MoS 2 @G hybrid was measured by annealing the sample in synthetic air upon TG/ DSC test. Assuming the complete formation of MoO 3 , SO 2 and CO 2 , 51 the graphene content is estimated to be 27.8 wt%, corresponding to the MoS 2 content of 72.2 wt% (Fig. 3e).
To further characterize the composite structure, the specic surface area and porous nature of the MoS 2 @G hybrid was quantied by Brunauer-Emmett-Teller (BET) method. Results from the full nitrogen adsorption and desorption isotherms (Fig. 3f) present typical type-IV characteristics with type-H4 hysteresis loop at a relative pressure above 0.5, indicating a nanoporous structure. Accordingly, the surface area of the MoS 2 @G hybrid is as high as 196.93 m 2 g À1 ; in contrast, the surface area of the MoS 2 sample is 6.22 m 2 g À1 (Fig. S4 †). It is also worth noting that the pore sizes of the MoS 2 @G hybrid, derived via the Barrett-Joyner-Halenda (BJH) method, are mainly distributed in the region of mesopores to macropores with a peak centered at 3.87 nm (inset in Fig. 3f). The high surface area of the MoS 2 @G hybrid together with the ample pores would be extremely favorable for energy storage applications such as batteries. The porosity in the Voronoi-structured framework can act as efficient electrolyte reservoirs to enlarge the contact areas between electrolyte and the active materials, and increase the active sites for sodiation/desodiation. Meanwhile, the porosity can also buffer the volume change to avoid structural pulverization during repeated charge/discharge cycles.
The above structural characterizations of our MoS 2 @G hybrid suggest the high potential of applying the product as electrode materials in NIBs. To verify this, systematic electrochemical measurements were performed with CR2032 coin cells. Fig. 4a shows the cyclic voltammograms (CVs) of MoS 2 @G hybrid during the initial ve cycles in the potential range of 0.01-3 V versus Na + /Na. In the rst cathodic scan (sodiation), the rst peak at 1.20 V can be ascribed to the intercalation of sodium ions into MoS 2 interlayer (refer to Fig. S5 † for the isolated rst scan). 24 The following two reduction peaks at 0.66 and 0.56 V are attributed to the two-step insertion of Na + into MoS 2 . 26,52 The fourth subtle peak located at $0.35 V is related to the conversion reaction from MoS 2 to Mo and Na 2 S. 18 The last sharp cathodic peak at 0.02 V is associated with the intercalation of Na + into the graphene interlayers. 53 In the subsequent anodic scan (desodiation), the peaks from 1.4 to 1.7 V should be attributed to the oxidation of Mo to MoS 2 . 27 In the subsequent cycles, the peaks at 0.66/0.56 V shi to 1.04/0.75 V with decreased intensity, corresponding to the progressive amorphization of MoS 2 @G hybrid. Notably, the CVs rapidly become overlapped in the later cycles, suggesting high reversibility for the electrode material. The stability of sodiation/desodiation processes was also conrmed by comparing the cycling performances of MoS 2 @G hybrid and MoS 2 under the galvanostatic mode at a current density of 0.1 A g À1 , as shown in Fig. 4b. First, both samples show initial capacity drops as well as low coulombic efficiencies (CE) at the rst cycle, which should be a result of the formation of solid electrolyte interface (SEI) lm. Second, the MoS 2 @G hybrid delivered a capacity as high as 484 mA h g À1 at the 2nd cycle and 474 mA h g À1 at the 100th cycle (corresponding to a small capacity decay of 0.02% per cycle). In contrast, the MoS 2 electrode delivered only a capacity of 268 mA h g À1 at the 2nd cycle and 97 mA h g À1 at the 50th cycle. Furthermore, the CE of the MoS 2 @G hybrid (>99%) is constantly higher than that of MoS 2 . Fig. 4c displays the galvanostatic discharge/charge voltage proles during the rst ve cycles of the MoS 2 @G hybrid at 0.05 A g À1 in the potential window of 0.01-3 V vs. Na + /Na. The distinct discharge/charge plateaus in the rst cycle are ascribed to the sample's high crystallinity. 23 The discharge and charge capacities for the rst cycle are 750 and 630 mA h g À1 , respectively, corresponding to a CE of 83.7% (in line with the low initial CE in Fig. 4b). In the subsequent cycles, the discharge and charge proles become identical, and no obvious discharge/charge plateau can be identied. These cells were then involved in the test of rate capability and the result is presented in Fig. 4d. The specic discharge capacities are 530, 475, 408, and 357 mA h g À1 at 0.05, 0.1, 0.25, and 0.5 A g À1 , respectively; i.e., when the current density is increased by ten times (from 0.05 to 0.5 A g À1 ), the electrode material can still retain $67% of its capacity (from 530 to 357 mA h g À1 ). Moreover, the MoS 2 @G hybrid is able to recover most of its original capacity when the current rate is restored back to 0.05 A g À1 aer forty deep cycles, indicating the high stability of the MoS 2 @G anode even upon high rate cycling (also refer to Fig. S6-S8 † for the CV proles, discharge/charge curves, and rate performance of the MoS 2 sample, as well as Fig. S9 † for the cycling performance of graphene). Therefore, another cycling test was conducted under the galvanostatic mode at a higher current density (0.5 A g À1 , Fig. 4e). Aer 200 cycles, the MoS 2 @G anode remarkably preserves its sodium storage capacity at as high as 371 mA h g À1 whereas the MoS 2 anode almost loses its electrochemical activity (nal capacity of 31 mA h g À1 ). It is worth noting that, during these repeated discharge/charge cycles, the volume change induced pulverization of the MoS 2 @G hybrid electrode has been signicantly suppressed, as reected by the constantly high CE (>99%) throughout the measurement and the comparative postmortem TEM study. As shown in Fig. 5, the postmortem TEM study of MoS 2 @G hybrid indicates the existence of small MoS 2 akes rmly decorated on graphene sheets. The MoS 2 content remains highly crystallized except the irregular outlines aer such long cycling. Moreover, the uniform distribution of MoS 2 on graphene sheets was also successful maintained. In contrast, the MoS 2 sample shows considerable cracks across the akes throughout the cycling (Fig. S10 †).
The above electrochemical characterizations substantiate the fact that our MoS 2 @G hybrid produced by the novel "selfbubbling" approach possesses high reversible capacity, excellent rate capability, as well as superior cycling stability as anode material in NIBs. Such impressive performance, to the best of our knowledge, is among one of the best values for MoS 2 -based anode materials for NIBs (refer to Table S1, † the comparison of electrochemical performances with selected MoS 2 -based anode materials for NIBs). We believe that the unique microstructural features of the MoS 2 @G hybrid itself have brought multiple advantages to act as high-performance electrode materials. First, the thin MoS 2 nanoakes (5-10 nm in thickness) with expanded interlayer spacing (0.65 nm) greatly reduces the strain caused by insertion and extraction of Na + . Second, the high surface area of the MoS 2 @G hybrid (196.93 m 2 g À1 ) provides a large electrode-electrolyte interface, in facilitating the critical charge-transfer process. Third, the monolithic graphene ensures a 3D porous and exible scaffold to protect MoS 2 from dissolution or detachment during repeated cycles, as well as 3D interconnected pathways for electron transport and Na-ion diffusion. Finally, the homogeneous distribution of MoS 2 nanoakes on graphene nanosheets together with their intimate contact guarantees good conductivity (for both Na + and electrons), and hence a high level of electrochemical activity and effective material utilization of the MoS 2 (as shown in the inset of Fig. 4b).

Conclusions
In summary, we have developed a simple and scalable "selfbubbling" approach for synthesizing an advanced MoS 2 @G hybrid composed of MoS 2 nanoakes entrapped in 3D macroporous monolithic graphene frameworks. Beneting from the unique microstructural characteristics, the MoS 2 @G hybrid has shown outstanding electrochemical performance including remarkable cycling stability and high rate capability upon used as anode material for NIBs. On account of the promising structural tunability of the MoS 2 @G hybrid, we also believe that the product might possess great potential in other application areas such as supercapacitors, catalysts, and sensors. Our onestep method may be also applicable in constructing other emerging graphene-based composites, and should therefore inspire further attempts to additional application scenes in future.

Synthesis of MoS 2 @G hybrid
The MoS 2 @G hybrid was synthesized from a novel "selfbubbling" approach. A mixed powder of (NH 4 ) 2 MoS 4 and glucose (weight ratio of 1 : 1.5) was directly subjected to thermal treatment in a horizontal furnace. As shown in Fig. 1b, the temperature was rst ramped from room temperature to 200 C in 40 minutes and this temperature was kept for 60 minutes. The temperature was then further increased to 1100 C in 180 minutes and hold for another 180 minutes. The product was harvested aer cooling the system to room temperature in 110 minutes. The whole annealing process was protected by a constant argon ow.

Material characterizations
The samples were characterized by different analytical techniques. Simultaneous TG/DSC analysis was performed on a NETZSCH STA 449 C Jupiter system. Optical image was captured on a Nikon Microphot-FXA microscope. SEM observations were made on a JEOL JSM-6700F eld-emission SEM. TEM images, SAED pattern, EELS, and EDS were obtained on a JEOL JEM-2100F STEM (200 kV, eld-emission gun) system equipped with an Oxford INCA x-sight EDS and an ENFINA 1000 EELS. XPS spectra were acquired on a Thermo Scientic Escalab 250Xi spectrometer. XRD measurement was conducted using a Rigaku SmartLab Intelligent X-ray diffraction system with ltered Cu K a radiation (l ¼ 1.5406Å, operating at 45 kV and 200 mA). Raman measurement was taken using a Horiba Jobin Yvon LabRAM HR system with a laser wavelength of 488 nm. The nitrogen adsorption and desorption isotherms were obtained at 77 K with a Micromeritics ASAP 2020 volumetric adsorption analyzer.

Electrochemical measurements
The working electrode slurry was prepared by mixing the active materials with Super P and carboxymethyl cellulose binder at a mass ratio of 8 : 1 : 1. The slurry was then spread on the surface of a copper foil and dried at 60 C for 12 h. Finally, the electrode was stamped into disks with a diameter of 10 mm and vacuum-dried at 60 C for another 6 h. With sodium tablets as the reference electrode and glass ber membrane as the separator, CR2032 coin cells were assembled in a glove box (MIKROUNA-Universal-2440-1750) lled with argon. 1 M NaClO 4 in the mixed solvent of ethylene carbonate/dimethyl carbonate (1 : 1 v/v ratio) with 5 wt% uoroethylene carbonate as additive was selected as the electrolyte for the coin cells. CV measurement was conducted on an electrochemical measurement system (PARSTAT 2273) with a scan rate of 0.2 mV s À1 from 3 to 0.01 V. Galvanostatic charge/discharge tests were performed by using a battery testing system (LAND CT2001A) within the potential of 0.01-3 V at room temperature. The capacities are given with respect to the total mass of the active materials throughout the work.