Direct Studies on the Lithium-Storage Mechanism of Molybdenum Disulfide

Transition metal sulfides are regarded as a type of high-performance anode materials for lithium ion batteries (LIBs). However, their electrochemical process and lithium-storage mechanism are complicated and remain controversial. This work is intended to give the direct observation on the electrochemical behavior and find out the lithium-storage mechanism of molybdenum disulfide (MoS2) using in situ transmission electron microscopy (TEM). We find that single-crystalline MoS2 nanosheets convert to Mo nanograins (~2 nm) embedded in Li2S matrix after the first full lithiation. After the delithiation, the Mo nanograins and Li2S transform to a large number of lamellar MoS2 nanocrystals. The discharge-charge cycling of MoS2 in LIBs is found to be a fully reversible conversion between MoS2 and Mo/Li2S rather than the electrochemical conversion between S and Li2S proposed by many researchers. The in situ real-time characterization results give direct evidence and profound insights into the lithium-storage mechanism of MoS2 as anode in LIBs.

couple in a deeply discharged MoS 2 /Li cell. Thus, the electrochemical reaction between MoS 2 and Li needs thorough enlightenment owing to these controversial results.
Considerable understandings of MoS 2 were achieved by examining the electrode materials after charge or discharge by disassembling the conventional LIBs (i.e. ex situ); no report provided direct reaction behavior of MoS 2 electrode during the discharge-charge cycling to analyze the lithium-storage mechanism. In situ TEM is an advanced technique that allows specific nanoscale site on the materials to be monitored in real-time [29][30][31][32][33][34][35][36][37] . The lithium intercalation into MoS 2 nanosheet has been reported by Bai's group through in situ TEM approach 36 , and they demonstrated a phase conversion from pristine 2H-MoS 2 to 1T-LiMoS 2 . Their finding confirms the initial Li + intercalation mechanism into MoS 2 . The controversial conversion mechanism involving the deep discharge of the battery (at approximate 0 V vs. Li/Li + ) is not resolved. Particularly, the microstructural evolution of MoS 2 electrode during continuous discharge-charge cycling remains unknown.
In this work, we investigated the lithium intercalation and conversion process of lamellar MoS 2 nanosheets in deep cycle (like the battery operation between 0-3 V) by in situ TEM, and realized a real-time observation of the electrochemical lithiation and delithiation process that greatly advances the understanding of the intercalation and conversion mechanism of MoS 2 in LIBs. The experimental results, for the first time, thoroughly demonstrate the structural transition of MoS 2 is reversible in the full discharge-charge cycling. Our findings shed light on the electrochemical reaction of MoS 2 with lithium and benefit for future material design and applications in energy conversion and storage devices.

Results
The morphology and microstructure of the synthesized MoS 2 /graphene were analyzed by SEM and TEM. Figure 1(a) presents a SEM image of the obtained MoS 2 /graphene used in this study; the MoS 2 /graphene is composed of many uniform MoS 2 microspheres with sizes of 1-2 μm. Figure 1(b) clearly reveals the microstructures of the MoS 2 microspheres anchored on graphene nanosheet. It can be seen that the hierarchical flower-like microspheres are regularly composed of numerous thin nanosheets. Figure 1 We prepared an all-solid nanoscale LIB inside the TEM that enabled the in situ electrochemical experiments of MoS 2 /graphene. Graphene nanosheets in the composite were used as substrate to support the MoS 2 nanostructures so that the in situ observation could be easily performed. The nanosized electrochemical cell consisted of three parts: MoS 2 /graphene working electrode, Li metal counter electrode, and naturally grown Li 2 O on Li metal surface used as solid electrolyte. The TEM images of Li/Li 2 O electrode are shown in Fig. S1 in Supporting Information. ED pattern and elemental mapping analysis confirm the existence of Li 2 O layer on Li metal. So the MoS 2 /graphene electrode is not directly contacted with lithium metals; the observed reaction is an electrochemical process rather than electrical short-circuit. Subsequently, the electrochemical conversion reaction of MoS 2 nanosheets during the first lithiation process was investigated. Figure 2 shows a series of time-resolved TEM images of a MoS 2 nanosheet during the first lithiation, which is a screenshot from Movie S1 in Supporting Information. The marked interplanar spacing of 0.61 nm corresponds to the (002) lattice plane of the hexagonal MoS 2 phase. Figure 2(a) was used as the starting point to monitor the progress of the lithiation reaction. Three stages in the first lithiation process can be discerned from Movie S1 in the Supporting Information. The first stage, from 0 to 14.0 s, indicates the initial Li + intercalating reaction with MoS 2 . We can see the slight variation of the orientation of lattice fringes in the TEM images and Movie S1 due to the strain induced by the lithium intercalation, leading to the formation of Li x MoS 2 phase via 2H-1T transformation 36 . The lattice fringes of 0.63 nm in Fig. 2 Figure 3(a) is a TEM image of a pristine MoS 2 microsphere; the initial size of the marked MoS 2 nanosheet was 133 nm. After the first full lithiation, the size of MoS 2 nanosheet increased to 145 nm as shown in Fig. 3(b), and the corresponding size expansion was ~8.8%. The TEM images of an individual MoS 2 nanoflake after the first lithiation are shown in Fig. 3(c,d). The surface and edges of the lithiated MoS 2 nanoflake were coated by a uniform layer of crystallite with a thickness of 8-9 nm, which was identified to be Li 2 S from the ED pattern in Fig. 3(f).   Fig. 3e), which is agreed with the lattice spacing of (110) plane of Mo (JCPDF No. 89-5156). The fringe spacing of the matrix was measured to be 3.3 Å, corresponding to the (111) plane of Li 2 S (77-2145). The dense Mo nanograins form an interconnected network that works as an efficient conductive pathway for electron transport into the MoS 2 , while the Li 2 S provides a similar pathway for Li ions during electrochemical reaction. Since Li 2 S has a much smaller density (1.66 g/cm 3 ) than that of metal Mo (10.28 g/cm 3 ), the volumetric expansion of MoS 2 electrode during the lithiation process is mainly caused by the formation of Li 2 S with lower density. Li 2 S is soft and the resulted Li 2 S in the nanoflakes could be partially squeezed out of the Mo network/framework due to volumetric expansion. So Li 2 S shells with a thickness of 8-10 nm were formed around the fully lithiated nanoflakes. Figure 3(f) is the ED pattern of the fully lithiated MoS 2 electrode. All the diffraction rings are well indexed into Mo and Li 2 S. The results demonstrate the lithiation reaction involves the reduction of MoS 2 to Mo nanograins accompanying with the formation of Li 2 S. The whole lithiation reaction can be expressed as the following equation: After the first lithiation process, a potential of +3.0 V was applied to the lithiated MoS 2 electrode to initiate the delithiation reaction. The time-resolved TEM images of the electrode during the delithiation are shown in Fig. 4. Figure 4(a) is a TEM image of the lithiated MoS 2 electrode, in which numerous Mo nanograins (black nanodots) around 2 nm are embedded in the Li 2 S matrix. After the delithiation reaction for 383 s (Fig. 4b), an obvious change could be observed in the nanosheet electrode in comparison to the image in Fig. 4a. The Mo nanograins became smaller due to the electrochemical reaction with Li 2 S during delithiation. As the delithiation reaction proceeded, black nanodots involving Mo nanograins almost disappeared, and the electrode turned to an approximately amorphous texture after 925 s (Fig. 4c). Obvious formation of lattice fringes in the electrode was observed at 2475 s during the delithiation process (Fig. 4d). After reaction for 3240 s (Fig. 4f), we found the perfect lattice fringes were formed, indicating the completion of delithiation reaction.
The delithiated MoS 2 electrode was analyzed using TEM. Figure 5(a) is a TEM image of the delithiated MoS 2 electrode. It can be clearly seen that the thin Li 2 S shell on lithiated nanosheets was disappeared. The magnified TEM image of the delithiated MoS 2 nanosheet is displayed in Fig. 5(b), suggesting that a large number of lamellar nanocrystals with a size of 3-5 nm formed during the delithiation process, as marked by the red circles. Figure 5(c) is a HRTEM image of the delithiated MoS 2 nanosheet; the interplanar distance of nanograins was measured to be 6.1 Å, which can be indexed as the (002) lattice planes of the hexagonal MoS 2 phase (JCPDF No. 87-2416). The ED pattern of the delithiated nanosheet is shown in Fig. 5(d); all the diffraction rings can be indexed as the hexagonal MoS 2 phase (JCPDF No. 87-2416). The HRTEM and ED results reveal that metallic Mo nanograins convert to MoS 2 nanograins in the delithiation process. Here molybdenum oxide was not observed during the delithiation process although Li 2 O electrolyte was contacted with the electrode. The electrochemical reaction in the delithiation process can be expressed as Mo + 2Li 2 S → MoS 2 + 4Li + + 4e − .
MoS 2 materials showed excellent cyclability during the charge-discharge cycles in LIBs [12][13][14] . In situ TEM experiments were performed to reveal the microstructure and phase evolution induced by the continual lithiation-delithiation in the discharge-charge cycles. Figure 6(a) is a TEM image of the pristine MoS 2 electrode, and the insets is a HRTEM image of the MoS 2 nanosheet. The marked interplanar spacing of 2.7 Å corresponds to the (100) and (1 10) lattice planes of the hexagonal MoS 2 phase, respectively. The ED pattern of the pristine MoS 2 nanosheet is displayed in Fig. 6(a 1 ). It can be indexed as (100), (010) and (1 10)   numerous Mo nanograins (~2 nm) were formed and embedded in Li 2 S matrix during the lithiation reaction. The corresponding ED pattern of the lithiated MoS 2 electrode is shown in Fig. 6(b 1 ). The diffraction rings can be well indexed to Mo and Li 2 S, suggesting the conversion of MoS 2 to Mo and Li 2 S in the first lithiation process. The potential was reversed to +3 V to initiate the delithiation process after the completed lithiation. Figure 6(c) is the TEM image of the delithiated MoS 2 electrode. It shows that the thin Li 2 S shell disappeared. The Mo nanograins transformed into a large number of MoS 2 nanograins with sizes of 3-5 nm during the delithiation reaction, as marked by the red circles. Figure 6(c 1 ) displays the ED pattern of the delithiated MoS 2 electrode; the diffraction rings all originate from lattice planes of the hexagonal MoS 2 phase (JCPDF No. 87-2416). The second lithiation process proceeded with the potential of −1.0 V again as shown in Fig. 6(d). Movie S2 in the Supporting Information also reveals the dynamic conversion of the electrode material in the second lithiation process. The Li 2 S layer formed and continued to cover the electrode like the first lithiation. The ED pattern of the secondly lithiated electrode is shown in Fig. 6(d 1 ) and can be indexed as Mo and Li 2 S. The Li 2 S layer and Mo nanograins disappeared again in the second delithiation process (Fig. 6e), showing a reversible phase change. The ED pattern in Fig. 6(e 1 ) indicated the resultant Mo/Li 2 S converted to MoS 2 nanograins again in the second delithiation process. The two cycles indicate that a reversible electrochemical conversion between MoS 2 and Mo/Li 2 S takes place in the whole cycling, and the electrochemical reaction in the discharge-charge cycling can be expressed as The electrochemical performances of MoS 2 /graphene electrode in coin cells were tested. As shown in Fig. S2, the initial discharge and charge capacities are 2018 mAh g −1 and 1609 mAh g −1 , respectively. After 100 cycles, the reversible capacity of the MoS 2 /graphene electrode remains 1002 mAh g −1 , demonstrating a high lithium-storage capability and excellent cycling performance. The reversible capacity is higher than the theoretic capacity of MoS 2 , which can be attributed to the synergistic effects of graphene and the reversible formation/decomposition of SEI membrane.
The in situ TEM results were further supported by ex situ TEM results, as displayed in Fig. S3 in Supporting Information. The TEM image of the MoS 2 /graphene electrode after 50 discharge-charge cycles shows that numerous ultrafine nanograins were formed (Fig. S3(a)). The stripe-width of the crystallites is 2.2 Å, which corresponds to the interplanar spacing of Mo (110) plane ( Fig. S3(b)). The HRTEM analysis confirms the formation of Mo nanograins after discharge. Meanwhile, Fig. R3(c) and (d) indicate that the MoS 2 phase was obtained after charge. The ex situ TEM results are well agreed with our in situ TEM results, and confirms the same conversion mechanism revealed by in situ TEM. The findings clarified the lithium-storage mechanism of MoS 2 involving the reversible phase transformation between Mo nanograins and MoS 2 nanograins along with the reversible growth/ decomposition of Li 2 S during cycling. The reactions are analogous to the electrochemical conversion of CoS 2 38 , and most transition-metal oxides 39 . Thus, the electrochemically active couple in MoS 2 electrode is not S/Li 2 S but MoS 2 and Mo/Li 2 S in a deeply discharged/charged MoS 2 /Li system.
Our in situ and ex situ TEM results reveal a reversible phase conversion during the lithitation-delithiaton processes of MoS 2 . However, the typical CV curves of MoS 2 in literatures generally show distinct 1st and subsequent discharge processes [40][41][42][43] . The difference in the CV curves may be caused by the irreversible processes such as the decomposition of electrolyte in the first discharge 40 , the formation of a solid-electrolyte interface (SEI) layer resulting from the continual reaction of freshly exposed surfaces of electrode materials with the organic electrolyte 41 , and particularly the structural rearrangement revealed by our TEM observation in the first discharge-charge process from micro-sized MoS 2 nanosheets to numerous MoS 2 nanograins of 3-5 nm, which possess distinct quantum and size effects. Besides, the MoS 2 nanostructures prepared by the hydrothermal or solution method contain many defect sites. A fraction of lithium ions can be trapped in the defect sites and hardly extracted in the following cycles 42 . The residual oxygen-containing groups in graphene for MoS 2 /graphene composite are also electrochemical reduced in the first discharge 43 . All these irreversible processes can cause different reaction thermodynamics between the 1st and subsequent discharge. The underlying mechanism still needs thorough investigation.

Conclusions
In summary, we have conducted a systematical study of the structural evolution of MoS 2 nanosheets during the lithiation-delithiation cycling using in situ TEM technique by constructing a nano-LIB device inside a TEM. In our experiments, the details of Li-ion intercalation and extraction-induced solid-state phase transformation in MoS 2 are clearly understood. The results demonstrate that single-crystalline MoS 2 nanosheets transform to multicrystalline nanosheets consisting of many Mo nanograins embedded in Li 2 S matrix during the first lithiation. Generally, a uniform layer of Li 2 S with thickness of 8-9 nm can be observed on the lithiated electrode. During the delithiation process, Mo nanograins and Li 2 S layer convert to a large number of lamellar MoS 2 nanograins with sizes of 3-5 nm. The charge-discharge processes of MoS 2 in LIBs are not the controversial electrochemical conversion between S and Li 2 S but a fully reversible phase conversion between MoS 2 nanograins and Mo nanograins along with the formation/decomposition of Li 2 S. Based on our in situ TEM results, the electrochemical conversion mechanism of MoS 2 in LIBs is: MoS 2 + 4Li + + 4e − ↔ Mo + 2Li 2 S. The in situ real-time characterization results provide direct evidence and achieve a profound understanding of the lithium-storage mechanism in MoS 2 .

Methods
Materials synthesis. Graphene Oxide (GO) was prepared by oxidization of natural graphite flakes using a modified Hummers method 44 . For the synthesis of MoS 2 /graphene, GO (0.06 g) was added into 20 mL of deionized water with sonication for 1 h to form a homogeneous dispersion. Then 0.3 g of Na 2 MoO 4 ·2H 2 O, 0.4 g of thiourea, and 0.15 g of PEG-20000 were added. After ultrasonication and stirring for 30 min, the solution was then transferred into a Teflon-lined stainless steel autoclave, sealed tightly, and heated at 200 °C for 24 h. The black products were collected by centrifugation, washed with deionized water and ethanol, and dried in a vacuum oven at 60 °C for 24 h. The obtained composites were annealed in a tube furnace at 800 °C for 2 h in a stream of hydrogen (20 sccm) and argon (180 sccm).

In situ electrochemical experiments inside TEM. The in situ nanoscale electrochemical experiments
were performed using a Nanofactory STM-TEM holder inside a TEM (JEOL JEM-2100F). MoS 2 /graphene samples were attached to a gold wire and served as the working electrode; a lithium particle on the tip of a W rod was served as the counter electrode and lithium source. Both the MoS 2 /graphene and lithium electrodes were loaded onto a Nanofactory STM-TEM holder inside a glovebox and sealed in an argon-filled bag. Then the holder was transferred into the TEM column quickly. A thin Li 2 O layer was formed on the surface of the metallic lithium particle due to the exposure to air during the transfer process and was served as the solid electrolyte. Once the Li 2 O-covered Li electrode driven by the mobile STM probe contacted the free end of the selected lamellar MoS 2 /graphene, a negative bias (−1.0 V) was applied to drive Li + transport through the solid-state Li 2 O layer to facilitate the lithiation. The current of about tens or hundreds nA passed through the sample during the reaction (dependent on the size of the selected sample). Then a bias was reversed to be positive (+3.0 V) to initiate the delithiation process. To minimize the electron beam irradiation effect on the microstructure and the electrochemical behavior, the experiments were carried out with the electron beam blanked except for imaging. During the image acquisition, the total dose of electron beam was reduced with sacrifice in resolution and contrast. In some cases, the beam radiation was performed in a sacrificial region and the TEM measurements were taken on a nearby region that had not been previously exposed to the electron beam.