Pseudocapacitive Behaviour and Ultra-fast Kinetics from Solvated Ion

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Pseudocapacitive Behaviour and Ultra-fast Kinetics from Solvated Ion Co-intercalation into MoS2 for its Quick Alkali Ion Charging Kai Zhang, Gabin Yoon, Jing Zhang, Mihui Park, Junghoon Yang, Kisuk Kang, and Yong-Mook Kang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00445 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 5, 2019

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ACS Applied Energy Materials

Pseudocapacitive Behaviour and Ultra-fast Kinetics from Solvated Ion Co-intercalation into MoS2 for its Alkali Ion Storage Kai Zhang,† Gabin Yoon,‡ Jing Zhang,† Mihui Park,† Junghoon Yang,† Kisuk Kang,‡ and Yong-Mook Kang*,† †Department

of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620, Republic of Korea of Materials Science and Engineering Research Institute of Advanced Materials, Seoul National University Seoul 08826, Republic of Korea ‡Department

KEYWORDS: MoS2, solvated ion co-intercalation, fast kinetics, anode material, Li-ion batteries

ABSTRACT: The popularization of electric vehicles and the increasing use of electronic devices highlight the importance of fast charging technology. The charging process of lithium secondary battery is basically limited by a series of processes on the anode side, which include desolvation of lithium ions as well as lithium diffusion through SEI and the anode material. These series of reactions are kinetically sluggish, leading to insufficient power density. Therefore, in order to unravel this problem, we need to either accelerate each step or skip over some of steps to make the whole charging process shorter. A solvated ion co-intercalation into graphite has turned out to successfully exclude both desolvation of lithium ions and SEI film formation to achieve high kinetics with graphite. Herein, the solvated ion co-intercalation into MoS2 demonstrated that it can help to remove desolvation of alkali ions as well as SEI formation and thereby ultra-high kinetics and long-term cyclability is attained by the characteristic pseudocapacitive behaviour irrespective of the charge/discharge mechanism of anode materials. This phenomenon occurred between 1 and 3 V with MoS2 anode in a novel electrolyte (i.e., 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane/tetraglyme (DME/TGM, v/v = 3:1 by volume)). In details, its capacity retentions slightly decreased from 95.9% to 91.8%, 89.7%, 87.7%, 84.8%, 77.0%, 67.9%, and 55.1% as current densities increased from 0.1 to 0.2, 0.5, 1, 2, 5, 10, and 20 A g−1, respectively. Meanwhile, it delivered a capacity retention of 90.6% even after 2000 cycles at 1 A g−1. Interestingly, MoS2 with solvated ion co-intercalation can display much higher capacity (273.5 mAh g−1) than that of graphite (less than 120 mAh g−1). As a result of the investigation on its reaction mechanism, the solvated ion co-intercalation was validated to occur during the discharge process in MoS2 anode, and thus induce pseudocapactive sodiation as well as exclude SEI film formation. This research may help to suggest an alternative way to enhance the kinetics of anode materials for high-power Alkali ion batteries.

1. Introduction Among various electrochemical energy storage systems, Liion batteries (LIBs) are regarded as one of the most efficient and important batteries for people’s daily life because of its merits such as high energy density, high working voltage, and long cycle life.1-10 However, commercial graphite anode shows an unsafe voltage plateau which is almost close to 0 V vs. Li+/Li. For the purpose of eliminating the related safety issue, some Tibased compounds were proposed such as Li4Ti5O12 and TiO2.1113 Unfortunately, these Ti-based compounds suffer from low conductivity and insufficient capacity (less than 200 mAh g−1). Thus, it is necessary and urgent to find another anode candidate with high safety, satisfied capacity, and fast kinetics. As well known, MoS2 is one of the most famous materials owing to its unique layered structure.14-25 Similarly to graphite, the adjacent layers in MoS2 are linked by weak van der Waals force and each layer consists of S-Mo-S sandwich structure with Mo-S covalent bonds. During the first discharge process, Li ions first insert into the interlayer of MoS2 and then bond with S ions to form Li2S. In other words, intercalation reaction (xLi + MoS2 → LixMoS2) first occurs and then is followed by the typical conversion reaction (LixMoS2 + (4−x)Li → Mo + 2Li2S).

Consequently, the theoretical specific capacity of MoS2 based on total four-electron transfer reaches 670 mAh g−1.26 Some researchers unveiled that MoS2 can be reversibly reformed during the following charge process,27 but this conversion reaction is clearly accompanied with large volume change and severe structural damage. Because it is difficult to procure the signal of MoS2 after the first cycle, many of other researches pointed out that only Li-S reaction (Li + S ↔ Li2S) takes place during the following charge/discharge.28 Therein, the insulating property of S and the shuttle effect of polysulfides inevitably lead to sluggish kinetics and poor cyclic stability.29-30 Furthermore, the discharging up to 0.01 V or 0.1 V, which may induce the growth of Li dendrite, can render the cells having MoS2 anode to be unstable as aforementioned. So, in order to not only avoid the critical conversion and Li-S reactions of MoS2 but also prevent the unstable dendrite formation, the modulation of voltage range looks essential to utilize the full potential of MoS2. This way, Kang’s group harnessed co-intercalation of [Lisolvent]+ ions into graphite to achieve rapid charge-transfer kinetics.31-32 Considering that solvated ion co-intercalation helps to skip over desolvation and solid electrolyte interface

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(SEI) film formation and the interaction between the intercalated solvated ions and graphite prevents the structural disruption of anode materials, this strategy deserves to be adopted for dichalcogenides anodes including MoS2 as well.3133 It is worth noting that the exfoliation of the graphite electrode would weaken the cycling performance; however, the exfoliation of the MoS2 electrode could effectively enhance the capacity with long life.34 In addition, the capacity of graphite with solvated ion co-intercalation is less than 120 mAh g−1, which is insufficient to meet the demands of anode materials. If the co-intercalation of solvated ions can work for MoS2 like the way for graphite, its capacity and reaction kinetics may be highly enhanced at the same time. Hence, herein, MoS2 was synthesized through a simple hydrothermal method, and its Li-storage properties were optimized and investigated to maximize its electrochemical properties. 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in dimethoxyethane/tetraglyme (DME/TGM, v/v = 3:1 by volume) and 1.0−3.0 V were chosen as the ideal electrolyte and voltage window, respectively. The optimized MoS2 with solvated ion co-intercalation delivered a capacity retention of 90.6% after 2000 cycles at 1 A g−1. Most importantly, the solvated ion co-intercalation also enabled fast kinetics to be realized. Actually, the capacity retentions based on the capacity at 0.05 A g−1 slightly decreased from 95.9% to 91.8%, 89.7%, 87.7%, 84.8%, 77.0%, 67.9%, and 55.1% as current densities increased from 0.1 to 0.2, 0.5, 1, 2, 5, 10, and 20 A g−1, respectively. The theoretical calculation investigation demonstrated that the solvated ion co-intercalation happened when introducing TGM solvent into electrolyte, and the [LiTGM]+ ions could effectively exfoliate the layer to increase the active sites and enhance the capacity. However, bare Li+ intercalation occurred when using traditional EC/DEC-based electrolytes, which faced to higher exfoliation energy compared with that of co-intercalation behaviours. Interestingly, the layered structure could be prevented in spite of enormous changes of lattice spacing and layer number during repeated [Li-TGM]+ co-intercalation. In addition, DME can remarkably accelerate the rate performance due to high pseudocapacitive contribution. In place, a partial pseudocapacitive behaviour was accompanied with the redox reaction resultantly facilitating fast reaction kinetics. 2. Experimental Section Material Synthesis: MoS2 was prepared by facile one-step hydrothermal route. In brief, 0.3 g NaMoO4∙2H2O (purity ≥ 99.0%, Sigma-Aldrich) and 0.4 g CS(NH2)2 (purity ≥ 98.0%, Junsei Chemical Co. Ltd.) were dissolved in 30 mL of deionic water to form transparent solution. Then, 1 mL concentrated HCl (35.0-37.0%, Samchun Chemical Co. Ltd.) was dropped into the above solution. After stirring for 0.5 h, the blue solution was transferred into a 50 mL of Teflon-lined stainless-steel autoclave and was heated at 180 °C for 24 h. After cooling down, the product was centrifuged and washed thoroughly with deionized water and absolute ethanol for several times. The black solid was dried in vacuum oven at 60 °C overnight and was then heated under argon atmosphere at 700 °C for 3 h. Material Characterization: Phase identification of the asprepared MoS2 was analyzed by powder X-ray diffractometer (XRD, Rigaku, D/Max-2500, Japan) with Cu Kα radiation at 40 kV and 30 mA. Morphology and microstructure of the asprepared MoS2 were observed by scanning electron microscopy

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(SEM, JSM-6700F field-emission, JEOL, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). The valences of Mo and S were investigated by X-ray photoelectron spectroscopy (XPS, Perkin Elmer PHI 1600 ESCA system, USA). Elemental weight percent was evaluated by inductively coupled plasma-optical emission spectroscopy (ICP-AES, OPTIMA 8300, Perkin-Elmer, USA). Raman spectra were performed with a Renishaw INVIA micro-Raman spectroscopy (UK). Fourier transform infrared spectroscopy (FT-IR) was carried out via using a Varian FTS 800 FTIR spectrometer (USA). Mo K-edge X-ray absorption near-edge structure (XANES) was collected in 10C beam line at Pohang Accelerating Laboratory (PAL) in Republic of Korea using a double Si (111) monochromator. All spectra were normalized to the main edge jump. Electrochemical Tests: The MoS2 electrode was prepared by mixing 80 wt% MoS2 powder, 10 wt% super P, and 10 wt% polyvinylidene fluoride (PVdF) in N-methyl pyrrolidone (NMP). After fully grinding, the as-obtained uniform slurry was casted onto Cu foil and dried at 110 °C for 12 h under vacuum. The mass loading was between 1.5 and 2.5 mg cm−2. Li foil disc with a diameter of 16 mm was harnessed as both reference and counter electrodes. The MoS2 electrode was cut into a disc with a diameter of 10 mm and was harnessed as the working electrode. Glass fiber filter paper with a diameter of 18 mm was used as a separator. Several electrolytes with different Li salts and solvents were selected. The Li salts include lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2, LiTFSI), lithium trifluoromethanesulfonate (LiCF3SO3), and lithium hexafluorophosphate (LiPF6). Ethylene carbonate/diethyl carbonate (EC/DEC, v/v = 1:1 by volume), dimethoxyethane (DME), diglyme (DGM), and tetraglyme (TGM) were used as the solvents. CR2032 coin half cells were assembled in an argon-filled glovebox. Galvanostatic discharge-charge tests, galvanostatic intermittent titration technique (GITT), and cyclic voltammetry (CV) were examined by using WBCS battery test system (WonATech). For GITT, the current density was set to 50 mA g−1. The time of charge or discharge process is 600 s, and the rest time is 3600 s. EIS was collected in a frequency range from 100 kHz to 100 mHz with a perturbation of plus or minus 5 mV (Iviumstat Electrochemical Interface). In order to observe the mass change during the discharge process, we first measured the mass of the pristine MoS2 electrode and then discharged it to SOD50% or SOD100%. After dissembled the cell, we measured the mass of the MoS2 electrode again and calculate the mass change. At each state, we choose five different samples and calculate the average value and corresponding error. The theoretical mass change (∆m) is calculated according to the following equations (1), (2): n(Li) = ∆m =

3.6𝑄𝑀𝑤(𝑀𝑜𝑆2)

𝐹 𝑚(𝑀𝑜𝑆2)𝑁𝐴𝑛(𝐿𝑖)𝑀𝑤(𝐿𝑖 𝑜𝑟 𝐿𝑖 ― 𝑠𝑜𝑙𝑣𝑒𝑛𝑡) 𝑀𝑤(𝑀𝑜𝑆2)

(1) (2)

where Q is the practical capacity, Mw(MoS2) is the molecular weight of MoS2, F is faraday constant, n(Li) is the transfer number of Li+ ions per MoS2 molecule, m(MoS2) is the mass of active materials, NA is the Avogadro’s number, and Mw(Li or Li-solvent) is the molecular weight of Li+ or [Li-solvent]+. Computational details: First-principles calculations using Vienna ab initio Simulation Package (VASP)35 were conducted to get information on the Li- and [Li-solvent]+-intercalated MoS2 structures. We used projector augmented wave


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pseudopotentials as implemented in VASP,36-37 and PerdewBurke-Ernzerhof parameterization of generalized gradient approximation was used for exchange-correlation functional.38 To better describe the interlayer interaction between MoS2 layers, we adopted DFT-D3 functional in all calculations 39. The validity of DFT-D3 functional was ensured by the similarity of the computed interlayer distance of 2H-MoS2 (6.05 Å) with the experimentally observed value (6.15 Å). We used a plane energy cutoff of 500 eV and all structure relaxations were performed until the remaining force in the system became less than 5 meV/Å. Exfoliation energies Eexf of the Li- and [Li-solvent]+intercalated MoS2 structures were calculated to see the structural stability upon charge and discharge, where Eexf is defined as: Eexf = Esys(d = ∞) − Esys(d = dε) (3) Here, Esys(d = dε) is an energy of pristine system and Esys(d = ∞) is an energy of a system where a single layer of MoS2 is isolated from pristine structure. Eexf was normalized by the number of MoS2 units for clear comparison. In this calculation, we adopted vacuum/slab geometries with a thickness of vacuum slab larger than 11 Å to ensure that slab layers do not interact each other.40 The thickness of Li- and [Li-solvent]+-intercalated MoS2 slabs were set to be sufficiently large so that the effect of the slab thickness on the exfoliation energy is negligible. 3. Results and discussion Figure 1a shows XRD pattern of the as-prepared product in a 2θ degree range of 10°−80°. All diffraction peaks are indexed to the hexagonal MoS2 with a standard JCPDS No. 24-513, which belongs to a space group of P63/mmc. The sharp peak at 14.2° corresponds to (002) crystal plane of the MoS2, and no impure phase is detected. Figure 1b exhibits XPS spectra in the S 2s and Mo 2d regions of the MoS2 in which the peaks at 225.9, 228.8, and 231.9 eV are assigned to S 2s, Mo 3d5/2, and Mo 3d3/2, respectively. In addition, there are two deconvolved peaks in Figure 1c, which correspond to S 2p1/2 and S 2p3/2, respectively. The XPS results reveal that the valences of Mo and S ions are +4 and −2, respectively. Figures 1d and 1e present SEM and TEM images of the as-prepared MoS2, and the MoS2 displays wrinkled nanoplate-assembled bulk morphology. Figure 1f depicts high-resolution TEM image of the as-prepared MoS2, and the graphene-like layered structure is observed. The layer spacing was measured by using Gatan DigitalMicrograph software, and the counted value is 0.64 nm as shown in inset of the Figure 1f. In order to optimize the electrochemical performance of the MoS2 electrode, two factors, namely, electrolyte and discharge terminal voltage are adjusted. Figure 2a shows the charge and discharge curves of the MoS2 when using same electrolyte (i.e., 1M LiTFSI in TGM) but different voltage windows. For a brief description, the MoS2 electrodes cycling in the voltage windows of 0.1−3.0 V and 1.0−3.0 V are named as MoS2-0.1V and MoS21.0V, respectively. Although the MoS2-0.1V delivers higher capacities in the first 30 cycles than those for MoS2-1.0V, it decays rapidly between 30 and 50 cycles. After 50 cycles, the discharge capacity of the MoS2-0.1V is only 93.1 mAh g−1, but the MoS2-1.0V still remains a discharge capacity of 179.7 mAh g−1. Typical carbonate-based electrolyte (i.e., 1M LiTFSI in EC/DEC) was investigated in different voltage ranges as shown in Figure S1. Noteworthily, the MoS2-1.0V shows better cycling performance than that of MoS2-0.1V no matter which

electrolyte is used, which is attributed to that only cointercalation reactions occur in the voltage range of 1.0−3.0 V. However, the Li-S reactions (S + Li ↔ Li2S) and the conversion reaction (MoS2 + 4Li ↔ Mo + 2Li2S) take place when cycling between 0.1 and 3.0 V (Figures S2 and S3). In addition, HRTEM images of the MoS2 electrodes discharged to 0.01 and 1.0 V were compared. As shown in Figure S4, the thin SEI film is obviously observed when discharged to 0.01 V, but there is no visible SEI film when discharged down to 1.0 V. 1.0−3.0 V is determined to be the appropriate voltage range according to the above cycling data. Then, the electrolytes were further optimized, and Figure 2b lists the molecular formula of the as-selected solvents. For the sake of discussion, the MoS2 electrodes with EC/DEC, DME, DGM, and TGM are labelled as MoS2-EC/DEC, MoS2-DME, MoS2-DGM, and MoS2-TGM, respectively. Figures 2c−2f present the discharge-charge curves of the as-prepared MoS2 with different electrolytes at a current density of 0.2 A g−1 in the 1st, 2nd, 40th, and 100th cycles. All samples deliver a long discharge plateaus at about 1.1 or 1.2 V in the first cycle and two discharge slopes in the voltage ranges of 2.5−1.5 V and 1.5−1.0 V in the second cycle. There are two charge slopes in the voltage ranges of 1.5−1.9 V and 2.0−2.5 V in the charge processes of the first two cycles. Notably, Li+ ions do not completely extract from the layered structure, and some of them are left in the interlayer according to their dischargecharge curve at the first cycle as well as ICP results (Figure S5). It may be the reason why their discharge-charge profiles gradually change during the initial cycles. After the initial 40 cycles, the four samples exhibit two kinds of discharge-charge curves. The carbonate-based and shortchain-like ether-based electrolytes (i.e., MoS2-EC/DEC and MoS2-DME) exhibit two couples of discharge-charge plateaus, but the long-chain-like ether-based electrolytes (i.e., MoS2DGM and MoS2-TGM) display only one couple of dischargecharge plateaus. The discharge-charge curves at the 100th cycle of the four samples are very similar to those at the 40th cycle, but the long-chain-like ether-based electrolytes deliver higher capacity at the 100th cycle compared with the carbonate-based and short-chain-like ether-based electrolytes. Although the charge and discharge profiles are quite different between DMEand EC/DEC-based electrolytes for graphite,32 the charge and discharge profiles for MoS2 with DME and DEC based solvent seem quite similar, which is probably attributed to that layer spacing of graphite (~0.34 nm) is much lower than that of MoS2. Figure 2g shows the middle voltage change of the four samples during the initial 100 cycles. The four samples also exhibit two different middle voltages. The carbonate-based and short-chain-like ether-based electrolytes show a middle voltage of 1.8 V, but the long-chain-like ether-based electrolytes display a relative low middle voltage of 1.6 V. Although large voltage hysteresis emerges in the initial cycles, the middlevoltage differences (∆E) between charge and discharge decrease to ~0.26 V for MoS2-DGM and MoS2-TGM after 10 cycles (Figure S6). During the following cycles, their ∆E values just slightly vary. Figure 2h compares the cycling performance of the four samples, and all samples possess no obvious capacity decay from the 2nd cycle. The carbonate-based and short-chain-like ether-based electrolytes deliver stable capacities from 10th cycle to 100th cycle, and their capacities maintain ~160 mAh g−1 after 100 cycles. However, the long-chain-like ether-based electrolytes display gradually increased capacities from the 10th



cycle, and their capacities reach ~190 mAh g−1 after 100 cycles. The effects of Li salts on the electrochemical performance of MoS2 were also estimated; however, the Li salts have slight effects on electrochemical performance compared with solvents (Figures S7 and S8). In addition, we tried to use TGM-based electrolyte for several cathode materials. As shown in Figure S9, LiCoO2, LiFePO4, and Li3V2(PO4)3 with the TGM-base electrolyte deliver similar charge-discharge curves and comparable capacities to those with the carbonate-based electrolyte (1M LiPF6 in EC/DEC). In order to gain an insight on the discharged structure of MoS2, we performed density functional theory calculations on the Li+- and [Li-solvent]+-intercalated MoS2 structures. Figure 3a describes the optimized structure of LiMoS2, which is a discharge product after one Li ion uptake per MoS2. Theoretically, reversible (de)intercalation of a single Li ion in MoS2 yields a gravimetric capacity of 167 mAh g−1. Upon Li intercalation, stacking order of MoS2 layers changes from 2H to 1T, and the interlayer distance increases from 6.05 Å to 6.36 Å. This value of interlayer distance well corresponds to Figures S10a and 3c, which is obtained in a system using EC/DECbased electrolyte. In addition, the theoretical capacity of LiMoS2 is also comparable to our electrochemical results (Figure 2h), both indicating that the Li intercalation behavior in EC/DEC solvent would be pure Li intercalation into MoS2, yielding 1T-LiMoS2. Contrary to the small increase in the interlayer distance in the case of pure Li ion intercalation, cointercalation of Li ion and bulky solvent molecule is expected to result in much larger interlayer expansion. Figure 3b depicts the structure of [Li-TGM]+-intercalated MoS2. It is shown that a single layer intercalation of [Li-TGM]+ in the interlayer space of MoS2 results in a large elongation of interlayer distance from 6.05 Å to ~10 Å, which matches to the observed value in TGM system (Figures S10b and 3d). Our structure calculations so far interpret that the Li ion intercalation behavior is dependent on the solvent species. EC/DEC system leads to the pure Li intercalation, whereas the use of linear ether solvents results in the co-intercalation of Li and solvent species. This observation is similar to the Li-graphite system, where the pure Li ion intercalation behavior with the carbonate solvents changes to the [Li-solvent]+ co-intercalation with linear ether solvents.32 In addition, HRTEM was also used to observe the surface of layer of MoS2-TGM at fully discharged and charged states, and there is no obvious SEI film around the layers (Figure S11). We also tested HRTEM of the MoS2-DME and MoS2-DGM samples after 100 cycles. As shown in Figure S12, the MoS2DGM show obvious layer exfoliation after 100 cycles, which is as same as MoS2-TGM; however, MoS2-DME maintain the layer stacking after 100 cycles like MoS2-EC/DEC. Thus, MoS2-EC/DEC and MoS2-DME show very similar dischargecharge profiles, but MoS2-DGM and MoS2-TGM exhibit another kind of discharge-charge curves. Besides, the layered structure can be maintained after 100 cycles, indicating that there is no Li-S reaction. Since the intercalation of Li+- or [Li-solvent]+ into MoS2 accompanies the expansion of MoS2 layers, the van der Waals interlayer binding which is inversely proportional to the interlayer distance could be weakened upon cycling, possibly leading to the exfoliation of MoS2 layers. In this respect, the exfoliation energies were calculated for Li+ and [Li-solvent]+intercalated MoS2 structures to investigate their structural stability. Although the Li+- intercalation results in the increase

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of interlayer distance (from 6.05 Å to 6.36 Å) which could decrease the interlayer van der Waals force, we found that the exfoliation energy of 1T-LiMoS2 (1106 meV) is much larger than 2H-MoS2 (277 meV). This is attributed to the increased interlayer binding because intercalated Li+ forms ionic bonds with adjacent MoS2 layers, as similarly observed in graphite system.40 On the other hand, the exfoliation energy of [LiTGM]+ intercalated MoS2 was 204 meV, which is smaller than pristine MoS2. The ionic interaction also exists in [Li-TGM]+intercalated MoS2, however, the degree of interlayer expansion upon the bulky [Li-TGM]+ is much large (from 6.05 Å to ~10 Å), leading to the significant decrease of interlayer binding energy. Besides using the HRTEM to detect the local layer spacing, XRD patterns and Raman spectra of both samples after 100 cycles were also performed to evaluate their layered structure. As shown in Figure S13a, the (002) peaks of MoS2-EC/DEC and MoS2-DME after 10 cycles slightly shift to low angle compared with the pristine MoS2 electrode, while the (002) peaks of MoS2-DGM and MoS2-TGM obviously shift to low angle compared with the pristine one. It indicates that layer spacings of MoS2-EC/DEC and MoS2-DME slightly increase, while the layer intervals of MoS2-DGM and MoS2-TGM markedly augment, which is in good agreement with HRTEM data. After 100 cycles, the (002) peaks of MoS2-EC/DEC and MoS2-DME still exist, while the (002) peaks of MoS2-DGM and MoS2-TGM almost disappear (Figure S13b). It reveals that the layer structures of MoS2-EC/DEC and MoS2-DME maintain but the layers of MoS2-DGM and MoS2-TGM are completely exfoliated after 100 cycles, which is also consistent with HRTEM results. In order to observe the (002) peak clearly, synchrotron XRD tests of MoS2-EC/DEC and MoS2-TGM electrodes after 100 cycles were performed. As shown in Figure 3e, the peak (002) around 14° is weaker for MoS2-TGM than for MoS2-EC/DEC. It indicates that the weak van der Waals force bonds are weakened in MoS2-TGM due to the layer exfoliation.34 There are two sharp peaks in their Raman spectra as shown in Figure 3f, and the peaks located at about 378 and 404 cm−1 correspond to Eg and A1g vibration mode of MoS2, respectively.34, 41 The distance between the two peaks is relative to the layer number of MoS2, and smaller distance means fewer layer number.41-42 For MoS2-EC/DEC, the distance between Eg and A1g peaks does not reduce compared with the pristine MoS2. In the case of MoS2-TGM, the distance between the two peaks decreases from 26.4 to 24 cm−1, indicating the reduced layer number. All characterizations demonstrate that the MoS2 layer is effectively exfoliated when using the long-chain-like ether-based electrolytes. In order to investigate the reaction mechanism of MoS2-TGM sample, mass variation of the electrodes during discharge process and FT-IR analyses were executed. Figures 3g and 3h show the weight variations of MoS2-EC/DEC and MoS2-TGM samples during the discharge process in which the dashed lines stand for the theoretical mass changes of the electrode when Li+ or [Li-TGM]+ intercalation happens. The experimental results of MoS2-TGM sample are much close to [Li-TGM]+ intercalation rather than Li+ intercalation; however, MoS2EC/DEC displays bare Li+ intercalation process. Similarly, MoS2-DEM and MoS2-DGM exhibit [Li-solvent]+ intercalation behavior (Figure S14). Figure S15 presents the FT-IR spectra of the 1M LiTFSI in TGM electrolyte and MoS2-TGM


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electrodes at fully discharged 1.0 V and charged 3.0 V. The both electrodes show the characteristic peaks of C-O bonding located between 1050 and 1250 cm−1, indicating that the ether-based solvent (i.e., TGM) also inserts into the interlayer. The C-O peaks turn weak but still exist at charged 3.0 V, revealing that the solvent still accompanies with Li+ after charging. The FTIR results confirm that the solvated ion co-intercalation take places during the charge-discharge processes. The valence changes of Mo and S ions during the chargedischarge process were also investigated by XANES and XPS. XANES spectra of MoS2-TGM (Figure S16a) reveal that the Mo K-edge positions shift to low energy value when discharged to 1.0 V in the first cycle and remain unchanged during the following cycles. It indicates that the Mo valences do not vary after the discharge process in the first cycle. Figure S16b exhibits the ex-situ Mo 2d5/2 and S 2p partial spectra of the charged and discharged electrodes at the first and 100th cycles. The Mo 2d5/2 peak positions at different charged and discharged states are almost unchanged. However, the S valences show obvious differences between fully charged state up to 3.0 V and discharged state down to 1.0 V. The S 2p peaks are located at lower binding energy when discharged to 1.0 V than charged to 3.0 V, indicating that S ions possess lower valence at discharged state than those at charged state. All results demonstrate that the reactions of MoS2-TGM mainly include anionic redox when cycling between 1.0 and 3.0 V. The solvated ion co-intercalation behaviour is beneficial for the rate performance of anode materials based on the previous reports,43-45 and it urged us to test the rate performance of the as-prepared MoS2 with different ether-based electrolytes (Figure 4a). Because the capacities of MoS2-DGM and MoS2TGM increase as the cycles undergo, the rate test was performed after cycling 100 cycles at 0.2 A g−1. As the current density gradually increases from 0.05 to 5 A g−1, the discharge capacities of MoS2-DGM and MoS2-TGM are higher than those of MoS2-DME. When the current density reaches to 10 A g−1, the capacity of MoS2-DME excesses the other samples. The discharge capacities of MoS2-DME, MoS2-DGM, and MoS2-TGM are 122.4, 77.8, and 102.3 mAh g−1, respectively. As the current density increase to 20 A g−1, the differences between MoS2-DME and the other two samples further enlarge. The discharge capacities of MoS2-DME, MoS2-DGM, and MoS2-TGM are 102.4, 46.5, and 35.7 mAh g−1, respectively. Even at an ultrahigh current density of 50 A g−1, the discharge capacity of MoS2-DME still remains 59.2 mAh g−1. However, MoS2-EC/DEC delivers awful rate capability, and its capacities at 10 and 20 A g−1 are only 27.6 and 16.7 mAh g−1, respectively (Figure S17). Figures 4b−4d exhibits their discharge-charge curves at different current densities. All samples show obvious chargedischarge plateaus at a current density of 0.2 A g−1. As current density increases to 2 A g−1, the MoS2 with ether-based electrolytes still have distinct charge-discharge plateaus, but the MoS2 with carbonate-based electrolytes (i.e., MoS2-EC/DEC) display severe polarization (Figure S18). Notably, only MoS2DME possesses charge-discharge plateaus at 20 A g−1. The GITT tests were performed to calculate the Li-ion diffusion coefficient of MoS2 with different electrolytes. Figure S19 shows GITT profiles of the as-prepared MoS2 with different electrolytes, and the plots of voltage against τ1/2 for the four samples exhibit good linear relationship (Figure S20) like previous report.46 Figures 4e and 4f exhibit the corresponding

Li+ diffusion coefficients when using different electrolytes. The Li-ion diffusion coefficient was calculated based on Equation 4 as follows:47-48 4 𝑛𝑚𝑉𝑚 2 ∆𝐸𝑠 2

D = 𝜋𝜏(

𝐴

) (∆𝐸 ) 𝜏

(4)

where D is the Li-ion diffusion coefficient τ represents the duration of the current pulse (s), nm and Vm denote the number of moles of active material (mol) and the molar volume of the electrode (cm3·mol−1), respectively, A stands for the contact area between the electrode and electrolyte (cm2), ΔΕs and ΔΕτ mean the steady-state voltage change owing to the current pulse (V) and the voltage change during the constant current pulse when neglecting the IR drop (V), respectively. The MoS2-DME delivers relative higher Li-ion diffusion coefficient compared with the other three samples, which may be attributed to small molecular volume of DME. Small [Li-DME]+ ions are beneficial for fast diffusion when they insert into the interlayer. In order to investigate the contribution of psuedocapacitive behaviour, the CV tests at different scan rates of MoS2-DME, MoS2-DGM, and MoS2-TGM are performed to calculate b values (Figure S21). If b = 1, the electrochemical reactions are dominated by pseudocapacitive behaviours, and if b = 0.5, the charge-discharge processes are controlled the ionic diffusion. The MoS2-DME shows higher b values than those of the other two samples, indicating its faster kinetics. Based on above results, MoS2-TGM shows high capacity because of layer exfoliation, while MoS2-DME exhibits excellent rate capability owing to small molecule of DME. Thus, it intuitively inspires us to combine TGM with DME to prepare mixture electrolytes. The MoS2 samples with 1M LiTFSI in DME/TGM (v/v = 1:1, 3:1, and 9:1 by volume) as the electrolyte are named as MoS2-DME/TGM-11, MoS2DME/TGM-31, and MoS2-DME/TGM-91, respectively. Figure 5a displays cyclability of MoS2-DME/TGM with different ratios of DME to TGM. When the ratio of DME to TGM reaches 3:1 or 1:1, the capacity-increase phenomenon emerges. However, the capacities of MoS2-DME/TGM-91 keep almost unchanged due to insufficient TGM. The HRTEM images and Raman spectra were used to observe the layer spacing and layer number of MoS2-DME/TGM-31 electrode. As shown in Figure S22, the layer spacing of MoS2-DME/TGM-31 electrode gradually increases. Its layer spacing excesses 1 nm after 100 cycles. From Raman spectra of MoS2-DME/TGM-31 (Figure S23), the distance between Eg and A1g peaks decreases from 26.4 to 24.7 cm−1, indicating the reduced layer number. The bottom right corner in Figure S23 depicts the process of layer exfoliation. The layers become more expanded and flexible compared with pristine MoS2 as the cycles undergo. Many layers are separated, and layer number reduces, leading to an enhanced specific surface area. Figure S24 displays the FT-IR spectra of the 1M LiTFSI in DME/TGM electrolyte and MoS2DME/TGM-31 electrodes at fully discharged 1.0 V and charged 3.0 V. The characteristic peaks of C-O bonding are detected at discharged 1.0 V and charged 3.0 V, revealing that the solvated ion co-intercalation take places during the charge-discharge processes. The rate plots of MoS2-DME, MoS2-TGM, and MoS2DME/TGM with different ratios of DME to TGM are shown in Figure 5b. As the current density gradually increases from 0.05 to 0.1, 0.2, 0.5, 1 A g−1, the discharge capacities slightly drop from 243 to 233, 223, 218, 213 mAh g−1. The capacity of MoS2-


(cm2·s−1),


DME/TGM-31 excesses those of the other four samples when the current density reaches 2 A g−1. The discharge capacities are 206, 187, 165, 134, and 77 mAh g−1 at current densities of 2, 5, 10, 20, and 50, respectively. Figure S25 exhibits dischargecharge curves of the MoS2-DME/TGM-31 at different current densities. As current density gradually increases from 0.5 to 20 A g−1, the MoS2 MoS2-DME/TGM-31 possesses distinct charge-discharge plateaus all the time. Figure S26 displays EIS curves of MoS2-DME, MoS2-TGM, and MoS2-DME/TGM with different ratios of DME to TGM. All curves are composed of one semicircle and one line, and no SEI resistance is detected in all samples. The diameter of the semicircle stands for the charge-transfer resistance (Rct), and the Rct values of MoS2DME, MoS2-TGM, MoS2-DME/TGM-91, MoS2-DME/TGM31, and MoS2-DME/TGM-11 are 165, 73, 78, 100, and 137 Ω, respectively. The MoS2-DME, MoS2-DME/TGM-91, and MoS2-DME/TGM-31 show relative low Rct compared with the other two samples, but only MoS2-DME/TGM-31 exhibits capacity increase during the cycles owing to its layer exfoliation. We also tested the rate performance of MoS2-DME/TGM-31 with different mass loadings. The rate performance of MoS2DME/TGM-31 is very similar when the mass loading increase from 1.59 mg cm−2 to 3.98 mg cm−2 but obviously weaken as the mass loading reaches to 4.42 mg cm−2 (Figure S27). In addition, we compared the rate capability of MoS2-DME/TGM31 with previous reports about high-safety Ti-based anodes for LIBs,12, 49-51 and the MoS2-DME/TGM-31 show obvious superior (Figure S28a). Meanwhile, MoS2 is superior to commercialized graphite anode32 in better rate performance (Figure S28b) and safer voltage range (Figure S29), which supports that MoS2 is a suitable anode for Li-ion batteries. The b-values of redox peak of MoS2-DME/TGM-31 were also calculated, which validates that its redox processes include both pseudocapacitive behaviours and co-intercalation reactions (Figure S30). The concrete pseudocapacitive contribution is calculated based on the equations (5, 6) as follows:44, 52-53 i = k1v + k2v0.5 (5) 𝑖

𝑣0.5

= 𝑘1𝑣0.5 + 𝑘2

where k1v and k2

(6) v0.5

stand for the pseudocapacitive and ionic i 0.5

diffusion contributions, respectively, and k1 is the slope of v ∼v0.5 plots. The pseudocapacitor contribution at various scan rates in MoS2-DME/TGM-31 sample are displayed in Figure 5c, and they are 47.6%, 48.2%, 51.7%, 55.3%, 59.3%, 63.3%, 68.7%, and 75.2% at scan rates of 0.1, 0.2, 0.3, 0.5, 0.8, 1, 1.5, and 2 mV s−1, respectively, indicating that the pseudocapacitive Li+-storage quantity occupies larger ratios as the scan rates increase. Figure S31 exhibits the detailed pseudocapacitive

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fraction at 1 mV s−1, which is analogous to some other metal dichalcogenides.52, 54 The long-term cyclability of MoS2-DME/TGM-31 also was tested. As shown in Figure 5d, capacity looks stable after 200 cycles, and the discharge capacity still reaches 273.5 mAh g−1 after 700 cycles at 0.1 A g−1. Notably, the discharge capacity could remain 193.1 mAh g−1 after 2000 cycles at 1 A g−1, which corresponds to capacity retentions of 103.7% and 94.6% calculated from 2nd and 200th cycles, respectively. In addition, the coulombic efficiency always keeps very close to 100% except for the first several cycles (Figure S32). The unprecedented high-rate capability and long cycling performance of the MoS2-DME/TGM-31 are attributed to twofactors (Figure 5e). Firstly, the reactions of MoS2DME/TGM-31 cycling between 1.0−3.0 V are co-intercalation reactions, which effectively avoids conversion reactions and LiS reactions. As well known, conversion reactions and Li-S reactions always accompany with inevitable capacity decay and sluggish kinetics. In addition, co-intercalation reactions inhibit the formation of SEI film to an extreme, leading to enhanced Li+ diffusion. Secondly, the reactions of MoS2-DME/TGM-31 are controlled by both pseudocapacitive behaviours and ionic diffusion, which combines the advantages of both capacitors and batteries. 4. Conclusion In summary, 1M LiTFSI in DME/TGM (v/v = 31 by volume) and 1.0−3.0 V voltage window serve as the optimized electrolyte and voltage window for MoS2/Li batteries, respectively. It delivers high capacities of 273.5 mAh g−1 after 700 cycles at 0.1 A g−1 and 193.1 mAh g−1 after 2000 cycles at 1 A g−1. Exhilaratingly, it also shows high-rate capability, and the discharge capacities are 206, 187, 165, 134, and 77 mAh g−1 at current densities of 2, 5, 10, 20, and 50 A g−1, respectively. Its unprecedented electrochemical performance is attributed to three factors. Firstly, the solvated ion co-intercalation reactions render MoS2 to effectively avoid the conversion reactions (MoS2 + 4Li ↔ 2Li2S + Mo) and Li-S reactions (2Li + S ↔ Li2S). As well known, the two kinds of reactions are always accompanied by capacity decay and sluggish kinetics. Secondly, the solvated ion co-intercalation reactions effectively skip the desolvation process and strongly inhibit the formation of SEI film, leading to enhanced Li+ diffusion. Finally, the overall battery performance is determined by its pseudocapacitive behaviour and enhanced ion diffusivity, which combines the advantages of both capacitors and batteries. As a result, the durable cyclabilty and fast kinetics are realized. This research may offer an alternative strategy for the utilization of MoS2 in high-power and high-safety lithium-ion batteries.


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Figure 1. XRD patterns (a), XPS spectra (b, c) as well as SEM (d), TEM (e), and high resolution TEM (HRTEM) (f) images of the as-prepared MoS2.

Figure 2. Voltage window and electrolyte optimization of MoS2/Li batteries. Cyclability in different voltage windows when using 1M LiTFSI in TGM electrolyte (a). The molecular formula of as-selected solvents (b). Discharge-charge curves at a current density of 0.2 A g−1 of different electrolytes when cycling between 1.0−3.0 V (c. the 1st cycle, d. the 2nd cycle, e. the 40th cycle, and f. the 100th cycle). Middle voltage change (g) and cyclability (h) of MoS2 with different electrolytes when cycling between 1.0−3.0 V.



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Figure 3. Density functional theory calculations on the (a) Li+- and (b) [Li-solvent]+-intercalated MoS2 structures. HRTEM images of the MoS2 electrodes with layer spacing measurement after 100 cycles when using different electrolytes (c. 1M LiTFSI in EC/DEC, d. 1M LiTFSI in TGM). XRD patterns of MoS2-EC/DEC and MoS2-TGM electrodes after 100 cycles (e). Raman spectra of the pristine MoS2 as well as MoS2-EC/DEC and MoS2-TGM electrodes after 100 cycles (f). Mass variations of the electrodes during discharge process when EC/DEC (g) and TGM (h) solvents are used.


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Figure 4. Rate capability of MoS2-DME, MoS2-DGM, and MoS2-TGM when cycling between 1.0−3.0 V (a). Discharge-charge curves at different current densities of MoS2 with different ether-based electrolytes when cycling between 1.0−3.0 V (b. 0.2 A g−1, c. 2 A g−1, d. 20 A g−1). Li-ion diffusion coefficient of MoS2 with different ether-based electrolytes during the discharge (e) and charge (f) processes.

Figure 5. Cyclability of MoS2-DME/TGM with different ratios of DME to TGM when cycling between 1.0−3.0 V (a). Rate capability plots of MoS2-DME, MoS2-TGM, and MoS2-DME/TGM with different ratios of DME to TGM (b). Bar chart showing the percent of pseudocapacitive contribution at different scan rates of MoS2-DME/TGM with a ratio of 3:1 (c). Long-term cycling performance of MoS2-DME/TGM with a ratio of 3:1 at current densities of 0.1 and 1 A g−1 (d). Illusration of reaction mechanism of MoS2 with the mixture ether-based solvents (e). Notes

ASSOCIATED CONTENT

The authors declare no competing financial interest.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.XXXX. Electrochemical measurement data, reaction mechanism analyses, electrochemical performance comparison.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: 82-2-2260-8674

ACKNOWLEDGMENT Y.–M. Kang would like to thank the National Research Foundation of Korea (NRF) grant through the Korean government (MSIP) for funding this work, whose grant numbers is NRF2017R1A2B3004383. Y.–M. Kang would also like to thank the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry and Energy (MOTIE) in South Korea; the number for the grant is 20152020105420. This research was also supported by Creative Materials Discovery Program through the National Research Foundation of Korea



(NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2017M3D1A1039553) as well as Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-TA1603-03. Finally, K. Zhang would like to thank the Korea Research Fellowship Program of the NRF, which was funded by the Ministry of Science and ICT; the number for this grant is 2016H1D3A1906790.

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MoS2 anode with a novel electrolyte, namely, 1M LiTFSI in DME/TGM (v/v = 31 by volume) and a voltage window of 1.0−3.0 V delivers unprecedented long-term cyclabilty and high-rate capability, which is attributed to the combination of the solvated ion co-intercalation reactions and partial pseudocapacitive behaviours.


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Pseudocapacitive Behaviour and Ultra-fast Kinetics from Solvated Ion

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