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Local Lattice Distortion Activate Metastable Metal Sulfide as Catalyst with Stable Full Discharge-Charge Capability for Li-O2 Batteries Sanpei Zhang, Zhennan Huang, Zhaoyin Wen, Linlin Zhang, Jun Jin, Reza Shahbazian-Yassar, and Jianhua Yang Nano Lett., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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Local Lattice Distortion Activate Metastable Metal Sulfide as Catalyst with Stable Full Discharge-Charge Capability for Li-O2 Batteries Sanpei Zhang,†,‡ Zhennan Huang,§ Zhaoyin Wen,†,* Linlin Zhang,† Jun Jin,† Reza Shahbazian-Yassar,§,* and Jianhua Yang† †

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics, Chinese

Academy of Sciences, Shanghai 200050, P. R. China E-mail: [email protected] ‡

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

§

Mechanical and Industrial Engineering Department, University of Illinois at Chicago, Chicago, Illi-

nois 60607, United States E-mail: [email protected] Key words: lattice distortion, metastable, two-dimensional materials, molybdenum, and electrochemical performance. Abstract The direct lattice strain, either distortion, compressive or tensile, can efficiently alter the intrinsic electrocatalytic property of the catalysts. In this work, we report a novel and effective strategy to distort the lattice structure by constructing a metastable MoSSe solid solution and thus, tune its catalytic activity for the Li-O2 batteries. The lattice distortion structure with inequivalent interplanar spacing between the same crystals plane were directly observed in individual MoSSe nanosheets with transmission electron microscopy and aberration-corrected transmission electron microscopy. In addition, in-situ transmission electron microscopy analysis revealed the fast Li+ diffusion across the whole metastable structure. As expected, when evaluated as oxygen electrode for deep-cycle Li-O2 batteries, the metastable MoSSe solid solution deliver a high specific capacity of ~730 mA h g-1 with stable discharge-charge overpotentials (0.17/0.49 V) over 30 cycles. 1 ACS Paragon Plus Environment

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Developing highly-efficient catalysts for energy storage and conversion process, such as Li-O2 batteries and hydrogen evolution reaction (HER), is still challenging but increasingly desirable.1-3 Numerous strategies have been investigated to systematically enhance the activity of known catalysts.4-7 Among these methods, modifying the surface electronic structure by lattice modulation has already led to the significant improvement in electrocatalytic performance.8-12 However, the strong interior bonding makes it difficult to induce the lattice distortion and most strategies are of limited use in practice because of their manipulating complexity and poor scalability. The emergency of novel two-dimensional (2D) nanomaterials, including boron, phosphorene, layered transition metal dichalcogenides (LTMDs) and oxides etc.13-15, pave a new solution on the road to develop alternative catalysts. Owing to the weak control of the interlayer van der Waals (vdW) on the lattice structure, the novel 2D crystals can serve as an ideal substrate to directly introduce lattice strain, either distortion, compressive or tensile, and further tune catalytic activity.10, 11 Molybdenum sulfides (MoS2), as a representative layered structure of the LTMDs with 2D S-Mo-S layers, presents exotic structure-property relationship in wide applications, including electrochemical hydrogen storage, transistors, lubricants and Li-ion batteries (LIBs) etc.3, 16-18 However, the application of 2D MoS2 still faces many obstacles, such as the low intrinsic catalytic activity and re-staking problems.19-21 Up to now, various methods have been exploited to modulate the atomic structure to get enhanced performance response, especially the catalytic properties. One effective way is to implant more active sites by modification on the surface of the few-layer MoS2. Owing to the poor catalytic activity of the thermodynamically stable (002) planes, the implanted active sites can significantly activate the preferentially exposed basal (002) planes of the nanosheets to get enhanced electrocatalytic performance.2224

Another method is to design MoS2-based vdW heterostructures by combining the multilayer

nanosheets with graphene or other 2D conductive nanosheets to change the intrinsic properties.7, 25 The 2 ACS Paragon Plus Environment

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peculiar emanating properties, especially the semi-metallic properties coming from its zero band gap, make graphene a most suitable conductive ingredient and matrix for the energy storage and conversion technologies.26, 27 The heterojunction of the designed interface between the few-layer MoS2 and graphene nanosheets can not only preserve the ultrathin nanosheets from restacking, but also provide the largest interface contact for ion transport and storage. Apart from the aforementioned MoS2-carbon nanosheets structures, very recently, some studies turned to the construction of all-semiconducting heterostructures, especially by vertically stacking the different LTMDs or switching them seamlessly together in-plane.28-30 Various physical properties have been investigated, and devices with enhanced performance have been demonstrated. First-principle calculations reveal that the total and partial density of states of the specific atoms in the formed heterojunction by two LTMDs monolayers delivers rather different properties from the initial LTMDs monolayer, enabling decreased band gap with semi-metallic or metallic characteristics and consequently the enhanced electrocatalytic activity.31 On the other hand, constructing solid-solution structure or heteroatoms doping are also commonly used to modulate the atomic structure or introduce active sites. The enhanced catalytic and electrochemical performance confirm the efficiency of these methods.32-34 However, it should be pointed out that the heteroatom and crystal strain effect is of great importance on activating the MoS2 catalysts, whereas most of these process are carried out by the high-temperature solid phase synthesis route. During the annealing process, the heteroatoms are introduced in the intrinsic crystal structure, but at the same time, the high-temperature triggers the atomic structure to achieve thermodynamic stability and form a new stable phase, which would reversely neutralize the designed activation effect. Thus, new methods that can effectively and flexibly induce the lattice distortion without introducing additional effects are needed. To achieve this, we develop a low-cost and potential scale-up route to synthesize three-dimensional metastable MoSSe solid solutions (3D M-MoSSe) with high-degree local lattice distortion. The as3 ACS Paragon Plus Environment

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prepared 3D M-MoSSe have a highly stable open microstructure with preserved ultrathin MoSSe layers. Furthermore, the lattice distortion structure with inequivalent interplanar spacing between the same crystals plane were directly obtained in individual MoSSe nanosheets. In-situ transmission electron microscopy analysis reveals the fast Li+ diffusion across the whole 3D metastable structure. As expected, the Li-O2 batteries with the 3D metastable MoSSe solid-solution can deliver a high specific capacity of ~730 mA h g-1 with stable discharge-charge overpotentials (0.17/0.49 V) over 30 cycles.

Figure 1 Schematics depicting the synthesis procedure of 3D M-MoSSe with distorted lattice structure. Results and Discussion Our strategy for inducing lattice distortion in the three-dimensional metastable MoSSe solid solution is schematically depicted in Figure 1. At the first step, hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) was mixed with sulfur (S) and selenium (Se) sources provided by thiourea (NH2CSNH2) and selenourea (NH2CSeNH2) with stationary Mo/S/Se molar ratio (1:1:1) to guarantee the fully bonding between Mo and S/Se. Assisted by the hydrothermal reactions, the (NH4)6Mo7O24·4H2O is decomposed to ultrathin MoOx nanosheets and, at the same time, reacts with 4 ACS Paragon Plus Environment

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NH2CSNH2 and NH2CSeNH2 to form ultrathin MoSSe nanosheets in the homogeneous aqueous solution.3, 35 Owing to the inconsistent thermal vibration frequency of Mo-S and Mo-Se36, the initial MoSSe nanosheets tend to form a disordered three-dimensional networks structure. After 20h, the 3D metastable solid-solution structure was obtained. In comparison, 3D stable MoSSe solid solution (3D S-MoSSe) was fabricated by a 300°C treatment of the 3D M-MoSSe. MoS2 (Figure S1) and MoSe2 (Figure S2) nanosheets (NSs) were also fabricated by the same procedure (the experimental details are shown in Experiment Section). Fourier transform infrared (FTIR) spectra comparison between the prepared MoS2, MoSe2 nanosheets and 3D M-MoSSe confirm the completion of the synthesis procedure (Figure S3). To probe the crystallinity structure and phase purity of the prepared samples, X-ray diffraction (XRD) was carried out. As shown in Figure 2a, the identified peaks of MoS2 (green) and MoSe2 (red) NSs are consistent with the standard pattern of MoS2 (JCPDS card No. 37-1492) and MoSe2 (JCPDS card No. 29-0914). In contrast, the XRD pattern of the prepared 3D M-MoSSe demonstrate similar diffraction characteristics to both the initial MoS2 and MoSe2 NSs, which may be related to the closely lattice constant of hexagonal MoS2 and MoSe2.37 The obvious broadening of all the diffraction peaks in 3D M-MoSSe suggests that the crystallites are at the nanoscale level. The peak intensity ratio of the (002) to other crystal faces of 3D M-MoSSe is close to the MoSe2, while far lower than the MoS2. This phenomenon indicates a high degree of the exposed edges in 3D M-MoSSe.21 Meanwhile, XRD pattern of the 3D S-MoSSe shows similar characteristics to that of 3D M-MoSSe, which confirms the stability of the stability of the crystal structure during the 300°C annealing process. The morphology of prepared 3D M-MoSSe was investigated by field-emission scanning electron microscopy (FESEM), as shown in Figure 2b and c. The 3D networks architecture is constructed by numerous ultrathin nanosheets and obvious ripples and corrugations can be observed. The enlarged SEM image in Figure 2c reveals that the 2D ultrathin nanosheets are randomly connected with the adjacent nanosheets and self-assembled into 5 ACS Paragon Plus Environment

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the continuous and interconnected 3D open wave-like architecture. This internal cross-linked structure would effectively prevent the lamellar reaggregation of the active few-layer nanosheets.3, 38 The elemental composition and purity of the products were further proved by energy-dispersive X-ray (EDX) analysis and elemental mapping images. As shown in Figure S4, strong signals from Mo, S and Se elements verify the high purity of the prepared samples. Corresponding element mapping images (Figure S5) reveal the highly uniform distribution of Mo, S and Se elements across the nanosheets and the whole 3D wave-like architecture, indicating the successful synthesis of homogeneous solid solution structure. Further transmission electron microscopy shown in Figure 2d confirms that the MoSSe are assembled by the ultrathin nanosheets in a disordered way, which is in accordance with the SEM observations. High-resolution transmission electron microscopy (HRTEM) of the representative region in the 3D M-MoSSe is also studied. As shown in Figure 2e, HRTEM image of the substrate part confirms the ultrathin nature of the nanosheets in the 3D M-MoSSe network. The corresponding HRTEM image of the lateral nanosheets in the bottom side view is shown in Figure 2f. It can be clearly observed that substantial 2D few-layer nanosheets are homogenously incorporated into the 3D M-MoSSe networks, offering rich edges for the active sites. A line-scan EDX analysis of the randomly selected few-layer nanosheets was carried out to investigate the distribution of the Mo, S and Se elements at the nanoscale. As can be seen from the Figure 2g, the equal dispersive intensity of all the three elements indicates the homogeneous distribution of Mo, S and Se elements across the few-layer nanosheets, further demonstrating the obtained uniform solid-solution structure. Impressively, careful investigation of layered crystal lattice structure in the HRTEM image, shown in Figure 2h and i, reveals that irregular interplanar spacing of the (002) plane with two main sizes of 0.92 and 0.74 nm commonly exist in the few-layer MoSSe nanosheets. Whereas the d spacing of the (002) crystal plane in the prepared MoS2 (Figure S2d) and MoSe2 (Figure S3d) is 6.2 and 6.5Å, respectively, agreeing well with the standard interplanar spacing of hexagonal MoS2 and MoSe2. The increasing interlayer spacing suggest the lattice tension in the 6 ACS Paragon Plus Environment

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prepared 3D M-MoSSe. In contrast, as shown in Figure S6a and b, the 3D S-MoSSe solid solution can maintain a stable three-dimensional structure. Meanwhile, the prepared 3D S-MoSSe display a condenser three-dimensional architecture than that of the 3D M-MoSSe. The obvious aggregation of the 3D structure is mainly attributed to the re-stacking of the active nanosheets. TEM and HRTEM images in Figure S6c and d confirm the thickness evolution of the nanosheets in the 3D M-MoSSe. Compared to the metastable structure, the 3D S-MoSSe delivers uniform interlayer spacing of (002) crystal plane.

Figure 2 a, XRD pattern of the prepared MoS2, MoSe2 nanosheets, 3D S-MoSSe and 3D M-MoSSe solid solution by the same route. b, SEM image of the 3D M-MoSSe solid solution. c, Corresponding enlarged FESEM image of the yellow area in (b). d, TEM image of the 3D M-MoSSe solid solution. e, HRTEM image of the area 2 in (d). f, HRTEM image of the area 1 in (d). g, Line-scan EDS analysis of 3D M-MoSSe solid solution along the yellow line in (f). h, i HRTEM image of 3D M-MoSSe solid solution, indicating the inconsistent d spacings of the (002) plane.

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To have an intuitive understanding of the distorted structure and further prove the existence of the modulation at the atomic scale, we also performed aberration-corrected scanning transmission electron microscope high-angle annular dark-field (STEM-HAADF) images of the 3D M-MoSSe. As shown in Figure 3a and b, the STEM-HAADF image from the (002) direction demonstrate a clear layered architecture and, more importantly, further demonstrates the existence of strong lattice strain out of the basal plane (Figure 3a), which is consistence with the aforementioned HRTEM results. The high degree outof-plane modulation is mainly related to the weak control of the vdW forces on the interlayer structure and the different Mo-S and Mo-Se bond lengths. Additionally, we also carried out the STEM-HAADF imaging on the surface of the basal plane, as shown in Figure 3b, a twisted lattice with departing atoms from the hexagonal structure can be clearly observed. To have a better understanding of this in-plane distortion, the train maps of atomic STEM-HAADF image was calculated by geometric phase analysis(GPA).39 As can be seen from the in-plane lattice distortion along x and y direction of the (002) plane shown in Figure 3c and d, a little distortion was shown on the x direction, while tremendous area of distortion on the y direction. Strain mapping along the y direction gives clear view of wave-like lattice expansion as depicted in Figure 3h. EDX-STEM of 3D M-MoSSe (Figure 3e-g) demonstrates that Mo, S, and Se are uniformly distributed across the sample at the atomic scale, indicating the uniform solidsolution structure. In contrast, after calcining at 300°C in Ar, the 3D S-MoSSe display more closely three-dimensional structure with uniform lattice structure (Figure S6). This phenomenon suggests the success of the reported hydrothermal method in this work on preserving the metastable solid-solution structure with high-degree distorted lattice structure. The special out-of-plane and in-plane distortion (Figure 3h) of the local atomic structure in the homogeneous solid solution could affect the intrinsic catalytic activity of the sample.


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Figure 3 STEM images of 3D M-MoSSe. HAADF-STEM images of the layers (a) and basal plane (b) of the 3D M-MoSSe. Strain map taken along the xx (c) and yy (d) direction calculated from the HAADF image in (b). The corresponding elemental mapping of Mo (e), Se (f), and S (g) of (b). h, Structural model of the in-plane and out-plane lattice distribution in the prepared 3D metastable MoSSe solid solution. The scale bars are 1 nm. 9 ACS Paragon Plus Environment

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Surface sensitive X-ray photoelectron spectroscopy (XPS) was used to study the chemical state of Mo, S and Se in the prepared 3D M-MoSSe. As can be seen from the high resolution XPS spectrum of Mo 3d in Figure 4a, the Mo 3d3/2 and 3d5/2 peaks of the 3D M-MoSSe are located at 232.42 and 229.24 eV, respectively, which is different from the standard Mo 3d3/2 and 3d5/2 of the pure MoS2 and MoSe2. Meanwhile, the high resolution XPS spectra of S 2p and Se 3d of the 3D M-MoSSe also show different characteristics from that of the prepared MoS2 and MoSe2 NSs. As depicted in Figure 4b, compared to the pure MoS2, the S 2p1/2 and S2p2/3 in 3D M-MoSSe occur a slight shift to lower binding energy, while the peaks of the Se 3d3/2 and 3d5/2 are located at a higher binding energy than that of the MoSe2 NSs. Moreover, compared to the pure MoSe2 NSs, the peak intensity ratio of the Se 3d3/2 and 3d5/2 in 3D M-MoSSe display opposite characteristics. The apparent peak shift of the S 2p and Se 3d indicate the change of the chemical environment around these atoms, which is mainly attributed to the metastable solid-solution structure in the 3D M-MoSSe networks. In contrast, the XPS result of the annealed 3D SMoSSe is in accordance with the solid solution prepared by solid state reactions or other hightemperature methods. As shown in Figure 4b and c, high-resolution S 2p and Se 3d XPS spectra of 3D S-MoSSe resembles that of pure MoS2 and MoSe2 samples, indicating a similar oxidation state and elemental environment of S and Se in 3D S-MoSSe to that of MoS2 and MoSe2, respectively. Therefore, the high resolution S 2p and Se 3d XPS spectra of 3D M-MoSSe further confirm the unique metastable solid-solution structure in the 3D M-MoSSe networks. The quantitative analysis results of EDX and XPS in Table S1 demonstrate that the atomic ratio of Mo, S and Se in the prepared MoSSe solid solution is about 1:1:1, which agree well with the molar ratio of the reaction precursor (shown in experimental section) and confirm the fully bonding between Mo and S/Se. In addition, full nitrogen adsorption and desorption isotherms of the 3D M-MoSSe solid solution were recorded to obtain the information of the specific surface area and pore size distribution of the MoSSe solid solutions. The nitrogen adsorption-desorption isotherms of MoSSe in Figure S8a shows typical IV curves of mesoporous mate10 ACS Paragon Plus Environment

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rials. A capillary condensation step at P/P0 = 0.60 to 0.9 reflects the existence of mesoporous channel resulted from the lateral stacking of MoSSe nanosheets (shown in Figure 2b). Based on the isotherms, the Brunauer-Emmett-Teller (BET) surface area of the MoSSe are calculated to be 55.21 m2/g, which is much higher than those of the prepared MoS2 and MoSe2 nanosheets by the same procedure (Figure S7a-d). Calculated by the Barrett-Joyner-Halenda (BJH) model, the pore size of the MoSSe centered at around 18 nm (Figure S8b), which is in accordance with that estimated from the SEM and TEM images. The high surface area with the appropriate pore size can offer large accessible space for the electrochemical reaction and serve as a substrate for the deposition of the intermediate products. Moreover, during the annealing process, the surface area along with the pore size distribution of 3D S-MoSSe solid solution (Figure S8c and d) have inconspicuous changes, indicating the stability of the 3D interconnected scaffold structure. The above results have conclusively demonstrated the success of the atomic structure modulation by the synthesis of the 3D M-MoSSe. The HRTEM, HADDF, XPS and BET results confirm the efficiency of the novel solvothermal procedure in this work on achieving metastable solid-solution structure with preserved distorted lattice structure, which is of great difference from the solid solution fabricated by solid phase or other high-temperature methods. The tailored lattice structure and different chemical environment may hold intriguing catalytic performance.


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Figure 4 XPS analysis of the 3D M-MoSSe, 3D S-MoSSe, MoS2 and MoSe2 nanosheets. a, Highresolution of Mo 3d of 3D M-MoSSe, 3D S-MoSSe, MoS2 and MoSe2. b, High-resolution of S 2p of 3D M-MoSSe, 3D S-MoSSe and MoS2. c, High-resolution of Se 3d of 3D M-MoSSe, 3D S-MoSSe and MoSe2. To verify our hypothesis, the performance of the 3D MoS2-MoSe2 entanglements as the cathode materials for Li-O2 batteries was examined using a Swagelok-type cell composed of a lithium metal anode, electrolyte (1 M LiClO4 in dimethyl sulfoxide (DMSO)) impregnated into a glass fiber separator, and a porous cathode. Figure 5a shows the fully discharge-charge profiles of the Li-O2 cell in the voltage window of 2.4 to 4.2 V. At the current density of 50 mA g-1, the lithium-oxygen battery can deliver a high discharge capacity of 708.24 mA h g-1. Notably, the specific capacity was calculated by the mass of the 3D M-MoSSe solid solution in the cathode. Low overpotentials of 0.17 and 0.49 V are obtained for the oxygen reduction (ORR) and evolution (OER) process, respectively, suggesting the highly electrochemical catalytic activity of the carbon-free 3D M-MoSSe cathode. In contrast, the Li-O2 cell with 12 ACS Paragon Plus Environment

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MoS2 and MoSe2 nanosheets cathode delivered low capacity with high overpotential. In addition, the LiO2 batteries with 3D S-MoSSe display a discharge capacity of only 515.7 mA h/g with a high overpotential of ~ 0.9 V. The obvious enhanced performance of the 3D M-MoSSe indicates the crucial role of the distorted lattice structure on improving the catalytic activity. The rate capability of the Li-O2 cells with 3D M-MoSSe cathode was also studied, as shown in Figure 5b. With increasing current density from 50 to 300 mA g-1, reversible discharge-charge capacities of 708.24, 642.65, and 321.32 mA h g-1 are obtained at 50, 100, and 300 mA g-1, respectively. The discharge overpotentials increase from 0.17 V at 50 mA g-1 to 0.23 V at 300 mA g-1, and the corresponding charging overpotentials from 0.49 to 0.73 V. Notably, all the discharge-charge curves at different testing current densities shown in Figure 5b end with sharp potential decrease or increase, suggesting the accomplishment of the corresponding discharging or charging processes. The reversibility of the Li-O2 batteries with 3D M-MoSSe cathode can also be proved by the electrochemical impedance spectroscopy (EIS) results. As shown in Figure S9, the 3D M-MoSSe nanosheet cathode shows low charge-transfer resistance before cycling. After discharged to 2.4 V, the insulating discharge products deposited on the surface of the cathode lead to an increase in the resistance of the 3D M-MoSSe cathode. The high resistance of the 3D M-MoSSe cathode recovered after the Li-O2 batteries was fully recharged to 4.2 V, indicating the decomposition of the Li2O2 discharge products.40 XRD (Figure S10) and XPS (Figure S11) also confirm the reversible formation of the Li2O2 discharge products during the discharge and charge process. The cycling performance of the deep discharge-charge Li-O2 batteries with 3D M-MoSSe cathode is tested at the current density of 50 mA/g in the voltage window of 2.4 ~ 4.2 V. As shown in Figure 5c, the discharge and charge potential over 30 cycles still remains unchanged with a very small overpotential, indicating a highly OER and ORR catalytic activity of the 3D M-MoSSe used in this work. Figure 5d reveals the increase discharge-charge capacities from the 1st to 6th cycle, which may be related to the construction of the stable three phase boundary between the O2, electrolyte, and cathode during cycling.40-42 After 30 cycles, the Li-O2 batter13 ACS Paragon Plus Environment

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ies with 3D M-MoSSe can still display a high discharge capacity of 725.33 mA h g-1, indicating the good reversibility during the deep cycling process. Therefore, the LTMDs is, for the first time, revealed to be efficient in catalyzing deep-cycling Li-O2 batteries.43, 44 6 5

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Figure 5 a, The first cycle discharge-charge profile of the MoS2, MoSe2, 3D S-MoSSe and 3D MMoSSe cathode at a current density of 50 mA g-1. b, Galvanostatic discharge and charge curves of the 3D metastable MoSSe cathode in the 1st cycle at current densities from 50 to 300 mA g-1. c, Full discharge-charge profiles of the Li-O2 batteries with 3D M-MoSSe cathode. d, The cycling stability of the Li-O2 cell with 3D metastable MoSSe cathode. TEM, STEM and XPS of the cycled 3D M-MoSSe cathode were also studied to investigate the stability of the distorted lattice structure. As shown in Figure S12a, TEM image of the cycled 3D MMoSSe confirms the stability of the 3D architecture during the cycles. The corresponding HRTEM image (Figure S12b) of the lateral nanosheets in Figure S11a shows the irregular interplanar spacing of the (002) plane with two main sizes of 0.92 and 0.74 nm in the few-layer MoSSe nanosheets, suggesting the stability of the out-of-plane distortion. Meanwhile, STEM-HAADF image of the basal (002) plane in Figure S12c demonstrates the twisted lattice with departing atoms from the hexagonal structure. The 14 ACS Paragon Plus Environment

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corresponding fast-inverse-Fourier-transform-filtered TEM (Inverse FFT) verify the obvious in-plane distorted lattice structure of the cycled 3D M-MoSSe cathode (Figure S12d). In addition, the cycled 3D M-MoSSe cathode were also probed by XPS to further confirm the stability of the metastable structure. As shown in Figure S13, high-resolution Mo 3d, S 2p and Se 3d XPS spectra of 3D M-MoSSe cathode after cycles resemble that of fresh 3D M-MoSSe, indicating the stability of the elemental environment of 3D M-MoSSe during the electrochemical process. From the combined analysis of XRD, HRTEM, STEM, Inverse FFT and XPS, the stability of the distorted lattice structure and the unique electronic structure of the metastable MoSSe solid solution cathode during the cycles can be proved. To have a deep insight into the 3D metastable solid solution on enhancing the electrochemical performance, we performed the in-situ TEM observation of the Li+ diffusion in the three-dimensional architecture. As shown in the in-situ TEM results (details shown in supporting video), we can clearly observe the fast Li+ diffusion at the inter-contact area between the Li and 3D M-MoSSe, which enable the improvement of the air-liquid-solid interface for the Li-O2 batteries. More importantly, the 3D MMoSSe can still maintain a stable structure even under the high working voltage of the in-situ TEM analysis and display good volume retention after lithium insertion. Thus, it can be concluded that the decreased overpotential and good cycling performance for 3D M-MoSSe are tightly associated with its unique modified atomic structure and high surface area that offers the following advantages. First, the open void space in the 3D architecture constructed by ultrathin nanosheets can significantly facilitate the transportation of electrons, ions and mass into the deep locations of the overall electrode, leading to enhanced capacities and rate capability. Second, the modulation of the atomic structure in the 3D MMoSSe can facilitate the fast transport of Li+ and provide numerous active sites for catalyzing the reversible Li-O2 batteries. Moreover, the stable three-dimensional structure effectively avoid the restacking of the active nanosheets during the application. The preserved ultrathin nanosheets can provide


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a large number of active sites and also can greatly shorten the diffusion distance for electrons and ions, which is beneficial to the good rate capability and cycling performance.3, 45, 46 To further study the effect of lattice distortion on enhancing electrocatalytic performance, the HER activities of the 3D metastable MoSSe are also evaluated in 0.5 M H2SO4 using a three-electrode electrochemical cell. Figure S14a exhibits the linear sweep voltammetry (LSV) for the prepared MoS2, MoSe2 and MoSSe. It is clearly observable that the 3D M-MoSSe have a small HER onset overpotential of ~125 mV, beyond which the recorded current density substantially increases under more negative potentials. This value is significantly less electronegative than that of the prepared MoS2 and MoSe2 and previous results of nanostructured highly crystalline solid solution.33, 34 Moreover, the 3D M-MoSSe displays a maximum iR-corrected cathodic current density of 29.95 mA cm-2 at an overpotential of 200 mV, suggesting a far greater production of the desired H2 (gas) compared to the pure MoS2 and MoSe2 electrodes. The highly catalytic activity of the 3D metastable MoSSe is mainly ascribed to the unique lattice distorted structure and the high surface area that facilitate the exposure of more active edge sites and provide more diffusion channels for ion and mass transport. The corresponding Tafel slopes of the 3D metastable MoSSe is 48 mV decade-1 (Figure S14b), while the prepared MoS2 and MoSe2 NSs deliver much higher Tafel slops of 91 and 70 mV decade-1, respectively. This value indicates a faster increment of HER, since the Tafel slope reflects the rate of the Volmer reaction to convert protons into adsorbed hydrogen atoms on the catalysts surface. In addition, as shown in Figure S14c, LSV of 3D SMoSSe suggests rather poor HER performance with high onset overpotential (200 mV) and low cathodic current density, demonstrating the dominant effect of the lattice distorted structure on activating the electrocatalytic activity of the 3D M-MoSSe. Table S2 gives the achieved exchange current density of all the tested samples by using the extrapolation method to the Tafel plots. The metastable MoSSe possesses the highest exchange current density of 7.76 µA cm-2, suggesting the best electrocatalytic activity. The long-term operating stability is another important criterion for the HER catalysts. To assess the 16 ACS Paragon Plus Environment

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durability of the 3D M-MoSSe solid solution in the acid environment, continuous cyclic voltammetry (CV) in the cathodic potential window at a scan rate of 50 mV s-1 over 1000 cycles was performed. Figure S14d shows the comparison of cathodic polarization curves collected before and after 1000 CV cycles ranging from 0 to -220 mV at a scan rate of 50 mV s-1. It can be clearly observed that there is only a negligible difference among the curves, which might be associated with the aggregation of H2 bubbles on the surface of the electrode or the consumption of H+.7 The enhanced electrocatalytic performance could be ascribed to the high-surface-area 3D metastable MoSSe solid solution with special distorted atomic structure that can provide more active edge sites and offer interconnected pathways for ion and mass transport.

Conclusion In summary, a highly general approach is developed to induce lattice distortion in the MoS2-based catalysts by constructing a metastable solid-solution structure. The properly tailored atomic structure are found to be efficient on significantly improving the electrochemical activity and facilitating the fast ion transport. When used as the catalysts for Li-O2 batteries and hydrogen evolution reactions, the 3D open metastable solid solution architecture manifests greatly enhanced electrochemical performance. The success of great modification on the atomic structure and construction of metastable solid-solution structure may pave the way to the design of next-generation catalytic materials. Experimental Section Samples Preparation Hexaammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O), thiourea (NH2CSNH2) and selenourea (NH2CSeNH2) were purchased from the Sigma-Aldrich Company. The autoclave (model number 4749) was ordered from the Nanjing RNK Science and tech Company.


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The synthesis of the metastable MoSSe was conducted in pure N2 atmosphere environment. Firstly, 3.5 mmol NH2CSeNH2 was dissolved in 30 mL deionized water under vigorous stirring for 30 min to form a homogeneous solution. Then 1 mmol (NH4)6Mo7O24·4H2O and 3.5 mmol NH2CSNH2 were added into the solution and stirred for 1 h. Subsequently, the solution was purged with pure O2 for 15 min prior to remove nitrogen and transferred into a 43 mL Teflon-lined stainless steel autoclave and kept at 230°C for 20 h. After being cooled to room temperature, the generated precipitates were filtered and washed with ethanol and water. The final products were obtained by freeze-drying for 12 h. In comparison, the same procedure was used to synthesize MoS2 and MoSe2 nanosheets. The stable MoSSe was obtained by a 300°C heat treatment of the metastable MoSSe solid solution. Structural Analyses XRD was conducted on a Rigaku D/Max ultima II with Cu Kα radiation. The FESEM images were taken on a JEOL JSM-6700F SEM. TEM images were taken on JEOL JEM-2100F microscopes. JEOL JEM-ARM 200CF equipped with a 200 kV cold-field emission gun, high angle annular dark field (HAADF) detector and an energy dispersive spectrometer (EDS) was utilized for the atomic image and image mapping. A 22-mrad probe convergence angle was used for all STEM images. The HAADF images were captured using a 90-mrad inner-detector angle. X-ray photoelectron spectroscopy analysis was conducted using a twin anode gun, Mg KR (1253.6 eV) (Microlab 310F Scanning Auger Microprobe, VG Scientific Ltd.). The specific surface area was further measured by the Brunauer–Emmett– Teller (BET) method using nitrogen adsorption–desorption isotherms on a Micromeritics Tristar II 3020 analyzer. The discharge products were analyzed by X-ray photoelectron spectroscopy (ESCALAB 250). Electrochemical measurements Li-O2 batteries: The electrochemical performance of all the samples as the oxygen electrode for rechargeable Li-O2 batteries was tested in Swagelok-type cells. The battery was assembled using highpurity lithium foil as the anode, a glass microfiber filter (GF/A, Whatman, Ø 18 mm) as the separator, 18 ACS Paragon Plus Environment

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and a solution of 1mol/L LiClO4 in Dimethyl sulfoxide (DMSO) as the electrolyte. The fabrication of the cells was conducted in an argon filled glove box with oxygen and water contents less than 0.1 ppm. The cells were electrochemically tested in a dried pure oxygen filled glovebox with water content less than 0.1 ppm. The test was conducted on a LANDCT2001A battery test system at room temperature. For the preparation of the oxygen electrode, the active materials (including MoS2, MoSe2, stable and metastable MoSSe solid solution) and Polytetrafluoroethylene (PTFE) were mixed with a weight ratio of 90:10 and dispersed in ethanol. The paste was coated onto a Ni mesh (φ =10 mm), and the mass loading of the active materials is ~ 0.9 mg/cm2. Hydrogen evolution reaction: All electrochemical measurements were performed in a three-electrode system at an electrochemical station (CHI660B). 4 mg of catalyst and 80 µl of 5 wt% Nafion solution were dispersed in 1 ml of 4:1 v/v water/ethanol by at least 30 min sonication to form a homogeneous ink. Then 5 µl of the catalyst ink (containing 20 µg of catalyst) was loaded onto a glassy carbon electrode of 3 mm in diameter (loading ~ 0.285 mg/cm2). Linear sweep voltammetry with a scan rate of 5 mV s-1 was conducted in 0.5 M H2SO4 (purged with pure N2) using Ag/AgCl (in 3 M KCl solution) electrode as the reference electrode, Pt foil (4.0 cm2) as the counter electrode, and the glassy carbon electrode with various samples as the working electrode. The working electrode was mounted on a rotating disc electrode with a rotating speed of 1000 rpm during the test. All the potentials were calibrated to a reversible hydrogen electrode (RHE). Supporting Information Available The Supporting Information is available free of charge online. FTIR, EDS spectra, elemental mapping images and BET results of the 3D M-MoSSe, SEM and TEM images of the prepared MoS2 and MoSe2 nanosheets, performance of the Li-O2 batteries with stable MoSSe cathode. XRD and XPS results of the discharged MoSSe cathode, exchange current density of the HER performance of MoS2, MoSe2 and MoSSe. 19 ACS Paragon Plus Environment

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Acknowledgement This work was supported by the National Natural Science Foundation of China under Grant No. 51432020 and fundamental research project from the Science and Technology Commission of Shanghai Municipality No. 14JC1493000 and 15DZ2281200. R. Shahbazian-Yassar and Z. Huang acknowledge financial support from the National Science Foundation (Award No. DMR-1620901). References 1.

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Table of Contents: In this work, we report a novel and effective strategy to distort the lattice structure by constructing a metastable MoSSe solid solution and thus, tune its catalytic activity for the deep-cycle Li-O2 batteries.


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In this work, we report a novel and effective strategy to distort the lattice structure by constructing a metastable MoSSe solid solution and thus, tune its catalytic activity for the deep-cycle Li-O2 batteries.

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Local Lattice Distortion Activate Metastable Metal ... - ACS Publications

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