Fast Energy Storage in Two-Dimensional MoO2 Enabled by Uniform

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Fast Energy Storage in Two-Dimensional MoO2 Enabled by Uniform Oriented Tunnels

Yuanyuan Zhu,†,⊥ Xu Ji,‡,⊥ Shuang Cheng,*,† Zhao-Ying Chern,∥ Jin Jia,# Lufeng Yang,§ Haowei Luo,† Jiayuan Yu,† Xinwen Peng,† Jenghan Wang,∥ Weijia Zhou,*,†,# and Meilin Liu*,§

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†

New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou 510006, China ‡ College of Automation, Zhongkai University of Agriculture and Engineering, Guangzhou 510225, China ∥ Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan # Institute for Advanced Interdisciplinary Research, University of Jinan, Jinan, Shandong 250022, China § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States S Supporting Information *

ABSTRACT: While pseudocapacitive electrodes have potential to store more energy than electrical double-layer capacitive electrodes, their rate capability is often limited by the sluggish kinetics of the Faradaic reactions or poor electronic and ionic conductivity. Unlike most transition-metal oxides, MoO2 is a very promising material for fast energy storage, attributed to its unusually high electronic and ionic conductivity; the one-dimensional tunnel is ideally suited for fast ionic transport. Here we report our findings in preparation and characterization of ultrathin MoO 2 sheets with oriented tunnels as a pseudocapacitive electrode for fast charge storage/release. A composite electrode consisting of MoO2 and 5 wt % GO demonstrates a capacity of 1097 C g−1 at 2 mV s−1 and 390 C g−1 at 1000 mV s−1 while maintaining ∼80% of the initial capacity after 10,000 cycles at 50 mV s−1, due to minimal change in structural features of the MoO2 during charge/ discharge, except a small volume change (∼14%), as revealed from operando Raman spectroscopy, X-ray analyses, and density functional theory calculations. Further, the volume change during cycling is highly reversible, implying high structural stability and long cycling life. KEYWORDS: capacitor, energy storage, MoO2, operando Raman, density functional theory calculations including oxides,11−17 nitrides,18−20 sulfides,21,22 and other effective materials.23−26 The desired properties of an ideal pseudocapacitive electrode material include variable valence state, high electronic and ionic conductivity, fast rate of Faradaic reactions, and small change in volume or crystal structure during the charge/discharge cycling. Monoclinic MoO2 has almost all of the desired properties and seems to be an excellent choice. Its low electrical resistivity (8.8 × 10−5 Ω·cm at 300 K)27,28 makes it advantageous among many transitionmetal oxides for fast charge storage.29,30 For example, the lithium storage capacity of MoO2 monolayer was highlighted by Zhou et al., predicting a theoretical capacity of 2513 mAh g−1 with six-layer Li+ on each side of the layer.31 Indeed, high

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ighly efficient and durable electrical energy storage devices are urgently needed for many emerging technologies, including portable electronics, electrical cars, and smart grids for the deployment of renewable energy.1 Batteries and supercapacitors are two of the most popular electrical energy storage devices.2−5 The former possesses higher energy density, while the latter has greater power density or higher rate capability. Both of them may be needed to meet the required energy and power demand. Pseudocapacitors, however, can store energy through both Faradaic reactions and electrical double layers, thus having potential to offer much higher energy density than conventional carbonbased electrical double-layer capacitors (EDLCs) while maintaining higher power density and durability than batteries. 6,7 Many efforts have been devoted to the optimization of structure, composition, and morphology of electrode materials in an effort to enhance the performance of pseudocapacitors;8−10 widely studied electrode materials are conductive polymers and various transition-metal compounds, © XXXX American Chemical Society

Received: April 30, 2019 Accepted: August 8, 2019 Published: August 8, 2019 A

DOI: 10.1021/acsnano.9b03324 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Crystal structure and morphology characterization. (a) Atomic model of the 2D MoO2: Atomic arrangement viewed down along “a” direction (bottom right image), the tunnels extend along “a” direction; atomic structure viewed down [100] (bottom left image) and side view (upper image) for the practical 2D MoO2 with a surface plane of (100); the tunnels exhibit an angle of 30° with the normal direction of (100) plane, which also can be seen clearly from the partial stereographic projection (bottom middle images). (b, c) XRD patterns (b) and Raman spectra (c) of MoO2 and MoO2/GO-5%. (d) SEM and HRTEM image of the 2D MoO2. (e, f) FFT pattern for a square area in HRTEM (e) and single crystal diffraction pattern of standard MoO2 crystal structure viewed down toward the (100) plane (f). (g, h) STEM (g) and AFM images (h) of a single 2D MoO2 sheet.

capacity (∼1500 mAh g−1) has been achieved when used as the anode in a Li-battery.29,30 While the electrochemical performance of MoO2 as a pseudocapacitive electrode has also been evaluated,14,32 in aqueous electrolytes, the observed capacity was more likely from the substrate, not from the MoO2 due to its low insertion potential; water would be decomposed before significant charge storage could take place in MoO2. Accordingly, most existing reports are about the energy storage properties of MoO2 as anodes for batteries, where phase conversion reactions always occur, accompanied by slow kinetics and large volume change. The fast charge storage properties of MoO2 as a pseudocapacitive electrode are yet to be explored. First, high electronic and ionic conductivities are required for high power density. MoO2 has not only high electronic conductivity but also considerable ionic conductivity due to the presence of one-dimensional (1D) tunnels for fast ion diffusion. Thus, controlling of MoO2 structure (especially the tunnels) is vital for fast energy storage. In the present work, a

high-quality two-dimensional (2D) MoO2 structure is synthesized through carefully tuning the reaction conditions. The 2D MoO2 possesses a thickness of only ∼1.3 nm, about three times of the (100) spacing, and the tunnels are oriented along the thickness direction, dramatically reducing the length of ion diffusion. Second, maintaining the structural stability is vital to achieving long cycling life because degradation in electrochemical performance is often associated with structural change or reconstruction of the electrode material during cycling. However, the structure evolution of MoO2 during lithiation is still not well understood. In this study, in situ/ operando Raman spectroscopy and X-ray diffraction (XRD) techniques, together with density functional theory (DFT)based calculations, are used to probe the structural changes of MoO2-based electrodes in a capacitor during cycling. Further, to minimize self-restacking of the 2D MoO2 layers, graphene or graphene oxide (GO) is introduced to form a composite or B


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Figure 2. Electrochemical characterization. (a, b) CV profiles (a) and cycling stability (b) of MoO2 and MoO2/GO-5% measured by CV at 10 mV s−1. (c) Long cycling stability of MoO2/GO-5% at 10 mV s−1. (d, e) CV profiles (d) and gravimetric capacitances (e) of MoO2/GO-5% at different sweep rates. The capacitive contribution of the MoO2/GO-5% at different scan rates of (f) 20, (g) 200, (h) 500 mV s−1.

362, 348, 229, 206, and 128 cm−1. The bands at 568 and 742 cm−1 are attributed to the stretching vibrations of the Mo−O (I) and Mo−O (II) groups in the lattice,14,38,39 and other bending vibrations have been detailed in the DFT calculations below. The D bands at 1348 cm−1 and G bands at 1596 cm−1 corresponds to the characteristic bands of carbon, confirming the presence of GO in the hybrid sample. Morphology and structure of the as-obtained MoO2 were characterized by SEM, TEM, STEM, and AFM. Quasihexagonal 2D sheets with a length in several hundred nanometers are shown in Figure 1d. The profile of the sheet even below the upper two layers still can be seen clearly, indicating an ultrathin structure, which is coincident with the AFM result (Figure 1h) where thickness is determined to be ∼1.3 nm (∼3 times of the (100) spacing or several atomic layers) for one typical sheet. After mixing with GO, morphology of the 2D sheets is not affected (Figure S1), while the specific surface area increases with the GO ratio (Table S1). HRTEM was employed to examine the crystal orientation of the sheet in Figure 1d and found a clear fringe spacing of 0.242 nm. In fact, there are several lattice planes that have similar spacing around 0.24 nm, as presented in the XRD pattern. To confirm the lattice plane observed, a fast Fourier transform (FFT) (Figure 1e) was made to a square area in the HRTEM, and the brightest spot should correspond to the plane whose fringe spacing is observed in HRTEM. Its distance from the central and the angles with other diffraction spots is consistent with those of the (020) plane in the standard crystal structure of MoO2; hence, the plane should be the (020) lattice plane. The FFT pattern is also consistent with the single crystal diffraction pattern of the standard crystal structure viewed down toward the (100) plane, as presented in Figure 1f; hence, it can be determined that the broad surface of the 2D sheet is the (100) plane and the 2D sheet is stacked by

hybrid electrode because of its good electrical conductivity and excellent structural stability.33−37 When tested in a capacitor, the hybrid electrode consisting of 2D MoO2 and GO demonstrates a capacity of 1097 C g−1 at 2 mV s−1 and 390 C g−1 at 1000 mV s−1 in an organic electrolyte, achieving both high capacity and good rate capability. Operando Raman spectroscopy reveals that the high-frequency vibrations of Mo−O stretching (742 cm−1), perpendicular to the tunnel, and low-frequency ones of Mo− Mo bending (128 and 206 cm−1), parallel to the tunnel, show redshift and blueshift during lithiation, respectively, implying the tunnels are the ion diffusion path, as confirmed by DFTbased calculations. In situ XRD analysis suggests that the crystal structure of MoO2 remains unchanged, but there is a small volume change (∼14%) during charge/discharge (the small volume change is critical to practical use of an electrode); the lattice axis parallel to the 1D tunnels has a slight shrinkage of ∼1%, whereas the ones perpendicular to the tunnels expand ∼7% after full lithiation.

RESULTS AND DISCUSSION Structural and Morphological Features. The structural features of MoO2 are schematically shown in Figure 1a (bottom right), consisting of [MoO6] octahedral connections that form 1D tunnels along a-axis. The XRD pattern of the assynthesized MoO2 sample via an one-step chemical vapor reduction process is shown in Figure 1b, suggesting that it has a monoclinic phase (space group P21/c), a distorted rutile-type structure, with lattice constants of a = 5.6109 Å, b = 4.8562 Å, c = 5.6285 Å, α = γ = 90°, β = 120.95° and cell volume of V = 131.52 Å3, consistent with the standard card (JCPDS no. 731249). The XRD pattern of the hybrid MoO2−GO electrode is similar to that of MoO2. The Raman spectrum of MoO2 (Figure 1c) shows characteristic bands at 742, 568, 496, 458, C


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Figure 3. Operando Raman test system and data. (a) Schematic illustration of the operando Raman measurement system. (b, c) Operando Raman signal evolution (b) and the corresponding mapping mode (c) of the MoO2/GO-5% electrode in a complete charge/discharge CV cycle at a scan rate of 1.667 mV s−1 with a potential window of 0.5−2.5 V vs Li/Li+.

(100) layers, which means that the openings of the 1 × 1 tunnels shown in Figure 1a are exactly on the broad surface of the sheet and oriented along a/x-axis having an angle of 30° with the normal direction. STEM image in Figure 1g exhibits a clear and doped-free surface with reasonable slight reconstruction. Surface plane and the side view of the 2D MoO2 as well as the orientation of the uniform oriented 1D tunnel seen from a partial stereographic projection are illustrated in Figure 1a according to the above analysis. Electrochemical Performance. Electrochemical performances of the as-made samples were investigated in a twoelectrode configuration. As shown in Figure 2a, there are two distinct pairs of redox peaks with anodic peaks at about 1.45 and 1.18 V (vs Li/Li+) and cathodic peaks at about 1.58 and 1.82 V (vs Li/Li+) in the CV curves, attributed to a two-step intercalation/deintercalation of Li+ (the lithiation/delithiation, MoO2 + xLi+ + xe− ↔ LixMoO2).40 The slightly larger CV area of MoO2/GO-5% (Figure S2) reveals a higher capacity than those of composite samples with a different GO ratio and pure MoO2 (Figure S3a−f) owing to the slight decline of interface charge-transfer resistance after the adding of GO (see

Figure S4). The cycling behavior of the MoO2/GO-5% at 10 mV s−1 is also better than that of pure MoO2 (Figure 2b) and the other composites (Figure S3g). The capacity of MoO2 declined rapidly with about 50% left. In contrast, the capacity of MoO2/GO-5% hybrid dropped by only 4% after 50 cycles at 10 mV s−1; furthermore, it exhibited a relative stable performance during 10,000 cycles (Figure 2c) accompanied by decreasing resistance (Figure S5e), suggesting that the addition of GO is beneficial to improve the durability due to the effective isolation to the ultrathin MoO2 sheets. As seen from Figure 2d, the CV profiles of MoO2/GO-5% electrode have similar shapes with a slight shift of the cathodic and anodic peaks, even when the scan rate was increased to 1000 mV s−1, implying an excellent rate performance. The corresponding capacities are shown in Figure 2e; the capacity can achieve 1097 C g−1 at a scan rate of 2 mV s−1 (according to an energy density of 259 Wh L−1 at 934 W L−1, as shown in Figure S5d) and still be able to reach 390 C g−1 at 1000 mV s−1, demonstrating high capacity and good rate performance, which should be ascribed to the high conductivity of MoO2 (Table S2) and the wide exposure of 1D tunnels’ opening. The D


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Figure 4. In situ XRD results. In situ XRD spectra and the corresponding contour plots in two complete charge/discharge GCD cycles of the MoO2/GO-5% electrode at a current density of 0.2 A g−1 in a potential window of 1−2.5 V vs Li/Li+.

Structural Evolution Revealed by Operando Raman and XRD Analyses. Figure 3a is a schematic illustration of the configuration used for operando Raman measurements, including a two-electrode model cell system with a quartz window to allow laser light to pass through. One Raman spectrum was acquired at an interval of 0.1 V when the electrode was subjected to a CV test at a scan rate of 1.667 mV s−1 in a potential range of 0.5−2.5 V vs Li/Li+. When the working electrode was discharged from 2.5 to 0.5 V, obvious changes in peak position and intensity were observed in the Raman spectra (Figure 3b). There is little change in spectral features in the potential range from 2.5 to 1.9 V. As the potential was changed from 1.8 to 0.9 V, however, the bands centered at 128 and 206 cm−1 shifted to higher frequencies (blueshift) and those at 348, 362, and 742 cm−1 follow an opposite trend (redshift), accompanied by a gradual weakening in intensity during the shift. Then two new bands at 288 and 636 cm−1 appeared when discharged to 0.8 V, together with a strengthening in intensity of the bands, and then there was change in a wide potential range of 0.8 V → 0.5 V → 2.0 V (maintained until charged to 2.0 V). The changes in these Raman signals imply that lithium ions are slowly embedded into the 2D MoO2 along the 1D channel during discharging, as explained in the DFT calculation below. This is in good agreement with the CV discharge curve, the reduction reaction started at ∼1.8 V and ended at ∼0.8 V, which means the lithiation is completed at ∼0.8 V accompanied by a band-structure change and unchanged after that. During the charging, on the other hand, the evolution in Raman bands reversed the direction as the potential was changed from 0.5 to 2.5 V; the two new bands began to disappear at 2.1 V, while the other bands began to appear and

capacity obtained here is higher than the theoretical value of MoO2 (756 C g−1, assuming a one-electron associated reaction) and better than those reported for most compounds (Table S3), which can be attributed to a high surface area of the sample that induces high eletronic double-layer (EDL) capacity. The rate performance (390 C g−1 at 1000 mV s−1, meaning nearly 52% of the theoretical capacity can be stored in ∼3 s) is even better than carbon-based electrodes that has been reported.41−44 In addition, the proportion of capacitive contribution of the sample is also analyzed with the power law in Figure 2f−h, the gray areas represent the capacitive contribution at 20, 200, and 500 mV s−1, which increases along with the sweep rate and is up to 90% at 500 mV s−1 (Figure S6). It can be concluded that the surface-controlled process, not the diffusion-controlled ones, dominates the charge storage process, indicating pseudocapacitive behavior. Furthermore, the galvanostatic charge/discharge curves and the corresponding capacities at different current densities from 0.5 to 100 A g−1 are shown in Figure S5a−c. At a current density of 0.5 A g−1, the capacity is up to 1311 C g−1 and still able to maintain a high capacity of 430 C g−1 with a high current density of 100 A g−1. To address the influence of loading amount, the thickness and electrochemical performance upon loading are presented in Figures S7−S12. Though there is an obvious decline in the capacitance, a high capacity of ∼1000 C g−1 still can be achieved when the thickness of active material is as high as ∼55 μm with a mass of ∼3 mg cm−2 on copper foil. In a parallel experiment, sphere-like MoO2 was prepared, and its electrochemical performance was also collected under the same conditions (Figures S13 and S14), which is much worse than that of the MoO2 sheets with oriented tunnels. E


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Figure 5. DFT calculation results. (a) The unit cell of MoO2 is plotted in the center; green, red, and pink balls represent Mo, O1, and O2 atoms, respectively. The principle a, b, and c axes are shown in red, green, and blue arrows. The Li+ (purple balls) adsorbed on the O1 and O2 sites are plotted in the left and right, respectively. (b) The changes of cell volume, a, b, and c axes for lithiated Li0.02MoO2, Li0.5MoO2, and LiMoO2 are listed in black, red, green, and blue numbers. (c) The high-frequency vibrations of Mo−O stretching modes. The atomic movements are shown in the black arrows. (d) The low-frequency vibrations of bending modes. The black circle and dots indicate the movement is in and out of the plane. (e) The additional vibrations of Li−O stretching and bending modes for lithiated LiMoO2.

coincided with the first one, exhibiting the excellent reversibility again. It is worth noting that only some peaks have undergone a slight shift and no new phase has been detected during charge/discharge, suggesting that there is no phase transition and the transition from monoclinic to orthotropic phase is suppressed in tunnel-structured MoO2 2D sheets. The small lattice expansion/recontraction caused by lithium ion intercalation/deintercalation indicates that the 1D tunnel along a axis and almost perpendicular to the sheets provides fast ion diffusion pathways without destroying the structure of MoO2, while the diffraction peaks of the pure MoO2 electrode can not exactly return to their pristine positions during charge/discharge (see the operando synchrotron XRD result in Figure S16), which should be caused by the fast aggregation of the ultrathin MoO2 sheets. DFT Calculations. In the computational study, we initially examined the adsorption energy (Eads) of the Li+ located in the transportation tunnel along a axis. At low concentration (Li0.02MoO2), Li+ has two stable adsorption sites, as Li+ can bond to either O1 or O2 (Figure 5a), with Eads of −3.44 and −3.27 eV, respectively, which should correspond to the two redox pairs in the CV curve. The similar and strong Eads indicates that Li+ can stably locate in the tunnel with no preferential locations. Also, the negligible activation barrier (Ea = 0.05 eV) for the lithium ion transporting between the two sites implies that MoO2 is an excellent ionic conductor for Li+ smoothly moving along the tunnel. At high concentration, Li+ can be fully occupied in the tunnel forming LiMoO2 with still high averaged Eads (−2.80 eV), in consistent with the Li0.98MoO2 observation in the complete discharge state. To further justify the capacity for MoO2 electrode, we examined the volume change of MoO2 varied with Li+ concentration, as schematically plotted in Figure 5b. The cell

pertained to MoO2 slowly. When charged to 2.5 V, all Raman bands belonging to MoO2 returned to their original positions. Along with the CV curves collected at the same time, our results indicate that the structural evolution of MoO2 is highly reversible during the charge/discharge processes, as presented in Figure 3c. We can clearly and concisely observe the reversible movements and intensity changes of the Raman bands. In situ/operando XRD technique is also a powerful tool for exploring the crystal structure change of materials under electrochemical processes. Figure 4 displays the changes in peak position and intensity of in situ XRD diffraction spectra in two complete discharge/charge processes of the MoO2/GO5% electrode; the corresponding discharge/charge curve is shown in Figure S15a. In the first discharge process from 2.5 to 1 V, the diffraction peak around 26° shifts to a lower angle, corresponding to the expansion of the (011) plane (see Figure 1a) due to the insertion of lithium ions. At the same time, the peak around 37.3° shifts to a higher angle, corresponding to the shrinkage of the (002) plane. At the completion of discharge, the XRD pattern coincides with JCPDS no. 84-0601 and indicates the formation of Li0.98MoO2 (Figure S15b) with the monoclinic phase (space group P21/c), lattice constants of a = 5.5654 Å, b = 5.2086 Å, c = 5.8587 Å, α = γ = 90°, β = 118.765°, and cell volume of V = 148.87 Å3. Compared with MoO2 (see Table S4), lattice parameters of b and c increase, α and β decrease, and the volume expands slightly (14%). Small volume changes of lithiated MoO2 are very advantageous for the long-term cycling stability of the electrode material. During the first charging process from 1 to 2.5 V, the corresponding diffraction peaks return to their pristine positions due to delithiation, revealing the outstanding reversibility. Also, in the second discharge/charge cycle, all the diffraction peaks F


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METHODS

volume shows a small expansion (2%) at rather low concentration of Li0.02MoO2, and b and c axes are slightly enlarged by 1%, whereas a axis, the tunnel, has no change. The cell volume of V further expands to 7%, as Li+ occupied half of the available sites (Li0.5MoO2), and b and c axes are enlarged by 4% and 3%, respectively, while a axis still has no change. As Li+ fully filled the tunnel at the maximum capacity of LiMoO2, the cell volume of V is expanded by 14%, and b and c axes are enlarged by 7% and 5%, respectively, while surprisingly, a axis shrinks 1%. The computed changes for LiMoO2 are comparable to the in situ XRD observation for Li0.98MoO2 that the volume expands 14% and a, b, and c axes changes by −1%, 7%, and 4%, respectively, implying that MoO2 electrode has a high capacity to completely accept Li+ in the fully discharge state. Finally, the vibrations observed from the operando Raman spectroscopy have been analyzed computationally. The selected vibrations are shown in Figure 5c−e. The complete vibrations before and after lithiation are demonstrated in the Supporting Information as animated Figure S17. Figure 5c shows the first two high-frequency vibrations at 751 and 545 cm−1 (experimentally observed as 742 and 568 cm−1, respectively), corresponding to the Mo−O stretching modes. Figure 5d shows the two lowest vibrations at 210 and 129 cm−1 (experimentally observed as 206 and 128 cm−1, respectively), which are related to the bending modes; specifically rocking and twisting of Mo−Mo pair, respectively. As Li+ occupied in the tunnel in the discharged state, two new vibrations of 653 and 288 cm−1 appear, corresponding to the Li−O stretching and bending modes, respectively, which are comparable to the 636 and 288 cm−1 peaks in the operando Raman spectra in the discharging process. Also, as observed from the experiment, the high-frequency vibration of 751 cm−1 redshifts significantly to 678 cm−1 in the lithiated LiMoO2, attributable to that its stretching direction is perpendicular to the Li+ transportation tunnel (a axis). Oppositely, the lowfrequency bending direction is parallel to the a axis and its frequency at 129 cm−1 dramatically blueshifts to 181 cm−1 in lithiated LiMoO2. The computational results are comparable to the experiments and mechanistically clarify the spectroscopic observation for the MoO2 electrode in the charge/discharge processes.

Preparation of MoO2 and MoO2/GO Composites. First, 2D MoO2 sheets were synthesized via a chemical vapor reduction of commercial MoO3 powders (99.9%, Aladdin) sublimated in quartz tube in Ar−H2 flow (10% H2, 200 sccm) at 900 °C for 60 min.45 Then the obtained MoO2 powder was magnetic stirred for 30 min in deionized water, and different proportions (1, 5, 10, 20, and 50 wt %) of graphene oxide (GO, Suzhou TANFENG graphene Tech Co.,Ltd., 5 mg/mL) were added dropwise and stirred for 24 h, sonicated for 1 h, and freeze drying for 24 h. The samples were identified as MoO2/ GO-1%, MoO2/GO-5%, MoO2/GO-10%, MoO2/GO-20%, and MoO2/GO-50%, respectively. In-situ XRD, Synchrotron XRD, and Raman Measurments. XRD patterns were determined by D8 Advance (Germany Bruker) Xray diffractometer using Cu Kα radiation (λ = 0.15406 nm) at a scanning rate of 0.16° s−1 in the range of 10° < 2θ < 70°. In situ XRD patterns were recorded between 20° and 40° at a scanning rate of 0.08° s−1, using a 0.02° step size. The in situ battery cell uses a Be foil as a visible window to transmit X-rays and carbon paper as current collector, and mass loading of active material was around 2.5 mg cm−2. Meanwhile, the in situ cell was charged/discharged at a current density of 0.2 A g−1 with the potential window of 1−2.5 V vs Li/Li+. Operando synchrotron SXRD was carried out at the same cycling condition with the X-ray beamline (λ = 0.24125 Å) at the National Synchrotron Light Source II (NSLS II) at Brookhaven National Laboratory (BNL). Operando Raman spectra obtained by a LabRAM HR800 spectrometer (Horiba Jobin Yvon, FR.) equipped with a diode pump solid-state laser (wavelength = 532 nm). Operando Raman spectra were continuously captured while a cyclic voltammetry test was performed at 1.667 mV s−1 in 0.5−2.5 V vs Li/Li+ on an in situ cell with quartz window. A spectrum is recorded every 60 s or every 0.1 V. Morphology, Conductivity, and Structure Characterization. Scanning electron microscopy (SEM, Hitachi SU8010) and transmission electron microscopy (TEM, JEM-2100F field emission electron microscope, JPN) were used to observe the morphologies and the structure. A scanning transmission electron microscope (STEM, FEI Tecnai G2 F30) was employed to acquire surface structure in atomic level. Thickness was analyzed by atomic force microscopy (AFM, Bruker Dimension Icon scanning probe microscope, Bruker Co., Germany), operated under ambient conditions. The Brunauer−Emmett−Teller specific surface area was attained on a Kubo X1000 instrument with nitrogen adsorption at 77 K using the Barrett−Joyner−Halenda method. Conductivity of the samples was detected with a four-probe conductivity meter. Electrochemical Measurements. The electrochemical performances of the MoO2 and MoO2/GO electrodes were characterized in a two-electrode system. Working electrodes were prepared by mixing active materials (MoO2 or MoO2/GO), acetylene black, and polyvinylidene difluoride in a weight ratio of 8:1:1 in N-methyl pyrrolidone solvent and then coated on copper foil serving as current collector (during in situ XRD test, the current collector is replaced by carbon paper). Lithium wafer is used as a counter electrode, and 1 M LiClO4 in propylene carbonate solution (1:1 in volume ratio) is used as an electrolyte. Mass loading of the active material is obtained by carefully measuring the weight of current collector used with and without active material using a precision balance with a minimum scale of 0.01 mg, which is averaged by more than 10 pieces and double checked by measuring each of them before it is assembled. After assembly in a CR 2032 coin cell, cyclic voltammetry (CV) and galvanostatic charge−discharge (GCD) were performed using a CHI 660E electrochemical workstation. Electrochemical impedance spectra were measured using a Solartron 1260 impedance analyzer in a frequency range from 0.01 Hz to 100 kHz with a perturbation voltage of 10 mV at open-circuit potential. DFT Calculations. The present study employed Vienna Ab Initio Simulation Package46−50 for the DFT calculation with a 3D periodic boundary condition. The exchange−correlation function utilized in the DFT is the generalized gradient approximation49 with Perdew−

CONCLUSION Benefiting from the high electronic conductivity and the fast ion diffusion ability, the hybrid electrode consisting of ultrathin 2D MoO2 sheets and GO shows good energy storage properties. When tested in a capacitor with an organic electrolyte, the MoO2/GO-5% electrode can deliver not only high capacity but also excellent rate capability, far better than other metal-oxide-based pseudocapacitive electrodes. XRD analysis shows that the crystal structure of MoO2 remains unchanged, although ∼14% volume expansion is observed at a fully discharged state, implying high stability during cycling. Further Raman analysis reveals that the structure changes during charge/discharge are highly reversible, which is also critical to stability. Besides, the electrolyte ion diffusion path, the 1D tunnel, and one Li+ accommodation per MoO2 unit to form LiMoO2 at fully discharge state are confirmed by both experimental measurements and DFT-based calculations. The excellent electrochemical performance of the 2D MoO2 sheetGO electrode makes it a very promising candidate for commercial pseudocapacitors. G


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ACS Nano Wang 1991 formulation, known as GGA-PW91.50 The core electrons are simulated by the cost-effective pseudopotential, and the valence electrons are expanded by the planewaves with their kinetic energies smaller than the cutoff energy of 600 eV, the projector-augmented wave method.51,52 The integration in the Brillouin zone is examined in the reciprocal space and sampled by the Monkhorst−Pack scheme53 at 0.05 × 2 (Å−1) interval. All the modeled structures are optimized by quasi-Newton method with the energetic and gradient convergences of 1 × 10−4 eV and 1 × 10−2 eV, respectively. Nudged elastic band method54 was used to locate the transition states in the Li+ transportation at the same energetic and gradient convergences. The vibrational modes and frequencies are analyzed by the finite displacement approach that the Hessian matrix (force constant matrix) is derived by, slightly displacing atoms from their optimized positions and truncated to the finite size of the modeled supercell. The MoO2 nanosheet is constructed by the 2 × 2 × 2 unit cells, in which all the atoms and cell volumes are free to relax to optimize the structure and lattice parameters. One Li+ is initially embedded in the constructed MoO2 model to examine the suitable adsorption positions and their related energetics in the ionic transportation. More Li+, two, four and saturated eight, are further embedded in the MoO2 models to examine the changes of the lattice parameters and cell volume and to compare with the observation in situ XRD. Also, the optimized structures of the clean and fully lithiated (8 Li+) MoO2 are applied to examine their vibrational modes and frequencies for the mechanistic understanding in the operando Raman spectra.

Key Support Major Research Project of China (no. 91745203), Tip-Top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program (2016TQ03N541), Guangdong Natural Science Funds for Distinguished Young Scholar (2017B030306001), and Guangdong Innovative and Entrepreneurial Research Team Program (grant 2014ZT05N200).

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b03324. Experimental methods, SEM, TEM, and the corresponding elemental mapping, CV and GCD curves, Nyquist plots, SEM images of the cross sections of MoO2/GO5% electrodes with different mass loading and the electrochemical performance, calculation methods for the charge storage ability, GCD curves during the in situ XRD test, XRD pattern after complete discharge, in situ SXRD, lattice parameters of MoO2 and Li0.98MoO2 (PDF) Raman vibrations of MoO2 (ZIP)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Shuang Cheng: 0000-0001-6301-175X Xinwen Peng: 0000-0002-4575-256X Jenghan Wang: 0000-0002-3465-4067 Weijia Zhou: 0000-0003-4339-0435 Meilin Liu: 0000-0002-6188-2372 Author Contributions ⊥

Y.Z. and X.J. contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for Central Universities of SCUT, China (nos. 2018ZD20, D2182400), Guangzhou Science and Technology Program (no. 20181002SF0115), the National Science Foundation for H


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Fast Energy Storage in Two-Dimensional MoO2 Enabled by Uniform

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