Sulfur Refines MoO2 Distribution Enabling Improved

Jul 18, 2014 - Studies also reveal that some interconnected MoO2 nano- strutures perform well without the help of carbons.25,32 For instance an ordere...
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Sulfur Refines MoO2 Distribution Enabling Improved Lithium Ion Battery Performance Zhanwei Xu,†,‡ Huanlei Wang,†,‡ Zhi Li,*,†,‡ Alireza Kohandehghan,†,‡ Jia Ding,†,‡ Jian Chen,‡ Kai Cui,‡ and David Mitlin*,†,‡,§ †

Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2 V4, Canada National Institute for Nanotechnology (NINT), National Research Council of Canada, Edmonton, Alberta T6G 2M9, Canada § Chemical & Biomolecular Engineering and Mechanical Engineering, Clarkson University, 8 Clarkson Avenue, Potsdam, New York 13699, United States ‡

S Supporting Information *

ABSTRACT: We employ a sulfur-assisted decomposition process to create agglomerates of large (200−500 nm) yet highly nanoporous threedimensional MoO2 single crystals partially covered with a few atomic layers of MoS2 (“MoS2/MoO2 nanonetworks”). These materials are highly promising as lithium ion battery anodes. At a current density of 100 mA g−1, the MoS2/MoO2 nanonetworks exhibit a reversible discharge specific capacity of 1233 mAh g−1, with only 5% degradation after 80 full charge/discharge cycles. Moreover at the relatively fast discharging rates of 200 and 500 mA g−1, the capacities are 1158 and 826 mAh g−1, respectively. A comparison with literature shows that these are among the more promising reversible capacity, cycling capacity, and rate capability values reported for MoO2. The electrochemical properties are attributed to the material’s nanoporous crystal morphology that allows for facile reversible transport of Li ions without either disintegration or agglomeration of the structure.

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transition (monoclinic 1 → orthorhombic → monoclinic 2) is “only” 11%, which is substantially less than for materials such as Si (∼300%). The performance of bulk MoO2 is generally poor due to sluggish lithiation/delithiation kinetics of micronscale particles.24 Recently, several MoO2 nanomaterials have been synthesized and employed as LIB anodes.4,25−27 To further improve the performance of MoO2, researchers have created carbon-containing nanocomposites. Carbons in various form, such as carbon cloth,28 carbon coating,29 graphite oxide,27 and graphene,26,30,31 have been successfully used for this purpose. As an example, a hierarchical MoO2/graphene structure possesses a specific capacity of 580 m Ah g−1 after 70 cycles, tested at 100 mA g−1.26 Not employing a carbon-based secondary phase is advantageous for maximizing the energy density of the electrode, since carbon allotropes will add significant volume to the system. Studies also reveal that some interconnected MoO2 nanostrutures perform well without the help of carbons.25,32 For instance an ordered mesoporous MoO2 prepared with mesoporous silica as a template exhibits a reversible capacity as high as 750 mAh g−1 after 30 cycles, tested at 42 mA g−1.25

INTRODUCTION With increasing requirements for electric vehicles and portable electronic devices, such as mobile phones, laptop computers, and video cameras, lithium ion batteries (LIBs) have attracted much interest.1−5 Commercial LIBs generally employ graphitebased anodes. However, the maximum specific capacity of graphite is 372 mAh g−1, which limits its potential applications. There is therefore a considerable effort to develop alternative anode materials with improved lithium ion storage capacity.6−12 Nanoscale metal oxides, such as Co3O4,13,14 NiO,15 Fe3O4,16−18 and SnO2,19 exhibit high initial charge storage capacities. However, in many cases the cycling capacity retention of these materials is not entirely satisfactory. This is caused by a number of factors, including the pulverization of the electrode due to the several hundred percent volume expansion associated with the conversion reaction.20,21 Carbon additives are often used to improve both the electrical conductivity and the structural stability of such materials.14,16,18,19 Different from most other metal oxides, molybdenum oxide (MoO2) has a relatively high electrical conductivity (8.8 × 10−5 Ω cm at 300 K in bulk vs 5.3 × 10−6 Ω cm for metallic Mo). It also possesses a high theoretical charge storage capacity (838 mAh g−1 with Li),22,23 which for yet to be fully explained reasons is actually surpassed in experimental studies. Moreover, the volume change associated with this insertion phase © 2014 American Chemical Society

Received: May 13, 2014 Revised: July 12, 2014 Published: July 18, 2014 18387

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Figure 1. (a) XRD patterns of the MoS2/MoO2 nanonetworks, baseline MoO2, baseline MoS2, and MoS2/MoO2 particulates. MoO2, open black squares; MoS2, filled red circles. XPS spectra of the MoS2/MoO2 nanonetworks: (b) entire XPS spectrum, (c) Mo 3d, (d) O 1s, and (e) S 2p.

(NH4)6Mo7O24·4H2O was first mechanically ground. The powder was then heated to 500 °C in air, held for 2 h, and cooled naturally. After washing and drying the final product yield was 2.62 g. A baseline MoS2 was obtained by combining 0.50 g of MoO3 and 1.50 g of S powder. While the heat treatment was also identical to that for the nanonetworks, the key difference was that the crucible employed for holding the mixture at temperature was covered. This prevented the S from escaping and ensured full sulfidation of the oxide. The final product yield was 0.57 g. X-ray diffraction (XRD) analysis was performed using a Bruker Discover 8 Diffractometer, with Cu Kα radiation (λ = 1.5406 Å) that was monochromatized using a single Gobel mirror. The X-ray photoelectron spectroscopy (XPS) spectra were recorded on an Axis Ultra spectrometer with Al (Mono) Kα X-ray source (1486.6 eV). The base pressure of the analysis chamber was below 10−9 mbar. Scanning electron microscopy (SEM) analysis was performed using a Hitachi S-4800 field emission scanning electron microscope. Transmission electron microscopy (TEM) analysis was performed using a JEOL 2100 LaB6 TEM, at a 200 kV accelerating voltage. For TEM analysis the samples were dispersed in alcohol with the aid of ultrasonic agitation for several minutes. A drop of the dispersion was deposited onto a copper grid covered by ultrathin carbon film supported by a lacey carbon film. Energy dispersive X-ray (EDS) analysis was employed by using a Hitachi S-3000 N scanning electron microscope. Thermal gravimetric analysis and differential scanning calorimetry analysis (TGA/DSC) was performed on a SDT Q600, TA Instruments DSC−TGA, in air with a heating rate of 10 °C min−1. Electrochemical characterization was performed in a CR2032-type coin cell. The electrode materials were prepared by mixing the active material with 15 wt % carbon black and 10 wt % PVDF (binder) in N-methylpyrrolidone (NMP) to form homogeneous slurry. The well-mixed slurry was then spread onto a copper foil and dried at 105 °C in a vacuum oven for 12 h. The foil was then punched into circular disk electrodes about 1.4 cm in diameter, with mass loading ∼2.5 mg on each electrode. The 2032 type button cell, copper foil, lithium foil, polyethene separator (porosity ∼36−44%, pore size ∼0.03

The success of such a material, as compared to the difficulty of working with micron-scale MoO2, highlight the importance of synthesizing the oxide in truly nanoscale dimensions. Our original hypothesis was that sulfur, being a highly reactive species toward Mo, would disrupt the orderly growth of large and fully dense MoO2 crystallites, rather promoting highly porous nanostructures. Such architectures would be then ideally suited for lithium ion battery applications. MoS2 is also a highly active LIB electrode material per se, which means that any secondary sulfide phase to form would also add to the overall capacity. Sulfur assisted synthesis has been successfully employed for refining the morphology of carbon nanotubes,33 SiC nanowires,34 and as a way for creating highly dispersed metal sulfide nanoparticles.35 However, it has never been attempted to template the growth of nanostructured oxides in general, or of MoO2 in particular. We demonstrate that despite the lack of secondary structural/conductive carbon our materials display one of the highest reversible charge storage capacities reported in literature for MoO2, while being able to survive 80 full lithiation/delithiation cycles with minimal capacity loss.

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EXPERIMENTAL SECTION Ammonium molybdate (NH4)6Mo7O24·4H2O and sulfur powder S were obtained from Sigma-Aldrich Co. The “MoS2/MoO2 nanonetworks” were synthesized from 1.00 g of (NH4)6Mo7O24·4H2O and 0.50 g of S powder, which were mechanically ground together for 5 min. The mixture was heated in a flowing (100 sccm) argon atmosphere to 500 °C using a rate of 10 °C min−1, followed by a 1 h hold and a natural cool to room temperature prior to removal. The resultant material was repeatedly washed with water and dried overnight under vacuum at 70 °C. The final product yield was 0.83g. Synthesis of a baseline “MoS2/MoO2 particulates” was performed by grinding a mixture of 0.50 g of MoO3 and 0.30 g Sof powder. The remaining fabrication approach was identical to that for the nanonetworks, with the final product yield being 0.45g. As a baseline microparticulate MoO3 was synthesized by decomposing (NH 4 ) 6 Mo 7 O 24 . A total of 3.00 g of 18388

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Figure 2. Morphology and structure of the MoS2/MoO2 nanonetworks. (a) Low magnification SEM micrograph, (b) higher magnification SEM micrograph, (c) bright field TEM micrograph, (d) corresponding indexed selected area diffraction (SAD) pattern. The single crystal SAD of the MoO2 is indexed directly on the experimental spots, while the fine polycrystalline MoS2 is shown on the left-hand simulation overlay. (e and f) Higher magnification bright field and HRTEM micrograph of the boxed region.

mm), and electrolyte (1 M LiPF6 in ethylene carbonate/ dimethyl carbonate, 1:1 in volume) were purchased from MTI Technologies. An argon glovebox with less than 0.2 ppm moisture and oxygen was employed for button cell assembly. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analysis were performed on a VersaSTAT3 potentiostat. CV were done at 1 mV s−1 scan rate in the potential range from 0.01 to 3.00 V (vs Li/Li+). Galvanostatic discharge/charge cycling was carried out using an BT2000 Arbin potentiostat, also in the range of 0.01 to 3.00 V.

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RESULTS AND DISCUSSION

X-ray diffraction (XRD) analysis was employed to investigate the product phases. The XRD patterns of the MoS2/MoO2 nanonetworks, baseline MoO2, baseline MoS2, and MoS2/ MoO2 particulates are shown in Figure 1a. The peaks at 2θ = 26.3, 37.1, 53.3, 60.4, and 66.8° (black hollow squares) correspond to the (011), (020), (220), (031), and (131) reflections of MoO2 (00-032-0671, P21/n (14), a = 5.607, b = 4.860, c = 5.537).26 The remaining weak peaks at higher 2θ values (Figure S1a) maybe also ascribed to MoO2. The weak peaks around 2θ = 14.4, 32.7, 39.6, and 58.3 o (solid red circles) are assigned to (002), (100), (103), and (110) reflections of MoS2 (03-065-0160, P63/mmc (194), a = 3.161, c = 12.295).36 The average (given that that interaction volume is in the range of a micron or more) composition of the MoS2/MoO2 nanonetworks was estimated by SEM EDS analysis. They were determined to contain approximately 61 atom %O, 33 atom %Mo and 6 atom %S, indicating a composite of 89 wt % MoO2 and 11 wt % MoS2.

Figure 3. MoS2/MoO2 particulates (a) SEM micrograph, (b) TEM micrograph, and (c) indexed corresponding SAD pattern showing the large crystallites of MoO2 indexed directly on the experimental SAD, and MoS2 indexed in the adjacent simulation. (d) HRTEM micrograph highlighting the morphology of the surface sulfides.

Figure S2a shows the air TGA curves of the baseline low surface area MoO2, which exhibits two main weight change steps. The first is a significant weight increase starting at around 18389

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Figure 4. Top row, MoS2/MoO2 nanonetwork electrodes; bottom row, MoS2 /MoO2 particulates. (a and b) CV curves, tested at 0.1 mV s−1. (c and d) Galvanostatic charge/discharge curves, tested at 100 mA g−1.

Figure 5. (a) Rate capability of the MoS2/MoO2 nanonetworks (black), baseline MoO2 (red), baseline MoS2 (green), and MoS2/MoO2 particulates (blue). In all graphs discharge is solid and charge is hollow. (b) Cycling performance of the MoS2/MoO2 nanonetworks, MoO2, MoS2, and MoS2/ MoO2 particulates, tested at 100 mA g−1. (c) Cycling performance of the MoS2/MoO2 nanonetworks at the current densities of 200 and 500 mA g−1. (d) Nyquist plots comparing MoS2/MoO2 nanonetworks and MoS2/MoO2 particulates, after 80 cycles.

300 °C and lasting to 600 °C. This is attributed to the oxidation of MoO2 to MoO3, with a corresponding 11.2% weight increase

that agrees well with theoretical 12.5%. The second step is a large weight loss starting at around 700 °C, being due to the 18390

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X-ray photoelectron spectroscopy (XPS) was employed to investigate the surface composition of the nanonetworks. Figure 1b shows the overall XPS spectrum, which reveals that the surface region of the obtained products is composed of Mo, O, and S. The Mo 3d XPS spectrum displays four peaks (Figure 1c). The strong peaks at 232.9 and 229.1 eV, are assigned to Mo 3d3/2 and Mo 3d5/2 of Mo4+.26,37 The weak peaks at 235.8 and 232.5 eV are assigned to Mo 3d3/2 and Mo 3d5/2 of Mo6+ species in an oxidic surrounding, respectively, showing that a small amount of Mo4+ on the surface was oxidized in air.26,37 The high resolution of O 1s XPS shows a peak at 530.1 eV, assigned to O 1s of MoO2 (Figure 1d).26 S 2p XPS spectrum shows two peaks at 163.7 and 162.3 eV, corresponding to S 2p1/2 and S 2p3/2 of MoS2 (Figure 1e).38 The obtained atomic composition of Mo, O, and S is 32.8%, 15.8%, and 51.4%, respectively. Therefore, it can be concluded that the nanonetworks’ surface is enriched in MoS2, having the mean composition of roughly 82 wt % MoS2, 15 wt % MoO2 and 3 wt % MoO3. The hierarchical architecture of the MoS2/MoO2 nanonetworks is illustrated in Figure 2. The “macro” structure consists of uniform particles that are around 200−500 nm in diameter. This is illustrated by the low magnification SEM micrograph in Figure 2a. As Figure 2b shows, each particle processes a sponge-like morphology with continuous ∼20 nm thick crystalline strands separated by an open network of nanopores that are on the same scale. The nitrogen adsorption isotherm of the nanonetworks is shown in Figure S1b. The calculated Brunauer−Emmett-Teller (BET) surface area is 39.6 m2 g−1. The pore size distribution plot also highlights the mesoporous structure of MoS2/MoO2 nanonetworks, with pores being distributed in the range of 5−35 nm. The TEM micrograph shown in Figure 2c further illustrates this complex hierarchical structure, which has never been previously reported for MoO2. The corresponding SAD, shown in Figure 2d, is a single crystal spot pattern corresponding to MoO2 and a ring pattern that is attributed to nanocrystalline

Figure 6. Comparison of the capacity versus current density of the MoS2/MoO2 nanonetworks (blue circles) versus the state-of-the-art MoO2 results from published literature.

sublimation of MoO3. Figure S2b shows the air TGA analysis of the baseline MoS2, which exhibits three weight loss steps. The initial weight loss of 2.1% occurs at 120 °C and is attributed to the evaporation of physically adsorbed water on the higher surface area sulfide. The second weight loss occurs between 400 and 600 °C. This 10.2% weight loss is attributed to the oxidation of MoS2 to MoO3, and agrees well with the theoretical 10.0%. Sublimation of MoO3 again begins to occur at 700 °C. TGA of the MoS2/MoO2 nanonetworks in air exhibits three weight change steps, as shown in Figure S2c. There is an initial weight loss of 0.7% at 120 °C that is associated with loss of adsorbed water. This is followed by a weight increase or 9.8% that occurs from 300 to 600 °C, which is attributed to the oxidation of both MoO2 and MoS2 to MoO3. Sublimation of MoO3 starts near 700 °C. It can thus be estimated that the MoS2/MoO2 nanonetworks are composed of 88 wt % MoO2 and 12 wt % MoS2, agreeing well with the SEM EDS analysis.

Table 1. Cycling Performance Comparison of MoS2/MoO2 Nanonetworks with State of the Art Literature for MoO2-Based Materials, All Tested As Half Cells vs Li/Li+ materials MoS2/MoO2 nanonetworks

sulfur assisted

Mesoporous crystalline MoO2 Hierarchical MoO2 Activated MoO2 Powder Amorphous MoO2 MoO2 hollow core−shell microspheres W-doped MoO2 Carbon-decorated WOx/MoO2 nanorods MoO2/graphene composite Annealed MoO2/ C nanocomposite MoO2/Graphene Nanoarchitectures MoO2/C nanowires

template nanocasting strategy templated thermal electrochemical activation solution process hydrolysis nanocasting method microwave-hydrothermal process

Carbon-coated MoO2 nanospheres MoO2/graphite oxide composite MoO2/graphene nanocomposite a

capacitya (mah g−1)

synthesis method

solution process hydrothermal method solution and annealed solution process solvothermally treating hydrothermal and thermal annealing solution process hydrothermal-calcination

current density (ma g−1)

voltage (V)

refs

1163(80) 1025(80) 654(80) 750 (30) 719 (20) 850 (30) 860 (50) 420 (30) 670 (20) 670 (50)

100 200 500 42 (0.05C) 200 100 100 50 75 (0.1C) 90 (0.1C)

0.01−3

this work

0.01−3 0.01−3 0.05−3 0.01−3 0.01−3 0.01−3 0.005−3

Shi et al.25 Sun et al.4 Ku et al.21 Ku et al.23 Lei et al.20 Fang et al.54 Yoon et al.53

530 629 598 500

540 200 1000 200

0.01−3 0.01−3 0.01−3 0.01−3

Bhaskar et al.30 Zhou et al.55 Sun et al.26 Gao et al.57

838 (1C) 100 100

0.1−3 0.005−3 0.01−2.5

Wang et al.58 Xu et al.27 Tang et al.56

(1000) (50) (70) (20)

675 (30) 726 (30) 1037 (60)

The capacity after (X) cycles. 18391

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The electrochemical charge storage performance of the MoS2/MoO2 nanonetworks and MoS2/MoO2 particulates was investigated through cyclic voltammetry (CV) and galvanostatic discharge/charge testing. These results are shown in Figure 4. The baseline MoO2 and MoS2 were also tested, being shown in Figures S8 and S9. Figure 4a shows the CV curves of the MoS2/ MoO2 nanonetwork electrodes, with the maximum of each peak being labeled on the figure. The CV curves of the MoS2/ MoO2 particulates are provided in Figure 4b. Figure 4c,d shows the constant current voltage profile of the nanonetworks and of the particulates, during the first, second, third and 10th cycles. The voltage profile of the MoS2/MoO2 particulate electrodes tested under identical conditions is provided in Figure 4d. The CV and voltage profiles of the baseline MoO2 and MoS2 are shown in Figure S8 and S9. For all systems the positions of the sloping plateaus generally agree with the CV results. In all cases a broad reduction peak in the range of 0.62 to 0.37 V (labeled at maximum of 0.49 V) is the voltage where solid electrolyte interface (SEI) formation occurs in most battery anodes during the first discharge, when tested in a half-cell configuration vs Li/Li+. For the nanonetworks and the particulates and baseline MoO2, there are two sets of reversible redox peaks at 1.49 V (red)/1.75 V (oxid) and 1.22 V (red)/1.47 V (oxid), which decrease in intensity with progressive cycling. While there is some literature debate regarding the phase transitions associated with these peaks, we agree with the dominant opinion that they correspond to insertion of Li into the MoO2 up to LiMoO2 (theoretical capacity 209 mAh g−1) and the associated monoclinic - orthorhombic - monoclinic phase transitions.25,26 However, this is not the complete story since the measured capacity is substantially above this value. The explanation to this discrepancy lies in the redox couple, consisting of a relatively narrow 0.25 V reduction peak and the substantially broader 0.5 V oxidation peak, which becomes progressively more pronounced with cycle number. The 0.25/ 0.5 V peaks are related to the reversible conversion reaction LiMoO2 + 3Li+ + 3e− → Mo + 4Li2O, which yields an additional 627 mAh g−1 of capacity, for a total of 836 mAh g−1 for the full voltage discharge.24 The MoO2 baseline and the MoS2/MoO2 particulates, which are coarser in microstructure, display the 0.25/0.5 V peaks of much lower intensity. Diffusional limitations would prevent a substantial portion of LixMoO2 in these large particles from disproportionating to metallic Mo and LiO2, reducing the kinetically accessible charge storage capacity. In the nanonetworks, we measured additional capacity beyond the theoretical 836 mAh g−1. The MoS2/MoO2 nanonetworks, the baseline MoO2 and the MoS2/MoO2 particulates show an activation behavior, with capacity beginning to slowly increase with cycling. This occurs anywhere from cycle 1 to cycle 10 in the materials, and is further illustrated in Figure 5. A capacity beyond the theoretical and a cycling-induced activation behavior has been previously reported for the MoO2 system.25,26,43 Surface adsorption was previously argued by researchers to be partially responsible for the extra capacity that they observed beyond the theoretical.22,43 A highly likely explanation for the extra capacity and activation is the cycling-induced formation of a polymer gel on the nanostructured conversion oxide electrode’s surface during lithiation, with its consequent dissolution upon delithation to potentials roughly above 1.5 V. Such a faradic process has been compared to the redox reactions in polymer-based electro-

MoS2. It may thus be concluded that the each of the large 200− 500 nm MoO2 macro particles is likely to be a single crystal, or at least an array of iso-oriented (i.e., very highly textured) polycrystals. The higher magnification TEM image (Figure 2e) further confirms that the nanonetworks are highly porous and shows that the surface MoS2 (arrowed) is several nanometers in thickness and is discontinuous. Figure 2f shows the zoomed in HRTEM image of the area marked in Figure 2e, highlighting the single crystal MoO2 covered by MoS2. Elemental sulfur experiences a phase change from solid to liquid at approximately 120 °C, and transforms into gas at approximately 370 °C. TGA-DSC curves of S are shown in Figure S3a. A large endothermal peak around 370 °C, with mass loss ∼100%, corresponds to the evaporation of S. When heating (NH4)6Mo7O24·4H2O from room temperature to 500 °C under an argon atmosphere, (NH4)6Mo7O24·4H2O experiences a three stage reaction (approximately 90, 220, and 340 °C) to eliminate the molecules of H2O and NH3 and finally to produce MoO3.39 The end point reaction sequence may be described as (NH4)6 Mo7O24 ·4H 2O(s) = 6NH3(g) + 7MoO3(s) + 7H 2O(g)

(1)

The TGA-DSC curves of this process are shown in Figure S3b, confirming the temperature range for these reactions. More details are provided in the Supporting Information. MoO3 is expected to transform to MoO2 via the following reaction with liquid and then gaseous sulfur: 2MoO3(s) + S(l org ) = 2MoO2 (s) + SO2 (g)

(2)

Because of the high reactivity of liquid and gaseous S, the chemical reaction step (2) will occur quite rapidly. The solid MoO2 will further react with S, partially transforming to MoS2: MoO2 (s) + 3S(l or g) = MoS2 (s) + SO2 (g)

(3)

The formation of porous nanonetworks benefits from the simultaneous addition of S and (NH4)6Mo7O24·4H2O. As a counterexample, MoS2/MoO2 particulates were prepared by directly sulfiding already synthesized MoO3. The XRD patterns of the MoS2/MoO2 particulates are shown in Figure S5a and are further discussed in the supplemental. EDS analysis demonstrated that the particulates had a similar MoS2/MoO2 ratio as the nanonetworks, being 88 wt % MoO2. The nitrogen adsorption isotherm of the particulates is shown in Figure S5b while the pore size distribution is shown in S5c. The calculated BET surface area of the MoS2/MoO2 particulates is 15.8 m2 g−1, much lower than that of the nanonetworks. The dense MoS2 /MoO2 particulates exhibit a fairly uninteresting morphology with pore-free MoO2 particles (up to 500 nm in diameter) that are partially sulfided and partially sintered together into 10+ micrometer-scale clumps (Figure 3a). Some of the smaller particles, such as the one labeled as MoS2 in Figure 3b, were identified as being almost entirely a sulfide. This agreed well SAD analysis shown in Figure 3c, which demonstrates spotty (rather than nearly continuous) ring patterns for both structures. The HRTEM image shown in Figure 3d shows that the MoO2 is too thick to obtain discernible lattice fringes even at its edge. It does highlight the morhology and structure of the surface MoS2, which are generally substantially coarser than in the case of the nanonetworks. 18392

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entire structure is electrochemically active. This is in turn likely associated with kinetic limitations of Li solid-state diffusion to the reactive interface through the LiMoO2. Such activation, combined with the reversible gel formation, will be responsible for the cycling-induced increase the in charge storage capacity of the oxides. Electrochemical impedance spectroscopy (EIS) measurements were carried out in order to compare the impedance differences between the MoS2/MoO2 nanonetworks and MoS2/MoO2 particulates. The Nyquist plots, shown in Figure 5d, were collected from 100 kHz to 0.01 Hz after the specimens underwent 80 charge/discharge cycles. The equivalent series resistance (highest frequency intercept of the abscissa) of the two materials is on par, as they both consist primarily of MoO2 with the same electrical conductivity. The high-frequency region of the Nyquist plot also consists of two overlapping semicircles, with their combined diameter being the total charge transfer resistance of the electrodes. In cycled materials the charge transfer resistance is normally attributed to a combination of the charge transfer resistance at the original electrode surface and through the multiple interfaces within the SEI. The charge transfer resistance in the cycled nanonetworks is roughly 50% lower than in the cycled particulates, which we believe is due to a more facile ion transfer at the nanonetworks’ surface. Another significant difference comes from the Warburg region, namely the slope of the 45° portion right after the semicircles. The overall length of the Warburg line is indicative of diffusional limitations in the material, with bulk ion insertion and conversion electrodes being solid-state diffusion limited. The nanonetworks demonstrate a much shorter Warburg region than the particulates, which is expected due to the markedly smaller diffusion lengths in the former. As we argued, the reduced diffusional limitations in the MoS 2 /MoO2 nanonetworks is a direct consequence of their architecture, i.e. the thin nm-scale oxide walls with interconnected mesoporosity wetted by the electrolyte. Figure 6 and Table 1 compare our results with the state-ofthe-art in previously published literature on the MoO2 system. A comparison with MoS2 is not included, since the MoS2 phase in the MoS2/MoO2 nanonetworks is a minority. The figure graphically contrasts the reversible capacity versus current density, and hence highlights the highly promising rate characteristics of our system. The figure demonstrates rates up to 1000 mAh g−1, since at higher rates there has been little data that was previously reported. The capacity values for the MoS2/MoO2 nanonetworks reported in Table 1 are slightly lower than they are in Figure 6. This is because the table reports the postcycling capacities, while the figure shows the stabilized capacity during a rate capability test. The table highlights that after 80 full charge−discharge cycles the MoS2/ MoO2 nanonetworks still demonstrate excellent capacity retention, among the highest in literature. Finally, it is important to point out that the operating voltage window must be also considered when evaluating the merit of a new electrode material. For the nanonetworks and for other MoO2-based structures, about one-third of the delithiation capacity occurs above 1 V. This would reduce the energy density of a full Li-ion cell. While a flat sub-1 V capacity profile is certainly more desirable for an anode, a sloping one does not necessarily disqualify an electrode material as long as its total capacity is large. In fact, many candidate materials to replace graphite demonstrate this issue to some extent. High surface area or heteratom doped carbons,7,61 nanocrystalline TiO26,62

chemical capacitors, being highly reversible and imparting a negligible Coulombic efficiency penalty.44−46 The MoS2 phase is also Li active, displaying characteristic features in the CV plots. According to the CVs of first cycle of the nanonetworks, the particulates and of the baseline MoS2, there is a slight irreversible reduction peak at 1.15 V, which is associated with the insertion reaction MoS2 + xLi+ + xe− → LixMoS2.40−42 The single phase LixMoS2 irreversibly decomposes at lower voltages, so this peak is not present at subsequent cycles. We attribute the relatively minor redox couple at 1.75 V (red)/2.25 V (oxid) to the reversible lithiation of sulfur (S + 2Li+ +2e− → Li2S), in turn present due to the irreversible decomposition of MoS2.47−49 Figure 5a shows the rate capabilities of the MoS2/MoO2 nanonetworks compared to the baseline MoO2, baseline MoS2 and to MoS2/MoO2 particulates. Among them, the nanonetworks clearly display the best performance at all the current densities. At a current density of 100 mA g−1 after 20 cycles, the MoS2/MoO2 nanonetworks, MoO2, MoS2 and MoS2/MoO2 particulates show capacities of 1213, 287, 606, and 534 mAh g−1, respectively. At 200 mA g−1 after 30 cycles, they show 1161, 236, 513, and 480 mAh g−1, respectively. At 500 mA g−1 after 40 cycles, nanonetworks, MoO2, MoS2 and particulates are at 814, 175, 436, and 394 mAh g−1. At a high current density of 1000 mA g−1, these values are 586, 118, 334, and 302 mAh g−1 after 50 cycles. At 2000 mA g−1, the capacities are 326, 71, 172, and 140 mAh g−1 after 60 cycles. When the current density was returned to 100 mA g−1, after 80 cycles the capacities of the nanonetworks, MoO2, MoS2 and particulates are 1142, 362, 510, and 654 mAh g−1. Figure 5b shows the cycling performance of the MoS2/MoO2 nanonetworks, MoO2, MoS2 and MoS2/MoO2 particulates, tested at 100 mA g−1. Figure 5c shows the cycling performance of the nanonetworks at higher rates. In the nanonetworks the overall capacity and rate capability are much higher than in the microstructurally coarser baselines. This is a direct result of the electrode structure, where the open nanoporosity reduces the effective oxide wall thickness and hence the solid-state diffusion distances. This would naturally promote more facile lithiation/ delithiation at a given current density.59 The thin and hence elastically compliant walls combined with dense nanoporosity should also buffer against volume changes associated with the insertion and conversion reactions, which would promote enhanced cycling stability.60 After 80 cycles at 100 mA g−1, the capacity still remains at 1163 mAh g−1, which is 40% higher than the initial capacity, and only a 5.6% off the peak value. The maximum literature reported reversible capacities of MoS2based electrodes, tested at a current density of ∼100 mAg−1, are in the range of 912 to 1290 mAh g−1.50−52 Knowing that MoS2/MoO2 nanonetworks are 89% MoO2 by weight, yields a reversible capacity of 1146−1163 mAh g−1 for the MoO2 constituent of the composite. Such values are among the highest capacities ever measured for any MoO2 based electrodes, including architectures based on amorphous MoO2, activated MoO2, various MoO2 nanoarrays,20,21,23,25,26 W-doped MoO2,53,54 MoO2-carbon nanocomposites.26,27,30,55−58 The differential curves of the voltage profiles of the MoS2/ MoO2 nanonetwork electrodes at the current densities of 100, 200, and 500 mA g−1, provided in Figure S10, show that the 0.25/0.5 V peaks from the first cycle to the 20th cycle increase. The cycling induced activation process of the LiMoO2 + 3Li+ + 3e− → Mo + 4Li2O reaction indicates that initially not the 18393

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and a range of conversion electrodes63 all display a substantial portion of their total capacity at higher voltage. Moreover, when coupled with typical cathode materials such as LiFePO4 in a full cell, anode materials with a higher redox potential will help to eliminate lithium plating and therefore improve the device safety.62

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CONCLUSIONS We employed a unique sulfur assisted synthesis process to create a composite of nanocrystalline−nanoporous MoO2 partially covered with few atomic layers of MoS2. The obtained mesoporous MoS2/MoO2 nanonetworks promote rapid lithiation kinetics due to short solid-state diffusion distances and excellent cycling stability due to thin and compliant walls of the structure. They exhibit a reversible capacity as high as 1163 mAh g−1 at the current density of 100 mA g−1 after 80 cycles. The nanonetworks also exhibit reversible capacities 1125 and 654 mAh g−1 at the current densities of 200 and 500 mA g−1 after 80 cycles, respectively. A comparison with state-of-the art scientific literature on MoO2 indicates that these performance matrices are highly favorable.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information about experimental details and additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 780-492-8793. *E-mail: [email protected]. Tel: 780-492-1542. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSERC Discovery and NINT NRC.

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REFERENCES

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