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Large-Scale Production of MoO3-Reduced Graphene Oxide Powders with Superior Lithium Storage Properties by Spray-Drying Process
Electrochimica Acta 173 (2015) 581–587
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Large-Scale Production of MoO3-Reduced Graphene Oxide Powders with Superior Lithium Storage Properties by Spray-Drying Process Gi Dae Park a , Jong Hwa Kim b , Yun Ju Choi c , Yun Chan Kang a, * a b c
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea Daegu Center, Korea Basic Science Institute, 80 Daehakro Bukgu, Daegu 702-701, Republic of Korea Suncheon Center, Korea Basic Science Institute, Suncheon 540-742, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 January 2015 Received in revised form 9 May 2015 Accepted 16 May 2015 Available online 19 May 2015
A MoO3-reduced graphene oxide (rGO) composite powder with spherical shape is prepared by a simple spray-drying process by applying a water-soluble metal salt. Ammonium molybdate–GO composite powders prepared by spray drying turn into a MoO3–rGO powder by heat treatment at 300 C under air atmosphere. The MoO3 nanocrystals are uniformly dispersed in a crumpled spherical rGO structure. MoO3 and MoO3–C powders, both with spherical particles, are also prepared by the same process for comparison of their electrochemical properties. The discharge capacities of the MoO3, MoO3–C, and MoO3–rGO powders after 100 cycles at a current density of 500 mA g 1 are 506, 738, and 1115 mA h g 1, respectively, and their corresponding capacity retentions measured from the second cycle are 53, 71, and 92%, respectively. The rGO layers improve the structural stability and electric conductivity of the MoO3–rGO composite powder. Therefore, the MoO3–rGO powder shows superior electrochemical properties as compared with the MoO3 and MoO3–C powders prepared by the same preparation procedure. ã2015 Elsevier Ltd. All rights reserved.
Keywords: Electrochemical properties Molybdenum oxide Reduced graphene oxide Spray drying Carbon composite
1. Introduction Nowadays, energy storage devices such as rechargeable lithium ion batteries (LIBs) are one of the most important energy sources [1–10]. Therefore, as the various synthetic methods have been developed, electrode materials having a variety of shapes and structures have also been developed [11–22]. Graphene is an impressive support material for active nanomaterials because of its high electric and thermal conductivity, flexibility, large surface area, and chemical stability [23–28]. Therefore, graphene–metal oxide composite materials with various compositions have been developed by various synthetic processes. Graphene-based composites prepared by liquid solution and spray pyrolysis processes have shown superior electrochemical properties as compared to bare metal oxides [29–35]. However, effective large-scale production of graphene–metal oxide composite materials having micronsize spherical particles for possible application in commercial LIBs has to be developed. In addition, the serious aggregation that occurs between the graphene sheets due to van der Waals forces has to be solved before large-scale production can be feasible.
* Corresponding author. E-mail address: [email protected] (Y.C. Kang). http://dx.doi.org/10.1016/j.electacta.2015.05.090 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
The spray-drying process has been applied to the large-scale production of various electrode materials [36–38]. Owing to the fact that it is a facile method to obtain dry powders from a liquid solution or slurry, spray drying is one of the most preferred commercial processes. Furthermore, this inexpensive and environmentally friendly process is economical for the large-scale production of electrode materials for LIBs. The spray drying of graphene prevents agglomeration of the nanoparticles and acts as a drying agent, without the need for other chemical drying agents [34,35,39]. Metal oxide–graphene oxide (GO) composite powders are mainly prepared from a colloidal solution of nonsoluble nanoparticles and well-dispersed graphene oxide (GO) nanosheets [40–46]. The well-dispersed nanoparticles, which are mainly prepared by liquid solution methods or purchased as commercial products, are applied as colloidal spray solutions. However, the uniform mixing of metal oxide nanoparticles and GO nanosheets cannot be achieved in the conventional spray-drying process [44–46]. In recent years, GO nanosheets decorated with metal oxides formed by liquid solution methods have been applied to the preparation of metal oxide–graphene composite powders by spray drying [47]. However, the liquid solution processes for metal oxide nanoparticles and GO nanosheets decorated with metal oxides are costly and time-consuming, and they result in the application of toxic materials. To the best of our knowledge, preparation of metal oxide–graphene composite powders by spray
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drying a solution containing water-soluble metal salts and GO nanosheets has rarely been reported. Nanostructured MoO3 materials such as nanorods, nanoparticles, yolk–shell particles, and hollow spheres have shown superior electrochemical properties as anode materials for LIBs [48–54]. The electrochemical properties of MoO3–graphene composite materials prepared by liquid solution and gas-phase reaction methods have also been studied [35,54–56]. In this study, an ammonium molybdate–GO composite powder was prepared by spray drying a colloidal solution containing water-soluble ammonium molybdate and well-dispersed GO nanosheets. The ammonium molybdate–GO composite then turned into MoO3-reduced graphene oxide (rGO) by heat-treatment at 300 C under air atmosphere. In addition, a bare MoO3 powder and a MoO3–carbon composite powder, both with spherical particles, were also prepared by the same spray-drying process. The electrochemical properties of the MoO3–rGO powder were compared with those of the bare MoO3 and MoO3–carbon powders. 2. Experimental The schematic diagram of the spray drying system is shown in Fig. S1. GO was synthesized from graphite flakes by a modified Hummers method described in our previous reports [34,35]. The as-obtained graphite oxide was re-dispersed in distilled water and then exfoliated to generate graphene oxide sheets by ultrasonication. 0.15 M of ammonium molybdate tetrahydrate [(NH4)6Mo7O244H2O] was dissolved into 300 mL of the exfoliated graphite oxide dispersion (3.3 mgmL 1). The precursor powders were prepared by using a commercial spray-drying system. The temperatures at the inlet and outlet of the spray dryer were 300 C and 130 C, respectively. A two-fluid nozzle with a diameter of 1.2 mm was used as an atomizer, and the atomization pressure was 2.4 bar. The spray-dried powders were heat treated at 300 C for 10 h under an air atmosphere. Bare MoO3 powder was prepared from a spray solution of ammonium molybdate tetrahydrate. 30 g (or 5 g) of dextrin was dissolved into a 300 mL spray solution of ammonium molybdate tetrahydrate to prepare the MoO3–C composite powder. For simplicity, the specimens prepared from the spray solutions with dextrin of 30 g and 5 g are referred to as ‘MoO3–C1’ and ‘MoO3–C2’, respectively. The crystal structures of the powders were investigated by X-ray diffractometry (XRD, X’pert PRO MPD) using Cu Ka radiation (l = 1.5418 Å) at the Korea Basic Science Institute (Daegu). The morphological features of the powder particles were investigated using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100F) operating at a working voltage of 200 kV. The specific surface areas of the powders were calculated from a Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption measurements (TriStar 3000). The decomposition characteristics of the precursor and post-treated powders were determined using thermogravimetric analysis (TGA, SDT Q600), which was performed in air at a heating rate of 20 C min 1. The capacities and cycling properties of the powders were determined using a 2032-type coin cell. The electrode was prepared from a mixture containing 70 wt% active material, 20 wt% Super P, and 10 wt% sodium carboxymethyl cellulose (CMC) binder. Lithium metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was a solution of 1 M LiPF6 in a 1:1 volume mixture of fluoroethylene carbonate–dimethyl carbonate (FEC–DMC). The charge/discharge characteristics of the samples were determined through cycling in the 0.001–3 V potential range at a set of fixed current densities. Cyclic voltammetry (CV) measurements were carried out at a scan rate of 0.07 mVs 1.
Scheme 1. Schematic diagram for formation mechanism of the MoO3-rGO composite powder in the spray drying process.
3. Results and discussion The formation mechanism of the MoO3–rGO composite powder applying the spray-drying process is shown in Scheme 1. The pneumatic nozzle formed droplets in which GO nanosheets were uniformly distributed in an ammonium molybdate tetrahydrate solution. Drying of the droplet produced the ammonium molybdate–GO composite powder. During the drying process, GO nanosheets were collapsed to form spherical structures, and ammonium molybdate was uniformly deposited over the spherical GO particles. Decomposition of the ammonium molybdate and thermal reduction of the GO did not occur during the spray-drying process; rather, heat treatment of the ammonium molybdate–GO composite powder at 300 C under air atmosphere produced the MoO3–rGO composite powder. The contents of MoO3 and rGO in the MoO3–rGO composite powder could be easily controlled by changing the concentration of ammonium molybdate tetrahydrate and the dispersing amount of GO nanosheets in the spray solution, respectively. The simple formation mechanisms of the bare MoO3 and MoO3–C1 powders are shown in Fig. S2. The drying of droplet containing ammonium molybdate tetrahydrate produced the ammonium molybdate powder. The formation of hollow particles during the spray drying and melting of the dried ammonium molybdate with a low melting temperature of 90 C resulted in particles with a pockmarked surface. The pockmarked structure of the ammonium molybdate particles was maintained even after heat treatment at 300 C. The drying of droplets containing ammonium molybdate and dextrin produced the ammonium molybdate–dextrin composite powder, and the carbonization of dextrin and decomposition of ammonium molybdate into MoO3 produced the MoO3–C composite powder. The morphologies of the precursor powder particles for the bare MoO3 and MoO3–C1 powders prepared directly by spray drying are shown in Fig. S3. The precursor powder particles for the bare MoO3 and MoO3–C1 powders had a spherical shape with a pockmarked structure. The fast drying rate of the droplets resulted in the powders with hollow structure. The collapse of the hollow powders during the drying stage resulted in the spherical precursor powder particles with a pockmarked structure. Aggregation between the particles was not observed in the SEM images shown in Fig. S3. The precursor powders prepared directly by spray drying had high stability in a high-humidity atmosphere. Therefore, aggregation between the particles did not occur inside the cyclone chamber for powder collection. The spray-dried
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powders shown in Fig. S3 were completely soluble in water. Therefore, decomposition of ammonium molybdate and dextrin did not occur inside the drying chamber. The morphologies of the MoO3 and MoO3–C1 powder particles heat treated at 300 C under air atmosphere are shown in Fig. S4. The spherical shapes of the precursor powder particles were maintained even after heat treatment. The mean sizes of the MoO3 and MoO3–C1 powder particles measured from the SEM images were 1.9 and 2.5 mm, respectively. The colors of the MoO3 and MoO3–C1 powders were light blue and black, respectively. Formation of amorphous
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carbon by the carbonization of dextrin produced the black MoO3–C powders. The morphologies of the precursor powder particles for the MoO3–rGO composite powder are also shown in Fig. S3. The precursor powder particles were several microns in size and showed a crumpled spherical structure. The collapse of GO nanosheets with high aspect ratios resulted in the crumpled structure of the spray-dried particles. The morphology of the MoO3–rGO composite powder particles is shown in Fig. 1. The structure of the precursor powder particles was maintained after
Fig. 1. Morphologies and elemental mapping images of the MoO3-rGO powders: (a) and (b) SEM images, (c) and (d) TEM images, (e) high resolution TEM image, and (f) elemental mapping images.
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heat treatment at 300 C. The TEM images shown in Figs. 1c and 1d reveal MoO3 nanocrystals uniformly dispersed in the crumpled spherical rGO particle structure. The MoO3 nanoplates were wrapped by rGO layers as shown by high resolution TEM image in Fig. 1e. The high resolution TEM image shown as inset in Fig. 1e depicts clear lattice fringes separated by 0.35 nm, corresponding to the (040) plane of a-MoO3 crystal. The size of the MoO3 nanoplates observed in the TEM image shown in Fig. 1d was approximately 200 nm. The elemental mapping images shown in Fig. 1f show the uniform distribution of carbon and Mo components covering the composite powder particles. Ammonium molybdate was uniformly deposited over the GO powder particles without phase separation during the drying process. The GO and rGO nanosheets restricted the crystal growth of the MoO3 nanocrystals during the heat treatment process. Therefore, crumpled spherical rGO powder particles uniformly decorated with ultrafine MoO3 nanocrystals were prepared by heat treatment of the spray-dried powders. The properties of the MoO3, MoO3–C1, MoO3–C2, and MoO3–rGO powders are shown in Fig. 2. The XRD patterns shown in Fig. 2a reveal a phase-pure a-MoO3 crystal structure irrespective of the type of sample. The XRD pattern of the bare MoO3 powder had sharper peaks than those of the MoO3–C1 and MoO3–rGO powders. The amorphous carbon formed by the carbonization of dextrin and rGO restricted the growth of the MoO3 nanocrystals of the MoO3–C1 and MoO3–rGO powders, respectively. Fig. S5 shows the C1s XPS spectrum measured for the MoO3-rGO powders. The C1s peak is made up of three components from namely, sp2 bonded carbon (C–C), epoxy and alkoxy groups (C–O), and carbonyl and carboxylic (C=O) groups, corresponding to peaks at 284.6, 286.2, and 288.5 eV, respectively [57,58]. The XPS spectrum of the MoO3-rGO powders showed a sharp peak at 284.6 eV, which was assigned to the C-C bond. Thermal reduction of GO nanosheets to rGO nanosheets occurred at a heat treatment temperature of 300 C. The high C/O ratio of the MoO3-rGO composite powders estimated as 6.0 after subtracting the oxygen species involved in Mo-O chemical bonds of MoO3 also revealed the thermal reduction of GO into rGO [59]. Recently, Wang et al. reported the facile, mild, and fast thermal decomposition reduction of graphene oxide in air atmosphere [60]. According
to their report, after heating at 300 C for 1 h in air atmosphere, GO was thermally decomposed into rGO. The Raman spectrum of the MoO3–rGO powder shown in Fig. 2b consisted of two peaks, one around 1340 cm 1 called D peak (disordered or amorphous), and another around 1605 cm 1 called G peak (graphite) [61]. The signal peak intensity of the D band was higher than that of the G band, which suggested a decrease in the average size of the sp2 domains upon reduction of the GO nanosheets [62]. D and G bands in the Raman spectrum of the MoO3–C1 powder were also observed at 1347 and 1563 cm 1, respectively. The large ID/IG ratio indicates the low graphitic degree in the carbon composite material. The N2 adsorption and desorption isotherms of the three samples are shown in Fig. S6. The isotherms of the MoO3–rGO powder show a clear hysteresis loop, indicating the existence of mesopores. Fig. 2c shows the Barrett–Joyner–Halenda (BJH) pore-size distribution of the MoO3–rGO powders. The MoO3–rGO powders had mesopores between 3 and 10 nm in size. The narrow peak at around 3 nm was attributed to the tensile strength effect of nitrogen desorption [63]. The inset figure shown in Fig. S6 showed a detail of the isotherm in the P/Po range 0.45-0.6. This result emphasized the tensile strength effect at P/Po 0.48. However, the MoO3 and MoO3–C1 powders had filled structures with low pore volumes, as shown in Fig. 2c. The BET surface areas of the MoO3, MoO3–C1, and MoO3–rGO powders were 2.9, 1.9, and 9.1 m2 g 1, respectively. The thermogravimetry (TG) curves of the precursor powders for the MoO3 and MoO3–rGO powders are shown in Fig. S7. The TG curves of the precursor powders for the MoO3 and MoO3–rGO powders had two- and four-step weight losses, respectively, between 40 and 450 C. The weight losses below 110 C in both samples were attributed to the evaporation of adsorbed water molecules. The weight loss between 110 and 324 C of the precursor powders for MoO3 due to the decomposition of ammonium molybdate into MoO3 was approximately 15%, a value that is similar to the theoretical weight loss by decomposition of ammonium molybdate into MoO3 [64]. These results indicate that the decomposition of ammonium molybdate did not occur during the spray-drying step. The strict weight loss at approximately 193 C in the TG curve of the precursor powders for MoO3–rGO was attributed to the decomposition of ammonium molybdate. The weight loss between 193 and 350 C was attributed to the
Fig. 2. Properties of the MoO3, MoO3-C, and MoO3-rGO powders: (a) XRD patterns, (b) Raman spectra, (c) pore size distributions, and (d) TG curves.
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decomposition of the oxygen-containing functional groups of GO and the weight loss between 350 to 550 C was attributed to the combustion of rGO. The weight increase by oxidation of MoO2 intermediate into MoO3 decreased the weight loss by combustion of rGO. The TG curves of the MoO3–C and MoO3–rGO composite powders are shown in Fig. 2d. The TG curves of the MoO3–C composite powders prepared from the spray solutions with low and high concentrations of dextrin showed two-step weight losses by evaporation of adsorbed water molecules and decomposition of amorphous carbon. However, the MoO3–rGO composite powder with a high surface area had a one-step weight loss by decomposition of rGO at approximately 400 C. The contents of amorphous carbon of the MoO3–C1 and MoO3–C2 composite powders were 8.4 and 29.1 wt%, respectively. The content of rGO of the MoO3–rGO composite powders was 6.0 wt%. The cyclic voltammogram (CV) curves of the MoO3, MoO3–C1, and MoO3–rGO powders performed at a scanning rate of 0.07 mV s 1 are shown in Figs. 3a–3c. The small reduction peaks observed at voltages above 2.0 V in the initial discharge curves for the three samples were due to the intercalation of Li+ into the crystalline MoO3 to form a LixMoO3 solid solution [53,54]. The MoO3 and MoO3–rGO powders had larger mean crystallite sizes and higher BET surface areas than those of the MoO3–C1 powders. Therefore, the CV curves of the MoO3 and MoO3–rGO powders showed distinct reduction peaks at voltages above 2.0 V in the initial discharge curves. The sharp reduction peaks observed at approximately 0.25 V were attributed to the reaction of the LixMoO3 solid solution with Li ions to form metallic Mo nanocrystals and amorphous Li2O [53,54]. The low mean crystallite size of the MoO3–C1 powders also resulted in the broad reduction peak at around 0.25 V in the first discharge curve. The three samples had two broad reduction peaks at approximately 0.14 and 1.3 V in the CV curves from the second cycle onward because crystalline MoO3 transformed into amorphous MoO3 after the first discharge process. The oxidation peaks in the CV curves also showed two broad peaks at approximately 1.4 and 1.8 V from the first cycle onward, irrespective of the type of sample. The well-overlapped CV curves of the MoO3–rGO powder from five cycles reveal the stable cycling performance of the powder. The shapes of the initial discharge curves of the samples, shown in Fig. 3d, were slightly affected by the type of
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sample. The MoO3 and MoO3–rGO powders with large mean crystallite sizes and high BET surface areas had potential plateaus at voltages above 2.0 V in the first discharge curve shown in Fig. 3d. However, the initial charge curves had similar shapes, irrespective of the type of sample. The initial discharge capacities of the MoO3, MoO3–C1, MoO3–C2, and MoO3–rGO powders at a current density of 500 mA g 1 were 1440, 1647, 1642, and 1675 mA h g 1, respectively, and their corresponding initial charge capacities were 931, 1040, 1133, and 1195 mA h g 1, respectively. The cycling and rate performances of the four samples are shown in Figs. 4a and 4b, respectively. The discharge capacities of the MoO3, MoO3–C1, MoO3–C2, and MoO3–rGO powders after 100 cycles at a current density of 500 mA g 1 were 506, 738, 645, and 1115 mA h g 1, respectively, and their corresponding capacity retentions measured from the second cycle were 53, 71, 56, and 92 %, respectively. The amorphous carbon and rGO improved the cycling performance of the MoO3–C1 and MoO3–rGO composite powders, respectively. The MoO3 and MoO3–C1 powders with low BET surface areas had similar rate performances, as shown in Fig. 4b. For each step, 10 cycles were measured to evaluate the rate performance. However, the MoO3–rGO powder showed superior rate performance as compared with the MoO3, MoO3–C1, and MoO3–C2 powders. The final discharge capacities of the MoO3–rGO powder at current densities of 200, 500, 1000, 1500, 2000, 2500, and 3000 mA g 1 (the current densities were increased in a stepwise manner) were 1176, 1118, 1060, 1019, 969, 938, and 894 mA h g 1, respectively. The discharge capacities slightly decreased when the current density was increased from 500 to 3000 mA g 1. In addition, after 70 cycles, the discharge capacity of the MoO3–rGO powder recovered well when the current density was decreased to 200 mA g 1. The impedance spectra of the three samples before and after 50 cycles measured in the potential range 0.001–3.0 V at a current density of 500 mA g 1 are shown in Figs. S8 and 4c, respectively. The impedance spectra comprise a semicircle and an inclined line. The medium-frequency semicircle was assigned to the charge-transfer resistance (Rct) and the line inclined at 45 to the real axis corresponded to the lithium diffusion process within the electrodes [65–67]. The MoO3–rGO powder with a high surface area had a slightly higher charge transfer resistance than
Fig. 3. CV curves and initial charge and discharge curves of the three samples: (a) CV curves of MoO3, (b) CV curves of MoO3-C1, (c) CV curves of MoO3-rGO, and (d) initial charge and discharge curves.
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Fig. 4. Electrochemical properties of the four samples: (a) cycling performances, (b) rate performances, (c) Nyquist plots after 50 cycles, and (d) relationships between Zre and v 1/2 after 50 cycles.
the MoO3 and MoO3–C1 powders before cycling, as shown in Fig. S8. However, the MoO3–rGO powder had the minimum charge–discharge resistance after 50 cycles, as shown in Fig. 4c. The charge transfer resistance of the MoO3–rGO powder decreased after cycling owing to the transformation of the crystalline structure into an amorphous structure after the first cycle. On the other hand, the charge transfer resistance of the bare MoO3 powder significantly increased after cycling owing to its structural instability during cycling. The relationships between the real part of the impedance spectra (Zre) and v 1/2 (where v is the angular frequency in the low-frequency region, given by v = 2pf) before cycling and after the 50th cycle are shown in Figs. S8 and 4d, respectively. The small slope gradient of the straight line is indicative of good Li-ion kinetics in the electrode materials. The MoO3–rGO powder exhibited lower slope than those of the MoO3 and MoO3–C1 powders before and after cycling. The MoO3–rGO powder, which had high electric conductivity and structural stability, had a faster lithium diffusion rate than the MoO3 and MoO3–C1 powders before and after cycling. 4. Conclusions The preparation process of a metal oxide–reduced graphene oxide (rGO) composite powder by a large-scale spray-drying of a solution containing water-soluble metal salt and GO nanosheets is reported. Details of the formation mechanism of the MoO3–rGO composite powder, which was selected as the first target material, were also investigated. The spray-drying process produced an ammonium molybdate–GO composite powder. Ammonium molybdate was uniformly deposited over the spherical GO powder particles during the spray-drying process. The heat treatment of the spray-dried powders at 300 C under air atmosphere produced a MoO3–rGO composite powder with superior electrochemical properties as an anode material for LIBs. Decomposition of the ammonium molybdate and thermal reduction of the GO occurred during the heat-treatment process. The contents of MoO3 and rGO in the MoO3–rGO composite powder were easily controlled by changing the concentration of ammonium molybdate tetrahydrate and the dispersing amount of GO nanosheets in the spray solution. Therefore, the process introduced in this study could be widely
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Large-Scale Production of MoO3-Reduced Graphene Oxide Powders with Superior Lithium Storage Properties by Spray-Drying Process Gi Dae Park a , Jong Hwa Kim b , Yun Ju Choi c , Yun Chan Kang a, * a b c
Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu, Seoul 136-713, Republic of Korea Daegu Center, Korea Basic Science Institute, 80 Daehakro Bukgu, Daegu 702-701, Republic of Korea Suncheon Center, Korea Basic Science Institute, Suncheon 540-742, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 January 2015 Received in revised form 9 May 2015 Accepted 16 May 2015 Available online 19 May 2015
A MoO3-reduced graphene oxide (rGO) composite powder with spherical shape is prepared by a simple spray-drying process by applying a water-soluble metal salt. Ammonium molybdate–GO composite powders prepared by spray drying turn into a MoO3–rGO powder by heat treatment at 300 C under air atmosphere. The MoO3 nanocrystals are uniformly dispersed in a crumpled spherical rGO structure. MoO3 and MoO3–C powders, both with spherical particles, are also prepared by the same process for comparison of their electrochemical properties. The discharge capacities of the MoO3, MoO3–C, and MoO3–rGO powders after 100 cycles at a current density of 500 mA g 1 are 506, 738, and 1115 mA h g 1, respectively, and their corresponding capacity retentions measured from the second cycle are 53, 71, and 92%, respectively. The rGO layers improve the structural stability and electric conductivity of the MoO3–rGO composite powder. Therefore, the MoO3–rGO powder shows superior electrochemical properties as compared with the MoO3 and MoO3–C powders prepared by the same preparation procedure. ã2015 Elsevier Ltd. All rights reserved.
Keywords: Electrochemical properties Molybdenum oxide Reduced graphene oxide Spray drying Carbon composite
1. Introduction Nowadays, energy storage devices such as rechargeable lithium ion batteries (LIBs) are one of the most important energy sources [1–10]. Therefore, as the various synthetic methods have been developed, electrode materials having a variety of shapes and structures have also been developed [11–22]. Graphene is an impressive support material for active nanomaterials because of its high electric and thermal conductivity, flexibility, large surface area, and chemical stability [23–28]. Therefore, graphene–metal oxide composite materials with various compositions have been developed by various synthetic processes. Graphene-based composites prepared by liquid solution and spray pyrolysis processes have shown superior electrochemical properties as compared to bare metal oxides [29–35]. However, effective large-scale production of graphene–metal oxide composite materials having micronsize spherical particles for possible application in commercial LIBs has to be developed. In addition, the serious aggregation that occurs between the graphene sheets due to van der Waals forces has to be solved before large-scale production can be feasible.
* Corresponding author. E-mail address: [email protected] (Y.C. Kang). http://dx.doi.org/10.1016/j.electacta.2015.05.090 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
The spray-drying process has been applied to the large-scale production of various electrode materials [36–38]. Owing to the fact that it is a facile method to obtain dry powders from a liquid solution or slurry, spray drying is one of the most preferred commercial processes. Furthermore, this inexpensive and environmentally friendly process is economical for the large-scale production of electrode materials for LIBs. The spray drying of graphene prevents agglomeration of the nanoparticles and acts as a drying agent, without the need for other chemical drying agents [34,35,39]. Metal oxide–graphene oxide (GO) composite powders are mainly prepared from a colloidal solution of nonsoluble nanoparticles and well-dispersed graphene oxide (GO) nanosheets [40–46]. The well-dispersed nanoparticles, which are mainly prepared by liquid solution methods or purchased as commercial products, are applied as colloidal spray solutions. However, the uniform mixing of metal oxide nanoparticles and GO nanosheets cannot be achieved in the conventional spray-drying process [44–46]. In recent years, GO nanosheets decorated with metal oxides formed by liquid solution methods have been applied to the preparation of metal oxide–graphene composite powders by spray drying [47]. However, the liquid solution processes for metal oxide nanoparticles and GO nanosheets decorated with metal oxides are costly and time-consuming, and they result in the application of toxic materials. To the best of our knowledge, preparation of metal oxide–graphene composite powders by spray
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drying a solution containing water-soluble metal salts and GO nanosheets has rarely been reported. Nanostructured MoO3 materials such as nanorods, nanoparticles, yolk–shell particles, and hollow spheres have shown superior electrochemical properties as anode materials for LIBs [48–54]. The electrochemical properties of MoO3–graphene composite materials prepared by liquid solution and gas-phase reaction methods have also been studied [35,54–56]. In this study, an ammonium molybdate–GO composite powder was prepared by spray drying a colloidal solution containing water-soluble ammonium molybdate and well-dispersed GO nanosheets. The ammonium molybdate–GO composite then turned into MoO3-reduced graphene oxide (rGO) by heat-treatment at 300 C under air atmosphere. In addition, a bare MoO3 powder and a MoO3–carbon composite powder, both with spherical particles, were also prepared by the same spray-drying process. The electrochemical properties of the MoO3–rGO powder were compared with those of the bare MoO3 and MoO3–carbon powders. 2. Experimental The schematic diagram of the spray drying system is shown in Fig. S1. GO was synthesized from graphite flakes by a modified Hummers method described in our previous reports [34,35]. The as-obtained graphite oxide was re-dispersed in distilled water and then exfoliated to generate graphene oxide sheets by ultrasonication. 0.15 M of ammonium molybdate tetrahydrate [(NH4)6Mo7O244H2O] was dissolved into 300 mL of the exfoliated graphite oxide dispersion (3.3 mgmL 1). The precursor powders were prepared by using a commercial spray-drying system. The temperatures at the inlet and outlet of the spray dryer were 300 C and 130 C, respectively. A two-fluid nozzle with a diameter of 1.2 mm was used as an atomizer, and the atomization pressure was 2.4 bar. The spray-dried powders were heat treated at 300 C for 10 h under an air atmosphere. Bare MoO3 powder was prepared from a spray solution of ammonium molybdate tetrahydrate. 30 g (or 5 g) of dextrin was dissolved into a 300 mL spray solution of ammonium molybdate tetrahydrate to prepare the MoO3–C composite powder. For simplicity, the specimens prepared from the spray solutions with dextrin of 30 g and 5 g are referred to as ‘MoO3–C1’ and ‘MoO3–C2’, respectively. The crystal structures of the powders were investigated by X-ray diffractometry (XRD, X’pert PRO MPD) using Cu Ka radiation (l = 1.5418 Å) at the Korea Basic Science Institute (Daegu). The morphological features of the powder particles were investigated using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100F) operating at a working voltage of 200 kV. The specific surface areas of the powders were calculated from a Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption measurements (TriStar 3000). The decomposition characteristics of the precursor and post-treated powders were determined using thermogravimetric analysis (TGA, SDT Q600), which was performed in air at a heating rate of 20 C min 1. The capacities and cycling properties of the powders were determined using a 2032-type coin cell. The electrode was prepared from a mixture containing 70 wt% active material, 20 wt% Super P, and 10 wt% sodium carboxymethyl cellulose (CMC) binder. Lithium metal and microporous polypropylene film were used as the counter electrode and separator, respectively. The electrolyte was a solution of 1 M LiPF6 in a 1:1 volume mixture of fluoroethylene carbonate–dimethyl carbonate (FEC–DMC). The charge/discharge characteristics of the samples were determined through cycling in the 0.001–3 V potential range at a set of fixed current densities. Cyclic voltammetry (CV) measurements were carried out at a scan rate of 0.07 mVs 1.
Scheme 1. Schematic diagram for formation mechanism of the MoO3-rGO composite powder in the spray drying process.
3. Results and discussion The formation mechanism of the MoO3–rGO composite powder applying the spray-drying process is shown in Scheme 1. The pneumatic nozzle formed droplets in which GO nanosheets were uniformly distributed in an ammonium molybdate tetrahydrate solution. Drying of the droplet produced the ammonium molybdate–GO composite powder. During the drying process, GO nanosheets were collapsed to form spherical structures, and ammonium molybdate was uniformly deposited over the spherical GO particles. Decomposition of the ammonium molybdate and thermal reduction of the GO did not occur during the spray-drying process; rather, heat treatment of the ammonium molybdate–GO composite powder at 300 C under air atmosphere produced the MoO3–rGO composite powder. The contents of MoO3 and rGO in the MoO3–rGO composite powder could be easily controlled by changing the concentration of ammonium molybdate tetrahydrate and the dispersing amount of GO nanosheets in the spray solution, respectively. The simple formation mechanisms of the bare MoO3 and MoO3–C1 powders are shown in Fig. S2. The drying of droplet containing ammonium molybdate tetrahydrate produced the ammonium molybdate powder. The formation of hollow particles during the spray drying and melting of the dried ammonium molybdate with a low melting temperature of 90 C resulted in particles with a pockmarked surface. The pockmarked structure of the ammonium molybdate particles was maintained even after heat treatment at 300 C. The drying of droplets containing ammonium molybdate and dextrin produced the ammonium molybdate–dextrin composite powder, and the carbonization of dextrin and decomposition of ammonium molybdate into MoO3 produced the MoO3–C composite powder. The morphologies of the precursor powder particles for the bare MoO3 and MoO3–C1 powders prepared directly by spray drying are shown in Fig. S3. The precursor powder particles for the bare MoO3 and MoO3–C1 powders had a spherical shape with a pockmarked structure. The fast drying rate of the droplets resulted in the powders with hollow structure. The collapse of the hollow powders during the drying stage resulted in the spherical precursor powder particles with a pockmarked structure. Aggregation between the particles was not observed in the SEM images shown in Fig. S3. The precursor powders prepared directly by spray drying had high stability in a high-humidity atmosphere. Therefore, aggregation between the particles did not occur inside the cyclone chamber for powder collection. The spray-dried
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powders shown in Fig. S3 were completely soluble in water. Therefore, decomposition of ammonium molybdate and dextrin did not occur inside the drying chamber. The morphologies of the MoO3 and MoO3–C1 powder particles heat treated at 300 C under air atmosphere are shown in Fig. S4. The spherical shapes of the precursor powder particles were maintained even after heat treatment. The mean sizes of the MoO3 and MoO3–C1 powder particles measured from the SEM images were 1.9 and 2.5 mm, respectively. The colors of the MoO3 and MoO3–C1 powders were light blue and black, respectively. Formation of amorphous
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carbon by the carbonization of dextrin produced the black MoO3–C powders. The morphologies of the precursor powder particles for the MoO3–rGO composite powder are also shown in Fig. S3. The precursor powder particles were several microns in size and showed a crumpled spherical structure. The collapse of GO nanosheets with high aspect ratios resulted in the crumpled structure of the spray-dried particles. The morphology of the MoO3–rGO composite powder particles is shown in Fig. 1. The structure of the precursor powder particles was maintained after
Fig. 1. Morphologies and elemental mapping images of the MoO3-rGO powders: (a) and (b) SEM images, (c) and (d) TEM images, (e) high resolution TEM image, and (f) elemental mapping images.
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heat treatment at 300 C. The TEM images shown in Figs. 1c and 1d reveal MoO3 nanocrystals uniformly dispersed in the crumpled spherical rGO particle structure. The MoO3 nanoplates were wrapped by rGO layers as shown by high resolution TEM image in Fig. 1e. The high resolution TEM image shown as inset in Fig. 1e depicts clear lattice fringes separated by 0.35 nm, corresponding to the (040) plane of a-MoO3 crystal. The size of the MoO3 nanoplates observed in the TEM image shown in Fig. 1d was approximately 200 nm. The elemental mapping images shown in Fig. 1f show the uniform distribution of carbon and Mo components covering the composite powder particles. Ammonium molybdate was uniformly deposited over the GO powder particles without phase separation during the drying process. The GO and rGO nanosheets restricted the crystal growth of the MoO3 nanocrystals during the heat treatment process. Therefore, crumpled spherical rGO powder particles uniformly decorated with ultrafine MoO3 nanocrystals were prepared by heat treatment of the spray-dried powders. The properties of the MoO3, MoO3–C1, MoO3–C2, and MoO3–rGO powders are shown in Fig. 2. The XRD patterns shown in Fig. 2a reveal a phase-pure a-MoO3 crystal structure irrespective of the type of sample. The XRD pattern of the bare MoO3 powder had sharper peaks than those of the MoO3–C1 and MoO3–rGO powders. The amorphous carbon formed by the carbonization of dextrin and rGO restricted the growth of the MoO3 nanocrystals of the MoO3–C1 and MoO3–rGO powders, respectively. Fig. S5 shows the C1s XPS spectrum measured for the MoO3-rGO powders. The C1s peak is made up of three components from namely, sp2 bonded carbon (C–C), epoxy and alkoxy groups (C–O), and carbonyl and carboxylic (C=O) groups, corresponding to peaks at 284.6, 286.2, and 288.5 eV, respectively [57,58]. The XPS spectrum of the MoO3-rGO powders showed a sharp peak at 284.6 eV, which was assigned to the C-C bond. Thermal reduction of GO nanosheets to rGO nanosheets occurred at a heat treatment temperature of 300 C. The high C/O ratio of the MoO3-rGO composite powders estimated as 6.0 after subtracting the oxygen species involved in Mo-O chemical bonds of MoO3 also revealed the thermal reduction of GO into rGO [59]. Recently, Wang et al. reported the facile, mild, and fast thermal decomposition reduction of graphene oxide in air atmosphere [60]. According
to their report, after heating at 300 C for 1 h in air atmosphere, GO was thermally decomposed into rGO. The Raman spectrum of the MoO3–rGO powder shown in Fig. 2b consisted of two peaks, one around 1340 cm 1 called D peak (disordered or amorphous), and another around 1605 cm 1 called G peak (graphite) [61]. The signal peak intensity of the D band was higher than that of the G band, which suggested a decrease in the average size of the sp2 domains upon reduction of the GO nanosheets [62]. D and G bands in the Raman spectrum of the MoO3–C1 powder were also observed at 1347 and 1563 cm 1, respectively. The large ID/IG ratio indicates the low graphitic degree in the carbon composite material. The N2 adsorption and desorption isotherms of the three samples are shown in Fig. S6. The isotherms of the MoO3–rGO powder show a clear hysteresis loop, indicating the existence of mesopores. Fig. 2c shows the Barrett–Joyner–Halenda (BJH) pore-size distribution of the MoO3–rGO powders. The MoO3–rGO powders had mesopores between 3 and 10 nm in size. The narrow peak at around 3 nm was attributed to the tensile strength effect of nitrogen desorption [63]. The inset figure shown in Fig. S6 showed a detail of the isotherm in the P/Po range 0.45-0.6. This result emphasized the tensile strength effect at P/Po 0.48. However, the MoO3 and MoO3–C1 powders had filled structures with low pore volumes, as shown in Fig. 2c. The BET surface areas of the MoO3, MoO3–C1, and MoO3–rGO powders were 2.9, 1.9, and 9.1 m2 g 1, respectively. The thermogravimetry (TG) curves of the precursor powders for the MoO3 and MoO3–rGO powders are shown in Fig. S7. The TG curves of the precursor powders for the MoO3 and MoO3–rGO powders had two- and four-step weight losses, respectively, between 40 and 450 C. The weight losses below 110 C in both samples were attributed to the evaporation of adsorbed water molecules. The weight loss between 110 and 324 C of the precursor powders for MoO3 due to the decomposition of ammonium molybdate into MoO3 was approximately 15%, a value that is similar to the theoretical weight loss by decomposition of ammonium molybdate into MoO3 [64]. These results indicate that the decomposition of ammonium molybdate did not occur during the spray-drying step. The strict weight loss at approximately 193 C in the TG curve of the precursor powders for MoO3–rGO was attributed to the decomposition of ammonium molybdate. The weight loss between 193 and 350 C was attributed to the
Fig. 2. Properties of the MoO3, MoO3-C, and MoO3-rGO powders: (a) XRD patterns, (b) Raman spectra, (c) pore size distributions, and (d) TG curves.
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decomposition of the oxygen-containing functional groups of GO and the weight loss between 350 to 550 C was attributed to the combustion of rGO. The weight increase by oxidation of MoO2 intermediate into MoO3 decreased the weight loss by combustion of rGO. The TG curves of the MoO3–C and MoO3–rGO composite powders are shown in Fig. 2d. The TG curves of the MoO3–C composite powders prepared from the spray solutions with low and high concentrations of dextrin showed two-step weight losses by evaporation of adsorbed water molecules and decomposition of amorphous carbon. However, the MoO3–rGO composite powder with a high surface area had a one-step weight loss by decomposition of rGO at approximately 400 C. The contents of amorphous carbon of the MoO3–C1 and MoO3–C2 composite powders were 8.4 and 29.1 wt%, respectively. The content of rGO of the MoO3–rGO composite powders was 6.0 wt%. The cyclic voltammogram (CV) curves of the MoO3, MoO3–C1, and MoO3–rGO powders performed at a scanning rate of 0.07 mV s 1 are shown in Figs. 3a–3c. The small reduction peaks observed at voltages above 2.0 V in the initial discharge curves for the three samples were due to the intercalation of Li+ into the crystalline MoO3 to form a LixMoO3 solid solution [53,54]. The MoO3 and MoO3–rGO powders had larger mean crystallite sizes and higher BET surface areas than those of the MoO3–C1 powders. Therefore, the CV curves of the MoO3 and MoO3–rGO powders showed distinct reduction peaks at voltages above 2.0 V in the initial discharge curves. The sharp reduction peaks observed at approximately 0.25 V were attributed to the reaction of the LixMoO3 solid solution with Li ions to form metallic Mo nanocrystals and amorphous Li2O [53,54]. The low mean crystallite size of the MoO3–C1 powders also resulted in the broad reduction peak at around 0.25 V in the first discharge curve. The three samples had two broad reduction peaks at approximately 0.14 and 1.3 V in the CV curves from the second cycle onward because crystalline MoO3 transformed into amorphous MoO3 after the first discharge process. The oxidation peaks in the CV curves also showed two broad peaks at approximately 1.4 and 1.8 V from the first cycle onward, irrespective of the type of sample. The well-overlapped CV curves of the MoO3–rGO powder from five cycles reveal the stable cycling performance of the powder. The shapes of the initial discharge curves of the samples, shown in Fig. 3d, were slightly affected by the type of
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sample. The MoO3 and MoO3–rGO powders with large mean crystallite sizes and high BET surface areas had potential plateaus at voltages above 2.0 V in the first discharge curve shown in Fig. 3d. However, the initial charge curves had similar shapes, irrespective of the type of sample. The initial discharge capacities of the MoO3, MoO3–C1, MoO3–C2, and MoO3–rGO powders at a current density of 500 mA g 1 were 1440, 1647, 1642, and 1675 mA h g 1, respectively, and their corresponding initial charge capacities were 931, 1040, 1133, and 1195 mA h g 1, respectively. The cycling and rate performances of the four samples are shown in Figs. 4a and 4b, respectively. The discharge capacities of the MoO3, MoO3–C1, MoO3–C2, and MoO3–rGO powders after 100 cycles at a current density of 500 mA g 1 were 506, 738, 645, and 1115 mA h g 1, respectively, and their corresponding capacity retentions measured from the second cycle were 53, 71, 56, and 92 %, respectively. The amorphous carbon and rGO improved the cycling performance of the MoO3–C1 and MoO3–rGO composite powders, respectively. The MoO3 and MoO3–C1 powders with low BET surface areas had similar rate performances, as shown in Fig. 4b. For each step, 10 cycles were measured to evaluate the rate performance. However, the MoO3–rGO powder showed superior rate performance as compared with the MoO3, MoO3–C1, and MoO3–C2 powders. The final discharge capacities of the MoO3–rGO powder at current densities of 200, 500, 1000, 1500, 2000, 2500, and 3000 mA g 1 (the current densities were increased in a stepwise manner) were 1176, 1118, 1060, 1019, 969, 938, and 894 mA h g 1, respectively. The discharge capacities slightly decreased when the current density was increased from 500 to 3000 mA g 1. In addition, after 70 cycles, the discharge capacity of the MoO3–rGO powder recovered well when the current density was decreased to 200 mA g 1. The impedance spectra of the three samples before and after 50 cycles measured in the potential range 0.001–3.0 V at a current density of 500 mA g 1 are shown in Figs. S8 and 4c, respectively. The impedance spectra comprise a semicircle and an inclined line. The medium-frequency semicircle was assigned to the charge-transfer resistance (Rct) and the line inclined at 45 to the real axis corresponded to the lithium diffusion process within the electrodes [65–67]. The MoO3–rGO powder with a high surface area had a slightly higher charge transfer resistance than
Fig. 3. CV curves and initial charge and discharge curves of the three samples: (a) CV curves of MoO3, (b) CV curves of MoO3-C1, (c) CV curves of MoO3-rGO, and (d) initial charge and discharge curves.
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Fig. 4. Electrochemical properties of the four samples: (a) cycling performances, (b) rate performances, (c) Nyquist plots after 50 cycles, and (d) relationships between Zre and v 1/2 after 50 cycles.
the MoO3 and MoO3–C1 powders before cycling, as shown in Fig. S8. However, the MoO3–rGO powder had the minimum charge–discharge resistance after 50 cycles, as shown in Fig. 4c. The charge transfer resistance of the MoO3–rGO powder decreased after cycling owing to the transformation of the crystalline structure into an amorphous structure after the first cycle. On the other hand, the charge transfer resistance of the bare MoO3 powder significantly increased after cycling owing to its structural instability during cycling. The relationships between the real part of the impedance spectra (Zre) and v 1/2 (where v is the angular frequency in the low-frequency region, given by v = 2pf) before cycling and after the 50th cycle are shown in Figs. S8 and 4d, respectively. The small slope gradient of the straight line is indicative of good Li-ion kinetics in the electrode materials. The MoO3–rGO powder exhibited lower slope than those of the MoO3 and MoO3–C1 powders before and after cycling. The MoO3–rGO powder, which had high electric conductivity and structural stability, had a faster lithium diffusion rate than the MoO3 and MoO3–C1 powders before and after cycling. 4. Conclusions The preparation process of a metal oxide–reduced graphene oxide (rGO) composite powder by a large-scale spray-drying of a solution containing water-soluble metal salt and GO nanosheets is reported. Details of the formation mechanism of the MoO3–rGO composite powder, which was selected as the first target material, were also investigated. The spray-drying process produced an ammonium molybdate–GO composite powder. Ammonium molybdate was uniformly deposited over the spherical GO powder particles during the spray-drying process. The heat treatment of the spray-dried powders at 300 C under air atmosphere produced a MoO3–rGO composite powder with superior electrochemical properties as an anode material for LIBs. Decomposition of the ammonium molybdate and thermal reduction of the GO occurred during the heat-treatment process. The contents of MoO3 and rGO in the MoO3–rGO composite powder were easily controlled by changing the concentration of ammonium molybdate tetrahydrate and the dispersing amount of GO nanosheets in the spray solution. Therefore, the process introduced in this study could be widely
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