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Reduced Graphene Oxide composite as anodes for lithium-ion batteries
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Hydrothermally synthesized Bi2MoO6/Reduced Graphene Oxide composite as anodes for lithium-ion batteries Manjunath Shettya, Murthy M.a, Mahesh Shastria, Sindhusree M.a, Nagaswarupa H.P.b, Prasanna D. Shivaramua, Dinesh Rangappaa,∗ a
Department of Nanotechnology and Visvesvaraya Center for Nanoscience and Technology, Center Post Graduation Studies, Visvesvaraya Technological University, Muddenahalli Campus, Chikkaballapura, India b PG Department of Chemistry, Davangere University, Davanagere, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Bi2MoO6 Irregular nanoplates RGO composite Anodes Lithium-ion batteries
Metal molybdates have been attracting the attention of researchers due to their potential application as electrodes in high performance energy storage systems. In this present paper, we are reporting the synthesis of single phase Bi2MoO6/Reduced Graphene Oxide (BMO/RGO) nanocomposite by one pot hydrothermal method. The synthesized BMO/RGO nanocomposite shows single phase orthorhombic crystal structure. Transmission electron microscopy (TEM) analysis revealed good irregular plates like morphology of as-prepared samples with an average particle size of 50 nm. The specific capacities measured for BMO nanoplates and BMO/RGO nanocomposite were 348 mA h g-1 and 750 mA h g-1 capacities at 0.1C with columbic efficiency of 90% and 99% respectively. The results show RGO content had the influence on the electrochemical performance of the electrodes. One-pot synthesis is shown to be a promising method for synthesis of BMO/RGO nanocomposite.
1. Introduction
[6,7]. Metal oxides like MoO2 [8], TiO2 [9], NiO [10], Fe3O4 [11], Co3O4 [12], V2O5 [13] have been studied widely to improve the stability and specific capacity as negative electrodes. Recently, the interest on multicomponent metal oxides such as CuCo2O4 [14], CoFe2O4 [15], NiCr2O4 [16], Sb2MoO6 [17], and NiMoO4 [18] has been increased due to their better electronic and ionic conductivities. Therefore, these materials are viewed as next-generation electrode materials for energy storage applications. Specifically, metal molybdates show the synergetic effect of metal and molybdate combination leading to better conductivity and stability, which are essential to study their ability to store Li-ions. Graphene based metal oxide composites have proven to be excellent candidates for energy storage applications owed to their excellent conductivity, chemical stability, flexibility, and large surface area [19,20]. Among the metal molybdate materials, Bi2MoO6 nanoparticles have been widely studied for their excellent photocatalytic, photoelectric and supercapacitor applications [21–23]. The Bi2MoO6 has a layered structure where (Bi2O2)2+ aurivilluis oxide layers are sandwitched between (MoO4)2- slabs. Slabs of (MoO4)2- are combination of uneven MoO6 octahedra and share four corners mutual with neighboring MoO6 octahedra. The layer structure offers stable and open channels for better Li+ intercalation/de intercalation during charge discharge-cycles [24].
With the exponential progress of lightweight electronic devices and electrical vehicles (EVs), demand for secondary energy storage devices has increased rapidly [1]. Due to extraordinary energy density, discharge capacity along with longer cycle life of Li-ion rechargeable batteries have emerged as major player in energy storage application [2]. Increasing specific capacity and energy density of electrode materials are major approaches for future batteries with reduced weight, cost and overall volume of the battery [3]. The increased demand for hybrid electric vehicles is driving the Li-ion battery technology to improve its energy density above 210 Wh kg-1 as of today [4]. The progress in the research on high specific capacity cathodes materials has made researchers look for better anode materials that would overcome the limitations of the graphite based anodes such as, low specific capacity and rate capacity. Inspite of their excellent conductivity, the great hierarchical structure for lithiation/de-lithiation and low cost, they are suffering from a lower theoretical capacity calculated 372 mA h g-1 [5]. In the last decade, metal oxides and sulfides are emerging as alternatives to the anodes material research owed to their great theoretical specific capacity (600 mA h g-1-1000 mAh g-1) and notable stability
∗
Corresponding author. E-mail address: [email protected] (D. Rangappa).
https://doi.org/10.1016/j.ceramint.2019.09.214 Received 7 June 2019; Received in revised form 12 September 2019; Accepted 21 September 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Manjunath Shetty, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.214
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As per calculations, BMO has a great theoretical capacity of 791 mA h g1 in comparision to other carbon or metal oxide based anode materials. However, this material is not much explored for energy storage application particularly for Li-ion batteries [24–26]. Zheng et al. reported Bi2MoO6 nanosheets showing 900 mA h g-1 capacity at 50 mA g-1 current with high cyclic stability and long life performance [25]. Yuan et al. synthesized porous 3-D hierarchical BMO with great structural stability, high rate capacity, and high coulombic efficiency [24]. Xiangang Zhai et al. have reported Bi2MoO6/RGO nanocomposite with different RGO ratios showing the optimum RGO composites for good capacity, coulombic efficiency and cyclic stability [26]. Generally, BMO nanoparticles are synthesized by solid state reactions, co-precipitation methods and sol-gel methods of synthesis [27–33]. However, the above mentioned techniques need high temperatures from 400 °C–700 °C, meanwhile controlling the properties such as purity, surface texture and grain shape have not been easy [34]. Recently, many advanced synthesis techniques have developed including hydrothermal method, solvothermal methods, microwave assisted hydrothermal, etc [35–37]. Among these nanoparticle growth methods solution-based growth routes like hydrothermal and solvothermal methods have gained great attention due to their flexibility in controlling size, anisotropic crystal growth, and producing different morphologies at low temperatures [38]. It is observed from the literature that different morphologies play important role in the performance of Bi2MoO6. Therefore, in this paper we report BMO/RGO nanocomposite with irregular nanoplate like morphology that was successfully synthesized using ethylene glycol (EG) assisted one pot hydrothermal method. As synthesized BMO/RGO nanocomposite were used as anodes in Li-ion battery.
2.3. Electrochemical characterization The electrochemical measurements of the BMO/RGO nanocomposite and BMO nanoplates were performed using CR2032 coin cells. The electrodes for the coin cell assembly were fabricated by using BMO/ RGO nanocomposite and BMO nanoplates as active material (80 wt % of the total weight), polyvinylidene fluoride (PVDF, 10 wt %, as binder), and acetylene black (10 wt %, as conductive agent) in N-methyl pyrrolidone (NMP) as organic solvent for slurry preparation. The combination was ground in mortar and pestle for 45 min. A copper foil was used as current collector to coat the slurry and dried at 90 °C for 12 h under vacuum. The electrodes used in the cell assembly were punched from the coated and dried copper foil. The half-cell assembly was done in an argon filled glove box (EMBRAUN glove box) with O2 and moisture content below 0.5 ppm. The assembly was composed of the coated electrode with separator (celgard polypropylene membrane) and lithium foil as counter electrode in the cell. Lithium hexafluorophosphate (LiPF6) in combination with ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC = 1:1 V/V) were used as electrolyte. Studies related Galvanostatic charge-discharge were conducted using 12 channel battery test system (ARBIN BATTERY TESTER).
3. Results and discussion XRD analysis was performed on as-prepared BMO/RGO nanocomposite sample to identify the crystal structure and phase formed. Fig. 1(b) shows the XRD pattern the of BMO/RGO nanocomposite has a characteristic peaks at 10.8°, 23.5°, 28.2°, 32.5°, 33.2°, 36.0°, 46.9°, 55.3°, 56.3° and 67.9° which can be assigned to (020), (111), (131), (002), (151), (240), (202), (133), (062) and (400) crystal planes respectively [26]. The XRD pattern of BMO/RGO nanocomposite reveals that sample has koechlinite phase with orthorhombic crystal structure and space group Pca21 (PDF Card no.: 9011351) for the synthesized samples. Fig. 1(a) illustrates the XRD pattern obtained for GO, which shows a characteristic peak at 10.8° and can be assigned to (001) crystal plane that disappears after hydrothermal treatment. Bi2MoO6 nanoplates in koechlinite phase are composed of perovskite-like slabs of (MoO4)2- located between (Bi2O2)2+ layers [24]. The average particle size for BMO irregular nanoplate like structures synthesized was calculated from Scherer's equation [40]. D= (0.9λ/(βcosθ)) Where λ = 0.154 nm, β = FWHM in radian, θ = angle of diffraction at highest intensity peak in degrees. The average crystallite size was found to be 44.5 nm.
2. Experimental section 2.1. Preparation of BMO/RGO nanocomposite BMO/RGO nanocomposite was synthesized using one pot ethylene glycol assisted hydrothermal method. At first Graphene oxide (GO) was synthesized by modified hummer's method as described elsewhere [39]. BMO/RGO nanocomposite was synthesized by dissolving 15 mmol Bi (NO3)3·5H2O (Karnataka fine Chemicals) was dissolved into 24 ml of deionized water and 8 ml of EG and solution was stirred until a transparent clear solution was formed. Then 7.5 mmol Na2MoO4·2H2O (Merck) was added stirring was continued for 1 h until a clear solution was formed. Later, 15 mg GO with 3 mg ml-1 concentration was dispersed into the above solution and stirred for 30 min followed by ultrasonication. The solution was then moved to Teflon lined stainless steel hydrothermal reactors and heated to a temperature of 140 °C for 6 h. For comparison the same experiment was conducted without GO with the same reaction conditions. The resultant products were collected and washed with ethanol and dried overnight at 70 °C to obtain the sample. 2.2. Characterization The crystal structure of the as-synthesized BMO/RGO nanocomposite was evaluated with Rigaku Ultima IV X-ray diffraction diffractometer (XRD) using copper radiation (λ = 0.15406 nm) over a range of 10° to 70° at 40 kV and 30 mA. Hitachi SU1510 Scanning electron microscopy (SEM) was used to observe the morphology of the as-synthesized samples. The size and morphology of nanoparticles in the sample were further studied by Hitachi transmission electron microscopy H7650 (TEM). The surface functional group analysis was performed using FTIR PerkinElmer at a range of 400 cm-1 to 4000 cm-1. Thermogravimetric – Differential thermal analysis (TG-DTA) was performed to determine the RGO content in BMO/RGO nanocomposite using PerkinElmer STA 8000 at a rate of heating of 10 °C min-1 in a nitrogen atmosphere.
Fig. 1. XRD patterns (a) as prepared GO by hummer's method and (b) as-synthesized BMO/RGO nanocomposite synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal synthesis. 2
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deformation mode and stretching mode of Bi-O [43]. At 543 cm-1 peaks can be ascribed to Bi-O, in the formation of Bi2O3 structures and very strong peaks at 695 cm-1 and 833 cm-1 are owed to the formation of MoO structures. In case of BMO/RGO nanocomposite, Peak at 1217 cm1 can be attributed to C-O and the peak at 1576 cm-1 of the sample can be attributing to bending of C]C in RGO [44]. The strong C]C peak observed for BMO/RGO nanocomposite indicates a stable graphene structure [45]. The down shift of the peak intensity in BMO/RGO nanocomposite shows a strong interaction between BMO irregular nanoplates like structures and RGO nanosheets [46]. Peaks at 1646 cm-1 is attributed to O-H bending vibration adsorbed by H2O molecules and 3390 cm-1 can be attributed to O-H in the hydroxyl group. The morphology of the as-synthesized BMO/RGO nanocomposite was studied using SEM. The SEM images clearly show formation BMO/ RGO nanocomposite shown at different magnifications in Fig. 4(a and b) but, do not conclusively confirm the plate like morphology due to lower magnifications. However, the TEM images of BMO/RGO nanocomposite reveal irregular nanoplate like morphology with good crystalline irregular nanoplate like structures ranging from 20 nm to 60 nm from Fig. 4(c and d). The formation of composite can be seen very clearly as the Bi2MoO6 irregular nanoplates are covered by RGO sheets as seen from Fig. 4(c). The average sizes of the irregular nanoplates observed are about 45 nm. This is in consistence with the crystallite size calculated using Scherrer's equation using XRD data from Fig. 1(b). The formation of BMO irregular nanoplates like structures are understood in the following simple phenomenon as illustrated in Scheme 1. Bi(NO3)5.H2O and Na2MoO4·2H2O get hydrolysed to form free Bi3+ ions, MoO4-2 ions. The electrostatic attraction causes the surface of graphene oxide (GO) sheets to adsorb the Bi3+ ions due to the negative charge on sheets surface which contain oxygen containing functional group in abundance at higher pH [26]. When precursor solution was hydrothermally treated in the presence of EG, the Bi3+ ions blend with MoO4-2 ions slowly. EG synchronizes with Bi3+ ions to produce Bi2O3 and decrease the Bi3+ ions concentration. This makes the entire reaction to slowdown, resulting in the splitting-up of growth and nucleation phase as reported [26]. Same time Bi2MoO6 formation takes place at a slow pace due to precipitation and tends to assemble into microsphere. Nanoparticles will prefer to grow like irregular plate like structure by Ostwald coarsening mechanism [47]. Thermal reduction of GO to RGO can be seen under hydrothermal treatment in the presence of EG, a mild reducing agent [48].
Fig. 2. TGA/DTA curves of as-synthesized BMO/RGO nanocomposite synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal method.
Fig. 2 Illustrates the TGA/DTA curves for BMO/RGO nanocomposite. The stability of the as-synthesized BMO/RGO nanocomposite was determined using the curves obtained from the TGA/DTA analysis with a heating range from 50 °C to 800 °C, and a heating rate of 10 min-1. The gradual decrease in the weight of sample was observed from a temperature of 100 °C–250 °C, this weight loss can be attributed to the loss of hydroxyl molecules and volatile organic groups if any. The endothermic peaks in DTA curve at 150 °C and 220 °C is due to the loss of water content and volatile organic groups. A weight loss of nearly 2.61% was from temperature of 250 °C–500 °C observed, which was due to decomposition of carbon molecules in RGO [41]. From TGA analysis, it was found that the BMO/RGO nanocomposite consisted of 2.6% of RGO and 97.4% BMO. An exothermic peak at 450 °C indicated the decomposition of carbon in RGO. From 500 °C to 800 °C, there was no notable weight loss with an increase in temperature inferring that the BMO irregular nanoplates were stable at a higher temperature. FTIR spectral analysis was performed to understand the nature of surface functional groups of the samples. Fig. 3 shows the FTIR spectra of as prepared samples. Strong peak is seen in both BMO irregular nanoplates and BMO/RGO nanocomposite at 695 cm-1 and 833 cm-1 can be ascribed to stretching vibrations of Mo-O and bending vibrations of Mo-O-Mo, correspondingly [42]. Bands at 543 cm-1 is attributed to
3.1. Electrochemical analysis Cyclic voltammetry measurements for BMO nanoplates at 10 mV cathodic sweep is as shown in Fig. 5(a). It can be observed that two peaks at 1.05 V and 2.17 V, that can be ascribed to the decomposition of Bi2O3 and MoO3 as per equation (1) and formation of Li3Bi as per equation (2), respectively. The anodic peaks at 0.25 V and 1.3 V could be attributed to de intercalation of Li-ions from the cathode and oxidation of Bi2O3 and MoO3 equations (3) and (4), respectively. Furthermore, it can be seen that with the increase in the scan rates, the current peaks is increased. Moreover, an increase in the potential was also observed, which may indicate that there is a fast kinetics and moderate polarization effect. This can be overcome by increasing RGO content to optimum as conductive coatings and reduction in particle size for faster electron transfer. The reactions at the electrodes are as follows:
Bi2 MoO6 +12Li+ +12e− → 2Bi + Mo + 6Li2 O
(1)
3Li+ +Bi
(2)
+
3e−
↔ Li3Bi
2Bi + 3Li2 O↔ Bi2 O3 +6Li+ +6e− Fig. 3. FTIR spectra of as-synthesized (a) BMO nanoplates and (b) BMO/RGO nanocomposite synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal method.
Mo +3Li2 O↔ MoO3 +6Li +
6e−
(3) (4)
The Galvanostatic charge-discharge performance of both BMO 3
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Fig. 4. SEM images of (a, b) as-synthesized BMO/RGO nanocomposite with different magnification. TEM images of (c, d) as-synthesized BMO/RGO nanocomposite (inset presents the corresponding enlarged TEM images) synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal method.
fade in the capacity during initial cycles can be attributed to the solid electrolyte interface (SEI) layer stabilization because of irreversible Liion insertion and adsorption of Li-ions onto the surface of electrodes [49]. The cycling stability was studied for 30 cycles as shown in Fig. 5(d) and the columbic efficiency was seen increasing from 58.8% to 89.57% at the end of the 30th cycle. This can be seen because of the progressive establishment of the SEI layer due delayed wetting of electrolyte into the pores of active electrode material. The BMO/RGO nanocomposite forms metal nanoparticles that are good candidates acting as substrates for conduction along with RGO, as they are dispersed in the electrodes to increase the performance of batteries [50]. Once the voltage of the cell reaches to 3.0 V, several nanoparticles are formed in electrodes these include Bi2O3, Li2O, MoO3 or Mo as reported earlier [51,52]. This formation of Li2O can reduce change in volume during Li-ion lithiation/de-lithiation by confining the agglomeration and diffusion of Bi and Mo, in turn resulting in molecules of smaller size of Bi2O3, Mo and alloys of the same [53]. The fading of the capacity
irregular nanoplates and BMO/RGO nanocomposite electrodes were performed between 0.2 V and 3.0 V at 0.1C current density. The chargedischarge curves are presented in Fig. 5(b and c) for the first, third and sixth discharge cycles. The reversible capacities of BMO/RGO nanocomposite obtained were 750 mA h g-1, 488 mA h g-1 and 407 mA h g-1 respectively. The observed values are much higher than the specific capacities of graphite (372 mA h g-1) [5]. Fig. 5(b) shows that discharge cycle having reversible capacities of BMO irregular nanoplates are 350 mA h g-1, 170 mA h g-1and 118 mA h g-1, respectively for 1st, 3rd and 6th cycle. This infer that BMO/RGO nanocomposite has a better cyclic stability when compared to earlier studies. At 30th cycle the capacity fade decreases and then stabilises at 249 mA h g-1 with an increase in columbic efficiency up to 90%. This can be owed to the presence of RGO, due to increase in the electronic conductivity and mechanical stability [19]. For Pure BMO irregular nanoplates the capacity fade was observed to be higher in comparison to BMO/RGO nanocomposite and 40 mA h g-1 at the end of 30th cycle. The gradual
Scheme 1. Schematic representation of the growth mechanism of BMO/RGO nanocomposite synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal method. 4
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Fig. 5. Electrochemical performance of as-synthesized samples (a) cyclic voltammetry curves for BMO nanoplates at different scan rates of 10 mV, 20 mV and 30 mV. (b) Charge-discharge curves of BMO nanoplates at 0.1C current density. (c) Charge-discharge curves of BMO/RGO nanocomposite at 0.1C current density (d) Cycling performances of BMO/RGO nanocomposite and BMO nanoplates for 30 cycles at 0.1C current density.
during cycling can be attributed to the reversible structural change of Bi2O3 and MoO3 that affects the morphology of the particles [24].
[6]
4. Conclusions [7]
In summary, the BMO/RGO nanocomposite synthesized by hydrothermal method have demonstrated one pot reliable approach for inorganic nanoparticle synthesis. Moreover, the BMO/RGO nanocomposite have shown high specific capacities up to 750 mA h g-1, 488 mA h g1 , and 407 mA h g-1 in the first six cycles, which is higher when compared to commercially used graphite anodes. Besides this, one pot synthesis approach is reliable for synthesis of RGO encapsulated nanoparticles showing a great potential in cost-effective Li-ion battery electrode synthesis and to move forward towards sustainable energy storage systems.
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Acknowledgement
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This work is partially supported by the Vision Group of Science and Technology, Department of Information Technology, Biotechnology and Science & Technology, Govt. of Karnataka, Grant No.: VGST/ CESEM/2012-13/281
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Hydrothermally synthesized Bi2MoO6/Reduced Graphene Oxide composite as anodes for lithium-ion batteries Manjunath Shettya, Murthy M.a, Mahesh Shastria, Sindhusree M.a, Nagaswarupa H.P.b, Prasanna D. Shivaramua, Dinesh Rangappaa,∗ a
Department of Nanotechnology and Visvesvaraya Center for Nanoscience and Technology, Center Post Graduation Studies, Visvesvaraya Technological University, Muddenahalli Campus, Chikkaballapura, India b PG Department of Chemistry, Davangere University, Davanagere, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Bi2MoO6 Irregular nanoplates RGO composite Anodes Lithium-ion batteries
Metal molybdates have been attracting the attention of researchers due to their potential application as electrodes in high performance energy storage systems. In this present paper, we are reporting the synthesis of single phase Bi2MoO6/Reduced Graphene Oxide (BMO/RGO) nanocomposite by one pot hydrothermal method. The synthesized BMO/RGO nanocomposite shows single phase orthorhombic crystal structure. Transmission electron microscopy (TEM) analysis revealed good irregular plates like morphology of as-prepared samples with an average particle size of 50 nm. The specific capacities measured for BMO nanoplates and BMO/RGO nanocomposite were 348 mA h g-1 and 750 mA h g-1 capacities at 0.1C with columbic efficiency of 90% and 99% respectively. The results show RGO content had the influence on the electrochemical performance of the electrodes. One-pot synthesis is shown to be a promising method for synthesis of BMO/RGO nanocomposite.
1. Introduction
[6,7]. Metal oxides like MoO2 [8], TiO2 [9], NiO [10], Fe3O4 [11], Co3O4 [12], V2O5 [13] have been studied widely to improve the stability and specific capacity as negative electrodes. Recently, the interest on multicomponent metal oxides such as CuCo2O4 [14], CoFe2O4 [15], NiCr2O4 [16], Sb2MoO6 [17], and NiMoO4 [18] has been increased due to their better electronic and ionic conductivities. Therefore, these materials are viewed as next-generation electrode materials for energy storage applications. Specifically, metal molybdates show the synergetic effect of metal and molybdate combination leading to better conductivity and stability, which are essential to study their ability to store Li-ions. Graphene based metal oxide composites have proven to be excellent candidates for energy storage applications owed to their excellent conductivity, chemical stability, flexibility, and large surface area [19,20]. Among the metal molybdate materials, Bi2MoO6 nanoparticles have been widely studied for their excellent photocatalytic, photoelectric and supercapacitor applications [21–23]. The Bi2MoO6 has a layered structure where (Bi2O2)2+ aurivilluis oxide layers are sandwitched between (MoO4)2- slabs. Slabs of (MoO4)2- are combination of uneven MoO6 octahedra and share four corners mutual with neighboring MoO6 octahedra. The layer structure offers stable and open channels for better Li+ intercalation/de intercalation during charge discharge-cycles [24].
With the exponential progress of lightweight electronic devices and electrical vehicles (EVs), demand for secondary energy storage devices has increased rapidly [1]. Due to extraordinary energy density, discharge capacity along with longer cycle life of Li-ion rechargeable batteries have emerged as major player in energy storage application [2]. Increasing specific capacity and energy density of electrode materials are major approaches for future batteries with reduced weight, cost and overall volume of the battery [3]. The increased demand for hybrid electric vehicles is driving the Li-ion battery technology to improve its energy density above 210 Wh kg-1 as of today [4]. The progress in the research on high specific capacity cathodes materials has made researchers look for better anode materials that would overcome the limitations of the graphite based anodes such as, low specific capacity and rate capacity. Inspite of their excellent conductivity, the great hierarchical structure for lithiation/de-lithiation and low cost, they are suffering from a lower theoretical capacity calculated 372 mA h g-1 [5]. In the last decade, metal oxides and sulfides are emerging as alternatives to the anodes material research owed to their great theoretical specific capacity (600 mA h g-1-1000 mAh g-1) and notable stability
∗
Corresponding author. E-mail address: [email protected] (D. Rangappa).
https://doi.org/10.1016/j.ceramint.2019.09.214 Received 7 June 2019; Received in revised form 12 September 2019; Accepted 21 September 2019 0272-8842/ © 2019 Published by Elsevier Ltd.
Please cite this article as: Manjunath Shetty, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.214
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As per calculations, BMO has a great theoretical capacity of 791 mA h g1 in comparision to other carbon or metal oxide based anode materials. However, this material is not much explored for energy storage application particularly for Li-ion batteries [24–26]. Zheng et al. reported Bi2MoO6 nanosheets showing 900 mA h g-1 capacity at 50 mA g-1 current with high cyclic stability and long life performance [25]. Yuan et al. synthesized porous 3-D hierarchical BMO with great structural stability, high rate capacity, and high coulombic efficiency [24]. Xiangang Zhai et al. have reported Bi2MoO6/RGO nanocomposite with different RGO ratios showing the optimum RGO composites for good capacity, coulombic efficiency and cyclic stability [26]. Generally, BMO nanoparticles are synthesized by solid state reactions, co-precipitation methods and sol-gel methods of synthesis [27–33]. However, the above mentioned techniques need high temperatures from 400 °C–700 °C, meanwhile controlling the properties such as purity, surface texture and grain shape have not been easy [34]. Recently, many advanced synthesis techniques have developed including hydrothermal method, solvothermal methods, microwave assisted hydrothermal, etc [35–37]. Among these nanoparticle growth methods solution-based growth routes like hydrothermal and solvothermal methods have gained great attention due to their flexibility in controlling size, anisotropic crystal growth, and producing different morphologies at low temperatures [38]. It is observed from the literature that different morphologies play important role in the performance of Bi2MoO6. Therefore, in this paper we report BMO/RGO nanocomposite with irregular nanoplate like morphology that was successfully synthesized using ethylene glycol (EG) assisted one pot hydrothermal method. As synthesized BMO/RGO nanocomposite were used as anodes in Li-ion battery.
2.3. Electrochemical characterization The electrochemical measurements of the BMO/RGO nanocomposite and BMO nanoplates were performed using CR2032 coin cells. The electrodes for the coin cell assembly were fabricated by using BMO/ RGO nanocomposite and BMO nanoplates as active material (80 wt % of the total weight), polyvinylidene fluoride (PVDF, 10 wt %, as binder), and acetylene black (10 wt %, as conductive agent) in N-methyl pyrrolidone (NMP) as organic solvent for slurry preparation. The combination was ground in mortar and pestle for 45 min. A copper foil was used as current collector to coat the slurry and dried at 90 °C for 12 h under vacuum. The electrodes used in the cell assembly were punched from the coated and dried copper foil. The half-cell assembly was done in an argon filled glove box (EMBRAUN glove box) with O2 and moisture content below 0.5 ppm. The assembly was composed of the coated electrode with separator (celgard polypropylene membrane) and lithium foil as counter electrode in the cell. Lithium hexafluorophosphate (LiPF6) in combination with ethylene carbonate (EC) and diethyl carbonate (DEC) (EC:DEC = 1:1 V/V) were used as electrolyte. Studies related Galvanostatic charge-discharge were conducted using 12 channel battery test system (ARBIN BATTERY TESTER).
3. Results and discussion XRD analysis was performed on as-prepared BMO/RGO nanocomposite sample to identify the crystal structure and phase formed. Fig. 1(b) shows the XRD pattern the of BMO/RGO nanocomposite has a characteristic peaks at 10.8°, 23.5°, 28.2°, 32.5°, 33.2°, 36.0°, 46.9°, 55.3°, 56.3° and 67.9° which can be assigned to (020), (111), (131), (002), (151), (240), (202), (133), (062) and (400) crystal planes respectively [26]. The XRD pattern of BMO/RGO nanocomposite reveals that sample has koechlinite phase with orthorhombic crystal structure and space group Pca21 (PDF Card no.: 9011351) for the synthesized samples. Fig. 1(a) illustrates the XRD pattern obtained for GO, which shows a characteristic peak at 10.8° and can be assigned to (001) crystal plane that disappears after hydrothermal treatment. Bi2MoO6 nanoplates in koechlinite phase are composed of perovskite-like slabs of (MoO4)2- located between (Bi2O2)2+ layers [24]. The average particle size for BMO irregular nanoplate like structures synthesized was calculated from Scherer's equation [40]. D= (0.9λ/(βcosθ)) Where λ = 0.154 nm, β = FWHM in radian, θ = angle of diffraction at highest intensity peak in degrees. The average crystallite size was found to be 44.5 nm.
2. Experimental section 2.1. Preparation of BMO/RGO nanocomposite BMO/RGO nanocomposite was synthesized using one pot ethylene glycol assisted hydrothermal method. At first Graphene oxide (GO) was synthesized by modified hummer's method as described elsewhere [39]. BMO/RGO nanocomposite was synthesized by dissolving 15 mmol Bi (NO3)3·5H2O (Karnataka fine Chemicals) was dissolved into 24 ml of deionized water and 8 ml of EG and solution was stirred until a transparent clear solution was formed. Then 7.5 mmol Na2MoO4·2H2O (Merck) was added stirring was continued for 1 h until a clear solution was formed. Later, 15 mg GO with 3 mg ml-1 concentration was dispersed into the above solution and stirred for 30 min followed by ultrasonication. The solution was then moved to Teflon lined stainless steel hydrothermal reactors and heated to a temperature of 140 °C for 6 h. For comparison the same experiment was conducted without GO with the same reaction conditions. The resultant products were collected and washed with ethanol and dried overnight at 70 °C to obtain the sample. 2.2. Characterization The crystal structure of the as-synthesized BMO/RGO nanocomposite was evaluated with Rigaku Ultima IV X-ray diffraction diffractometer (XRD) using copper radiation (λ = 0.15406 nm) over a range of 10° to 70° at 40 kV and 30 mA. Hitachi SU1510 Scanning electron microscopy (SEM) was used to observe the morphology of the as-synthesized samples. The size and morphology of nanoparticles in the sample were further studied by Hitachi transmission electron microscopy H7650 (TEM). The surface functional group analysis was performed using FTIR PerkinElmer at a range of 400 cm-1 to 4000 cm-1. Thermogravimetric – Differential thermal analysis (TG-DTA) was performed to determine the RGO content in BMO/RGO nanocomposite using PerkinElmer STA 8000 at a rate of heating of 10 °C min-1 in a nitrogen atmosphere.
Fig. 1. XRD patterns (a) as prepared GO by hummer's method and (b) as-synthesized BMO/RGO nanocomposite synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal synthesis. 2
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deformation mode and stretching mode of Bi-O [43]. At 543 cm-1 peaks can be ascribed to Bi-O, in the formation of Bi2O3 structures and very strong peaks at 695 cm-1 and 833 cm-1 are owed to the formation of MoO structures. In case of BMO/RGO nanocomposite, Peak at 1217 cm1 can be attributed to C-O and the peak at 1576 cm-1 of the sample can be attributing to bending of C]C in RGO [44]. The strong C]C peak observed for BMO/RGO nanocomposite indicates a stable graphene structure [45]. The down shift of the peak intensity in BMO/RGO nanocomposite shows a strong interaction between BMO irregular nanoplates like structures and RGO nanosheets [46]. Peaks at 1646 cm-1 is attributed to O-H bending vibration adsorbed by H2O molecules and 3390 cm-1 can be attributed to O-H in the hydroxyl group. The morphology of the as-synthesized BMO/RGO nanocomposite was studied using SEM. The SEM images clearly show formation BMO/ RGO nanocomposite shown at different magnifications in Fig. 4(a and b) but, do not conclusively confirm the plate like morphology due to lower magnifications. However, the TEM images of BMO/RGO nanocomposite reveal irregular nanoplate like morphology with good crystalline irregular nanoplate like structures ranging from 20 nm to 60 nm from Fig. 4(c and d). The formation of composite can be seen very clearly as the Bi2MoO6 irregular nanoplates are covered by RGO sheets as seen from Fig. 4(c). The average sizes of the irregular nanoplates observed are about 45 nm. This is in consistence with the crystallite size calculated using Scherrer's equation using XRD data from Fig. 1(b). The formation of BMO irregular nanoplates like structures are understood in the following simple phenomenon as illustrated in Scheme 1. Bi(NO3)5.H2O and Na2MoO4·2H2O get hydrolysed to form free Bi3+ ions, MoO4-2 ions. The electrostatic attraction causes the surface of graphene oxide (GO) sheets to adsorb the Bi3+ ions due to the negative charge on sheets surface which contain oxygen containing functional group in abundance at higher pH [26]. When precursor solution was hydrothermally treated in the presence of EG, the Bi3+ ions blend with MoO4-2 ions slowly. EG synchronizes with Bi3+ ions to produce Bi2O3 and decrease the Bi3+ ions concentration. This makes the entire reaction to slowdown, resulting in the splitting-up of growth and nucleation phase as reported [26]. Same time Bi2MoO6 formation takes place at a slow pace due to precipitation and tends to assemble into microsphere. Nanoparticles will prefer to grow like irregular plate like structure by Ostwald coarsening mechanism [47]. Thermal reduction of GO to RGO can be seen under hydrothermal treatment in the presence of EG, a mild reducing agent [48].
Fig. 2. TGA/DTA curves of as-synthesized BMO/RGO nanocomposite synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal method.
Fig. 2 Illustrates the TGA/DTA curves for BMO/RGO nanocomposite. The stability of the as-synthesized BMO/RGO nanocomposite was determined using the curves obtained from the TGA/DTA analysis with a heating range from 50 °C to 800 °C, and a heating rate of 10 min-1. The gradual decrease in the weight of sample was observed from a temperature of 100 °C–250 °C, this weight loss can be attributed to the loss of hydroxyl molecules and volatile organic groups if any. The endothermic peaks in DTA curve at 150 °C and 220 °C is due to the loss of water content and volatile organic groups. A weight loss of nearly 2.61% was from temperature of 250 °C–500 °C observed, which was due to decomposition of carbon molecules in RGO [41]. From TGA analysis, it was found that the BMO/RGO nanocomposite consisted of 2.6% of RGO and 97.4% BMO. An exothermic peak at 450 °C indicated the decomposition of carbon in RGO. From 500 °C to 800 °C, there was no notable weight loss with an increase in temperature inferring that the BMO irregular nanoplates were stable at a higher temperature. FTIR spectral analysis was performed to understand the nature of surface functional groups of the samples. Fig. 3 shows the FTIR spectra of as prepared samples. Strong peak is seen in both BMO irregular nanoplates and BMO/RGO nanocomposite at 695 cm-1 and 833 cm-1 can be ascribed to stretching vibrations of Mo-O and bending vibrations of Mo-O-Mo, correspondingly [42]. Bands at 543 cm-1 is attributed to
3.1. Electrochemical analysis Cyclic voltammetry measurements for BMO nanoplates at 10 mV cathodic sweep is as shown in Fig. 5(a). It can be observed that two peaks at 1.05 V and 2.17 V, that can be ascribed to the decomposition of Bi2O3 and MoO3 as per equation (1) and formation of Li3Bi as per equation (2), respectively. The anodic peaks at 0.25 V and 1.3 V could be attributed to de intercalation of Li-ions from the cathode and oxidation of Bi2O3 and MoO3 equations (3) and (4), respectively. Furthermore, it can be seen that with the increase in the scan rates, the current peaks is increased. Moreover, an increase in the potential was also observed, which may indicate that there is a fast kinetics and moderate polarization effect. This can be overcome by increasing RGO content to optimum as conductive coatings and reduction in particle size for faster electron transfer. The reactions at the electrodes are as follows:
Bi2 MoO6 +12Li+ +12e− → 2Bi + Mo + 6Li2 O
(1)
3Li+ +Bi
(2)
+
3e−
↔ Li3Bi
2Bi + 3Li2 O↔ Bi2 O3 +6Li+ +6e− Fig. 3. FTIR spectra of as-synthesized (a) BMO nanoplates and (b) BMO/RGO nanocomposite synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal method.
Mo +3Li2 O↔ MoO3 +6Li +
6e−
(3) (4)
The Galvanostatic charge-discharge performance of both BMO 3
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Fig. 4. SEM images of (a, b) as-synthesized BMO/RGO nanocomposite with different magnification. TEM images of (c, d) as-synthesized BMO/RGO nanocomposite (inset presents the corresponding enlarged TEM images) synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal method.
fade in the capacity during initial cycles can be attributed to the solid electrolyte interface (SEI) layer stabilization because of irreversible Liion insertion and adsorption of Li-ions onto the surface of electrodes [49]. The cycling stability was studied for 30 cycles as shown in Fig. 5(d) and the columbic efficiency was seen increasing from 58.8% to 89.57% at the end of the 30th cycle. This can be seen because of the progressive establishment of the SEI layer due delayed wetting of electrolyte into the pores of active electrode material. The BMO/RGO nanocomposite forms metal nanoparticles that are good candidates acting as substrates for conduction along with RGO, as they are dispersed in the electrodes to increase the performance of batteries [50]. Once the voltage of the cell reaches to 3.0 V, several nanoparticles are formed in electrodes these include Bi2O3, Li2O, MoO3 or Mo as reported earlier [51,52]. This formation of Li2O can reduce change in volume during Li-ion lithiation/de-lithiation by confining the agglomeration and diffusion of Bi and Mo, in turn resulting in molecules of smaller size of Bi2O3, Mo and alloys of the same [53]. The fading of the capacity
irregular nanoplates and BMO/RGO nanocomposite electrodes were performed between 0.2 V and 3.0 V at 0.1C current density. The chargedischarge curves are presented in Fig. 5(b and c) for the first, third and sixth discharge cycles. The reversible capacities of BMO/RGO nanocomposite obtained were 750 mA h g-1, 488 mA h g-1 and 407 mA h g-1 respectively. The observed values are much higher than the specific capacities of graphite (372 mA h g-1) [5]. Fig. 5(b) shows that discharge cycle having reversible capacities of BMO irregular nanoplates are 350 mA h g-1, 170 mA h g-1and 118 mA h g-1, respectively for 1st, 3rd and 6th cycle. This infer that BMO/RGO nanocomposite has a better cyclic stability when compared to earlier studies. At 30th cycle the capacity fade decreases and then stabilises at 249 mA h g-1 with an increase in columbic efficiency up to 90%. This can be owed to the presence of RGO, due to increase in the electronic conductivity and mechanical stability [19]. For Pure BMO irregular nanoplates the capacity fade was observed to be higher in comparison to BMO/RGO nanocomposite and 40 mA h g-1 at the end of 30th cycle. The gradual
Scheme 1. Schematic representation of the growth mechanism of BMO/RGO nanocomposite synthesized at 140 °C for 6 h by ethylene glycol assisted hydrothermal method. 4
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Fig. 5. Electrochemical performance of as-synthesized samples (a) cyclic voltammetry curves for BMO nanoplates at different scan rates of 10 mV, 20 mV and 30 mV. (b) Charge-discharge curves of BMO nanoplates at 0.1C current density. (c) Charge-discharge curves of BMO/RGO nanocomposite at 0.1C current density (d) Cycling performances of BMO/RGO nanocomposite and BMO nanoplates for 30 cycles at 0.1C current density.
during cycling can be attributed to the reversible structural change of Bi2O3 and MoO3 that affects the morphology of the particles [24].
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4. Conclusions [7]
In summary, the BMO/RGO nanocomposite synthesized by hydrothermal method have demonstrated one pot reliable approach for inorganic nanoparticle synthesis. Moreover, the BMO/RGO nanocomposite have shown high specific capacities up to 750 mA h g-1, 488 mA h g1 , and 407 mA h g-1 in the first six cycles, which is higher when compared to commercially used graphite anodes. Besides this, one pot synthesis approach is reliable for synthesis of RGO encapsulated nanoparticles showing a great potential in cost-effective Li-ion battery electrode synthesis and to move forward towards sustainable energy storage systems.
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Acknowledgement
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This work is partially supported by the Vision Group of Science and Technology, Department of Information Technology, Biotechnology and Science & Technology, Govt. of Karnataka, Grant No.: VGST/ CESEM/2012-13/281
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