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DOI: 10.1039/C4NR05373G
Cite this: DOI: 10.1039/c0xx00000x
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Young Jun Hong and Yun Chan Kang*
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Core-shell structured Zn2SnO4–carbon microspheres with various carbon contents are prepared by one-pot spray pyrolysis without further heating process. One Zn2SnO4–carbon composite microsphere is prepared from one droplet containing Zn and Sn salts and polyvinylpyrrolidone (PVP). Melted PVP moves out to the outside of the composite microsphere during the drying stage of the droplet. In addition, melting of phase separated metal salts forms the core part with dense structure. Carbonization of phase separated PVP forms the textured and porous thick carbon shell. The discharge capacities of the core-shell structured Zn2SnO4-carbon microspheres for the 2nd and 120th cycles at a current density of 1 A g-1 are 864 and 770 mA h g-1, respectively. However, the discharge capacities of the bare Zn2SnO4 microspheres prepared by the same process without PVP for the 2nd and 120th cycles are 1106 and 81 mA h g-1, respectively. The stable reversible discharge capacities of the Zn2SnO4-carbon composite microspheres prepared from the spray solution with 15 g PVP decrease from 894 to 528 mA h g-1 with an increase in the current density from 0.5 to 5 A g-1.
Introduction
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SnO2 materials have been widely studied as anode materials for lithium ion batteries (LIBs) because of their low potential, high theoretical reversible specific capacity, and low cost.1-30 However, poor cyclability arising from the large volume change (of 300%) during the repetitive charging and discharging of the battery hinders the commercialization of SnO2.1-30 To overcome the poor cycling performances of SnO2 materials, Sn-based multicomponent materials including transition metal components, which accomodate the huge volume change during cycling, have been studied. Spinel oxides of M2SnO4 (M = Zn, Mg, Mn, Co) materials prepared by solid state and liquid solution methods have been mainly studied.9-30 Especially, Li alloy forming Zn was known as an efficient matrix element because it can contribute to capacity by alloying-dealloying reaction and to buffering the large volume change of Sn component during cycling.10-32 Nanostructured Zn2SnO4 materials with and without carbon coating material have been studied to obtain the good electrochemical properties.18-30 Cherian et al. prepared Zn2SnO4 nanowires by a vapor transport method. In the voltage range 0.005-3V, nanowire electrode retained a capacity of 660 mA h g-1 after 50 cycles at a current density of 120 mA g-1.24 Rong et al. prepared Zn2SnO4 nanocubes by hydrothermal method.25 The cube-shaped Zn2SnO4 materials showed a discharge capacity of 580 mA h g-1 after 50 cycles at a current density of 100 mA g-1. Zhu et al. reported the cubic shaped Zn2SnO4 materials showing a discharge capacity of 645 mA h g-1 after 20 cycles at a current density of 50 mA g-1.26 Hou et al. prepared inverse spinel structure Zn2SnO4 crystals by a solid state reaction having the This journal is © The Royal Society of Chemistry [year]
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specific discharge capacity of 689 mA h g−1 after 50 cycles at a current density of 50 mA g-1.27 Feng et al. reported a hydrothermal route to prepare monodisperse single crystal Zn2SnO4 cubes, and this anode material showed a discharge capacity of 775 mA h g-1 after 20 cycles at a current density of 50 mA g-1.28 Ji et al. prepared carbon-coated Zn2SnO4 nanomaterials showing a discharge capacity of 400 mA h g-1 after 40 cycles at a current density of 200 mA g-1.29 Zhao et al. reported the N-doped carbon-coated Zn2SnO4 hollow boxes with an enhanced electrochemical properties (616 mA h g-1 after 45 cycles at a current density of 300 mA g-1).30 However, the reported Zn2SnO4 materials had lower capacities and poorer cycling performances compared with those of the single component SnO2 materials.33,34 In addition, to the best of our knowledge, the simple one-pot method for carbon-coated Zn2SnO4 nanomaterials has not been reported. In this study, carbon-coated Zn2SnO4 microspheres were prepared by one-pot spray pyrolysis without further heating process. One composite microsphere was formed from one droplet containing metal salts of Zn and Sn components and polyvinylpyrrolidone (PVP). The detail formation mechanism of the carbon-coated Zn2SnO4 microsphere by one-pot method was studied. The electrochemical properties of the carbon-coated Zn2SnO4 microspheres were compared with those of the bare Zn2SnO4 microspheres prepared by the same preparation conditions without PVP.
Experimental Synthesis of carbon-coated Zn2SnO4 microspheres Carbon-coated Zn2SnO4 microspheres were prepared
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Nanoscale Accepted Manuscript
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Formation of core–shell-structured Zn2SnO4–carbon microspheres with superior electrochemical properties by one-pot spray pyrolysis
Nanoscale
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DOI: 10.1039/C4NR05373G
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Characterizations The crystal structures of the microspheres were investigated by X-ray diffractometry (XRD, X’pert PRO MPD) using Cu Kα radiation (λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). The morphologies of the microspheres were characterized using scanning electron microscopy (SEM, JEOL JSM-6060) 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 Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption measurements (TriStar 3000). The decomposition characteristics of the microspheres were determined using thermogravimetric analysis (TGA, SDT Q600), which was performed in air at a heating rate of 10 °C min-1. Electrochemical Measurements The capacities and cycling properties of the microspheres 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 0.1 mV s–1.
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Fig. 2 Morphologies and elemental mapping images of the bare Zn2SnO4 microspheres prepared from the spray solution without PVP: (a) SEM image, (b)-(d) TEM images, and (e) elemental mapping images.
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The crystal structures of the microspheres prepared from the spray solution with different amount of PVP are shown in Figure 1. The microspheres prepared from the spray solution without PVP had pure crystal structure of spinel Zn2SnO4 phase. However, the microspheres prepared from the spray solution with a large amount of 25 g PVP in 250 mL had main crystal structure of spinel Zn2SnO4 phase with impurity phases of metallic Sn (JCPDS card No. 04-0673) and tetragonal SnO (JCPDS card No. 06-0395). The impurity phases of the microspheres prepared from the spray solution with 15 g PVP were tetragonal SnO and tetragonal SnO2 (JCPDS card No. 41-1445). The microspheres prepared from the spray solution with small amount of 5 g PVP in 250 mL had negligible impurity peak of SnO.
Fig. 1 XRD patterns of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres prepared by one-pot spray pyrolysis.
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The morphologies of the bare Zn2SnO4 microspheres prepared from the spray solution without PVP are shown in Figure 2. The Zn2SnO4 microspheres had completely spherical shape and nonaggregated characteristics. The TEM image shown in Figure 2c had porous structure with ultrafine nanocrystals. The highresolution TEM image shown in Figure 2d depicts clear lattice fringes separated by 0.306 nm, corresponding to the (220) plane of spinel Zn2SnO4. The elemental mapping images shown in Figure 2e revealed the uniform distributions of Zn and Sn components all over the microsphere. The morphologies of the Zn2SnO4–C composite microspheres prepared from the spray solution with small amount of 5 g PVP are shown in Figure 3. The microspheres had core/shell structure with several voids within the microsphere. The spherical core part with several voids was fully covered with porous shell as shown in TEM images. Some of the melted PVP moved out to the This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
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ultrasonic spray pyrolysis process at 700 oC; a schematic of the apparatus is shown in Fig. S1. A quartz reactor with a length of 1,200 mm and a diameter of 50 mm was used. A 1.7-MHz ultrasonic spray generator with six vibrators was used to generate a large quantity of droplets. The flow rate of the N2 gas used as the carrier gas was 5 L min-1. The concentrations of the zinc oxide and tin oxalate were 0.2 and 0.1 M, respectively. Appropriate amount of nitric acid was used to dissolve the zinc oxide powders. The amount of PVP dissolved into the spray solution of 250 mL was changed from 5 to 25 g. In this manuscript, the amount of PVP dissolved into the spray solution was based on 250 mL.
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Fig. 3 Morphologies and elemental mapping images of the Zn2SnO4–carbon composite microspheres prepared from the spray solution with 5 g PVP: (a) SEM image, (b)-(d) TEM images, and (e) elemental mapping images. 5
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outside of the microsphere during the drying stage of droplet. The decomposition of the phase separated PVP formed the carbonrich thin shell part of the Zn2SnO4–C composite microspheres. In addition, the decomposition of the phase separated PVP inside the microsphere formed the void-like space. The high resolution TEM image of the shell part shown in Figure 3d revealed the amorphous structure of the carbon layer. The ultrasmall ZnO nanocrystal was observed within the amorphous carbon layer as shown in Figure 3d. The elemental mapping images shown in Figure 3e revealed the uniform distributions of Sn and Zn components all over the porous core part. However, carbon component is uniformly distributed all over the Zn2SnO4–C composite microspheres. The void-like space formed by decomposition of phase separated PVP was filled as amorphous carbon. The morphologies of the Zn2SnO4–C composite microspheres prepared from the spray solution with 15 g PVP are shown in Figure 4. The microspheres had core/shell structure, in which dense and spherical core part was covered with textured and porous thick shell layer as shown in Figures 3a and 3b. The carbon component was observed mainly in the shell part of the composite microsphere in the elemental mapping images shown in Figure 4e. A large amount of melted PVP resulted in the phase separation between PVP and metal salts. Almost melted PVP moved out to the outside of the composite microsphere during the drying stage of the droplet. Melting of phase separated metal salts formed the core part with dense structure. Carbonization of phase separated PVP formed the textured thick carbon shell. The high resolution TEM image shown in Figure 4d revealed the amorphous structure of the carbon shell. Decomposition of metal salts into each oxide materials and reaction between the metal This journal is © The Royal Society of Chemistry [year]
Fig. 4 Morphologies and elemental mapping images of the Zn2SnO4–carbon composite microspheres prepared from the spray solution with 15 g PVP: (a) SEM image, (b)-(d) TEM images, and (e) elemental mapping images.
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Scheme 1. Schematic diagram for the formation mechanism of the Zn2SnO4-carbon composite microsphere from the spray solution with 15 g PVP.
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oxides formed the Zn2SnO4 core part with impurity phases of SnO and SnO2. The zinc component was also observed in the shell part as shown in the elemental mapping images in Figure 4e. The morphologies of the Zn2SnO4–C composite microspheres prepared from the spray solution with 25 g PVP are shown in Figure S2. The Zn2SnO4–C composite microspheres prepared from the spray solution with a large amount of PVP had also core-shell structure. The zinc component was also observed in the shell part in the elemental mapping images shown in Figure S2e. The some zinc ions are bound by the strong ionic bonds with the amide groups in a PVP.35 Therefore, some zinc component Journal Name, [year], [vol], 00–00 | 3
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DOI: 10.1039/C4NR05373G
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moved out to the outside of the microsphere during the formation of core-shell structured microsphere. The phase separated zinc component formed the ultrasmall ZnO nanocrystal as shown in Figure 4d. Carbon monoxide gas released by decomposition of PVP resulted in the reducing atmosphere around the microspheres during the formation inside the reactor maintained at 700 oC.36 Reduction of abundant SnO2 into oxygen deficient metallic Sn and SnO occurred. Therefore, Zn2SnO4 microspheres with oxygen deficient Sn metal and SnO impurities were prepared from the spray solution with a large amount of 25 g PVP. Fortunately, oxygen deficient Sn metal and SnO are efficient anode materials for LIBs. The Li-ion storage mechanisms with Sn metal and SnO in a Li-ion battery can be described by the following equations: SnO + 2 Li+ + 2 e- → Sn + Li2O and Sn + x Li+ + x e- ↔ LixSn. The schematic diagram of the formation mechanism of the Zn2SnO4-carbon composite microsphere from the spray solution with 15 g PVP is shown in Scheme 1. The thermogravimetric (TG) curves of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres are shown in Figure S3. The TG curves of the Zn2SnO4-carbon composite microspheres had two-step weight losses. The first weight loss observed at temperatures below 200 oC was attributed to the evaporation of adsorbed water molecules. The second-step weight loss observed at around 400 oC was attributed to the decomposition of carbon component. Weight increase due to oxidations of Sn metal and SnO impurities diminished the weight loss by decomposition of carbon component. Therefore, the minimum carbon contents of the Zn2SnO4-carbon composite microspheres prepared from the spray solutions with 5, 15, and 25 g PVP were 10, 17, and 25 wt%, respectively. Figure S4 shows the N2 adsorption and desorption isotherms and the Barrett-Joyner-Halenda (BJH) pore-
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Fig. 5 Electrochemical properties of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres: (a) cyclic voltammogram (CV) curves, (b) initial charge and discharge curves, (c) cycling performances, and (d) rate performances.
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size distributions of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres. The clear hysteresis loops in the N2 adsorption and desorption isotherms revealed the existence of the mesopores in the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres. The bare Zn2SnO4 microspheres had mesopore sizes between 7 and 11 nm. However, the Zn2SnO4-carbon composite microspheres had sharp mesopore at around 3.5 nm. The BET surface areas of the microspheres prepared from the spray solutions with 0, 5, 15, and 25 g PVP were 32, 36, 20, and 20 m2 g-1, respectively. The melting of metal salts during the formation process of the microspheres decreased the BET surface areas of the Zn2SnO4-carbon composite microspheres when the amount of PVP dissolved into the spray solution was high above 15 g. The electrochemical properties of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres are shown in Figure 5. The cyclic voltammogram (CV) curves of the microspheres measured in the voltage range of 0.001-3.0 V vs. Li/Li+ at a scan rate of 0.1 mA s-1 are shown in Figure 5a. The CV curves had two reduction peaks at 0.27 V and 0.05 V in the first discharging process. The first reduction peak at around 0.27 V corresponds to the formation of Li2O and metallic Sn and Zn by lithium insertion reaction with Zn2SnO4: 8Li+ + Zn2SnO4 + 8e- => 2Zn + Sn + 4Li2O.15-19 The second reduction peak at around 0.05 V corresponds to the formation of Li-Sn and Li-Zn alloys by the reaction of lithium ions with Sn and Zn metals.15-19 After the first cycle, the reduction peak of 0.27 V disappeared, and was replaced by the peak of 0.8 V due to the formation of Li2O and metallic Zn by lithium insertion reaction with ZnO.15-19 The two oxidation peaks at around 0.4 and 1.4 V, which correspond to the delithiation processes of Li-Sn and Li-Zn alloys were observed from the first cycle onward.15-19 The first broad oxidation peak at around 0.4 V was attributed to the delithiation of Li-Sn and Li-Zn alloys to from Sn and Zn metal nanograins via several steps.15-19 The second oxidation peak at around 1.4 V was attributed to the regeneration of ZnO by reaction of Zn metal and Li2O.10,15-19,31 The CV peak intensities of the bare Zn2SnO4 powders decreased with increasing the cycle numbers as shown by arrow. However, the CV curves of the Zn2SnO4–carbon composite powders prepared from the spray solution with 15 g PVP were well overlapped after the first several cycles. The initial charge and discharge curves of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres are shown in Figure 5b. The initial discharge curves had several plateaus corresponding the reduction peaks in the CV curves. The initial discharge capacities of the powders prepared from the spray solutions with 0, 5, 15, and 25 g PVP were 1580, 1553, 1526, and 1506 mA h g-1, respectively, and their initial Coulombic efficiencies were 64, 64, 57, and 50 %. The initial Coulombic efficiencies of the Zn2SnO4-carbon composite microspheres decreased with increasing the amount of PVP dissolved into the spray solutions due to the formation of a large amount of amorphous carbon with low initial Coulombic efficiency. The cycling performances of the microspheres at a current density of 1 A g-1 are shown in Figure 5c. The discharge capacities of the bare Zn2SnO4 microspheres decreased strictly from the second cycle onward to the 100 mA h g-1 after 60 cycles. However, the Zn2SnO4-carbon composite microspheres showed This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR05373G
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extremely high capacities and good cycling performances. The discharge capacities of the microspheres prepared from the spray solutions with 0, 5, 15, and 25 g PVP after 120 cycles were 81, 814, 770, and 693 mA h g-1, respectively, and their capacity retentions measured from the second cycles were 7, 81, 89, and 91%. The void-like space and amorphous carbon coating layer improved the cycling performances of the Zn2SnO4-carbon composite microspheres. The Zn2SnO4-carbon composite microspheres with Sn metal and SnO impurities prepared from the spray solution with 25 g PVP showed the best cycling performance. However, the large amount of carbon decreased the capacities of the Zn2SnO4-carbon composite microspheres prepared from the spray solution with 25 g PVP. The Coulombic efficiencies of the Zn2SnO4-carbon composite microspheres prepared from the spray solution with 15 g PVP reached to 99.6 % after 5 cycles and then constantly maintained during further cycling. The rate performances of the Zn2SnO4-carbon composite microspheres are shown in Figure 5d, in which the current densities were increased step wise from 0.5 to 5 A g-1. For each step, ten cycles were measured to evaluate the rate performance. The Zn2SnO4-carbon composite microspheres had good rate performances irrespective of carbon contents of the composite microspheres. The carbon coating layer with high electronic conductivity improved the rate performances of the Zn2SnO4carbon composite microspheres.37-39 The stable reversible discharge capacities of the Zn2SnO4-carbon composite microspheres prepared from the spray solution with 15 g PVP decreased from 856 to 502 mA h g-1 with an increase in the current density from 0.5 to 5 A g-1. The discharge capacities of the Zn2SnO4-carbon composite microspheres were well recovered when the current density was returned to 0.5 A g-1.
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metal nanocrystals during the first cycles. The charge-transfer resistance of the Zn2SnO4-carbon composite microspheres slightly increased after 200 cycles. On the other hand, the chargetransfer resistance of the bare Zn2SnO4 microspheres abruptly increased after 200 cycles. Figure S5 shows the relationship between Zre and ω-1/2 in the low frequency region, where ω is the angular frequency in the low frequency region (ω = 2πf ). The lower slope (σ, Warburg impedance coefficient) of the real part of the impedance spectra (Zre) versus ω-1/2 for the Zn2SnO4carbon composite microspheres revealed a higher lithium-ion diffusion rate than the bare Zn2SnO4 microspheres.40,41 Figures S6-S8 show the morphologies of the bare Zn2SnO4 and Zn2SnO4carbon composite microspheres after 200 cycles. The spherical morphology of the bare Zn2SnO4 microspheres was destroyed after cycling. However, the spherical morphologies of the Zn2SnO4-carbon composite microspheres were maintained even after 200 cycles at a high current density of 1 A g-1. The zinc and tin components were well distributed within the carbon shell as shown in the elemental mapping images in Figure S8. The Zn2SnO4-carbon composite microspheres with thick carbon coating later had good structural stability during repeated lithium insertion and desertion processes at a high current density. In addition, the carbon layer formed at a high temperature heattreatment process improved the electronic conductivity of the composite microspheres. Therefore, the Zn2SnO4-carbon composite microspheres had superior electrochemical properties compared with those of the bare Zn2SnO4 microspheres. The electrochemical properties of the Zn2SnO4-carbon composite microspheres prepared by one-pot spray pyrolysis are compared to those of Zn2SnO4 materials with various morphologies reported previously in the literature and the results are summarized in Table S1. The Zn2SnO4-carbon composite microspheres obtained in the present study seem to show markedly superior cycling and rate performances compared to values previously reported for Zn2SnO4 materials. In addition, the carbon-coated Zn2SnO4 composite microspheres prepared by onepot spray pyrolysis had superior electrochemical performances compared with those of the SnO2 and SnO2-carbon composite powders prepared by the same process.42
Conclusions Fig. 6 Electrochemical impedance spectroscopy (EIS) of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres: (a) before cycling and (b) after cycling. 35
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Electrochemical impedance spectroscopy (EIS) analysis was conducted to determine the Li+ transfer behavior in the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres at room temperature before and after the first, 100th, and 200th cycles. The Zn2SnO4-carbon composite microspheres were prepared from the spray solution with 15 g PVP. The medium-frequency semicircles observed in the Nyquist plots shown in Figure 6 were assigned to the charge-transfer resistance (Rct) of the electrodes.40,41 The diameters of the semicircles obtained before and after first cycling in the medium-frequency region for the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres shown in Figures 6a and 6b were similar. The charge-transfer resistances of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres decreased after 1st cycles due to formation of amorphous-like This journal is © The Royal Society of Chemistry [year]
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The bare Zn2SnO4 and carbon-coated Zn2SnO4 microspheres were prepared by one-pot spray pyrolysis for anode applications in lithium ion batteries. The detail formation mechanism of the carbon-coated Zn2SnO4 microsphere by one-pot method was studied. PVP used as the carbon source material played a key role in the direct preparation of the carbon-coated Zn2SnO4 microspheres in the spray pyrolysis process. A large amount of melted PVP resulted in the phase separation between PVP and metal salts. Carbonization of PVP and decomposition of phase separated metal salts resulted in the core-shell structured Zn2SnO4–carbon microsphere. The carbon coating layer could be easily controlled by changing the concentration of PVP dissolved into the spray solution. The carbon-coated Zn2SnO4 microspheres having high electrical conductivity and good structural stability showed superior electrochemical properties compared with those of the bare Zn2SnO4 microspheres prepared by the same preparation conditions without PVP. The formation process of the Journal Name, [year], [vol], 00–00 | 5
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DOI: 10.1039/C4NR05373G
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carbon-coated Zn2SnO4 microsphere by one pot spray pyrolysis could be applied to the preparation of carbon-coated multicomponent metal oxides with various compositions. 70 5
Acknowledgements
Published on 10 November 2014. Downloaded by University of Windsor on 12/11/2014 11:07:10.
"This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012R1A2A2A02046367)."
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Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu Seoul 136-713, Republic of Korea E-mail: [email protected] Fax: +82-2-928-3584; Tel: +82-2-3290-3268 † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/ 1 2
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28 N. Feng, S. Peng, X. Sun, L. Qiao, X. Li, P. Wang, D. Hu, D. He, Mater. Lett., 2012, 76, 66. 29 X. Ji, X. Huang, Q. Zhao, A. Wang, X. Liu, J. Nanomater., 2014, 169373. 30 Y. Zhao, Y. Huangn, Q. Wang, K. Wang, M. Zong, L. Wang, X. Sun, Ceram. Int., 2014, 140, 2275. 31 S. J. Yang, S. Nam, T. Kim, J. H. Im, H. Jung, J. H. Kang, S. Wi, B. Park, C. R. Park, J. Am. Chem. Soc., 2013, 135, 7394. 32 F. Han, W. C. Li, C. Lei, B. He, K. Oshida, A. H. Lu, Small, 2014, 10, 2637. 33 Y. J. Hong, M. Y. Son, Y. C. Kang, Adv. Mater., 2013, 25, 2279. 34 S. H. Choi, Y. C. Kang, Chem. –Eur. J., 2014, 20, 5835. 35 H. M. Kamari, M. G. Naseri, E. B. Saion, Metals, 2014, 4, 118. 36 Y. Zhang, J. Yang, Q. Li, X. Cao, J. Crystal Growth, 2007, 308, 180. 37 Y. Luo, X. Xu, Y. Zhang, Y. Pi, Y. Zhao, X. Tian, Q. An, Q. Wei, L. Mai, Adv. Energy Mater., DOI: 10.1002/aenm.201400107. 38 Q. Wei, Q. An, D. Chen, L. Mai, S. Chen, Y. Zhao, K. M. Hercule, L. Xu, A. Minhas-Khan, Q. Zhang, Nano Lett., 2014, 14, 1042. 39 L. He, M. Toda, Y. Kawai, H. Miyashita, M. Omori, T. Hashida, R. Berger, T. Ono, Microsys. Technol., 2014, 20, 201. 40 N. Takami, A. Satoh, M. Hara, T. J. Ohsaki, J. Electrochem. Soc.,1995, 142, 371. 41 Y. Shi, J. Z. Wang, S. L. Chou, D. Wexler, H. J. Li, K. Ozawa, H. K. Liu, Y. P. Wu, Nano Lett., 2013, 13, 4715. 42 S. M. Lee, S. H. Choi, Y. C. Kang, Chem. –Eur. J., 2014, DOI: 10.1002/chem.201404077.
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Carbon-coated Zn2SnO4 microspheres are prepared by one-pot spray pyrolysis. The carbon-coated Zn2SnO4 microspheres having high electrical conductivity and good structural stability showed superior electrochemical properties compared with those of the bare Zn2SnO4 microspheres prepared by the same preparation conditions without PVP.
Keyword: energy storage materials; synthesis design; energy conversion; nanostructures; carbon composite Young Jun Hong and Yun Chan Kang * 15
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Formation of core–shell-structured Zn2SnO4–carbon microspheres with superior electrochemical properties by one-pot spray pyrolysis 20
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DOI: 10.1039/C4NR05373G
Cite this: DOI: 10.1039/c0xx00000x
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Young Jun Hong and Yun Chan Kang*
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x Core-shell structured Zn2SnO4–carbon microspheres with various carbon contents are prepared by one-pot spray pyrolysis without further heating process. One Zn2SnO4–carbon composite microsphere is prepared from one droplet containing Zn and Sn salts and polyvinylpyrrolidone (PVP). Melted PVP moves out to the outside of the composite microsphere during the drying stage of the droplet. In addition, melting of phase separated metal salts forms the core part with dense structure. Carbonization of phase separated PVP forms the textured and porous thick carbon shell. The discharge capacities of the core-shell structured Zn2SnO4-carbon microspheres for the 2nd and 120th cycles at a current density of 1 A g-1 are 864 and 770 mA h g-1, respectively. However, the discharge capacities of the bare Zn2SnO4 microspheres prepared by the same process without PVP for the 2nd and 120th cycles are 1106 and 81 mA h g-1, respectively. The stable reversible discharge capacities of the Zn2SnO4-carbon composite microspheres prepared from the spray solution with 15 g PVP decrease from 894 to 528 mA h g-1 with an increase in the current density from 0.5 to 5 A g-1.
Introduction
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SnO2 materials have been widely studied as anode materials for lithium ion batteries (LIBs) because of their low potential, high theoretical reversible specific capacity, and low cost.1-30 However, poor cyclability arising from the large volume change (of 300%) during the repetitive charging and discharging of the battery hinders the commercialization of SnO2.1-30 To overcome the poor cycling performances of SnO2 materials, Sn-based multicomponent materials including transition metal components, which accomodate the huge volume change during cycling, have been studied. Spinel oxides of M2SnO4 (M = Zn, Mg, Mn, Co) materials prepared by solid state and liquid solution methods have been mainly studied.9-30 Especially, Li alloy forming Zn was known as an efficient matrix element because it can contribute to capacity by alloying-dealloying reaction and to buffering the large volume change of Sn component during cycling.10-32 Nanostructured Zn2SnO4 materials with and without carbon coating material have been studied to obtain the good electrochemical properties.18-30 Cherian et al. prepared Zn2SnO4 nanowires by a vapor transport method. In the voltage range 0.005-3V, nanowire electrode retained a capacity of 660 mA h g-1 after 50 cycles at a current density of 120 mA g-1.24 Rong et al. prepared Zn2SnO4 nanocubes by hydrothermal method.25 The cube-shaped Zn2SnO4 materials showed a discharge capacity of 580 mA h g-1 after 50 cycles at a current density of 100 mA g-1. Zhu et al. reported the cubic shaped Zn2SnO4 materials showing a discharge capacity of 645 mA h g-1 after 20 cycles at a current density of 50 mA g-1.26 Hou et al. prepared inverse spinel structure Zn2SnO4 crystals by a solid state reaction having the This journal is © The Royal Society of Chemistry [year]
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specific discharge capacity of 689 mA h g−1 after 50 cycles at a current density of 50 mA g-1.27 Feng et al. reported a hydrothermal route to prepare monodisperse single crystal Zn2SnO4 cubes, and this anode material showed a discharge capacity of 775 mA h g-1 after 20 cycles at a current density of 50 mA g-1.28 Ji et al. prepared carbon-coated Zn2SnO4 nanomaterials showing a discharge capacity of 400 mA h g-1 after 40 cycles at a current density of 200 mA g-1.29 Zhao et al. reported the N-doped carbon-coated Zn2SnO4 hollow boxes with an enhanced electrochemical properties (616 mA h g-1 after 45 cycles at a current density of 300 mA g-1).30 However, the reported Zn2SnO4 materials had lower capacities and poorer cycling performances compared with those of the single component SnO2 materials.33,34 In addition, to the best of our knowledge, the simple one-pot method for carbon-coated Zn2SnO4 nanomaterials has not been reported. In this study, carbon-coated Zn2SnO4 microspheres were prepared by one-pot spray pyrolysis without further heating process. One composite microsphere was formed from one droplet containing metal salts of Zn and Sn components and polyvinylpyrrolidone (PVP). The detail formation mechanism of the carbon-coated Zn2SnO4 microsphere by one-pot method was studied. The electrochemical properties of the carbon-coated Zn2SnO4 microspheres were compared with those of the bare Zn2SnO4 microspheres prepared by the same preparation conditions without PVP.
Experimental Synthesis of carbon-coated Zn2SnO4 microspheres Carbon-coated Zn2SnO4 microspheres were prepared
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Formation of core–shell-structured Zn2SnO4–carbon microspheres with superior electrochemical properties by one-pot spray pyrolysis
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Characterizations The crystal structures of the microspheres were investigated by X-ray diffractometry (XRD, X’pert PRO MPD) using Cu Kα radiation (λ = 1.5418 Å) at the Korea Basic Science Institute (Daegu). The morphologies of the microspheres were characterized using scanning electron microscopy (SEM, JEOL JSM-6060) 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 Brunauer–Emmett–Teller (BET) analysis of nitrogen adsorption measurements (TriStar 3000). The decomposition characteristics of the microspheres were determined using thermogravimetric analysis (TGA, SDT Q600), which was performed in air at a heating rate of 10 °C min-1. Electrochemical Measurements The capacities and cycling properties of the microspheres 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 0.1 mV s–1.
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Fig. 2 Morphologies and elemental mapping images of the bare Zn2SnO4 microspheres prepared from the spray solution without PVP: (a) SEM image, (b)-(d) TEM images, and (e) elemental mapping images.
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The crystal structures of the microspheres prepared from the spray solution with different amount of PVP are shown in Figure 1. The microspheres prepared from the spray solution without PVP had pure crystal structure of spinel Zn2SnO4 phase. However, the microspheres prepared from the spray solution with a large amount of 25 g PVP in 250 mL had main crystal structure of spinel Zn2SnO4 phase with impurity phases of metallic Sn (JCPDS card No. 04-0673) and tetragonal SnO (JCPDS card No. 06-0395). The impurity phases of the microspheres prepared from the spray solution with 15 g PVP were tetragonal SnO and tetragonal SnO2 (JCPDS card No. 41-1445). The microspheres prepared from the spray solution with small amount of 5 g PVP in 250 mL had negligible impurity peak of SnO.
Fig. 1 XRD patterns of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres prepared by one-pot spray pyrolysis.
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The morphologies of the bare Zn2SnO4 microspheres prepared from the spray solution without PVP are shown in Figure 2. The Zn2SnO4 microspheres had completely spherical shape and nonaggregated characteristics. The TEM image shown in Figure 2c had porous structure with ultrafine nanocrystals. The highresolution TEM image shown in Figure 2d depicts clear lattice fringes separated by 0.306 nm, corresponding to the (220) plane of spinel Zn2SnO4. The elemental mapping images shown in Figure 2e revealed the uniform distributions of Zn and Sn components all over the microsphere. The morphologies of the Zn2SnO4–C composite microspheres prepared from the spray solution with small amount of 5 g PVP are shown in Figure 3. The microspheres had core/shell structure with several voids within the microsphere. The spherical core part with several voids was fully covered with porous shell as shown in TEM images. Some of the melted PVP moved out to the This journal is © The Royal Society of Chemistry [year]
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ultrasonic spray pyrolysis process at 700 oC; a schematic of the apparatus is shown in Fig. S1. A quartz reactor with a length of 1,200 mm and a diameter of 50 mm was used. A 1.7-MHz ultrasonic spray generator with six vibrators was used to generate a large quantity of droplets. The flow rate of the N2 gas used as the carrier gas was 5 L min-1. The concentrations of the zinc oxide and tin oxalate were 0.2 and 0.1 M, respectively. Appropriate amount of nitric acid was used to dissolve the zinc oxide powders. The amount of PVP dissolved into the spray solution of 250 mL was changed from 5 to 25 g. In this manuscript, the amount of PVP dissolved into the spray solution was based on 250 mL.
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Fig. 3 Morphologies and elemental mapping images of the Zn2SnO4–carbon composite microspheres prepared from the spray solution with 5 g PVP: (a) SEM image, (b)-(d) TEM images, and (e) elemental mapping images. 5
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outside of the microsphere during the drying stage of droplet. The decomposition of the phase separated PVP formed the carbonrich thin shell part of the Zn2SnO4–C composite microspheres. In addition, the decomposition of the phase separated PVP inside the microsphere formed the void-like space. The high resolution TEM image of the shell part shown in Figure 3d revealed the amorphous structure of the carbon layer. The ultrasmall ZnO nanocrystal was observed within the amorphous carbon layer as shown in Figure 3d. The elemental mapping images shown in Figure 3e revealed the uniform distributions of Sn and Zn components all over the porous core part. However, carbon component is uniformly distributed all over the Zn2SnO4–C composite microspheres. The void-like space formed by decomposition of phase separated PVP was filled as amorphous carbon. The morphologies of the Zn2SnO4–C composite microspheres prepared from the spray solution with 15 g PVP are shown in Figure 4. The microspheres had core/shell structure, in which dense and spherical core part was covered with textured and porous thick shell layer as shown in Figures 3a and 3b. The carbon component was observed mainly in the shell part of the composite microsphere in the elemental mapping images shown in Figure 4e. A large amount of melted PVP resulted in the phase separation between PVP and metal salts. Almost melted PVP moved out to the outside of the composite microsphere during the drying stage of the droplet. Melting of phase separated metal salts formed the core part with dense structure. Carbonization of phase separated PVP formed the textured thick carbon shell. The high resolution TEM image shown in Figure 4d revealed the amorphous structure of the carbon shell. Decomposition of metal salts into each oxide materials and reaction between the metal This journal is © The Royal Society of Chemistry [year]
Fig. 4 Morphologies and elemental mapping images of the Zn2SnO4–carbon composite microspheres prepared from the spray solution with 15 g PVP: (a) SEM image, (b)-(d) TEM images, and (e) elemental mapping images.
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Scheme 1. Schematic diagram for the formation mechanism of the Zn2SnO4-carbon composite microsphere from the spray solution with 15 g PVP.
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oxides formed the Zn2SnO4 core part with impurity phases of SnO and SnO2. The zinc component was also observed in the shell part as shown in the elemental mapping images in Figure 4e. The morphologies of the Zn2SnO4–C composite microspheres prepared from the spray solution with 25 g PVP are shown in Figure S2. The Zn2SnO4–C composite microspheres prepared from the spray solution with a large amount of PVP had also core-shell structure. The zinc component was also observed in the shell part in the elemental mapping images shown in Figure S2e. The some zinc ions are bound by the strong ionic bonds with the amide groups in a PVP.35 Therefore, some zinc component Journal Name, [year], [vol], 00–00 | 3
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moved out to the outside of the microsphere during the formation of core-shell structured microsphere. The phase separated zinc component formed the ultrasmall ZnO nanocrystal as shown in Figure 4d. Carbon monoxide gas released by decomposition of PVP resulted in the reducing atmosphere around the microspheres during the formation inside the reactor maintained at 700 oC.36 Reduction of abundant SnO2 into oxygen deficient metallic Sn and SnO occurred. Therefore, Zn2SnO4 microspheres with oxygen deficient Sn metal and SnO impurities were prepared from the spray solution with a large amount of 25 g PVP. Fortunately, oxygen deficient Sn metal and SnO are efficient anode materials for LIBs. The Li-ion storage mechanisms with Sn metal and SnO in a Li-ion battery can be described by the following equations: SnO + 2 Li+ + 2 e- → Sn + Li2O and Sn + x Li+ + x e- ↔ LixSn. The schematic diagram of the formation mechanism of the Zn2SnO4-carbon composite microsphere from the spray solution with 15 g PVP is shown in Scheme 1. The thermogravimetric (TG) curves of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres are shown in Figure S3. The TG curves of the Zn2SnO4-carbon composite microspheres had two-step weight losses. The first weight loss observed at temperatures below 200 oC was attributed to the evaporation of adsorbed water molecules. The second-step weight loss observed at around 400 oC was attributed to the decomposition of carbon component. Weight increase due to oxidations of Sn metal and SnO impurities diminished the weight loss by decomposition of carbon component. Therefore, the minimum carbon contents of the Zn2SnO4-carbon composite microspheres prepared from the spray solutions with 5, 15, and 25 g PVP were 10, 17, and 25 wt%, respectively. Figure S4 shows the N2 adsorption and desorption isotherms and the Barrett-Joyner-Halenda (BJH) pore-
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Fig. 5 Electrochemical properties of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres: (a) cyclic voltammogram (CV) curves, (b) initial charge and discharge curves, (c) cycling performances, and (d) rate performances.
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size distributions of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres. The clear hysteresis loops in the N2 adsorption and desorption isotherms revealed the existence of the mesopores in the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres. The bare Zn2SnO4 microspheres had mesopore sizes between 7 and 11 nm. However, the Zn2SnO4-carbon composite microspheres had sharp mesopore at around 3.5 nm. The BET surface areas of the microspheres prepared from the spray solutions with 0, 5, 15, and 25 g PVP were 32, 36, 20, and 20 m2 g-1, respectively. The melting of metal salts during the formation process of the microspheres decreased the BET surface areas of the Zn2SnO4-carbon composite microspheres when the amount of PVP dissolved into the spray solution was high above 15 g. The electrochemical properties of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres are shown in Figure 5. The cyclic voltammogram (CV) curves of the microspheres measured in the voltage range of 0.001-3.0 V vs. Li/Li+ at a scan rate of 0.1 mA s-1 are shown in Figure 5a. The CV curves had two reduction peaks at 0.27 V and 0.05 V in the first discharging process. The first reduction peak at around 0.27 V corresponds to the formation of Li2O and metallic Sn and Zn by lithium insertion reaction with Zn2SnO4: 8Li+ + Zn2SnO4 + 8e- => 2Zn + Sn + 4Li2O.15-19 The second reduction peak at around 0.05 V corresponds to the formation of Li-Sn and Li-Zn alloys by the reaction of lithium ions with Sn and Zn metals.15-19 After the first cycle, the reduction peak of 0.27 V disappeared, and was replaced by the peak of 0.8 V due to the formation of Li2O and metallic Zn by lithium insertion reaction with ZnO.15-19 The two oxidation peaks at around 0.4 and 1.4 V, which correspond to the delithiation processes of Li-Sn and Li-Zn alloys were observed from the first cycle onward.15-19 The first broad oxidation peak at around 0.4 V was attributed to the delithiation of Li-Sn and Li-Zn alloys to from Sn and Zn metal nanograins via several steps.15-19 The second oxidation peak at around 1.4 V was attributed to the regeneration of ZnO by reaction of Zn metal and Li2O.10,15-19,31 The CV peak intensities of the bare Zn2SnO4 powders decreased with increasing the cycle numbers as shown by arrow. However, the CV curves of the Zn2SnO4–carbon composite powders prepared from the spray solution with 15 g PVP were well overlapped after the first several cycles. The initial charge and discharge curves of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres are shown in Figure 5b. The initial discharge curves had several plateaus corresponding the reduction peaks in the CV curves. The initial discharge capacities of the powders prepared from the spray solutions with 0, 5, 15, and 25 g PVP were 1580, 1553, 1526, and 1506 mA h g-1, respectively, and their initial Coulombic efficiencies were 64, 64, 57, and 50 %. The initial Coulombic efficiencies of the Zn2SnO4-carbon composite microspheres decreased with increasing the amount of PVP dissolved into the spray solutions due to the formation of a large amount of amorphous carbon with low initial Coulombic efficiency. The cycling performances of the microspheres at a current density of 1 A g-1 are shown in Figure 5c. The discharge capacities of the bare Zn2SnO4 microspheres decreased strictly from the second cycle onward to the 100 mA h g-1 after 60 cycles. However, the Zn2SnO4-carbon composite microspheres showed This journal is © The Royal Society of Chemistry [year]
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extremely high capacities and good cycling performances. The discharge capacities of the microspheres prepared from the spray solutions with 0, 5, 15, and 25 g PVP after 120 cycles were 81, 814, 770, and 693 mA h g-1, respectively, and their capacity retentions measured from the second cycles were 7, 81, 89, and 91%. The void-like space and amorphous carbon coating layer improved the cycling performances of the Zn2SnO4-carbon composite microspheres. The Zn2SnO4-carbon composite microspheres with Sn metal and SnO impurities prepared from the spray solution with 25 g PVP showed the best cycling performance. However, the large amount of carbon decreased the capacities of the Zn2SnO4-carbon composite microspheres prepared from the spray solution with 25 g PVP. The Coulombic efficiencies of the Zn2SnO4-carbon composite microspheres prepared from the spray solution with 15 g PVP reached to 99.6 % after 5 cycles and then constantly maintained during further cycling. The rate performances of the Zn2SnO4-carbon composite microspheres are shown in Figure 5d, in which the current densities were increased step wise from 0.5 to 5 A g-1. For each step, ten cycles were measured to evaluate the rate performance. The Zn2SnO4-carbon composite microspheres had good rate performances irrespective of carbon contents of the composite microspheres. The carbon coating layer with high electronic conductivity improved the rate performances of the Zn2SnO4carbon composite microspheres.37-39 The stable reversible discharge capacities of the Zn2SnO4-carbon composite microspheres prepared from the spray solution with 15 g PVP decreased from 856 to 502 mA h g-1 with an increase in the current density from 0.5 to 5 A g-1. The discharge capacities of the Zn2SnO4-carbon composite microspheres were well recovered when the current density was returned to 0.5 A g-1.
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metal nanocrystals during the first cycles. The charge-transfer resistance of the Zn2SnO4-carbon composite microspheres slightly increased after 200 cycles. On the other hand, the chargetransfer resistance of the bare Zn2SnO4 microspheres abruptly increased after 200 cycles. Figure S5 shows the relationship between Zre and ω-1/2 in the low frequency region, where ω is the angular frequency in the low frequency region (ω = 2πf ). The lower slope (σ, Warburg impedance coefficient) of the real part of the impedance spectra (Zre) versus ω-1/2 for the Zn2SnO4carbon composite microspheres revealed a higher lithium-ion diffusion rate than the bare Zn2SnO4 microspheres.40,41 Figures S6-S8 show the morphologies of the bare Zn2SnO4 and Zn2SnO4carbon composite microspheres after 200 cycles. The spherical morphology of the bare Zn2SnO4 microspheres was destroyed after cycling. However, the spherical morphologies of the Zn2SnO4-carbon composite microspheres were maintained even after 200 cycles at a high current density of 1 A g-1. The zinc and tin components were well distributed within the carbon shell as shown in the elemental mapping images in Figure S8. The Zn2SnO4-carbon composite microspheres with thick carbon coating later had good structural stability during repeated lithium insertion and desertion processes at a high current density. In addition, the carbon layer formed at a high temperature heattreatment process improved the electronic conductivity of the composite microspheres. Therefore, the Zn2SnO4-carbon composite microspheres had superior electrochemical properties compared with those of the bare Zn2SnO4 microspheres. The electrochemical properties of the Zn2SnO4-carbon composite microspheres prepared by one-pot spray pyrolysis are compared to those of Zn2SnO4 materials with various morphologies reported previously in the literature and the results are summarized in Table S1. The Zn2SnO4-carbon composite microspheres obtained in the present study seem to show markedly superior cycling and rate performances compared to values previously reported for Zn2SnO4 materials. In addition, the carbon-coated Zn2SnO4 composite microspheres prepared by onepot spray pyrolysis had superior electrochemical performances compared with those of the SnO2 and SnO2-carbon composite powders prepared by the same process.42
Conclusions Fig. 6 Electrochemical impedance spectroscopy (EIS) of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres: (a) before cycling and (b) after cycling. 35
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Electrochemical impedance spectroscopy (EIS) analysis was conducted to determine the Li+ transfer behavior in the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres at room temperature before and after the first, 100th, and 200th cycles. The Zn2SnO4-carbon composite microspheres were prepared from the spray solution with 15 g PVP. The medium-frequency semicircles observed in the Nyquist plots shown in Figure 6 were assigned to the charge-transfer resistance (Rct) of the electrodes.40,41 The diameters of the semicircles obtained before and after first cycling in the medium-frequency region for the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres shown in Figures 6a and 6b were similar. The charge-transfer resistances of the bare Zn2SnO4 and Zn2SnO4-carbon composite microspheres decreased after 1st cycles due to formation of amorphous-like This journal is © The Royal Society of Chemistry [year]
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The bare Zn2SnO4 and carbon-coated Zn2SnO4 microspheres were prepared by one-pot spray pyrolysis for anode applications in lithium ion batteries. The detail formation mechanism of the carbon-coated Zn2SnO4 microsphere by one-pot method was studied. PVP used as the carbon source material played a key role in the direct preparation of the carbon-coated Zn2SnO4 microspheres in the spray pyrolysis process. A large amount of melted PVP resulted in the phase separation between PVP and metal salts. Carbonization of PVP and decomposition of phase separated metal salts resulted in the core-shell structured Zn2SnO4–carbon microsphere. The carbon coating layer could be easily controlled by changing the concentration of PVP dissolved into the spray solution. The carbon-coated Zn2SnO4 microspheres having high electrical conductivity and good structural stability showed superior electrochemical properties compared with those of the bare Zn2SnO4 microspheres prepared by the same preparation conditions without PVP. The formation process of the Journal Name, [year], [vol], 00–00 | 5
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carbon-coated Zn2SnO4 microsphere by one pot spray pyrolysis could be applied to the preparation of carbon-coated multicomponent metal oxides with various compositions. 70 5
Acknowledgements
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"This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. 2012R1A2A2A02046367)."
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Department of Materials Science and Engineering, Korea University, Anam-Dong, Seongbuk-Gu Seoul 136-713, Republic of Korea E-mail: [email protected] Fax: +82-2-928-3584; Tel: +82-2-3290-3268 † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/ 1 2
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This journal is © The Royal Society of Chemistry [year]
Nanoscale Accepted Manuscript
DOI: 10.1039/C4NR05373G
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DOI: 10.1039/C4NR05373G
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Carbon-coated Zn2SnO4 microspheres are prepared by one-pot spray pyrolysis. The carbon-coated Zn2SnO4 microspheres having high electrical conductivity and good structural stability showed superior electrochemical properties compared with those of the bare Zn2SnO4 microspheres prepared by the same preparation conditions without PVP.
Keyword: energy storage materials; synthesis design; energy conversion; nanostructures; carbon composite Young Jun Hong and Yun Chan Kang * 15
Nanoscale Accepted Manuscript
Published on 10 November 2014. Downloaded by University of Windsor on 12/11/2014 11:07:10.
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Formation of core–shell-structured Zn2SnO4–carbon microspheres with superior electrochemical properties by one-pot spray pyrolysis 20
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 7
