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The ZnSn(OH)6 nanocube–graphene composite as an anode material for Li-ion batteries† Caiyun Chen,ab Xiangzhen Zheng,ab Jie Yangab and Mingdeng Wei*ab ZnSn(OH)6 (ZSH) nanocubes with a uniform size of 40–80 nm were synthesized by using a simple
Received 28th June 2014, Accepted 4th August 2014 DOI: 10.1039/c4cp02842b
hydrothermal route and then combined with graphene sheets (rGO) via the electrostatic interaction. The formed composite of ZnSn(OH)6 nanocube–graphene (ZSH–rGO) was used as an anode material for Li-ion batteries and it exhibited significantly enhanced electrochemical performance. For instance, a capacity of 540 mA h g1 at 500 mA g1 was retained after 40 cycles.
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Introduction Li-ion batteries (LIBs), some of the most promising electrical energy storage systems with the merits of continuous and efficient storage of electrochemical energy, nil memory, environmental benignity and long lifespan,1,2 have become the main power source for portable electronics and are being actively applied to propel electric vehicles in the near future. Although graphite anodes have been widely used in commercial mobile applications, their relatively low theoretical capacity (372 mA h g1) is insufficient to meet future market requirements. Therefore, there is still an urgent need to exploit novel anode materials for LIBs with larger specific capacities and higher power densities.3,4 In the past few years, Sn-based oxides have become attractive anode materials due to their feasible low potentials for Li+ insertion and high theoretical capacities.5 However, the poor capacity retention of these materials during the cycling process, as well as the large irreversible capacity loss during the initial discharge–charge cycle, has limited them in the practical application. Designing the synthesis of novel nanostructures has been an effective route to alleviate the problem of capacity fading. Recently, tin-based ternary oxides M2SnO4 (M = Zn, Co, Ni, Mn, and Ca) have been widely investigated as anode materials for LIBs and they exhibited good electrochemical properties.6–10 These results encouraged us to extend our studies to the investigation of ZnSn(OH)6 (ZSH). To the best of our knowledge,
however, the electrochemical performance of ZSH has not been reported. As is well known, tin-based oxides have a major drawback of the severe electrode pulverization caused by the drastic volume variation (up to 200%) during repeated electrochemical cycling, which greatly hampers their long-term cycling stability. To overcome this issue, tin-based oxides were often loaded onto a matrix. It has been found that the electrochemical performance of tin-based oxides can be enhanced obviously when carbon materials were used as the matrices.11–16 Compared with conventional carbon materials, graphene, with a more flexible structure, superior electrical conductivity and higher surface area, has recently attracted much attention in LIBs. For instance, Qian et al.17 synthesized the hybrid materials of Co2SnO4 and graphene, which exhibited enhanced electrochemical performance. In the present work, the composites of ZSH nanocubes and rGO were synthesized via a simple hydrothermal route and then used as the anode materials for LIBs. It was found that these composites exhibited an obviously improved electrochemical performance compared with bulk ZSH, indicating their promising application as anode materials for LIBs. Furthermore, the relationship between these composites and electrochemical properties was also investigated in detail.
Experimental Synthesis of ZSH
a
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China. E-mail: [email protected]; Tel: +86-591-83753180 b Institute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002, China † Electronic supplementary information (ESI) available: Cyclic voltammograms of samples. See DOI: 10.1039/c4cp02842b
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The ZSH nanocubes were synthesized by a simple hydrothermal route.18 In a typical synthesis 2 mmol of Zn(Ac)22H2O was dissolved in 80 mL of deionized water under intense agitation, and then 2 mmol K2SnO33H2O was added into the above solution. After stirring for several minutes, the resulting solution was poured into a Teflon-lined stainless-steel autoclave with a 100 mL capacity
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and heated at 160 1C for 18 h. After being cooled to room temperature, the resulting white precipitate was separated by centrifugation and washed with deionized water and absolute ethanol several times. Finally, the products were dried at 70 1C overnight in a vacuum oven.
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Synthesis of ZSH–rGO composites Graphene oxide (GO) was prepared according to a modified Hummer method.19,20 The synthesis of ZSH–rGO composites was typically performed as follows. By soaking in 25 mL of HCl solution (pH = 4), ZSH nanocubes were functionalized to have a positively charged surface which can be adsorbed onto the negatively charged GO surface by the electrostatic attraction. When GO was dispersed into 25 mL of deionized water completely by ultrasonication and ZSH suspension was added, a homogeneous mixture was formed. After vigorous stirring for 4 h, the mixture solution was transferred to a 75 mL Teflonlined autoclave and kept at 120 1C for 6 h, which allows sufficient reduction of GO to form reduced GO (rGO). After being cooled to room temperature, the product was recovered by filtration and washed with water, and then dried at 70 1C in an oven to obtain the final product, ZSH–rGO composites.
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The EIS data were collected with an AC voltage of 10 mV amplitude in the frequency range from 1 MHz to 100 MHz.
Results and discussion The crystal structures of pure GO, bulk ZSH and ZSH–rGO composites were measured by X-ray diffraction (XRD). As shown in Fig. 1, except for the pattern of GO, all the diffraction peaks are perfectly indexed to the cubic phase of ZSH (JCPDS No. 20-1455). It manifested that ZSH retains the intrinsic crystal structure after combining with rGO. However, the typical diffraction peaks of GO or reduced GO (rGO) nanocrystals have not been detected mainly due to their noncrystalline structure.21 Fig. 2(a and b) shows SEM images of ZSH–rGO composites. It can be seen that the sample consists of nanocubes with a homogeneous size of about 40–80 nm. It can also be found that the nanocubes are uniformly covered by the graphene sheets. In fact, the graphene sheets with obvious wrinkles can act as
Materials characterization X-ray diffraction (XRD) patterns were recorded on a PANalytical X’Pert spectrometer using the Co Ka radiation (k = 1.788 Å), and the data were changed to Cu Ka data. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were taken on a Hitachi S4800 instrument and a Tecnai G2 F20 S-TWIN instrument, respectively. TG analyses were performed on a Perkin-Elmer TGA 7 thermal analyzer in air at a heating rate of 10 1C min1. Infrared spectra of the samples pelletized with KBr were recorded on a Fourier transform infrared spectrometer (FT-IR spectrometer, Spectrum 2000). Raman spectroscopy was performed on a microscopic confocal laser Raman spectrometer (inVia Reflex, Renishaw Co.) using a 532 nm laser at room temperature.
Fig. 1 Typical XRD patterns of pristine GO, bulk ZSH and ZSH–rGO composites.
Electrochemical measurements To evaluate the electrochemical performance, the working electrode was constructed by mixing the active material, acetylene back carbon (AB) powder and polyvinylidene fluoride (PVDF) powder in a weight ratio of 70 : 20 : 10. The mixture was added to N-methyl-2-pyrrolidinone solvent to form a homogeneous slurry and pressed on Cu foil circular flakes, and then dried at 120 1C for 24 h under vacuum conditions. Metallic lithium foils were used as the negative electrodes. The electrolyte was 1 M LiPF6 in a 1 : 1 : 1 (volume ratio) mixture of ethyl-carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC). The separator was a UP 3093 (Japan) micro-porous polypropylene membrane. The cells were assembled in a glove box filled with highly pure argon gas, and charge–discharge tests were performed in the voltage range of 0.01 to 3 V (Li+/Li) at a current density of 0.5 A g1 on a Land automatic battery tester (Land CT 2001A, Wuhan, China). The electrochemical impedance spectroscopy (EIS) was performed on an IM6 Electrochemical Workstation (Zahner).
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Fig. 2
(a and b) SEM and (c and d) TEM images of ZSH–rGO composites.
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Fig. 4 Raman spectra of pristine GO and ZSH–rGO composites. Fig. 3 The Fourier transform infrared spectra (FT-IR) of original GO, bulk ZSH and ZSH–GO, ZSH–rGO composites.
the support matrix of nanocubes. As depicted in Fig. 2(c and d), TEM images also confirm that the sample was composed of uniform nanocubes which were wrapped by rGO sheets. FTIR was carried out and the spectra are shown in Fig. 3. The band at 3390 cm1 present in the spectrum of GO is attributed to H2O molecules physically absorbed on the surface and the band at 2370 cm1 is related to CO2 molecules. The stretching vibrations of carbonyl and carboxyl CQO bonds are assigned to the peak at 1740 cm1. The bands at 1050, 1210 and 1630 cm1 are contributed to C–O–C, carboxyl C–O and CQC stretching vibrations, respectively, suggesting that the graphite was oxidized sufficiently into hydrophilic GO with multitudinous oxygen-containing functional groups. For the other three curves, the band at 3230 cm1 is attributed to the bending and stretching vibration modes of the OH group in ZSH. The five bands at 542, 661, 787, 1170 and 2360 cm1 arise from the vibration of M–OH or M–OH–M groups in ZSH, which is similar to the consequences described in the previous report.22 These results indicate that the structure of ZSH was not changed during the hybridization and reduction of GO, which is also in accordance with the XRD results. Raman spectroscopy is another effective tool to monitor the structural changes of carbonaceous materials in the graphene-based composites. Of particular note is the intensity ratio of the D (1348 cm1) and G (1591 cm1) bands, ID/IG, which is a measure of the relative concentration of local defects or disorders (particularly the sp3-hybridized defects) compared to the sp2-hybridized rGO domains.23,24 It can be seen from Fig. 4 that the ratio of ID/IG is 0.93 for GO and it increased to 1.01 for ZSH–rGO after the hydrothermal reaction; this is due to the decrease of average size of the sp2 domains during the reduction of GO.25 Nevertheless, the number of the sp2 domains was increased during the reduction reactions.26 TGA was performed to determine the number of graphene sheets in the composite of ZSH–rGO. As shown in Fig. 5, the
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Fig. 5
TG curves of ZSH and ZSH–rGO composites.
weight loss below about 420 1C is generally attributed to the decomposition of ZSH:27 ZnSn(OH)6 - ZnSnO3 + 3H2O
(1)
The weight loss of 20.7 wt% was almost consistent with the theoretical value of 19 wt% calculated according to eqn (1). The weight loss at about 420 1C can be attributed to the removal of graphene from the composite.28 Above 550 1C, TGA traces are stable with no further mass loss, indicating that graphene in the composite was completely removed. Therefore, the graphene content in the composite of ZSH–rGO was determined to be about 8.3 wt%. The electrochemical performances of ZSH–rGO composites and ZSH were investigated, and the results are shown in Fig. 6. Fig. 6(a) shows the representative discharge–charge profiles of two samples at a current density of 500 mA g1 in the initial three cycles in a potential window of 0.01–3.0 V. The initial charge and discharge capacities of ZSH–rGO composites were 776 and 1836 mA h g1, respectively, while the bulk ZSH sample exhibited the initial charge and discharge capacities of 310 and
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Fig. 6 (a) Discharge–charge voltage profiles of ZSH and ZSH–rGO composites for the first three cycles, (b) Cycling performance of pure rGO, bulk ZSH and ZSH–rGO composites at a current density of 500 mA g1, (c) Rate performance of bulk ZSH and ZSH–rGO composites, and (d) CV curves of the ZSH–rGO composite electrode at a scan rate of 0.5 mV s1 in the voltage range 0.01–3.0 V.
1954 mA h g1. According to the lithium ion storage mechanism of ZnO29 and SnO2,30 the following electrochemical reactions were suggested for lithium insertion into the anode of ZSH nanocubes: 4Li+ + ZnSn(OH)6 + 4e - Sn + 2Li2O + 2ZnO + 3H2O
(2a)
6Li+ + ZnSn(OH)6 + 6e - Zn + Sn + 3Li2O + 3H2O
(2b)
xLi+ + Sn + xe 2 LixSn, x r 4.4 yLi+ + Zn + ye 2 LiyZn,
yr1
(3) (4) 1
The theoretical reversible capacity of about 506 mA h g is derived from the maximum uptake of 5.4 mol Li for ZSH based on eqn (3) and (4). The theoretical value from eqn (2) and (4) is calculated to be about 1068 mA h g1. The extra Li consumption via a pseudo-capacitance mechanism,31,32 such as a trace amount of water in the assembled cells, accounts mostly for the excess capacity in the initial cycle. However, the large capacity loss in the first cycle is mainly attributed to the initial irreversible formation of Li2O, and other irreversible processes such as trapping of some Li-ions in the lattice, inevitable formation of a solid electrolyte interface (SEI) layer and electrolyte decomposition, which are all quite common for most anode materials.33–39 The difference in voltage profiles of ZSH and ZSH–rGO composites suggests that some extra reaction occurs in the composite, which may be responsible for the extra capacity. As can be seen in Fig. 6(a), the steepness of the voltage profile of the ZSH–rGO composite basically unchanged in the range of 0.01–3 V, while the voltage increased sharply when the bulk ZSH cell was further charged to over 1.0 V. These facts suggest that some extra conversion reaction may occur in the ZSH–rGO composite
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electrode based on the previous report.40 Besides, the synergistic effect between ZSH nanocubes and the conductive graphene likely contributed to the extra capacity. The spreading graphene coated on the ZSH nanocubes maximizes the contact of graphene with the electrolyte, which adds the capacity of graphene contributed by Li-ion absorption.41 To demonstrate the advantages of ZSH–rGO composites for Li-ion storage, the cycling performance of bulk ZSH is also investigated under identical test conditions, as shown in Fig. 6(b). The cells are charged and discharged at 500 mA g1 in a potential range of 0.01–3.0 V. Compared with bulk ZSH, ZSH–rGO composites exhibited an increased capacity and an enhanced cycling stability. It was also found that the charge capacity of bulk ZSH dropped rapidly from 310 to 197 mA h g1 after 40 cycles at 500 mA g1. For ZSH–rGO composites, a charge capacity of 540 mA h g1 can still be retained. As a reference sample, the cell made up of graphene only exhibited a capacity below 200 mA h g1 after 40 cycles. Thus, the cycling stability of ZSH–rGO composites might be ascribed to the buffering effect and the enhanced conductivity in the presence of graphene, which effectively alleviates the large volume changes and provides a highly conductive medium for electron transport during Li-ion insertion/extraction into/out of the ZSH anode. Furthermore, the aggregation of ZSH nanocubes can be inhibited by the immobilization effect of graphene. Fig. 6(c) compares the rate capability between ZSH and ZSH–rGO composites. The latter demonstrates much better electrochemical performance and high rate capacity than those of the former, suggesting that the improved rate capability mainly arises from the following two factors. Firstly, the highly conductive graphene offers 2D conductive channels for the ZSH anode. Secondly, the unique structure of ZSH–rGO composites facilitates the better wetting of the active materials by the electrolyte, leading to the faster Li-ion transport across the electrode/electrolyte interface. When being cycled at different current densities of 500, 1000, 1500, 2000 and 500 mA g1, the ZSH–rGO composite could deliver discharge capacities of 755.2, 578.7, 514.8, 469.4 and 657.5 mA h g1, respectively. After being cycled at different current densities, the capacity of ZSH–rGO can be recovered to a high value when the current density was returned to 500 mA g1, suggesting that the integrity of the electrode can be maintained during the cycling process. These results show a considerable or even better electrochemical performance when compared with the graphene wrapped Zn2SnO4 boxes.42 This might be related to hydroxyl groups on the ZnSn(OH)6 surface, which can combine with graphene oxides more intimately. Typical cyclic voltammogram (CV) curves of ZSH–rGO are depicted in Fig. 6(d). In the first cycle, the cathodic peak at B0.68 V (vs. Li/Li+), disappearing in the following cycles, can be ascribed to the decomposition of ZSH into Zn and Sn, and the formation of amorphous Li2O and the SEI film.43–45 Another two peaks located near 0.43 and 0.2 V correspond to the multistep electrochemical lithiation process with the decomposition of ZSH (eqn (2a) and (2b)) and the formation of alloys (eqn (3) and (4)). At the same time, an anodic peak at 0.68 V corresponds
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to the dealloying reaction. Two weak anodic peaks located at 1.1 V and 1.6 V correspond to the oxidation of Sn46 and Zn.47 In the second and subsequent cycles, the anodic peak at 0.68 V shifts to 0.64 V and the cathodic peak at 0.2 V shifts to 0.26 V, illustrating the different lithium reaction process. The redox couple indexed as 0.26/0.64 V is related to the alloying/dealloying process of LixSn and LiyZn. For a more clear explanation, the CV curves of bulk ZSH and rGO are presented in Fig. S1 (ESI†). There is no peak of pure rGO, and bulk ZSH only showed a redox peak in the initial cycle, indicating that the capacity contribution arises from ZSH nanocubes with the assistance of rGO sheets. EIS is a technique that is used to analyze the electrode kinetic process as well as impedance of the cell. Impedance analysis illustrates the contribution of electrolyte resistance, surface film resistance and solid state diffusion of Li ions through the bulk of the active material. EIS analysis was performed using fresh cells under open circuit voltage conditions. Fig. 7(a) compares the electrochemical impedance spectra of ZSH and ZSH–rGO composites. It can be observed that the impedance plots of both samples consisted of an inclined line in the low-frequency region and one compressed semicircle in the medium-frequency region. The impedance plots were fitted by using an equivalent circuit model, as shown in the
Paper Table 1 model
Impedance parameters calculated from an equivalent circuit
Samples
Rs (O)
Rct (O)
s (O s1/2)
ZSH ZSH–rGO
6.51 5.38
140 36.9
192 192
inset of Fig. 7(a),48 which includes the Warburg impedance assigned to the diffusion of Li-ions into the bulk of the electrode materials, the charge-transfer resistance Rct, the SEI resistance Rf, the electrolyte resistance Rs, and two constant phase elements (CPE1 and CPE2) corresponding to the charge-transfer resistance and interfacial resistance, respectively. EIS fitting values are shown in Table 1, which indicate that electrolyte resistance (Rs) is almost the same, but the different Rct values determine the electrochemical performance of the active material. The Rct values of ZSH and ZSH–rGO are 140 and 36.9 O, respectively. Obviously, the Rct of the latter was much smaller than that of the former, suggesting that the conductive capability of the electrode material can be greatly increased after ZSH was coated by rGO. EIS is also used to evaluate the diffusion coefficient of Li+ ions within the active material. The diffusion coefficient was calculated using the following eqn (5):49 DLi+ = R2T 2/2A2n4F 4C2s2
(5)
where R is the gas constant (8.314 J K1 mol1), F is the Faraday constant (96 500 C mol1), T is the room temperature in our experiment (absolute temperature), A is the surface area, n is the number of the electrons per molecule attending the electron transfer reaction, C is the concentration, D is the diffusion coefficient, s is the Warburg factor, which can be obtained from the line of Z 0 B o1/2 in the low frequency region, respectively. The relationship between Z 0 and o1/2 in the low frequency region is shown in Fig. 7(b). The Warburg factors s are on the whole the same as the value of 192 for both ZSH–rGO composites and ZSH. If other parameters are fixed, the diffusion coefficient of the former is the same as that of the latter, indicating that the ZSH sample retains the original state after hybridized with rGO. In fact, the results of EIS simulation analysis which was carried out using two models mentioned above are all consistent with our experimental results.
Conclusions
Fig. 7 (a) The EIS spectra of ZSH and ZSH–rGO and their corresponding equivalent circuits (inset) and (b) Randles plot of ZSH and ZSH–rGO.
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In summary, we have designed a unique hybrid nanostructure, ZSH nanocubes coated by rGO, for highly reversible lithium storage. When firstly evaluated as an anode material for LIBs, the composite of ZSH–rGO exhibited high capacity, cycling stability and good rate performance. The improved electrochemical performance might be attributed to the fact that the incorporation of the flexible rGO acted as the buffer to alleviate the volume changes and inhibit the aggregation of ZSH nanocubes. Thus, such a class of functional materials may hold great promise for the development of high performance LIBs.
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Acknowledgements This work was financially supported by National Natural Science Foundation of China (NSFC 21173049).
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The ZnSn(OH)6 nanocube–graphene composite as an anode material for Li-ion batteries† Caiyun Chen,ab Xiangzhen Zheng,ab Jie Yangab and Mingdeng Wei*ab ZnSn(OH)6 (ZSH) nanocubes with a uniform size of 40–80 nm were synthesized by using a simple
Received 28th June 2014, Accepted 4th August 2014 DOI: 10.1039/c4cp02842b
hydrothermal route and then combined with graphene sheets (rGO) via the electrostatic interaction. The formed composite of ZnSn(OH)6 nanocube–graphene (ZSH–rGO) was used as an anode material for Li-ion batteries and it exhibited significantly enhanced electrochemical performance. For instance, a capacity of 540 mA h g1 at 500 mA g1 was retained after 40 cycles.
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Introduction Li-ion batteries (LIBs), some of the most promising electrical energy storage systems with the merits of continuous and efficient storage of electrochemical energy, nil memory, environmental benignity and long lifespan,1,2 have become the main power source for portable electronics and are being actively applied to propel electric vehicles in the near future. Although graphite anodes have been widely used in commercial mobile applications, their relatively low theoretical capacity (372 mA h g1) is insufficient to meet future market requirements. Therefore, there is still an urgent need to exploit novel anode materials for LIBs with larger specific capacities and higher power densities.3,4 In the past few years, Sn-based oxides have become attractive anode materials due to their feasible low potentials for Li+ insertion and high theoretical capacities.5 However, the poor capacity retention of these materials during the cycling process, as well as the large irreversible capacity loss during the initial discharge–charge cycle, has limited them in the practical application. Designing the synthesis of novel nanostructures has been an effective route to alleviate the problem of capacity fading. Recently, tin-based ternary oxides M2SnO4 (M = Zn, Co, Ni, Mn, and Ca) have been widely investigated as anode materials for LIBs and they exhibited good electrochemical properties.6–10 These results encouraged us to extend our studies to the investigation of ZnSn(OH)6 (ZSH). To the best of our knowledge,
however, the electrochemical performance of ZSH has not been reported. As is well known, tin-based oxides have a major drawback of the severe electrode pulverization caused by the drastic volume variation (up to 200%) during repeated electrochemical cycling, which greatly hampers their long-term cycling stability. To overcome this issue, tin-based oxides were often loaded onto a matrix. It has been found that the electrochemical performance of tin-based oxides can be enhanced obviously when carbon materials were used as the matrices.11–16 Compared with conventional carbon materials, graphene, with a more flexible structure, superior electrical conductivity and higher surface area, has recently attracted much attention in LIBs. For instance, Qian et al.17 synthesized the hybrid materials of Co2SnO4 and graphene, which exhibited enhanced electrochemical performance. In the present work, the composites of ZSH nanocubes and rGO were synthesized via a simple hydrothermal route and then used as the anode materials for LIBs. It was found that these composites exhibited an obviously improved electrochemical performance compared with bulk ZSH, indicating their promising application as anode materials for LIBs. Furthermore, the relationship between these composites and electrochemical properties was also investigated in detail.
Experimental Synthesis of ZSH
a
State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou, Fujian 350002, China. E-mail: [email protected]; Tel: +86-591-83753180 b Institute of Advanced Energy Materials, Fuzhou University, Fuzhou, Fujian 350002, China † Electronic supplementary information (ESI) available: Cyclic voltammograms of samples. See DOI: 10.1039/c4cp02842b
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The ZSH nanocubes were synthesized by a simple hydrothermal route.18 In a typical synthesis 2 mmol of Zn(Ac)22H2O was dissolved in 80 mL of deionized water under intense agitation, and then 2 mmol K2SnO33H2O was added into the above solution. After stirring for several minutes, the resulting solution was poured into a Teflon-lined stainless-steel autoclave with a 100 mL capacity
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and heated at 160 1C for 18 h. After being cooled to room temperature, the resulting white precipitate was separated by centrifugation and washed with deionized water and absolute ethanol several times. Finally, the products were dried at 70 1C overnight in a vacuum oven.
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Synthesis of ZSH–rGO composites Graphene oxide (GO) was prepared according to a modified Hummer method.19,20 The synthesis of ZSH–rGO composites was typically performed as follows. By soaking in 25 mL of HCl solution (pH = 4), ZSH nanocubes were functionalized to have a positively charged surface which can be adsorbed onto the negatively charged GO surface by the electrostatic attraction. When GO was dispersed into 25 mL of deionized water completely by ultrasonication and ZSH suspension was added, a homogeneous mixture was formed. After vigorous stirring for 4 h, the mixture solution was transferred to a 75 mL Teflonlined autoclave and kept at 120 1C for 6 h, which allows sufficient reduction of GO to form reduced GO (rGO). After being cooled to room temperature, the product was recovered by filtration and washed with water, and then dried at 70 1C in an oven to obtain the final product, ZSH–rGO composites.
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The EIS data were collected with an AC voltage of 10 mV amplitude in the frequency range from 1 MHz to 100 MHz.
Results and discussion The crystal structures of pure GO, bulk ZSH and ZSH–rGO composites were measured by X-ray diffraction (XRD). As shown in Fig. 1, except for the pattern of GO, all the diffraction peaks are perfectly indexed to the cubic phase of ZSH (JCPDS No. 20-1455). It manifested that ZSH retains the intrinsic crystal structure after combining with rGO. However, the typical diffraction peaks of GO or reduced GO (rGO) nanocrystals have not been detected mainly due to their noncrystalline structure.21 Fig. 2(a and b) shows SEM images of ZSH–rGO composites. It can be seen that the sample consists of nanocubes with a homogeneous size of about 40–80 nm. It can also be found that the nanocubes are uniformly covered by the graphene sheets. In fact, the graphene sheets with obvious wrinkles can act as
Materials characterization X-ray diffraction (XRD) patterns were recorded on a PANalytical X’Pert spectrometer using the Co Ka radiation (k = 1.788 Å), and the data were changed to Cu Ka data. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were taken on a Hitachi S4800 instrument and a Tecnai G2 F20 S-TWIN instrument, respectively. TG analyses were performed on a Perkin-Elmer TGA 7 thermal analyzer in air at a heating rate of 10 1C min1. Infrared spectra of the samples pelletized with KBr were recorded on a Fourier transform infrared spectrometer (FT-IR spectrometer, Spectrum 2000). Raman spectroscopy was performed on a microscopic confocal laser Raman spectrometer (inVia Reflex, Renishaw Co.) using a 532 nm laser at room temperature.
Fig. 1 Typical XRD patterns of pristine GO, bulk ZSH and ZSH–rGO composites.
Electrochemical measurements To evaluate the electrochemical performance, the working electrode was constructed by mixing the active material, acetylene back carbon (AB) powder and polyvinylidene fluoride (PVDF) powder in a weight ratio of 70 : 20 : 10. The mixture was added to N-methyl-2-pyrrolidinone solvent to form a homogeneous slurry and pressed on Cu foil circular flakes, and then dried at 120 1C for 24 h under vacuum conditions. Metallic lithium foils were used as the negative electrodes. The electrolyte was 1 M LiPF6 in a 1 : 1 : 1 (volume ratio) mixture of ethyl-carbonate (EC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC). The separator was a UP 3093 (Japan) micro-porous polypropylene membrane. The cells were assembled in a glove box filled with highly pure argon gas, and charge–discharge tests were performed in the voltage range of 0.01 to 3 V (Li+/Li) at a current density of 0.5 A g1 on a Land automatic battery tester (Land CT 2001A, Wuhan, China). The electrochemical impedance spectroscopy (EIS) was performed on an IM6 Electrochemical Workstation (Zahner).
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Fig. 2
(a and b) SEM and (c and d) TEM images of ZSH–rGO composites.
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Fig. 4 Raman spectra of pristine GO and ZSH–rGO composites. Fig. 3 The Fourier transform infrared spectra (FT-IR) of original GO, bulk ZSH and ZSH–GO, ZSH–rGO composites.
the support matrix of nanocubes. As depicted in Fig. 2(c and d), TEM images also confirm that the sample was composed of uniform nanocubes which were wrapped by rGO sheets. FTIR was carried out and the spectra are shown in Fig. 3. The band at 3390 cm1 present in the spectrum of GO is attributed to H2O molecules physically absorbed on the surface and the band at 2370 cm1 is related to CO2 molecules. The stretching vibrations of carbonyl and carboxyl CQO bonds are assigned to the peak at 1740 cm1. The bands at 1050, 1210 and 1630 cm1 are contributed to C–O–C, carboxyl C–O and CQC stretching vibrations, respectively, suggesting that the graphite was oxidized sufficiently into hydrophilic GO with multitudinous oxygen-containing functional groups. For the other three curves, the band at 3230 cm1 is attributed to the bending and stretching vibration modes of the OH group in ZSH. The five bands at 542, 661, 787, 1170 and 2360 cm1 arise from the vibration of M–OH or M–OH–M groups in ZSH, which is similar to the consequences described in the previous report.22 These results indicate that the structure of ZSH was not changed during the hybridization and reduction of GO, which is also in accordance with the XRD results. Raman spectroscopy is another effective tool to monitor the structural changes of carbonaceous materials in the graphene-based composites. Of particular note is the intensity ratio of the D (1348 cm1) and G (1591 cm1) bands, ID/IG, which is a measure of the relative concentration of local defects or disorders (particularly the sp3-hybridized defects) compared to the sp2-hybridized rGO domains.23,24 It can be seen from Fig. 4 that the ratio of ID/IG is 0.93 for GO and it increased to 1.01 for ZSH–rGO after the hydrothermal reaction; this is due to the decrease of average size of the sp2 domains during the reduction of GO.25 Nevertheless, the number of the sp2 domains was increased during the reduction reactions.26 TGA was performed to determine the number of graphene sheets in the composite of ZSH–rGO. As shown in Fig. 5, the
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Fig. 5
TG curves of ZSH and ZSH–rGO composites.
weight loss below about 420 1C is generally attributed to the decomposition of ZSH:27 ZnSn(OH)6 - ZnSnO3 + 3H2O
(1)
The weight loss of 20.7 wt% was almost consistent with the theoretical value of 19 wt% calculated according to eqn (1). The weight loss at about 420 1C can be attributed to the removal of graphene from the composite.28 Above 550 1C, TGA traces are stable with no further mass loss, indicating that graphene in the composite was completely removed. Therefore, the graphene content in the composite of ZSH–rGO was determined to be about 8.3 wt%. The electrochemical performances of ZSH–rGO composites and ZSH were investigated, and the results are shown in Fig. 6. Fig. 6(a) shows the representative discharge–charge profiles of two samples at a current density of 500 mA g1 in the initial three cycles in a potential window of 0.01–3.0 V. The initial charge and discharge capacities of ZSH–rGO composites were 776 and 1836 mA h g1, respectively, while the bulk ZSH sample exhibited the initial charge and discharge capacities of 310 and
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Fig. 6 (a) Discharge–charge voltage profiles of ZSH and ZSH–rGO composites for the first three cycles, (b) Cycling performance of pure rGO, bulk ZSH and ZSH–rGO composites at a current density of 500 mA g1, (c) Rate performance of bulk ZSH and ZSH–rGO composites, and (d) CV curves of the ZSH–rGO composite electrode at a scan rate of 0.5 mV s1 in the voltage range 0.01–3.0 V.
1954 mA h g1. According to the lithium ion storage mechanism of ZnO29 and SnO2,30 the following electrochemical reactions were suggested for lithium insertion into the anode of ZSH nanocubes: 4Li+ + ZnSn(OH)6 + 4e - Sn + 2Li2O + 2ZnO + 3H2O
(2a)
6Li+ + ZnSn(OH)6 + 6e - Zn + Sn + 3Li2O + 3H2O
(2b)
xLi+ + Sn + xe 2 LixSn, x r 4.4 yLi+ + Zn + ye 2 LiyZn,
yr1
(3) (4) 1
The theoretical reversible capacity of about 506 mA h g is derived from the maximum uptake of 5.4 mol Li for ZSH based on eqn (3) and (4). The theoretical value from eqn (2) and (4) is calculated to be about 1068 mA h g1. The extra Li consumption via a pseudo-capacitance mechanism,31,32 such as a trace amount of water in the assembled cells, accounts mostly for the excess capacity in the initial cycle. However, the large capacity loss in the first cycle is mainly attributed to the initial irreversible formation of Li2O, and other irreversible processes such as trapping of some Li-ions in the lattice, inevitable formation of a solid electrolyte interface (SEI) layer and electrolyte decomposition, which are all quite common for most anode materials.33–39 The difference in voltage profiles of ZSH and ZSH–rGO composites suggests that some extra reaction occurs in the composite, which may be responsible for the extra capacity. As can be seen in Fig. 6(a), the steepness of the voltage profile of the ZSH–rGO composite basically unchanged in the range of 0.01–3 V, while the voltage increased sharply when the bulk ZSH cell was further charged to over 1.0 V. These facts suggest that some extra conversion reaction may occur in the ZSH–rGO composite
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electrode based on the previous report.40 Besides, the synergistic effect between ZSH nanocubes and the conductive graphene likely contributed to the extra capacity. The spreading graphene coated on the ZSH nanocubes maximizes the contact of graphene with the electrolyte, which adds the capacity of graphene contributed by Li-ion absorption.41 To demonstrate the advantages of ZSH–rGO composites for Li-ion storage, the cycling performance of bulk ZSH is also investigated under identical test conditions, as shown in Fig. 6(b). The cells are charged and discharged at 500 mA g1 in a potential range of 0.01–3.0 V. Compared with bulk ZSH, ZSH–rGO composites exhibited an increased capacity and an enhanced cycling stability. It was also found that the charge capacity of bulk ZSH dropped rapidly from 310 to 197 mA h g1 after 40 cycles at 500 mA g1. For ZSH–rGO composites, a charge capacity of 540 mA h g1 can still be retained. As a reference sample, the cell made up of graphene only exhibited a capacity below 200 mA h g1 after 40 cycles. Thus, the cycling stability of ZSH–rGO composites might be ascribed to the buffering effect and the enhanced conductivity in the presence of graphene, which effectively alleviates the large volume changes and provides a highly conductive medium for electron transport during Li-ion insertion/extraction into/out of the ZSH anode. Furthermore, the aggregation of ZSH nanocubes can be inhibited by the immobilization effect of graphene. Fig. 6(c) compares the rate capability between ZSH and ZSH–rGO composites. The latter demonstrates much better electrochemical performance and high rate capacity than those of the former, suggesting that the improved rate capability mainly arises from the following two factors. Firstly, the highly conductive graphene offers 2D conductive channels for the ZSH anode. Secondly, the unique structure of ZSH–rGO composites facilitates the better wetting of the active materials by the electrolyte, leading to the faster Li-ion transport across the electrode/electrolyte interface. When being cycled at different current densities of 500, 1000, 1500, 2000 and 500 mA g1, the ZSH–rGO composite could deliver discharge capacities of 755.2, 578.7, 514.8, 469.4 and 657.5 mA h g1, respectively. After being cycled at different current densities, the capacity of ZSH–rGO can be recovered to a high value when the current density was returned to 500 mA g1, suggesting that the integrity of the electrode can be maintained during the cycling process. These results show a considerable or even better electrochemical performance when compared with the graphene wrapped Zn2SnO4 boxes.42 This might be related to hydroxyl groups on the ZnSn(OH)6 surface, which can combine with graphene oxides more intimately. Typical cyclic voltammogram (CV) curves of ZSH–rGO are depicted in Fig. 6(d). In the first cycle, the cathodic peak at B0.68 V (vs. Li/Li+), disappearing in the following cycles, can be ascribed to the decomposition of ZSH into Zn and Sn, and the formation of amorphous Li2O and the SEI film.43–45 Another two peaks located near 0.43 and 0.2 V correspond to the multistep electrochemical lithiation process with the decomposition of ZSH (eqn (2a) and (2b)) and the formation of alloys (eqn (3) and (4)). At the same time, an anodic peak at 0.68 V corresponds
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to the dealloying reaction. Two weak anodic peaks located at 1.1 V and 1.6 V correspond to the oxidation of Sn46 and Zn.47 In the second and subsequent cycles, the anodic peak at 0.68 V shifts to 0.64 V and the cathodic peak at 0.2 V shifts to 0.26 V, illustrating the different lithium reaction process. The redox couple indexed as 0.26/0.64 V is related to the alloying/dealloying process of LixSn and LiyZn. For a more clear explanation, the CV curves of bulk ZSH and rGO are presented in Fig. S1 (ESI†). There is no peak of pure rGO, and bulk ZSH only showed a redox peak in the initial cycle, indicating that the capacity contribution arises from ZSH nanocubes with the assistance of rGO sheets. EIS is a technique that is used to analyze the electrode kinetic process as well as impedance of the cell. Impedance analysis illustrates the contribution of electrolyte resistance, surface film resistance and solid state diffusion of Li ions through the bulk of the active material. EIS analysis was performed using fresh cells under open circuit voltage conditions. Fig. 7(a) compares the electrochemical impedance spectra of ZSH and ZSH–rGO composites. It can be observed that the impedance plots of both samples consisted of an inclined line in the low-frequency region and one compressed semicircle in the medium-frequency region. The impedance plots were fitted by using an equivalent circuit model, as shown in the
Paper Table 1 model
Impedance parameters calculated from an equivalent circuit
Samples
Rs (O)
Rct (O)
s (O s1/2)
ZSH ZSH–rGO
6.51 5.38
140 36.9
192 192
inset of Fig. 7(a),48 which includes the Warburg impedance assigned to the diffusion of Li-ions into the bulk of the electrode materials, the charge-transfer resistance Rct, the SEI resistance Rf, the electrolyte resistance Rs, and two constant phase elements (CPE1 and CPE2) corresponding to the charge-transfer resistance and interfacial resistance, respectively. EIS fitting values are shown in Table 1, which indicate that electrolyte resistance (Rs) is almost the same, but the different Rct values determine the electrochemical performance of the active material. The Rct values of ZSH and ZSH–rGO are 140 and 36.9 O, respectively. Obviously, the Rct of the latter was much smaller than that of the former, suggesting that the conductive capability of the electrode material can be greatly increased after ZSH was coated by rGO. EIS is also used to evaluate the diffusion coefficient of Li+ ions within the active material. The diffusion coefficient was calculated using the following eqn (5):49 DLi+ = R2T 2/2A2n4F 4C2s2
(5)
where R is the gas constant (8.314 J K1 mol1), F is the Faraday constant (96 500 C mol1), T is the room temperature in our experiment (absolute temperature), A is the surface area, n is the number of the electrons per molecule attending the electron transfer reaction, C is the concentration, D is the diffusion coefficient, s is the Warburg factor, which can be obtained from the line of Z 0 B o1/2 in the low frequency region, respectively. The relationship between Z 0 and o1/2 in the low frequency region is shown in Fig. 7(b). The Warburg factors s are on the whole the same as the value of 192 for both ZSH–rGO composites and ZSH. If other parameters are fixed, the diffusion coefficient of the former is the same as that of the latter, indicating that the ZSH sample retains the original state after hybridized with rGO. In fact, the results of EIS simulation analysis which was carried out using two models mentioned above are all consistent with our experimental results.
Conclusions
Fig. 7 (a) The EIS spectra of ZSH and ZSH–rGO and their corresponding equivalent circuits (inset) and (b) Randles plot of ZSH and ZSH–rGO.
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In summary, we have designed a unique hybrid nanostructure, ZSH nanocubes coated by rGO, for highly reversible lithium storage. When firstly evaluated as an anode material for LIBs, the composite of ZSH–rGO exhibited high capacity, cycling stability and good rate performance. The improved electrochemical performance might be attributed to the fact that the incorporation of the flexible rGO acted as the buffer to alleviate the volume changes and inhibit the aggregation of ZSH nanocubes. Thus, such a class of functional materials may hold great promise for the development of high performance LIBs.
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Acknowledgements This work was financially supported by National Natural Science Foundation of China (NSFC 21173049).
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