- Email: [email protected]
carbon nanotubes composite with enhanced electrochemical performance as anode materials for lithium-ion batteries
Materials Letters 164 (2016) 44–47
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Zn2SnO4/carbon nanotubes composite with enhanced electrochemical performance as anode materials for lithium-ion batteries Liping Qin a,b, Shuquan Liang a,n, Anqiang Pan a, Xiaoping Tan a a b
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China Department of Science & Technology, Guangxi University of Science and Technology, Liuzhou, Guangxi 545006, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 August 2015 Received in revised form 22 October 2015 Accepted 26 October 2015 Available online 26 October 2015
Zn2SnO4/carbon nanotubes (Zn2SnO4/CNT) composite has been prepared by a facile hydrothermal process. The CNTs intertwine with Zn2SnO4 nanoparticles to form a 3D network wiring. Some Zn2SnO4 nanoparticles are anchored on CNTs and the diameter of Zn2SnO4 nanoparticles is about 20 nm. As anode materials for lithium-ion batteries, Zn2SnO4/CNT composite shows high initial discharge/charge capacities of 1925.4/1064.9 mAh g 1 at the current density of 100 mA g 1. Compared with bare Zn2SnO4, the Zn2SnO4/CNT exhibits enhanced electrochemical performance, including higher capacities, better capacity retention and higher rate capability, due to the 3D network conductive channels. & 2015 Elsevier B.V. All rights reserved.
Keywords: Zn2SnO4/CNT Anode Lithium-ion batteries Energy storage and conversion Nanocomposite
1. Introduction Lithium-ion batteries (LIBs) are widely used as rechargeable power sources for various consumer electronics appliance because of their high energy density, fast charge/discharge rate, and excellent cycle performance. Graphite and lithium titanate (Li4Ti5O12), which are used as the anode materials for commercial lithium ion batteries, suffer from the drawback of low capacity [1]. Among the alternative anode materials, Zn2SnO4 is considered as a promising anode material for LIBs owing to its low lithium insertion potentials and high specific capacities [2,3]. During the discharge process, about 14.4 mol lithium atoms will react with per formula Zn2SnO4 and go through phase conversion and alloying process as the equations: Z n2S nO4 + 8Li+ + 8e → 2Zn + Sn + 4Li2 O , Sn + xLi+ + xe ↔ Lix Sn (0 ≤ x ≤ 4.4), Zn + yLi+ + ye ↔ Liy Zn (0 ≤ y ≤ 1) [4]. However, the large volume change upon alloying/dealloying processes results in structural pulverization and poor cycle performance. In addition, the worse conductivity of bare Zn2SnO4 causes the poor rate performance. Many strategies have been developed to improve their electrochemical properties including nanostructured materials [5,6] and hybridization of active materials and carbonaceous ones [2,7,8], which will enhance electronic conductivity, mechanical strength, and buffer huge volume changes. n
Corresponding author. Fax: þ86 731 88876692. E-mail address: [email protected] (S. Liang).
http://dx.doi.org/10.1016/j.matlet.2015.10.125 0167-577X/& 2015 Elsevier B.V. All rights reserved.
Herein, we first report the synthesis of Zn2SnO4/CNT composite by one-pot facile hydrothermal process. As anode materials for lithium-ion batteries, the Zn2SnO4/CNT composite exhibits enhanced electrochemical performance than bare Zn2SnO4.
2. Experimental methods For a typical synthesis, 0.4 g commercial multiwall carbon nanotubes (CNTs) were dispersed in 50 ml deionized water by sonication. Then, 10 mmol ZnCl2 and 5 mmol SnCl4 5H2O were added into the above solution under stirring for 2 h, followed by the dropwise adding of 20 mL Na2CO3 solution (3.18 g Na2CO3). Continue stirring for 2 h, the obtained mixture was transferred to a 100 mL autoclave and maintained at 200 °C for 20 h. After that, the precipitate was washed and centrifuged with deionized water and ethanol for several times before drying in air. For comparison, bare Zn2SnO4 was prepared without CNTs in the mixture solution. The crystal structures and morphologies of the samples were characterized by X-ray diffraction analysis (XRD) (Rigaku D/ max2500), Raman spectrum (LabRAM HR-800), scanning electron microscopy (SEM, Nova NanoSEM230) and transmission electron microscopy (TEM, HEM-2100F/UHR). The electrochemical measurements were conducted by assembly into coin cells. The working electrode was prepared by mixing sample with acetylene black and poly (vinylidene fluoride) at a weight ratio of 70:20:10 in N-methyl-2-pyrrolidone to form
L. Qin et al. / Materials Letters 164 (2016) 44–47
45
Fig. 1. (a) XRD patterns and (b) Raman spectra of Zn2SnO4//CNT and Zn2SnO4.
Fig. 2. SEM images of Zn2SnO4 (a) and Zn2SnO4/CNT (b). TEM images of Zn2SnO4/CNT (c and d).
slurry. Then, the resultant slurry was uniformly pasted on copper foil and dried to make the anode. The cells were assembled in a glove box (Mbraun, Germany) filled with ultra-high purity argon
using polypropylene membrane as the separator, and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) (EC:DMC ¼1:1 v/v) as the electrolyte. Li foil as a counter electrode. The discharge/
46
L. Qin et al. / Materials Letters 164 (2016) 44–47
Fig. 3. (a and b) The first two discharge/charge voltage profiles of Zn2SnO4 and Zn2SnO4/CNT. (c) Cycling performance of Zn2SnO4/CNT and Zn2SnO4 at 100 mA g 1. (d) Rate performances of Zn2SnO4/CNT and Zn2SnO4 at different current densities.
Fig. 4. (a) EIS of Zn2SnO4/CNT and Zn2SnO4 electrodes. (b) The equivalent circuit diagram.
charge cycle and rate performances were tested on Land Battery Tester (Land CT 2001A) in the potential range 0.005–3.0 V. The electrochemical impedance spectrometry (EIS) was measured on electrochemical workstation (ZAHNER-IM6ex) in the frequency range of 100 kHz to 10 mHz.
3. Results and discussion Fig. 1a shows the XRD patterns of prepared Zn2SnO4/CNT and bare Zn2SnO4 samples. The sharp diffractive peaks are in good
agreement with cubic structure Zn2SnO4 (JCPDS 24-1470). In addition, a broad diffractive peak at 26 ° in Zn2SnO4/CNT sample is attributed to the (002) crystal plane of CNTs. The Raman spectra of Zn2SnO4/CNT and Zn2SnO4 are presented in Fig. 1b. Note that Zn2SnO4/CNT exhibits two spectrum peaks at 1350 and 1580 cm 1, corresponding to the D and G bands of carbon materials, respectively. Zn2SnO4 displays two spectrum peaks at 545 and 646 cm 1, associated respectively with the stretching vibrations of short M–O bonds in the MO octahedron (M ¼Sn or Zn) sticking out into the structure spaces and internal vibrations of oxygen tetrahedron of Zn2SnO4 [9]. In the Raman
L. Qin et al. / Materials Letters 164 (2016) 44–47
spectra of Zn2SnO4/CNT, the peak at 646 cm 1 still exists, the peak at 545 cm 1 is very weak, which can be assigned to the introduction of CNTs in the composite. Therefore, the Zn2SnO4/CNT composite was successfully prepared by the facile hydrothermal process. The amount of CNTs in Zn2SnO4/CNT composite is about 23.5% according to the thermal gravimetric measurement. The morphologies of the bare Zn2SnO4 and Zn2SnO4/CNT are characterized and the results are shown in Fig. 2. According to the SEM images, the bare Zn2SnO4 particles intend to aggregate to form the secondary particles with a diameter around 200 nm. From Fig. 2b, it can be seen that the CNTs intertwine with Zn2SnO4 nanoparticles to form a three dimensional (3D) network wiring, acting as branches to connect Zn2SnO4 nanoparticles, and the agglomeration of Zn2SnO4 nanoparticles is adequately prevented by the deposition of Zn2SnO4 nanoparticles in the CNTs. The TEM images (Fig. 2c and d) further confirm that some Zn2SnO4 nanoparticles are anchored on CNTs and the diameter of Zn2SnO4 nanoparticles in Zn2SnO4/CNT composite is about 20 nm. The nanostructure Zn2SnO4 anchored on CNTs and the 3D network wiring can not only efficiently relax the volume changes of Zn2SnO4 nanoparticles during Li þ insertion/extraction process, but enhance the electrical conductivity, contributing to improve the transmission of Li þ and the electrochemical performance of Zn2SnO4/CNT. Fig. 3a and b show the first two discharge/charge voltage profiles of Zn2SnO4 and Zn2SnO4/CNT at a current density of 100 mA g 1. The first discharge and charge capacities of Zn2SnO4 are 1216.7 and 625 mAh g 1, respectively. For Zn2SnO4/CNT, the first discharge and charge capacities are 1925.4 and 1064.9 mAh g 1, respectively, corresponding to the coulombic efficiency of 55.31%. The theoretical initial discharge capacity of Zn2SnO4 is 1231 mAh g 1. The reasons for much higher initial discharge capacity of composite are as follows: (1) The side reactions of electrolyte and subsequent formation of solid electrolyte interface (SEI) film on the surface of nanoparticles account mostly for the excess capacity [10]. (2) The CNTs consume some lithium ions [11]. (3) The agglomeration of Zn2SnO4 nanoparticles can efficiently be prevented as CNTs intertwine with Zn2SnO4 nanoparticles, so most nanoparticles can attend the change/discharge process. The large irreversible capacity loss is mainly attributed to the formation of SEI film and the irreversible reaction process of metal oxides to metal with Li2O formation [4]. The second discharge/charge capacities and coulombic efficiency of Zn2SnO4/CNT electrode are 1191.7/1093.7 mAh g 1 and 91.77%, respectively. From the second cycle, the irreversible capacity loss decreases and the coulombic efficiency much increases, illustrating high efficient Li þ insertion/extraction. Fig. 3c shows the cycling performance of Zn2SnO4/CNT and Zn2SnO4 electrodes at the current density of 100 mA g 1. It is evident that Zn2SnO4/CNT exhibits an improved capacity and cycling stability compared with bare Zn2SnO4. The discharge/charge capacities of Zn2SnO4 drop rapidly from 1216.7/625 to 273.4/272.2 mAh g 1 after 30 cycles. For Zn2SnO4/CNT, capacities of 703.8/686.3 mAh g 1 can be retained after 30 cycles. Much higher capacities can be obtained for Zn2SnO4/CNT. Fig. 3d compares the rate performances of Zn2SnO4/CNT and Zn2SnO4 electrodes. After 5 cycles, the Zn2SnO4/CNT electrode delivers discharge/charge capacities of 1372.5/1307.8, 1190.6/1156.5, 947.1/939.3, 652.7/658.4, 503.3/499.8, 274/251.7 mAh g 1 at different current densities of 50, 100, 200, 400, 600 and 1000 mA g 1, respectively. When the current density is reset to 50 mA g 1 after 30 cycles, a high discharge capacity of 991.4 mA h g 1 can be recovered. However, the corresponding capacities of Zn2SnO4 are 655.6/635.6, 558.2/542.7, 474.4/450.3, 373.3/342.5, 265.9/254.1, 97.4/93.2 mAh g 1. The results demonstrate that Zn2SnO4/CNT shows better rate capability than bare Zn2SnO4.
47
Fig. 4a illustrates the EIS of Zn2SnO4/CNT and Zn2SnO4 electrodes. Both the impedance spectra consist of one semicircle in the medium frequency region and an inclined line in the low frequency region, which can be fitted to the equivalent circuit diagram (Fig. 4b). In the diagram, Re is the electrolyte resistance, Rf is the resistance of the surface film and contact, C1 and C2 are two constant phase elements associated with the interfacial resistance and charge transfer resistance, respectively. W is the Warburg impedance related to the diffusion of lithium ions into the bulk of materials electrodes. Rct is the charge transfer resistance. Based on the simulation results, the Rct of Zn2SnO4/CNT and Zn2SnO4 electrodes are 384.8 and 168.2 Ω, respectively. Obviously, the Zn2SnO4/CNT electrode has much lower charge transfer resistance attributed to the introduction of highly conductive CNTs.
4. Conclusion Zn2SnO4/CNT composite was synthesized by one-pot facile hydrothermal process. As anode materials for lithium-ion batteries, the Zn2SnO4/CNT composite exhibits enhanced electrochemical performance including higher specific capacity and better rate capability compared with bare Zn2SnO4. The improved electrochemical performance can be attributed to the introduction of CNTs. The highly conductive CNTs offer 3D conductive channels for Zn2SnO4 nanoparticles, and reduce the aggregation of Zn2SnO4 nanoparticles, and better accommodate the volume changes during discharge/charge process.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51374255, 51302323) and Natural Science Foundation of Guangxi Province, China (2015GXNSFBA139226).
References [1] N. Alias, A.A. Mohamad, Advances of aqueous rechargeable lithium-ion battery: a review, J. Power Sour. 274 (2015) 237–251. [2] C. Yan, J. Yang, Q. Xie, Z. Lu, B. Liu, C. Xie, et al., Novel nanoarchitectured Zn2SnO4 anchored on porous carbon as high performance anodes for lithium ion batteries, Mater. Lett. 138 (2015) 120–123. [3] Y. Zhao, Y. Huang, Q. Wang, K. Wang, M. Zong, L. Wang, et al., Preparation of hollow Zn2SnO4 boxes for advanced lithium-ion batteries, RSC Adv. 3 (2013) 14480. [4] K. Kim, A. Annamalai, S.H. Park, T.H. Kwon, M.W. Pyeon, M.-J. Lee, Preparation and electrochemical properties of surface-charge-modified Zn2SnO4 nanoparticles as anodes for lithium-ion batteries, Electrochim. Acta 76 (2012) 192–200. [5] L. Qin, S. Liang, A. Pan, X. Tan, Facile solvothermal synthesis of Zn2SnO4 nanoparticles as anode materials for lithium-ion batteries, Mater. Lett. 141 (2015) 255–258. [6] C.T. Cherian, M. Zheng, M.V. Reddy, B.V. Chowdari, C.H. Sow, Zn2SnO4 nanowires versus nanoplates: electrochemical performance and morphological evolution during Li-cycling, ACS Appl. Mater. Interfaces 5 (2013) 6054–6060. [7] B.-Y. Wang, H.-Y. Wang, Y.-L. Ma, X.-H. Zhao, W. Qi, Q.-C. Jiang, Facile synthesis of fine Zn2SnO4 nanoparticles/graphene composites with superior lithium storage performance, J. Power Sour. 281 (2015) 341–349. [8] W. Song, J. Xie, W. Hu, S. Liu, G. Cao, T. Zhu, et al., Facile synthesis of layered Zn2SnO4/graphene nanohybrid by a one-pot route and its application as highperformance anode for Li-ion batteries, J. Power Sour. 229 (2013) 6–11. [9] S. Shi, Effect of grain size on raman and surface photovoltage spectra of zn2sno4 powders, J. Light Scatt. 24 (2012) 271–275. [10] A. Rong, G.R. Li, T.Y. Yan, H.Y. Zhu, J.Q. Qu, D.Y. Song, Hydrothermal Synthesis of Zn2SnO4 as Anode Materials for Li-Ion Battery, J. Phys. Chem. B 110 (2006) 14754–14760. [11] S. Yoon, S. Lee, S. Kim, K.-W. Park, D. Cho, Y. Jeong, Carbon nanotube film anodes for flexible lithium ion batteries, J. Power Sour. 279 (2015) 495–501.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Zn2SnO4/carbon nanotubes composite with enhanced electrochemical performance as anode materials for lithium-ion batteries Liping Qin a,b, Shuquan Liang a,n, Anqiang Pan a, Xiaoping Tan a a b
School of Materials Science and Engineering, Central South University, Changsha, Hunan 410083, China Department of Science & Technology, Guangxi University of Science and Technology, Liuzhou, Guangxi 545006, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 August 2015 Received in revised form 22 October 2015 Accepted 26 October 2015 Available online 26 October 2015
Zn2SnO4/carbon nanotubes (Zn2SnO4/CNT) composite has been prepared by a facile hydrothermal process. The CNTs intertwine with Zn2SnO4 nanoparticles to form a 3D network wiring. Some Zn2SnO4 nanoparticles are anchored on CNTs and the diameter of Zn2SnO4 nanoparticles is about 20 nm. As anode materials for lithium-ion batteries, Zn2SnO4/CNT composite shows high initial discharge/charge capacities of 1925.4/1064.9 mAh g 1 at the current density of 100 mA g 1. Compared with bare Zn2SnO4, the Zn2SnO4/CNT exhibits enhanced electrochemical performance, including higher capacities, better capacity retention and higher rate capability, due to the 3D network conductive channels. & 2015 Elsevier B.V. All rights reserved.
Keywords: Zn2SnO4/CNT Anode Lithium-ion batteries Energy storage and conversion Nanocomposite
1. Introduction Lithium-ion batteries (LIBs) are widely used as rechargeable power sources for various consumer electronics appliance because of their high energy density, fast charge/discharge rate, and excellent cycle performance. Graphite and lithium titanate (Li4Ti5O12), which are used as the anode materials for commercial lithium ion batteries, suffer from the drawback of low capacity [1]. Among the alternative anode materials, Zn2SnO4 is considered as a promising anode material for LIBs owing to its low lithium insertion potentials and high specific capacities [2,3]. During the discharge process, about 14.4 mol lithium atoms will react with per formula Zn2SnO4 and go through phase conversion and alloying process as the equations: Z n2S nO4 + 8Li+ + 8e → 2Zn + Sn + 4Li2 O , Sn + xLi+ + xe ↔ Lix Sn (0 ≤ x ≤ 4.4), Zn + yLi+ + ye ↔ Liy Zn (0 ≤ y ≤ 1) [4]. However, the large volume change upon alloying/dealloying processes results in structural pulverization and poor cycle performance. In addition, the worse conductivity of bare Zn2SnO4 causes the poor rate performance. Many strategies have been developed to improve their electrochemical properties including nanostructured materials [5,6] and hybridization of active materials and carbonaceous ones [2,7,8], which will enhance electronic conductivity, mechanical strength, and buffer huge volume changes. n
Corresponding author. Fax: þ86 731 88876692. E-mail address: [email protected] (S. Liang).
http://dx.doi.org/10.1016/j.matlet.2015.10.125 0167-577X/& 2015 Elsevier B.V. All rights reserved.
Herein, we first report the synthesis of Zn2SnO4/CNT composite by one-pot facile hydrothermal process. As anode materials for lithium-ion batteries, the Zn2SnO4/CNT composite exhibits enhanced electrochemical performance than bare Zn2SnO4.
2. Experimental methods For a typical synthesis, 0.4 g commercial multiwall carbon nanotubes (CNTs) were dispersed in 50 ml deionized water by sonication. Then, 10 mmol ZnCl2 and 5 mmol SnCl4 5H2O were added into the above solution under stirring for 2 h, followed by the dropwise adding of 20 mL Na2CO3 solution (3.18 g Na2CO3). Continue stirring for 2 h, the obtained mixture was transferred to a 100 mL autoclave and maintained at 200 °C for 20 h. After that, the precipitate was washed and centrifuged with deionized water and ethanol for several times before drying in air. For comparison, bare Zn2SnO4 was prepared without CNTs in the mixture solution. The crystal structures and morphologies of the samples were characterized by X-ray diffraction analysis (XRD) (Rigaku D/ max2500), Raman spectrum (LabRAM HR-800), scanning electron microscopy (SEM, Nova NanoSEM230) and transmission electron microscopy (TEM, HEM-2100F/UHR). The electrochemical measurements were conducted by assembly into coin cells. The working electrode was prepared by mixing sample with acetylene black and poly (vinylidene fluoride) at a weight ratio of 70:20:10 in N-methyl-2-pyrrolidone to form
L. Qin et al. / Materials Letters 164 (2016) 44–47
45
Fig. 1. (a) XRD patterns and (b) Raman spectra of Zn2SnO4//CNT and Zn2SnO4.
Fig. 2. SEM images of Zn2SnO4 (a) and Zn2SnO4/CNT (b). TEM images of Zn2SnO4/CNT (c and d).
slurry. Then, the resultant slurry was uniformly pasted on copper foil and dried to make the anode. The cells were assembled in a glove box (Mbraun, Germany) filled with ultra-high purity argon
using polypropylene membrane as the separator, and 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC) (EC:DMC ¼1:1 v/v) as the electrolyte. Li foil as a counter electrode. The discharge/
46
L. Qin et al. / Materials Letters 164 (2016) 44–47
Fig. 3. (a and b) The first two discharge/charge voltage profiles of Zn2SnO4 and Zn2SnO4/CNT. (c) Cycling performance of Zn2SnO4/CNT and Zn2SnO4 at 100 mA g 1. (d) Rate performances of Zn2SnO4/CNT and Zn2SnO4 at different current densities.
Fig. 4. (a) EIS of Zn2SnO4/CNT and Zn2SnO4 electrodes. (b) The equivalent circuit diagram.
charge cycle and rate performances were tested on Land Battery Tester (Land CT 2001A) in the potential range 0.005–3.0 V. The electrochemical impedance spectrometry (EIS) was measured on electrochemical workstation (ZAHNER-IM6ex) in the frequency range of 100 kHz to 10 mHz.
3. Results and discussion Fig. 1a shows the XRD patterns of prepared Zn2SnO4/CNT and bare Zn2SnO4 samples. The sharp diffractive peaks are in good
agreement with cubic structure Zn2SnO4 (JCPDS 24-1470). In addition, a broad diffractive peak at 26 ° in Zn2SnO4/CNT sample is attributed to the (002) crystal plane of CNTs. The Raman spectra of Zn2SnO4/CNT and Zn2SnO4 are presented in Fig. 1b. Note that Zn2SnO4/CNT exhibits two spectrum peaks at 1350 and 1580 cm 1, corresponding to the D and G bands of carbon materials, respectively. Zn2SnO4 displays two spectrum peaks at 545 and 646 cm 1, associated respectively with the stretching vibrations of short M–O bonds in the MO octahedron (M ¼Sn or Zn) sticking out into the structure spaces and internal vibrations of oxygen tetrahedron of Zn2SnO4 [9]. In the Raman
L. Qin et al. / Materials Letters 164 (2016) 44–47
spectra of Zn2SnO4/CNT, the peak at 646 cm 1 still exists, the peak at 545 cm 1 is very weak, which can be assigned to the introduction of CNTs in the composite. Therefore, the Zn2SnO4/CNT composite was successfully prepared by the facile hydrothermal process. The amount of CNTs in Zn2SnO4/CNT composite is about 23.5% according to the thermal gravimetric measurement. The morphologies of the bare Zn2SnO4 and Zn2SnO4/CNT are characterized and the results are shown in Fig. 2. According to the SEM images, the bare Zn2SnO4 particles intend to aggregate to form the secondary particles with a diameter around 200 nm. From Fig. 2b, it can be seen that the CNTs intertwine with Zn2SnO4 nanoparticles to form a three dimensional (3D) network wiring, acting as branches to connect Zn2SnO4 nanoparticles, and the agglomeration of Zn2SnO4 nanoparticles is adequately prevented by the deposition of Zn2SnO4 nanoparticles in the CNTs. The TEM images (Fig. 2c and d) further confirm that some Zn2SnO4 nanoparticles are anchored on CNTs and the diameter of Zn2SnO4 nanoparticles in Zn2SnO4/CNT composite is about 20 nm. The nanostructure Zn2SnO4 anchored on CNTs and the 3D network wiring can not only efficiently relax the volume changes of Zn2SnO4 nanoparticles during Li þ insertion/extraction process, but enhance the electrical conductivity, contributing to improve the transmission of Li þ and the electrochemical performance of Zn2SnO4/CNT. Fig. 3a and b show the first two discharge/charge voltage profiles of Zn2SnO4 and Zn2SnO4/CNT at a current density of 100 mA g 1. The first discharge and charge capacities of Zn2SnO4 are 1216.7 and 625 mAh g 1, respectively. For Zn2SnO4/CNT, the first discharge and charge capacities are 1925.4 and 1064.9 mAh g 1, respectively, corresponding to the coulombic efficiency of 55.31%. The theoretical initial discharge capacity of Zn2SnO4 is 1231 mAh g 1. The reasons for much higher initial discharge capacity of composite are as follows: (1) The side reactions of electrolyte and subsequent formation of solid electrolyte interface (SEI) film on the surface of nanoparticles account mostly for the excess capacity [10]. (2) The CNTs consume some lithium ions [11]. (3) The agglomeration of Zn2SnO4 nanoparticles can efficiently be prevented as CNTs intertwine with Zn2SnO4 nanoparticles, so most nanoparticles can attend the change/discharge process. The large irreversible capacity loss is mainly attributed to the formation of SEI film and the irreversible reaction process of metal oxides to metal with Li2O formation [4]. The second discharge/charge capacities and coulombic efficiency of Zn2SnO4/CNT electrode are 1191.7/1093.7 mAh g 1 and 91.77%, respectively. From the second cycle, the irreversible capacity loss decreases and the coulombic efficiency much increases, illustrating high efficient Li þ insertion/extraction. Fig. 3c shows the cycling performance of Zn2SnO4/CNT and Zn2SnO4 electrodes at the current density of 100 mA g 1. It is evident that Zn2SnO4/CNT exhibits an improved capacity and cycling stability compared with bare Zn2SnO4. The discharge/charge capacities of Zn2SnO4 drop rapidly from 1216.7/625 to 273.4/272.2 mAh g 1 after 30 cycles. For Zn2SnO4/CNT, capacities of 703.8/686.3 mAh g 1 can be retained after 30 cycles. Much higher capacities can be obtained for Zn2SnO4/CNT. Fig. 3d compares the rate performances of Zn2SnO4/CNT and Zn2SnO4 electrodes. After 5 cycles, the Zn2SnO4/CNT electrode delivers discharge/charge capacities of 1372.5/1307.8, 1190.6/1156.5, 947.1/939.3, 652.7/658.4, 503.3/499.8, 274/251.7 mAh g 1 at different current densities of 50, 100, 200, 400, 600 and 1000 mA g 1, respectively. When the current density is reset to 50 mA g 1 after 30 cycles, a high discharge capacity of 991.4 mA h g 1 can be recovered. However, the corresponding capacities of Zn2SnO4 are 655.6/635.6, 558.2/542.7, 474.4/450.3, 373.3/342.5, 265.9/254.1, 97.4/93.2 mAh g 1. The results demonstrate that Zn2SnO4/CNT shows better rate capability than bare Zn2SnO4.
47
Fig. 4a illustrates the EIS of Zn2SnO4/CNT and Zn2SnO4 electrodes. Both the impedance spectra consist of one semicircle in the medium frequency region and an inclined line in the low frequency region, which can be fitted to the equivalent circuit diagram (Fig. 4b). In the diagram, Re is the electrolyte resistance, Rf is the resistance of the surface film and contact, C1 and C2 are two constant phase elements associated with the interfacial resistance and charge transfer resistance, respectively. W is the Warburg impedance related to the diffusion of lithium ions into the bulk of materials electrodes. Rct is the charge transfer resistance. Based on the simulation results, the Rct of Zn2SnO4/CNT and Zn2SnO4 electrodes are 384.8 and 168.2 Ω, respectively. Obviously, the Zn2SnO4/CNT electrode has much lower charge transfer resistance attributed to the introduction of highly conductive CNTs.
4. Conclusion Zn2SnO4/CNT composite was synthesized by one-pot facile hydrothermal process. As anode materials for lithium-ion batteries, the Zn2SnO4/CNT composite exhibits enhanced electrochemical performance including higher specific capacity and better rate capability compared with bare Zn2SnO4. The improved electrochemical performance can be attributed to the introduction of CNTs. The highly conductive CNTs offer 3D conductive channels for Zn2SnO4 nanoparticles, and reduce the aggregation of Zn2SnO4 nanoparticles, and better accommodate the volume changes during discharge/charge process.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51374255, 51302323) and Natural Science Foundation of Guangxi Province, China (2015GXNSFBA139226).
References [1] N. Alias, A.A. Mohamad, Advances of aqueous rechargeable lithium-ion battery: a review, J. Power Sour. 274 (2015) 237–251. [2] C. Yan, J. Yang, Q. Xie, Z. Lu, B. Liu, C. Xie, et al., Novel nanoarchitectured Zn2SnO4 anchored on porous carbon as high performance anodes for lithium ion batteries, Mater. Lett. 138 (2015) 120–123. [3] Y. Zhao, Y. Huang, Q. Wang, K. Wang, M. Zong, L. Wang, et al., Preparation of hollow Zn2SnO4 boxes for advanced lithium-ion batteries, RSC Adv. 3 (2013) 14480. [4] K. Kim, A. Annamalai, S.H. Park, T.H. Kwon, M.W. Pyeon, M.-J. Lee, Preparation and electrochemical properties of surface-charge-modified Zn2SnO4 nanoparticles as anodes for lithium-ion batteries, Electrochim. Acta 76 (2012) 192–200. [5] L. Qin, S. Liang, A. Pan, X. Tan, Facile solvothermal synthesis of Zn2SnO4 nanoparticles as anode materials for lithium-ion batteries, Mater. Lett. 141 (2015) 255–258. [6] C.T. Cherian, M. Zheng, M.V. Reddy, B.V. Chowdari, C.H. Sow, Zn2SnO4 nanowires versus nanoplates: electrochemical performance and morphological evolution during Li-cycling, ACS Appl. Mater. Interfaces 5 (2013) 6054–6060. [7] B.-Y. Wang, H.-Y. Wang, Y.-L. Ma, X.-H. Zhao, W. Qi, Q.-C. Jiang, Facile synthesis of fine Zn2SnO4 nanoparticles/graphene composites with superior lithium storage performance, J. Power Sour. 281 (2015) 341–349. [8] W. Song, J. Xie, W. Hu, S. Liu, G. Cao, T. Zhu, et al., Facile synthesis of layered Zn2SnO4/graphene nanohybrid by a one-pot route and its application as highperformance anode for Li-ion batteries, J. Power Sour. 229 (2013) 6–11. [9] S. Shi, Effect of grain size on raman and surface photovoltage spectra of zn2sno4 powders, J. Light Scatt. 24 (2012) 271–275. [10] A. Rong, G.R. Li, T.Y. Yan, H.Y. Zhu, J.Q. Qu, D.Y. Song, Hydrothermal Synthesis of Zn2SnO4 as Anode Materials for Li-Ion Battery, J. Phys. Chem. B 110 (2006) 14754–14760. [11] S. Yoon, S. Lee, S. Kim, K.-W. Park, D. Cho, Y. Jeong, Carbon nanotube film anodes for flexible lithium ion batteries, J. Power Sour. 279 (2015) 495–501.