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Hierarchal mesoporous SnO2@C@TiO2 nanochains for anode material of lithium-ion batteries with excellent cycling stability
Accepted Manuscript Title: Hierarchal mesoporous SnO2 @C@TiO2 nanochains for anode material of lithium-ion batteries with excellent cycling stability Author: Guoen Luo Weijian Liu Songshan Zeng Congcong Zhang Xiaoyuan Yu Yueping Fang Luyi Sun PII: DOI: Reference:
S0013-4686(15)30645-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.10.062 EA 25865
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
5-6-2015 3-10-2015 13-10-2015
Please cite this article as: Guoen Luo, Weijian Liu, Songshan Zeng, Congcong Zhang, Xiaoyuan Yu, Yueping Fang, Luyi Sun, Hierarchal mesoporous SnO2@C@TiO2 nanochains for anode material of lithium-ion batteries with excellent cycling stability, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.10.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hierarchal mesoporous SnO2@C@TiO2 nanochains for anode material of lithium-ion batteries with excellent cycling stability Guoen Luoa† ,Weijian Liua†, Songshan Zengb, Congcong Zhang a, Xiaoyuan Yua* a
b**
[email protected], Yueping Fang , Luyi Sun
[email protected]
a
Institute of Biomaterial, College of Materials and Energy, South China Agricultural University,
Guangzhou 510642, China b
Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials
Science, University of Connecticut, Storrs, Connecticut 06269, United States *Corresponding authors: Tel.: (+86) 20-8528 0323; fax: (+86) 20-8528 5026. *Corresponding authors.(L. Y. Sun) Tel: 860-486-6895; Fax: 860-486-4745. †
These authors contribute equally.
Highlights
Hierarchal mesoporous SnO2@C@TiO2 nanochains (SCT) were prepared via a facile hydrothermal method.
TiO2 shell and carbon shell act as double-decker protection to buffer the volume change of SnO2.
Cells with the Hierarchal mesoporous SCT nanochains anode showed excellent cycle performances.
ABSTRACT A new type of hierarchal mesoporous SnO2@C@TiO2 nanochains (SCT) as anode material for the lithium-ion batteries was prepared via a facile hydrothermal and kinetics-controlled coating method. The TiO2 shell and carbon shell act as double-decker protection to buffer the huge volume change of SnO2 during the charged and discharged process. The resulting batteries equipped with this novel anode material showed excellent cycling stability. SCT-250 with 8 nm TiO2 external shell was demonstrated for optimal electrochemical performances. A high initial capacity of 807 mAh g-1 was achieved at the current density of 100 mA g-1 and maintained 369 mAh g-1 after 100 cycles. The outstanding stable cycling and well rate performance suggest that SnO2@C@TiO2 core-shell-shell nanochains have great potential to be applied as anode material for lithium-ion batteries.
KEYWORDS: mesoporous; SnO2@C@TiO2; nanochains; Lithium-ion batteries; anode material
1. Introduction The increasing huge demand for energy and the depletion of oil energy have stimulated intensive researches on the next-generation energy sources [1, 2]. Lithium-ion batteries are considered as one of the most promising candidates to address to this challenge in the 21st century [3-5]. Comparing with the traditional electronic equipment, the portable electronic devices (such as mobile phones, tablet computer, digital cameras, and so on) and electric vehicles require such higher power density [6]. However, the commercial graphite materials (the theoretical capacity of 372 mAh g-1 [7]) are difficult to satisfy the increasing harsh demands. Therefore, it is highly urgent to develop the next generation anode material with high theoretical capacity. Recently, tin-based materials especially tin dioxides (SnO2) receive particular attentions due to their high theoretical capacity (786 mAh g-1), environmental friendliness and abundance [8-10]. However, the commercialization of SnO2 is retrained by two major issues: (1) the huge volume change (about 300%) during discharge/charge process [11-13], leading to disintegration and pulverization of the electrodes with quick capacity fading; (2) the poor electrical conductivity constrains the migration of electrons [14]. To mitigate these problems, many research papers were focused on controlling the nanosized [15] and constructing various nanostructures (such as hollow spheres [16, 17], nanoboxes [18], nanotubes [19-21], and so on of SnO2. Also, the composites of SnO2 and carbon (carbon [22-25], carbon nanotube [26, 27] and graphene [28-30]) show brilliant electrochemical performances, which not only enhance the conductivity but also buffer the volume change. In addition, Jiang [31] propose that advanced metal oxide-base hybrid nanostructure is superior to single-phase oxides, which can synergistically enhance the electrical conductivity and mechanical stability. Thus, a number of metal oxides can be applied to prepare SnO2-base hybrid to enhance the electrochemical performance. Among these, TiO2 is an excellent candidate due to its structural stability and chemical safety during cycling, which only has small volume change throughout the lithium insertion and extraction process [32-34]. At the same time, TiO2 can show an outstanding rate performance [35, 36]. In this research, we report a new type hierarchal mesoporous SnO2@C@TiO2 core-shell-shell nanochains (SCT), prepared by a facile hydrothermal and kinetics-controlled coating method. Here, SnO2 was coated by carbon via a facile hydrothermal in order to providing a buffering
layer and a perfect electrical conductivity. Then, TiO2 nanoparticles were monodispersed on the surface of SnO2@C core-shell nanochains, which could protect the structure of SnO2@C core-shell nanochains effectively. Generally speaking, the TiO2 shell and the carbon shell play a double-decker protection to buffer the huge volume change of SnO2. The mesoporous SnO2@C@TiO2 core-shell-shell nanochains exhibit excellent electrochemical performances.
2. Experimental 2.1 Preparation of SnO2@C core-shell nanochains The SnO2@C core-shell nanochains were synthesized via a facile hydrothermal method based on our previous reports [37]. 0.324 g of Na2SnO3·3H2O and 6 g glucose were dissolved in 40 mL of deionized water. The solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed, and kept at 180 ℃ for 4h in an oven prior to cooling down to room temperature. The product was harvested by centrifugation, washed with deionized water and ethanol, and dried at 60 ℃ for 12 h. 2.2 Preparation of mesoporous SnO2@C@TiO2 nanochains The mesoporous SnO2@C@TiO2 nanochains were obtained via a kinetics-controlled coating method [38]. For the synthesis, appropriate amount of SnO2@C nanochains were first sonicated in 400 mL of anhydrous ethanol. Ammonia solution (1.2 mL, 28 wt%) was added to the mixture solution, stirred for 15 minutes. Afterward, 3.0 mL tetrabutyl titanate was added dropwise, and the reaction was allowed to proceed for 24 h at 45 ℃ under continuous mechanical stirring. The product was centrifuged, and washed with deionized water and ethanol for three times, respectively. Subsequently, the obtained powders were dried at 60 ℃ overnight. Finally, the resulting samples were calcined at 500 ℃ for 3 h under low flow rate argon atmosphere. The uniform mesoporous SnO2@C@TiO2 nanochains are prepared with variable amount of SnO2@C (mass of SnO2@C is 50, 150, 250, 350 and 450 mg were investigated, respectively.), which are denoted as SCT-50, SCT-150, SCT-250, SCT-350 and SCT-450 (mass content of TiO2 is about 93.3, 82.3, 73.7, 66.7 and 60.8% ,respectively). 2.3 Materials characterization The as-prepared samples are characterized with X-ray powder diffraction (Rigaku, D/max 2500v/pc). Transmission electron microscopy (TEM) and high resolution transmission electron
microscopy (HRTEM) measurements were conducted on a FEI-Tecnai 12 electron microscope operated at 100 kv and a JEOL JEM 2010 electron microscopy operated at 200 kv, respectively. Fourier transform infrared (FTIR) spectra are recorded using an FTIR analyzer (Nicolet Avatar 360 FT-IR). N2 adsorption-desorption isotherms were measured by using a Gemini-2360 analyzer (Micromeritics Co., USA) at the testing temperature of 77 K. Brunauer-Emmett-Teller (BET) was used to calculate the surface area. The pore-size distributions were derived from the desorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) method. The total pore volumes, Vp, were estimated from the amount adsorbed at a relative pressure of P/P0 = 0.95. 2.4 Electrochemical measurements The electrochemical performance was tested by assembling CR2025 coin cells in a glove box filled with ultra-pure argon, using lithium metal as a counter anode. The working electrode was composed of active material (SCT), conductive material (acetylene black) and binder (PVDF) at a weight ratio of 8:1:1. A micro-porous polypropylene film (Celgard 2400) was elected as a separator, and a 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) was used as the electrolyte. The galvanostatic charge-discharge was conducted with a Battery Tester (Neware, Shenzhen, China). Cyclic voltammetry (CV) tests were performed on a CHI660C Electrochemical Workstation over the potential range of 0.05-3.0 V at a scanning rate of 0.2 mV s-1.
3. Results and discussion 3.1 Formation of mesoporous SnO2@C@TiO2 nanochains Scheme 1 illustrates the schematic process for the synthesis of mesoporous SnO2@C@TiO2 nanochains. First, SnO2@C nanochains were harvested by a facile hydrothermal. Dozens of SnO2 nanoparticles with diameters of 2-6 nm assemble into ca. 50 nm sized nanoclusters as the core, and then uniformly coated with a 6-12 nm glucose-derived carbon-rich polysaccharide (GCP) to form ideal core-shell nanostructures. Importantly, these SnO2@C nanochains were constructed into unique nanochains by linear self-assembly. It is well known that GCP contains plentiful hydroxyl groups.[39] Thus, TiO2, hydrolyzed from tetrabutyl titanate slowly at a low temperature, can be monodispersed on the surface of SnO2@C nanochains. Finally, to improve crystallinity, the resulting samples were calcined at a relatively low temperature under argon atmosphere. The nanochains structure can be well preserved due to their strong structure interaction and the mild reaction conditions. This
geometry has several advantages and is favorable for the superior electrochemical performance as described below: Firstly, the SnO2 nanoparticles can partially accommodate the huge volume change to slow down capacity declines with the assistance of core-cell configuration. Secondly, except for being as an effective physical buffering layer for volume variation, the carbon layer can also provide the electrical conductivity to the system. Thirdly, the TiO2 layer can further strengthen the buffering layer, leading to double-decker protection. Fourthly, the one-dimensional nanochains can shorten the pathway of the electron transfer effectively [38]. The FTIR spectra of SnO2@C are shown in Fig 1a. The absorption peaks at about 3402 and 1651 cm-1 could be ascribed to the O-H vibration of the absorbed water in the sample. The bands at 1079 and 952 cm-1 are confirmed as surface hydroxyl groups of the carbon layer [1]. The absorption peaks at 2918 and 1401 cm-1 are attributed to the C-H stretching vibration. The peak at 617 cm-1 corresponding to the Sn-O-Sn vibration is observed at low wavenumber region [40]. Thus, this indicated that there is high density of functional groups on the surface of SnO2@C and they acted as active sites for reaction of the TiO2 coating. Surface area of the SnO2@C nanochains was characterized by nitrogen adsorption and desorption isotherms (Figure 1b). The surface area of the SnO2@C nanochains is 10.46 m2 g-1 and the low surface area is attributed to the chaining of the SnO2@C nanobeads. In addition, the average pore diameter is about 56.7 nm, including two main pore sizes distributed in 2.3 nm (derived from carbon) and 92.5 nm (derived from SnO2 core and carbon shell) which provide the coating space for the TiO2. Transmission electron microscopy (TEM) image of SnO2@C nanochains is shown in Fig 2a. It clearly demonstrates that SnO2@C nanochains are linked by nanobeads which are the composites of carbon depositing on SnO2 core. The SnO2@C core-shell nanobeads are about 80 nm. The morphology and microstructure of the SCT-250 were investigated by TEM (Fig 2b). It clearly reveals that the SnO2@C nanochains are coated by TiO2 perfectly with the appearance of nanochains morphology due to the mild reaction. In Fig 2c, it can be seen that the later dimension of the nanobeads are about 90 nm. It is also confirmed that the SnO2 core is assembled by the SnO2 nanoparticles with diameters of 2-6 nm and coated by the carbon layer (6-12nm) and the thin TiO2 layer (about 8nm). Furthermore, the thin TiO2 layer is assembled by the TiO2 nanoparticles with diameters of 2-6 nm. The HRTEM image distinctly shows the lattice fringes with spacing of 0.35 nm which correspond to the (101) planes of TiO2. Also, the SnO2 core shows clear lattice fringes for the
(110) planes tetragonal SnO2 with spacings of 0.33 nm in another HRTEM image. As expected, nitrogen adsorption/desorption measurement for the Pore size distributions of SCT-250 are given in the Fig 3. The result shows that the SCT-250 possess a high BET specific surface area of 69.9 m2 g-1, which is far larger than the SnO2@C nanochains, due to the outer TiO2 shell. The N2 adsorption-desorption isotherm demonstrate an IV type isotherm. The pore-size distribution calculated by the BJH method is presented in Fig 3e (inset). The relatively narrow pore-size distribution at 3.0 nm is attributed to the TiO2 layer assembled by the TiO2 nanoparticles. It indicates that SCT-250 has a mesoporous structure and high specific surface area which can provide a large number of active sites for Li+ intercalation/deintercalation and improve the capacity [11]. The composition of the as-prepared products was characterized by X-ray diffraction (XRD) (Fig 4). The XRD patterns exhibits the characteristic diffraction peaks, which can be perfectly indexed to anatase TiO2 (JCPDS card no. 21-1272) and tetragonal rutile structure of SnO2 (JCPDS card no. 41-1445). It is well known that the diffraction peak of the (101) planes of TiO2 is closed to the diffraction peak of the (110) planes of SnO2. The diffraction peak of SCT-50 located at 2θ=25.2º (the (101) planes of TiO2) covers up 2θ=26.6º (the (110) planes of SnO2). Also, the low intensity diffraction peak located at 2θ=33.9º is identified as the (101) planes of SnO2. The intensity of characteristic diffraction peaks of SnO2 increases with the concentration of SnO2@C nanochains. The diffraction peak of SCT-450 located at 2θ=26.6º (the (110) planes of SnO2) covers up 2θ=25.2º (the (101) planes of TiO2). TEM images of SCT-50, SCT-150, SCT-350 and SCT-450 are shown in Fig 5. As shown in the image of SCT-50 (Fig 5a), there are numerous TiO2 nanoparticles coated on the surface of the SnO2@C nanochains. It is clear that the size of the TiO2 nanoparticle is ca. 6 nm. The structure of SnO2@C core-shell is not able to identify due to the high concentration of TiO2. With the increasing concentration of SnO2@C nanochains, the TiO2 layer become thinner and the SnO2 core and TiO2 shell can be seen. The TiO2 shell thickness of SCT-150 is about 30 nm, which is 4 times of SCT-250. However, segments of the carbon shell can’t be coated by TiO2 when the mass content of SnO2@C nanochains increase to 350 mg and 450 mg. Then, the specific surface area would decrease which will finally deteriorate the capacity performance. Thus, It is important that the advisable concentration of SnO2@C nanochains is necessary for the perfect structure of core-shell-shell. The initial discharge/charge voltage profiles of the intermediate SnO2@C and as-prepared SCT
samples are illustrated in Fig 6a. The cycling tests were conducted at a current density of 100 mA g -1 between 0.05 and 3.0 V. It is remarkable that the initial discharge capacity of SnO2@C is higher than those of SCT samples, and SCT-350 (1071 mAh g-1) is also higher than SCT-50 (605 mAh g-1), SCT-150 (617 mAh g-1), SCT-250 (807 mAh g-1) and SCT-450 (981 mAh g-1). It is obvious that the initial discharge capacity increases with the increase of concentration of SnO2@C nanochains except for SCT-450. And the initial discharge capacity of SCT-450 is close to SCT-250, because SCT-450 has the low surface area as mentioned in the TEM image discussion. From the profiles of SCT-50, the characteristic discharge and charge voltage platforms of TiO2 occur at 1.75 V and 2 V, respectively[41]. Then, the region which is below 1.75 V is the characteristic of SnO2 in the discharge profile. When the concentration of SnO2@C nanochains increases, the capacities are mainly provide by SnO2. The cyclic performances of the intermediate SnO2@C and SCT samples are shown in Fig 6b. The discharge capacities of SCT-50, SCT-150, SCT-250, SCT-350 and SCT-450 decrease to 253 mAh g-1, 298 mAh g-1, 408 mAh g-1, 465 mAh g-1 and 387 mAh g-1 after 1st cycle, respectively. The irreversible capacities are attributed to the irreversible reaction, such as the formation of SEI. However, all of the products express a decent cycling stability and have a marginal capacity fading in the initial several cycles. SCT-50, SCT-150, SCT-250, SCT-350 and SCT-450 remain 196 mAh g-1, 277 mAh g-1, 369 mAh g-1, 285 mAh g-1 and 278 mAh g-1 after 100 cycles, respectively. The well cycling stability of SCT is attributed to the mesoporous TiO2 layer and the double-decker protection. There is also an interesting phenomenon shown in the curves that the reverible capacity of SCT-150, SCT-250, SCT-350 and SCT-450 decrease slightly in its initial 20 cycles, and then increase significantly. The capacity decrease is attributed to the pulverization of original aggregation of SnO2 during the Li+ intercalation/deintercalation process. After the pulverization, the particle size of SnO2 become smaller and smaller and the particles attaches to the carbon shell more tightly [14, 42]. The reversible reaction of the electrode will be facilitated by the decrease of particle size. Thus, the discharge capacity increases after 20 cycles. However, the curve of SCT-50 is so flat due to the low concentration of SnO2@C nanochains and this further confirms the above analysis. Compared with the intermediate SnO2@C and others SCT samples, it is clear that SCT-250 plays a better cyclic performance than others SCT samples. In order to investigate the influence of the carbon shell and mesoporous TiO2 shell on the capacity and cyclability. Fig.6(d ) displays the TEM image of SCT-250 after 50 charge/discharge cycles. It is observed that the
structural integrity of the SnO2@C@TiO2 core-shell nanostructure is still retained during electrochemical cycling. It is also obvious that critical evidence for the improved electrochemical performance of SCT has been confirmed due to the physical buffer function of carbon shells and TiO2 shell to prevent the large volume change of SnO2 anodes. The rate performance of SCT is given in Fig 6(c). As seen in Fig 6c, the SCT-250 exhibited an excellent rate performance. The discharge capacities are 350 mAh g-1, 220 mAh g-1, 165 mAh g-1, 124 mAh g-1 and 92 mAh g-1 at the current density of 100 mA g-1, 200 mA g-1, 400 mA g-1, 800 mA g-1 and 1600 mA g-1, respectively. Even at the high current density, the capacities maintain steadily due to the mesoporous TiO2 layer. Moreover, the capacity recovers to 340 mAh g-1 when the current density returns to 100 mA g-1. More interestingly, when the current density increases, the SCT-50, SCT-150 and SCT-250, which have the complete TiO2 coating layer, express a better performance than SCT-350 and SCT-450. These give further evidences that the TiO2 play a significant role in the rate performance. And the capacities of as-prepared products can be recovered when the current density returns to 100 mA g-1. The capacities in every current density are so steady and have no capacity fading. In order to understand the Li+ intercalation/deintercalation process, cyclic voltammetry (CV) of the SCT-250 was conducted at 0.2 mV s-1 between 0.05 and 3.0V (Fig 8). Obviously, the first cycle is distinguished from the others. Consistent with the reports of TiO2 [41], two prominent current peaks are observed at about 1.72 V and 2.11V during cathodic and anodic sweeps, respectively. The cathodic peak at 1.72 V marks the formation of orthorhombic Li0.5TiO2. And the anodic peak at 2.11 V marks the process from Li0.5TiO2 to TiO2 which corresponds to the initial charge and discharge curves. Compared with the first curve, there are two weak peaks occurred at 0.44 V and 0.73 V in the second one which indicates that the electrochemical reaction of Li+ and SnO2 begins at the second cycle. This result shows that only the electrochemical reaction of Li+ and TiO2 takes place at the beginning and the pathway to SnO2 core has not been opened. With the deep discharge/charge process, the pathway to SnO2 core begins to be opened. After three cycles, the reduction/oxidation peaks of SnO2 begin to be stable and have small shift. The reduction/oxidation peaks at about 0.26 V and 0.62 V are attributed to Li+ alloying/dealloying with SnO2 in the third and fourth cycles. Furthermore, the decent symmetry of the reduction/oxidation peak pairs shows the high reversibility of the intercalation/deintercalation reaction.
4. Conclusions In conclusion, we design a new type of mesoporous SnO2@C@TiO2 core-shell-shell nanochains in order to overcome the issue of SnO2 such as huge volume change and poor electrical conductivity. The SCT-250 exhibits the prefect structure and excellent electrochemical performance with delivering a high initial capacity of 807 mAh g-1 and maintaining 369 mAh g-1 after 100 cycles at the current density of 100 mA g-1. The outstanding stable cycling and well rate performance suggest that SnO2@C@TiO2 core-shell-shell nanochains are very promising as anode material for lithium-ion batteries.
Acknowledgement This research was supported financially by the Guangdong Natural Science Foundation (No. 9151064201000039),
the
Guangdong
Science
and
Technology
Planning
Project
(No.
2009B010900025), the National Natural Science Foundation of China (Nos. 51003034 and 21173088), the Key Academic Program of the 3rd phase ‘211 Project’ (No. 2009B010100001), the President of South China Agricultural University (No. K09140), the State Key Laboratory of Motor Vehicle Biofuel Technology (No. 2013025)，and the Air Force Office of Scientific Research (No. FA9550-12-1-0159).
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Figure Captions
Scheme 1. The formation process of mesoporous SnO2@C@TiO2 nanochains.
Figure 1. (a) FTIR spectra of SnO2@C nanochains, (b) Nitrogen isotherm adsorption-desorption curves and Pore size distributions of the of SnO2@C nanochains.
Figure 2. (a) The TEM image of SnO2@C nanochains, (b, c) TEM images, (d) HRTEM and Enlarged HRTEM (inset) images of mesoporous SnO2@C@TiO2 nanochains.
Figure 3. Nitrogen isotherm adsorption-desorption curves and Pore size distributions of the of mesoporous SnO2@C@TiO2 nanochains.
Figure 4. XRD patterns of as-prepared SCT materials.
Figure 5. The TEM images of (a) SCT-50, (b) SCT-150, (c) SCT-350 and (d) SCT-450.
Figure 6. (a)Initial charge and discharge curves, (b) cycling performances of as-prepared SCT anode materials, (c)Rate performances of as-prepared SCT anode materials and (d) TEM image of SCT-250 after 50 charge/discharge cycles.
Figure 7. Cyclic voltammetry curves of SCT-250 at 0.2 mV s-1 between 0.05 and 3.0V.
S0013-4686(15)30645-9 http://dx.doi.org/doi:10.1016/j.electacta.2015.10.062 EA 25865
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
5-6-2015 3-10-2015 13-10-2015
Please cite this article as: Guoen Luo, Weijian Liu, Songshan Zeng, Congcong Zhang, Xiaoyuan Yu, Yueping Fang, Luyi Sun, Hierarchal mesoporous SnO2@C@TiO2 nanochains for anode material of lithium-ion batteries with excellent cycling stability, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.10.062 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Hierarchal mesoporous SnO2@C@TiO2 nanochains for anode material of lithium-ion batteries with excellent cycling stability Guoen Luoa† ,Weijian Liua†, Songshan Zengb, Congcong Zhang a, Xiaoyuan Yua* a
b**
[email protected], Yueping Fang , Luyi Sun
[email protected]
a
Institute of Biomaterial, College of Materials and Energy, South China Agricultural University,
Guangzhou 510642, China b
Department of Chemical & Biomolecular Engineering and Polymer Program, Institute of Materials
Science, University of Connecticut, Storrs, Connecticut 06269, United States *Corresponding authors: Tel.: (+86) 20-8528 0323; fax: (+86) 20-8528 5026. *Corresponding authors.(L. Y. Sun) Tel: 860-486-6895; Fax: 860-486-4745. †
These authors contribute equally.
Highlights
Hierarchal mesoporous SnO2@C@TiO2 nanochains (SCT) were prepared via a facile hydrothermal method.
TiO2 shell and carbon shell act as double-decker protection to buffer the volume change of SnO2.
Cells with the Hierarchal mesoporous SCT nanochains anode showed excellent cycle performances.
ABSTRACT A new type of hierarchal mesoporous SnO2@C@TiO2 nanochains (SCT) as anode material for the lithium-ion batteries was prepared via a facile hydrothermal and kinetics-controlled coating method. The TiO2 shell and carbon shell act as double-decker protection to buffer the huge volume change of SnO2 during the charged and discharged process. The resulting batteries equipped with this novel anode material showed excellent cycling stability. SCT-250 with 8 nm TiO2 external shell was demonstrated for optimal electrochemical performances. A high initial capacity of 807 mAh g-1 was achieved at the current density of 100 mA g-1 and maintained 369 mAh g-1 after 100 cycles. The outstanding stable cycling and well rate performance suggest that SnO2@C@TiO2 core-shell-shell nanochains have great potential to be applied as anode material for lithium-ion batteries.
KEYWORDS: mesoporous; SnO2@C@TiO2; nanochains; Lithium-ion batteries; anode material
1. Introduction The increasing huge demand for energy and the depletion of oil energy have stimulated intensive researches on the next-generation energy sources [1, 2]. Lithium-ion batteries are considered as one of the most promising candidates to address to this challenge in the 21st century [3-5]. Comparing with the traditional electronic equipment, the portable electronic devices (such as mobile phones, tablet computer, digital cameras, and so on) and electric vehicles require such higher power density [6]. However, the commercial graphite materials (the theoretical capacity of 372 mAh g-1 [7]) are difficult to satisfy the increasing harsh demands. Therefore, it is highly urgent to develop the next generation anode material with high theoretical capacity. Recently, tin-based materials especially tin dioxides (SnO2) receive particular attentions due to their high theoretical capacity (786 mAh g-1), environmental friendliness and abundance [8-10]. However, the commercialization of SnO2 is retrained by two major issues: (1) the huge volume change (about 300%) during discharge/charge process [11-13], leading to disintegration and pulverization of the electrodes with quick capacity fading; (2) the poor electrical conductivity constrains the migration of electrons [14]. To mitigate these problems, many research papers were focused on controlling the nanosized [15] and constructing various nanostructures (such as hollow spheres [16, 17], nanoboxes [18], nanotubes [19-21], and so on of SnO2. Also, the composites of SnO2 and carbon (carbon [22-25], carbon nanotube [26, 27] and graphene [28-30]) show brilliant electrochemical performances, which not only enhance the conductivity but also buffer the volume change. In addition, Jiang [31] propose that advanced metal oxide-base hybrid nanostructure is superior to single-phase oxides, which can synergistically enhance the electrical conductivity and mechanical stability. Thus, a number of metal oxides can be applied to prepare SnO2-base hybrid to enhance the electrochemical performance. Among these, TiO2 is an excellent candidate due to its structural stability and chemical safety during cycling, which only has small volume change throughout the lithium insertion and extraction process [32-34]. At the same time, TiO2 can show an outstanding rate performance [35, 36]. In this research, we report a new type hierarchal mesoporous SnO2@C@TiO2 core-shell-shell nanochains (SCT), prepared by a facile hydrothermal and kinetics-controlled coating method. Here, SnO2 was coated by carbon via a facile hydrothermal in order to providing a buffering
layer and a perfect electrical conductivity. Then, TiO2 nanoparticles were monodispersed on the surface of SnO2@C core-shell nanochains, which could protect the structure of SnO2@C core-shell nanochains effectively. Generally speaking, the TiO2 shell and the carbon shell play a double-decker protection to buffer the huge volume change of SnO2. The mesoporous SnO2@C@TiO2 core-shell-shell nanochains exhibit excellent electrochemical performances.
2. Experimental 2.1 Preparation of SnO2@C core-shell nanochains The SnO2@C core-shell nanochains were synthesized via a facile hydrothermal method based on our previous reports [37]. 0.324 g of Na2SnO3·3H2O and 6 g glucose were dissolved in 40 mL of deionized water. The solution was transferred into a 50 mL Teflon-lined stainless steel autoclave, sealed, and kept at 180 ℃ for 4h in an oven prior to cooling down to room temperature. The product was harvested by centrifugation, washed with deionized water and ethanol, and dried at 60 ℃ for 12 h. 2.2 Preparation of mesoporous SnO2@C@TiO2 nanochains The mesoporous SnO2@C@TiO2 nanochains were obtained via a kinetics-controlled coating method [38]. For the synthesis, appropriate amount of SnO2@C nanochains were first sonicated in 400 mL of anhydrous ethanol. Ammonia solution (1.2 mL, 28 wt%) was added to the mixture solution, stirred for 15 minutes. Afterward, 3.0 mL tetrabutyl titanate was added dropwise, and the reaction was allowed to proceed for 24 h at 45 ℃ under continuous mechanical stirring. The product was centrifuged, and washed with deionized water and ethanol for three times, respectively. Subsequently, the obtained powders were dried at 60 ℃ overnight. Finally, the resulting samples were calcined at 500 ℃ for 3 h under low flow rate argon atmosphere. The uniform mesoporous SnO2@C@TiO2 nanochains are prepared with variable amount of SnO2@C (mass of SnO2@C is 50, 150, 250, 350 and 450 mg were investigated, respectively.), which are denoted as SCT-50, SCT-150, SCT-250, SCT-350 and SCT-450 (mass content of TiO2 is about 93.3, 82.3, 73.7, 66.7 and 60.8% ,respectively). 2.3 Materials characterization The as-prepared samples are characterized with X-ray powder diffraction (Rigaku, D/max 2500v/pc). Transmission electron microscopy (TEM) and high resolution transmission electron
microscopy (HRTEM) measurements were conducted on a FEI-Tecnai 12 electron microscope operated at 100 kv and a JEOL JEM 2010 electron microscopy operated at 200 kv, respectively. Fourier transform infrared (FTIR) spectra are recorded using an FTIR analyzer (Nicolet Avatar 360 FT-IR). N2 adsorption-desorption isotherms were measured by using a Gemini-2360 analyzer (Micromeritics Co., USA) at the testing temperature of 77 K. Brunauer-Emmett-Teller (BET) was used to calculate the surface area. The pore-size distributions were derived from the desorption branches of the isotherms using the Barrett-Joyner-Halenda (BJH) method. The total pore volumes, Vp, were estimated from the amount adsorbed at a relative pressure of P/P0 = 0.95. 2.4 Electrochemical measurements The electrochemical performance was tested by assembling CR2025 coin cells in a glove box filled with ultra-pure argon, using lithium metal as a counter anode. The working electrode was composed of active material (SCT), conductive material (acetylene black) and binder (PVDF) at a weight ratio of 8:1:1. A micro-porous polypropylene film (Celgard 2400) was elected as a separator, and a 1 M solution of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 by volume) was used as the electrolyte. The galvanostatic charge-discharge was conducted with a Battery Tester (Neware, Shenzhen, China). Cyclic voltammetry (CV) tests were performed on a CHI660C Electrochemical Workstation over the potential range of 0.05-3.0 V at a scanning rate of 0.2 mV s-1.
3. Results and discussion 3.1 Formation of mesoporous SnO2@C@TiO2 nanochains Scheme 1 illustrates the schematic process for the synthesis of mesoporous SnO2@C@TiO2 nanochains. First, SnO2@C nanochains were harvested by a facile hydrothermal. Dozens of SnO2 nanoparticles with diameters of 2-6 nm assemble into ca. 50 nm sized nanoclusters as the core, and then uniformly coated with a 6-12 nm glucose-derived carbon-rich polysaccharide (GCP) to form ideal core-shell nanostructures. Importantly, these SnO2@C nanochains were constructed into unique nanochains by linear self-assembly. It is well known that GCP contains plentiful hydroxyl groups.[39] Thus, TiO2, hydrolyzed from tetrabutyl titanate slowly at a low temperature, can be monodispersed on the surface of SnO2@C nanochains. Finally, to improve crystallinity, the resulting samples were calcined at a relatively low temperature under argon atmosphere. The nanochains structure can be well preserved due to their strong structure interaction and the mild reaction conditions. This
geometry has several advantages and is favorable for the superior electrochemical performance as described below: Firstly, the SnO2 nanoparticles can partially accommodate the huge volume change to slow down capacity declines with the assistance of core-cell configuration. Secondly, except for being as an effective physical buffering layer for volume variation, the carbon layer can also provide the electrical conductivity to the system. Thirdly, the TiO2 layer can further strengthen the buffering layer, leading to double-decker protection. Fourthly, the one-dimensional nanochains can shorten the pathway of the electron transfer effectively [38]. The FTIR spectra of SnO2@C are shown in Fig 1a. The absorption peaks at about 3402 and 1651 cm-1 could be ascribed to the O-H vibration of the absorbed water in the sample. The bands at 1079 and 952 cm-1 are confirmed as surface hydroxyl groups of the carbon layer [1]. The absorption peaks at 2918 and 1401 cm-1 are attributed to the C-H stretching vibration. The peak at 617 cm-1 corresponding to the Sn-O-Sn vibration is observed at low wavenumber region [40]. Thus, this indicated that there is high density of functional groups on the surface of SnO2@C and they acted as active sites for reaction of the TiO2 coating. Surface area of the SnO2@C nanochains was characterized by nitrogen adsorption and desorption isotherms (Figure 1b). The surface area of the SnO2@C nanochains is 10.46 m2 g-1 and the low surface area is attributed to the chaining of the SnO2@C nanobeads. In addition, the average pore diameter is about 56.7 nm, including two main pore sizes distributed in 2.3 nm (derived from carbon) and 92.5 nm (derived from SnO2 core and carbon shell) which provide the coating space for the TiO2. Transmission electron microscopy (TEM) image of SnO2@C nanochains is shown in Fig 2a. It clearly demonstrates that SnO2@C nanochains are linked by nanobeads which are the composites of carbon depositing on SnO2 core. The SnO2@C core-shell nanobeads are about 80 nm. The morphology and microstructure of the SCT-250 were investigated by TEM (Fig 2b). It clearly reveals that the SnO2@C nanochains are coated by TiO2 perfectly with the appearance of nanochains morphology due to the mild reaction. In Fig 2c, it can be seen that the later dimension of the nanobeads are about 90 nm. It is also confirmed that the SnO2 core is assembled by the SnO2 nanoparticles with diameters of 2-6 nm and coated by the carbon layer (6-12nm) and the thin TiO2 layer (about 8nm). Furthermore, the thin TiO2 layer is assembled by the TiO2 nanoparticles with diameters of 2-6 nm. The HRTEM image distinctly shows the lattice fringes with spacing of 0.35 nm which correspond to the (101) planes of TiO2. Also, the SnO2 core shows clear lattice fringes for the
(110) planes tetragonal SnO2 with spacings of 0.33 nm in another HRTEM image. As expected, nitrogen adsorption/desorption measurement for the Pore size distributions of SCT-250 are given in the Fig 3. The result shows that the SCT-250 possess a high BET specific surface area of 69.9 m2 g-1, which is far larger than the SnO2@C nanochains, due to the outer TiO2 shell. The N2 adsorption-desorption isotherm demonstrate an IV type isotherm. The pore-size distribution calculated by the BJH method is presented in Fig 3e (inset). The relatively narrow pore-size distribution at 3.0 nm is attributed to the TiO2 layer assembled by the TiO2 nanoparticles. It indicates that SCT-250 has a mesoporous structure and high specific surface area which can provide a large number of active sites for Li+ intercalation/deintercalation and improve the capacity [11]. The composition of the as-prepared products was characterized by X-ray diffraction (XRD) (Fig 4). The XRD patterns exhibits the characteristic diffraction peaks, which can be perfectly indexed to anatase TiO2 (JCPDS card no. 21-1272) and tetragonal rutile structure of SnO2 (JCPDS card no. 41-1445). It is well known that the diffraction peak of the (101) planes of TiO2 is closed to the diffraction peak of the (110) planes of SnO2. The diffraction peak of SCT-50 located at 2θ=25.2º (the (101) planes of TiO2) covers up 2θ=26.6º (the (110) planes of SnO2). Also, the low intensity diffraction peak located at 2θ=33.9º is identified as the (101) planes of SnO2. The intensity of characteristic diffraction peaks of SnO2 increases with the concentration of SnO2@C nanochains. The diffraction peak of SCT-450 located at 2θ=26.6º (the (110) planes of SnO2) covers up 2θ=25.2º (the (101) planes of TiO2). TEM images of SCT-50, SCT-150, SCT-350 and SCT-450 are shown in Fig 5. As shown in the image of SCT-50 (Fig 5a), there are numerous TiO2 nanoparticles coated on the surface of the SnO2@C nanochains. It is clear that the size of the TiO2 nanoparticle is ca. 6 nm. The structure of SnO2@C core-shell is not able to identify due to the high concentration of TiO2. With the increasing concentration of SnO2@C nanochains, the TiO2 layer become thinner and the SnO2 core and TiO2 shell can be seen. The TiO2 shell thickness of SCT-150 is about 30 nm, which is 4 times of SCT-250. However, segments of the carbon shell can’t be coated by TiO2 when the mass content of SnO2@C nanochains increase to 350 mg and 450 mg. Then, the specific surface area would decrease which will finally deteriorate the capacity performance. Thus, It is important that the advisable concentration of SnO2@C nanochains is necessary for the perfect structure of core-shell-shell. The initial discharge/charge voltage profiles of the intermediate SnO2@C and as-prepared SCT
samples are illustrated in Fig 6a. The cycling tests were conducted at a current density of 100 mA g -1 between 0.05 and 3.0 V. It is remarkable that the initial discharge capacity of SnO2@C is higher than those of SCT samples, and SCT-350 (1071 mAh g-1) is also higher than SCT-50 (605 mAh g-1), SCT-150 (617 mAh g-1), SCT-250 (807 mAh g-1) and SCT-450 (981 mAh g-1). It is obvious that the initial discharge capacity increases with the increase of concentration of SnO2@C nanochains except for SCT-450. And the initial discharge capacity of SCT-450 is close to SCT-250, because SCT-450 has the low surface area as mentioned in the TEM image discussion. From the profiles of SCT-50, the characteristic discharge and charge voltage platforms of TiO2 occur at 1.75 V and 2 V, respectively[41]. Then, the region which is below 1.75 V is the characteristic of SnO2 in the discharge profile. When the concentration of SnO2@C nanochains increases, the capacities are mainly provide by SnO2. The cyclic performances of the intermediate SnO2@C and SCT samples are shown in Fig 6b. The discharge capacities of SCT-50, SCT-150, SCT-250, SCT-350 and SCT-450 decrease to 253 mAh g-1, 298 mAh g-1, 408 mAh g-1, 465 mAh g-1 and 387 mAh g-1 after 1st cycle, respectively. The irreversible capacities are attributed to the irreversible reaction, such as the formation of SEI. However, all of the products express a decent cycling stability and have a marginal capacity fading in the initial several cycles. SCT-50, SCT-150, SCT-250, SCT-350 and SCT-450 remain 196 mAh g-1, 277 mAh g-1, 369 mAh g-1, 285 mAh g-1 and 278 mAh g-1 after 100 cycles, respectively. The well cycling stability of SCT is attributed to the mesoporous TiO2 layer and the double-decker protection. There is also an interesting phenomenon shown in the curves that the reverible capacity of SCT-150, SCT-250, SCT-350 and SCT-450 decrease slightly in its initial 20 cycles, and then increase significantly. The capacity decrease is attributed to the pulverization of original aggregation of SnO2 during the Li+ intercalation/deintercalation process. After the pulverization, the particle size of SnO2 become smaller and smaller and the particles attaches to the carbon shell more tightly [14, 42]. The reversible reaction of the electrode will be facilitated by the decrease of particle size. Thus, the discharge capacity increases after 20 cycles. However, the curve of SCT-50 is so flat due to the low concentration of SnO2@C nanochains and this further confirms the above analysis. Compared with the intermediate SnO2@C and others SCT samples, it is clear that SCT-250 plays a better cyclic performance than others SCT samples. In order to investigate the influence of the carbon shell and mesoporous TiO2 shell on the capacity and cyclability. Fig.6(d ) displays the TEM image of SCT-250 after 50 charge/discharge cycles. It is observed that the
structural integrity of the SnO2@C@TiO2 core-shell nanostructure is still retained during electrochemical cycling. It is also obvious that critical evidence for the improved electrochemical performance of SCT has been confirmed due to the physical buffer function of carbon shells and TiO2 shell to prevent the large volume change of SnO2 anodes. The rate performance of SCT is given in Fig 6(c). As seen in Fig 6c, the SCT-250 exhibited an excellent rate performance. The discharge capacities are 350 mAh g-1, 220 mAh g-1, 165 mAh g-1, 124 mAh g-1 and 92 mAh g-1 at the current density of 100 mA g-1, 200 mA g-1, 400 mA g-1, 800 mA g-1 and 1600 mA g-1, respectively. Even at the high current density, the capacities maintain steadily due to the mesoporous TiO2 layer. Moreover, the capacity recovers to 340 mAh g-1 when the current density returns to 100 mA g-1. More interestingly, when the current density increases, the SCT-50, SCT-150 and SCT-250, which have the complete TiO2 coating layer, express a better performance than SCT-350 and SCT-450. These give further evidences that the TiO2 play a significant role in the rate performance. And the capacities of as-prepared products can be recovered when the current density returns to 100 mA g-1. The capacities in every current density are so steady and have no capacity fading. In order to understand the Li+ intercalation/deintercalation process, cyclic voltammetry (CV) of the SCT-250 was conducted at 0.2 mV s-1 between 0.05 and 3.0V (Fig 8). Obviously, the first cycle is distinguished from the others. Consistent with the reports of TiO2 [41], two prominent current peaks are observed at about 1.72 V and 2.11V during cathodic and anodic sweeps, respectively. The cathodic peak at 1.72 V marks the formation of orthorhombic Li0.5TiO2. And the anodic peak at 2.11 V marks the process from Li0.5TiO2 to TiO2 which corresponds to the initial charge and discharge curves. Compared with the first curve, there are two weak peaks occurred at 0.44 V and 0.73 V in the second one which indicates that the electrochemical reaction of Li+ and SnO2 begins at the second cycle. This result shows that only the electrochemical reaction of Li+ and TiO2 takes place at the beginning and the pathway to SnO2 core has not been opened. With the deep discharge/charge process, the pathway to SnO2 core begins to be opened. After three cycles, the reduction/oxidation peaks of SnO2 begin to be stable and have small shift. The reduction/oxidation peaks at about 0.26 V and 0.62 V are attributed to Li+ alloying/dealloying with SnO2 in the third and fourth cycles. Furthermore, the decent symmetry of the reduction/oxidation peak pairs shows the high reversibility of the intercalation/deintercalation reaction.
4. Conclusions In conclusion, we design a new type of mesoporous SnO2@C@TiO2 core-shell-shell nanochains in order to overcome the issue of SnO2 such as huge volume change and poor electrical conductivity. The SCT-250 exhibits the prefect structure and excellent electrochemical performance with delivering a high initial capacity of 807 mAh g-1 and maintaining 369 mAh g-1 after 100 cycles at the current density of 100 mA g-1. The outstanding stable cycling and well rate performance suggest that SnO2@C@TiO2 core-shell-shell nanochains are very promising as anode material for lithium-ion batteries.
Acknowledgement This research was supported financially by the Guangdong Natural Science Foundation (No. 9151064201000039),
the
Guangdong
Science
and
Technology
Planning
Project
(No.
2009B010900025), the National Natural Science Foundation of China (Nos. 51003034 and 21173088), the Key Academic Program of the 3rd phase ‘211 Project’ (No. 2009B010100001), the President of South China Agricultural University (No. K09140), the State Key Laboratory of Motor Vehicle Biofuel Technology (No. 2013025)，and the Air Force Office of Scientific Research (No. FA9550-12-1-0159).
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Figure Captions
Scheme 1. The formation process of mesoporous SnO2@C@TiO2 nanochains.
Figure 1. (a) FTIR spectra of SnO2@C nanochains, (b) Nitrogen isotherm adsorption-desorption curves and Pore size distributions of the of SnO2@C nanochains.
Figure 2. (a) The TEM image of SnO2@C nanochains, (b, c) TEM images, (d) HRTEM and Enlarged HRTEM (inset) images of mesoporous SnO2@C@TiO2 nanochains.
Figure 3. Nitrogen isotherm adsorption-desorption curves and Pore size distributions of the of mesoporous SnO2@C@TiO2 nanochains.
Figure 4. XRD patterns of as-prepared SCT materials.
Figure 5. The TEM images of (a) SCT-50, (b) SCT-150, (c) SCT-350 and (d) SCT-450.
Figure 6. (a)Initial charge and discharge curves, (b) cycling performances of as-prepared SCT anode materials, (c)Rate performances of as-prepared SCT anode materials and (d) TEM image of SCT-250 after 50 charge/discharge cycles.
Figure 7. Cyclic voltammetry curves of SCT-250 at 0.2 mV s-1 between 0.05 and 3.0V.