Accepted Manuscript SnO2/TiO2 nanocomposites embedded in porous carbon as a superior anode material for lithium-ion batteries Xiao Shi, Sitong Liu, Bo Tang, Xieji Lin, Ang Li, Xiaohong Chen, Jisheng Zhou, Zhaokun Ma, Huaihe Song PII: DOI: Reference:
S1385-8947(17)31311-6 http://dx.doi.org/10.1016/j.cej.2017.07.164 CEJ 17430
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
4 May 2017 7 July 2017 28 July 2017
Please cite this article as: X. Shi, S. Liu, B. Tang, X. Lin, A. Li, X. Chen, J. Zhou, Z. Ma, H. Song, SnO2/TiO2 nanocomposites embedded in porous carbon as a superior anode material for lithium-ion batteries, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.07.164
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SnO2/TiO2 nanocomposites embedded in porous carbon as a superior anode material for lithium-ion batteries
Xiao Shi, Sitong Liu, Bo Tang, Xieji Lin, Ang Li, Xiaohong Chen, Jisheng Zhou, Zhaokun Ma, Huaihe Song* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, 100029, Beijing (China) *Corresponding author: [email protected]
Abstract With the objective of improving the capacity and cycling stability of SnO2-based anode materials, a new material of SnO2/TiO2 nanocomposites dispersed at porous carbon was synthesized via a route of absorption and carbonization by using Ti-MOF (MIL-125) as both titanium and carbon sources and SnCl2 as tin source. The composite shows micro-/nano-scale combined configuration: a monodisperse sub-micro cylinder structure formed by well-dispersed SnO2 and TiO2 nanoparticles in amorphous carbon matrix. Benefiting from the synergistic effect of TiO2 nanoparticle and amorphous carbon on maintaining the robust architecture as well as the uniform and stable nano-sized particles, the composite exhibits excellent performance as anode material for lithium-ion batteries: a high specific capacity of 1177 mA h g-1 and 88.7 % capacity retention after 100 cycles at 100 mA h g-1, and a stable capacity of 487 at high current density of 2 A g-1. Keywords: SnO2/[email protected]
nanocomposites, configuration, lithium-ion batteries
1. Introduction Nowadays lithium-ion batteries (LIBs) are the most widely used secondary batteries and the materials used in LIBs is in continuous evolution. Gradually, conventional graphite anode material cannot meet the requirements of high power and high energy densities of advanced energy storage systems such as in electric vehicles and wind power storage [1-4]. Among all the substitutes for traditional graphite anodes, SnO2 has drawn much attention because of its high theoretical specific capacity (1493mA h g-1 for nano SnO2), low discharge potential and abundant sources [5-7]. However, the large volume change during the charge-discharge process is the main obstacle that affects the cycling stability and blocks its applications . Generally, several techniques can be used to overcome this hurdle. Carbon coating is the most common used [9-11]. Carbon as a buffer layer can accommodate the strain caused by lithium insertion/desertion and improve the electrode/electrolyte wettability 1 ·
as well. In addition with carbon coating, the fabrication of SnO2 with special morphologies especially with nanostructures is another popular approach to improve the property of SnO2 anode [12-15]. In this route, nanostructured SnO2 is expected to reduce the strain caused by volume change and thus to reduce the cracking and pulverization during cycle process. For example, Zhou  used Fe2O3 as sacrificial templates and fabricated hollow submicroboxes composed of untra-small SnO2. After carbon coating by using dopamine as carbon source, the material exhibited a capacity of 491 mA h g-1 after 100 cycles at a current density of 500 mA g-1. Besides above techniques, incorporation of other constituents to form compound or composite with SnO2 is another choice [7, 17-21]. In this field, TiO2 is believed to be an ideal additive because of its low volume change during cycling, high reversibility, and better safety, although its theoretical capacity (178 mA h g-1) is relatively low [22-26]. As an example, Zhang  reported [email protected]
double-shell nanotubes on carbon cloth material, and found that this material performed a reversible capacity of 616 mA h g-1. Rational design of SnO2/TiO2/carbon composite materials is believed to be an effective way to improve the property, however, finding a simple way to fabricate well-dispersed and structure-stabilized material is still the facing problem. These years, metal–organic frameworks (MOFs) have attracted extensive attention because of their special porous structures and the corresponding wide applications such as in electrochemical fields [28-32]. Recently, Wang  used Cu-MOF (HKUST-1) as template to prepare [email protected]
nanocomposites. The well-dispersed SnO2 nanoparticles in porous carbon gave a stable capacity of 880 mA h g-1 at 100 mA g-1 after 200 cycles. But in that fabrication, the generation of SnO2 needs the subsequent oxidation of Sn-Cu alloy and the removing of inactive Cu. Here, in this work, we choose one type of Ti-MOF (MIL-125) as the TiO2 and carbon sources to prepare SnO2/TiO2/C composite anode materials by introducing SnCl2 to Ti-MOFs and further carbonization. The composite exhibits “Eccles Cake” morphology, that is, TiO2 nanoparticles along with SnO2 nanoparticles were uniformly embedded in porous carbon matrix. This special structure is beneficial for the electrochemical performance: SnO2 nanoparticles contributes the major capacity and TiO2 contributes some. Most significantly, TiO2 acts as a more stable material to isolate SnO2 and has a combined effect with carbon matrix to stabilize the structure. As a result, the composite as an anode material exhibits a stable capacity of 1045 mA h g-1 after 100 charge-discharge cycles, which is among the highest reported SnO2 based materials in LIBs. 2. Experimental 2.1. Materials preparation MIL-125 crystals were synthesized through solvothermal method . Typically, 2 g of terephthalic acid and 1.04 mL of tetra-n-butyl titanate were added into a mixed solution of anhydrous DMF (36 mL) and methanol (4 mL). After stirring for 20 min, the solution was transferred into a Teflon-sealed autoclave and kept at 150 ℃ for 24 h. After cooling to ambient temperature naturally, the white precipitates were collected by centrifugation and washed with DMF for 1 time and ethanol for 3 times 2 ·
followed by drying at 80℃ under vacuum for 4h to obtain MIL-125 white crystal particles. The obtained powders were heated at 350 ℃ under Ar for 1h to further remove the absorbed solvent and stabilize the structure. Then 100 mg of SnCl2·2H2O was dissolved in 200 µL of ethanol and dropped into 100 mg of the MIL-125 powders. The mixture was annealed at 180 ℃ under air for 2 h followed by heat treated at 550 ℃ for 4 h under Ar atmosphere to get SnO2/[email protected]
was obtained under the same condition except double the amount of SnCl2·2H2O to 200 mg. For comparison, [email protected]
was prepared without any SnCl2·2H2O added and bare SnO2 was prepared by heating SnCl2·2H2O under air condition at 550 ℃ for 2 h. 2.2 Characterization The structures of the materials were characterized by XRD (Rigaku D/max-2500B2+/PCX, Cu Kα, λ=1.54056 Å) and X-ray photoelectron spectroscopy (XPS , ESCALAB 250 ,Thermo Electron Corporation, Al Kα X-ray source,1486.6 eV). The field-emission scanning electron microscopy (FE-SEM, Hitachi S-4700) and transmission electron microscopy (TEM, Tecnai G2 F20, FEI) were used to investigate the morphology of the material. Thermogravimetric analysis (TGA, STA449C Jupiter, NETZSCH) was performed on air condition with a heating rate of 10 ℃/min. The pore structure was analyzed by nitrogen adsorption–desorption measurement (performed on Automated Physisorption and Chemisorption Analyzer, micromeritics ASAP 2020). 2.3 Electrochemical measurements The electrochemical measurements were tested in CR2025 coin-type cell with a Li metal counter electrode. The electrode was composed of active material, acetylene black and binder (polyvinylidene difluoride, PVDF with a mass ratio of 8:1:1. And 1 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate(1:1, V/V) was used as electrolyte. Cyclic voltammograms (CV) and electrochemical impedance spectroscopy (EIS) were performed on a ZAHNER ENNIUM electrochemical workstation. Galvanostatic charge/discharge was performed on Land CT2001A system in the voltage ranging from 0.01 to 3.0 V.
3. Results and discussion
Fig. 1 Schematic view of the fabrication of SnO2/TiO2 @C composites. 3 ·
Fig. 1 shows the formation process of the SnO2/TiO2 nanocomposites embedded in porous carbon. Using the absorption properties of MOFs (MIL-125), SnCl2·2H2O was introduced to the structure. After the following carbonization, SnO2 nanoparticles were generated in MIL-125 and the Ti ions in MIL-125 transform to TiO2 nanoparticles, and the organic ligands transform to carbon matrix simultaneously. Thus, in a simple absorption and carbonization process, the composite of SnO2/TiO2 nanoparticles in porous carbon (SnO2/[email protected]
) was synthesized.
Fig. 2 XRD patterns of different samples.
The XRD patterns of the samples are exhibited in Fig. 2. [email protected]
derived from the carbonization of pure MIL-125 and the diffraction peaks are indexed to phase of TiO2 (anatase). In SnO2/[email protected]
and SnO2/[email protected]
, the peaks of SnO2 are distinguished as well as the peaks of anatase TiO2. What’s more, the peaks of the SnO2 phase become stronger and sharper in SnO2/[email protected]
than that in SnO2/[email protected]
, indicating that the amount of SnO2 is greater. To determine the amounts of the components in each sample quantitatively, thermogravimetric analysis (TGA) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) are performed. From the TGA curves (Fig. S1), the main weight loss takes place in the temperature between 300 and 400℃, implying the decomposition of carbon layers and the remaining components are metal oxides. The TiO2 content in [email protected]
is calculated to be 59.9 %. ICP-AES can be used to determine the amount of SnO2. Combining the data of TGA and ICP-AES, the SnO2 and TiO2 contents are 21.5 %, 50.6 % in SnO2/[email protected]
and 41.2 %, 43.9 % in SnO2/[email protected]
Fig. 3 XPS survey spectra of samples (a), and the corresponding high-resolution XPS spectra of C 1s (b), Sn 3d (c) and Ti 2p (d) of SnO2/[email protected]
The chemical constitutions and element chemical states of the composites were further confirmed by XPS. As shown in Fig 3, the overall XPS spectrum detects the carbon, oxygen, tin, and titanium elements in both SnO2/[email protected]
and SnO2/[email protected]
The overlapped high-resolution C1s peak was fitted by four components (284.6 eV for C–C&C=C, 286.1 eV for C-O, 287.9 eV for C=O and 289.1 eV for C-O-C) . The high-resolution spectrum of Sn 3d of SnO2/[email protected]
has two species: Sn 3d5/2 (487.3 eV) and Sn 3d3/2 (495.7 eV) which are in tune with SnO2 . And the binding energy of Ti 2p1/2 (465.2 eV) and Ti 2p3/2 (459.4 eV) are in accordance with TiO2 .
Fig. 4 SEM images (a) and TEM image (inset of (a)) of [email protected]
; SEM images of 5 ·
(b,c); TEM image (d), HRTEM image (e) and SAED pattern (f) of SnO2/[email protected]
; STEM image (g), and corresponding elemental mapping images (h-k) of SnO2/[email protected]
The morphologies of the samples were investigated by SEM and TEM. Fig 4 (a-c) shows that both [email protected]
and SnO2/[email protected]
can maintain the tablet morphology of MIL-125. And the cylinder are uniformly distributed with the diameter of about 500 nm and thickness of 200 nm. The surface of SnO2/[email protected]
is not as smooth as [email protected]
as a result of the absorption of tin source before carbonization. From the TEM and HRTEM images, the inner structure of the tablet can be seen. In the carbon matrix, the nanoparticle was embedded in carbon matrix and the size has a narrow distribution with the mean diameter of 6.8 nm (Fig. S4). For further observation, the HRTEM was taken from the edge of the tablet (Fig. 4e). The obvious interplanar fringes indicates good crystalization. SnO2 and TiO2 nanoparticles can be distinguished according to different interplanar distance (0.352 nm for (100) crystal planes of anatase TiO2 and 0.335 nm for (100) crystal planes of SnO2), and the selected area electron diffraction pattern (SAED, Fig. 4f)) of SnO2/[email protected]
clearly shows spotted diffraction rings which confirms the coexistence of polycrystalline TiO2 and SnO2 with spotted diffraction rings belonging to them, which is consistent with the result of XRD. What separates them from each other is a thin layer of amorphous carbon. The high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) is shown in Fig. 4g. The corresponding element mapping (Fig. 4h-4k) and the linear sweep (Fig. S4) shows uniform distribution of C, O, Ti and Sn which reveals a uniform components over the entire composite tablet. The mono-dispersed nanocomposite can be ascribed to the uniform structure of Ti-MOF, which provides a confined space for the growth of SnO2. As a comparison, if heat-treating SnCl2·2H2O under air condition without Ti-MOF added, bare SnO2 with much bigger size and inhomogenous disperse was gotten (Fig. S2).
Fig. 5 (a) N2 adsorption-desorption isotherms of different samples and (b) Pore size distribution obtained by the NLDFT.
The N2 adsorption-desorption isotherms were performed on the obtained composites to investigate the porous structure which has a crucial influence on the electrochemical performance. The isotherms of all the samples showed slight hysteresis loops (Fig. 5a), which are typical for Type IV isotherms . According to the results, the surface areas of [email protected]
, SnO2/[email protected]
and SnO2/[email protected]
are 310.2 m2 g-1, 280.9 m2 g-1, 239.7 m2 g-1, respectively. And the pore volume decreases from 0.29 cm3/g ([email protected]
) to 0.19 cm3/g (SnO2/[email protected]
). The successive decreases of both specific surface area and pore volume can be ascribed to the addition of SnCl2 and the formation of SnO2. The SnO2 nanoparticles occupied some pores in MIL-125. It is noticed that the pore size distribution shows differences mainly in mesopore range (>2nm), that is SnO2-contained materials show narrow pore distribution in mesopore zone as shown in Fig. 5b, indicating the reduction of mesopores when tin source was added. This may be because micropores are too small for Sn source to permeate in and the growth of SnO2 is confined within mesopores. The MOFs (MIL-125) provides abundant and uniform pore space for the growth of SnO2 and at the same time the SnO2 was confined within a small range (<10nm, as shown in TEM images). Moreover, some pores in carbon matrix remained. A proper decrease in surface area could have a good effect in first coulombic efficiency and a certain amounts of mesopores remained provide abundant channels for electrolyte to improve the electronic performance. 7 ·
Fig. 6 Cyclic voltammograms of (a) [email protected]
, (b) SnO2/[email protected]
electrodes with scanning rate of 0.2 mV s−1 in the range of 0.01-3.0 V; charge-discharge curves of (c) TiO2 @C and (d) SnO2/[email protected]
at current density of 100 mA g-1.
To evaluate the electrochemical behavior of our materials, the composites were tested as electrodes in a half cell system using lithium metal as counter electrode. Cyclic voltammetry (CV) was performed at a scan rate of 200 µV/m between 0.01 and 3V. For [email protected]
(Fig. 6a), the first Li+ insertion into TiO2 results in the cathodic peak at 1.2 V and in the following scans, the cathodic peak at 1.5 V and anodic peak at 2.2 V can be ascribed to reversible Li+ insertion and extraction in TiO2 , respectively. As a comparison, besides the pair of peak at 1.7V/2.2V belonging to the insertion and exertion of Li+ with TiO2, more peaks can be observed in SnO2/[email protected]
(Fig 6b). In the cathodic scan, the peak at 0.8 V on the first cycle is ascribed to the reduction of SnO2 to Sn as well as the formation of SEI layer . The peak at 0.2 V indicates the alloying of Li with Sn which is a reversible reaction as ascribed in Eq. (2) and the corresponding anodic peak can be observed at 0.6 V . The reduction peak at 1.1 V and the oxidation peak at 1.25V is referenced to the partial reversible reaction described in Eq. (1) . 4 4 → 2 ↔ 0 4.4
As a correlative results of CV curves, Fig.6c and 6d show the charge–discharge voltage profiles of [email protected]
and SnO2/[email protected]
for the first 3 cycles at a current density of 100 mA g-1 with a voltage range of 0.01~3 V. It is obvious that SnO2/[email protected]
exhibits a more retarded plateau below 1 V as a result of SnO2 addition which contributes most of the capacity. In the first cycle, the [email protected]
delivered a discharge and charge capacities of 892 and 578 mA h g-1, giving an initial Coulombic efficiency of 64.2 %. As a comparision, SnO2/[email protected]
exhibits a more retarded plateau below 1 V because of addition of SnO2 that contributes most of the capacity. As a result, it shows a higher first specific capacity of 1755 mA h g-1 for discharge (Li insertion) and 1177 mA h g-1 for charge. Notably, the curves of the 2nd and 3rd cycle are almost overlapped in Fig. 6c and 6d, suggesting that both of the electrodes have good cycling stability. 8 ·
Fig. 7 (a) CV curves of SnO2/[email protected]
at different scan rates from 0.1~2.0 mV s-1 and (b) log-log plot of the peak currents at 0.5~0.7 V and 2.1~2.5 V at different scan rates.
As is known, SnO2 and TiO2 exhibit different electrochemical mechanisms: the capacity of SnO2 derives from the alloying/dealloying between Sn and Li but the capacity of TiO2 is from the insertion/extraction of Li ion in TiO2 crystal. Moreover, nano-sized particles may also provide some surface reaction and Li-storage capacity. For further exploration of the essence of the chare-discharge process of SnO2/[email protected]
, CV tests at different scan rates were carried out (Fig. 7a). For a redox reaction, the peak current obey the power law : (3) If the peak currents were plotted vs scan rate on a log-log scale, the slope of the fitted line is the value of b which reflects different charge storage mechanism. For the diffusion-limited process, b=0.5 and for capacitive behavior, b=1. Fig. 7b shows the b value of the peaks between 0.5 V and 0.7 V that belongs to dealloying process of LixSn is 0.62. And the b value is 0.70 for the peaks of 2.1~2.5 V which can be ascribed to the extraction of Li+ from TiO2. That is to say the storage mechanism exhibits a mixture of diffusion-limited and capacitive process. This is because, a large fraction of lithium storage can be achieved for nanosized particles in the surface layer as a fast Faradaic pseudocapacitive process . This process is believed to have a contribution to improve the rate capacity of materials. The cycling performances of different materials were evaluated by galvanostatic charge-discharge (Fig. 8a). For bare SnO2, the capacity decreases continuously through the cycling as a result of volume effect. Other electrodes exhibit stable cycling performances at the current density of 100 mA g-1. The [email protected]
gives a capacity of 387 mA h g-1 after 100 charge-discharge cycles showing good cycle stability. Intriguingly, when tin source added, the material can show a capacity as high as 1045 mA h g-1 (SnO2/[email protected]
). It can be roughly estimated that the SnO2 attributes 813 mA h g-1 and others (TiO2 and carbon) attribute 232 mA h g-1 considering the SnO2 contents (41.2 %) calculated from ICP-AES. It is clear that SnO2 provides the major capacity, but the role of TiO2 is non-negligible. It is noteworthy to highlight that the entire structure maintained well after cycling which benefits for the superior stability of the electronic performance. The SEM image of SnO2/[email protected]
electrode (Fig. 9b) after 100 cycles shows the tablet integrity maintains well. And from the TEM image (Fig. 9c), the SEI layer formed on the surface of tablet can be observed and the interior nano particles are still uniformly dispersed. Fig. 9d shows the XRD patterns of the electrode before and after cycles. 9 ·
Besides the Ni peaks belonging to the nickel current collector, the TiO2 peaks after cycling remain well showing the hold of TiO2 active part in SnO2/[email protected]
during cycling. The peaks of SnO2 disappear and new peaks belonging to tin appear, which can be explained by the lithium storage of SnO2 described in Eq. (1) and Eq. (2). TiO2 plays an important role in maintaining the structure and stabilizing the capacity. This is because that TiO2, as a low volume change (<4%) anode material, can act as a “matrix” during the LixSn alloying formation , and more importantly, TiO2 may strongly combine onto carbon [44, 45] and separate SnO2 nanoparticles from each other to avoid or reduce the possible aggression and crumbling caused by volume change of SnO2 during charge-discharge. Moreover, superior to other buffer metals (Co, Fe, Cu…) used in tin-based electrodes , TiO2 has a substantial capacity. These features give the material high capacity and excellent stability.
Fig. 8 Cycling performance and Coulombic efficiency at 100 mA g-1 (a), rate performance (b) of different samples, and cycling performance of different samples at high current density of 2 A g-1 (c).
Fig. 9 SEM images of the SnO2/TiO2 @C-2 electrode before (a) and after 100 charge/discharge cycles (b), TEM image image of SnO2/[email protected]
after 100 cycles (c) and XRD patterns of the SnO2/[email protected]
electrode before and after cycles (d).
In addition, SnO2/[email protected]
shows outstanding rate performances. As the current density increases, the specific capacity decreases somewhat, but the capacity can keep stable at each current stage and shows a considerable capacity of 487 mA h g-1 at a high current rate of 2 A g-1. And when the cycling current density was again reduced back to 100 mA g-1, the reversible capacity can change back to the original value, demonstrating a stable rate performance. Moreover, the performance of the electrode during long cycles at high current density was also evaluated. Fig. 8c gives the performance of the electrodes at 2 A g-1. Compared with other reported SnO2/TiO2 composite based materials or SnO2 based materials (Table S1), our material exhibits competitive performance. It is reasonable to believe that the improved rate capacity of our materials could be ascribed to the unique micro-/nano-scale combined configuration. Firstly, micro-sized tablet give the material enhanced strain-accommodation capability and structural stability [47, 48] and nano-sized particles not only shorten the pathway of lithium ions and provide capacitive process . Secondly, TiO2 owns negligible volume change (<4%) during cycling and moreover, TiO2 may strongly bind with carbon thus it has an “anchor” effect to hold the structure integrity and isolate SnO2 which has big volume change during lithium insertion/extraction . Thirdly, uniform porous carbon layer not only provides higher electronic conductivity, buffers the volume change of SnO2 during electrochemical reactions effectively but also improves the electrolyte wettability of the electrolyte.
Fig. 10 AC impedance spectra of the electrodes after 10 cycles and Randles equivalent circuit for the EIS spectra (inlet).
For a better insight into the activity of the materials, electrochemical impedance spectroscopy (EIS) measurements were performed on the electrodes after 10 charge-discharge cycles (Fig. 10). The straight line with a slope near to 45° in low frequency region is associate with the lithium ions diffusion in the bulk. An approximate semicircle in the mid-frequency region corresponds to charge transfer resistance of the electrode and can be quantified by the fitted value Rct . It is clear that the SnO2/[email protected]
possesses the smallest semicircle radius and the Rct is calculated to be 18.3 Ω compared to 35.2 Ω for [email protected]
and 20.4 Ω for SnO2/[email protected]
The EIS results also validate the addition of SnO2 increases the electrochemistry activity of the material and result in the best performance of the SnO2/[email protected]
4. Conclusions In summary, using the absorption property of MOFs, we have successfully synthesized a composite composed of SnO2/TiO2 nanoparticles embedded in porous carbon. The anode material shows high stable capacity and superior rate performance which can be ascribed to the synergistic effect of TiO2 nanoparticle and amorphous carbon on buffering the volume change of SnO2 and maintaining the architecture integrity. Hopefully, this design gives a new choice to improve the stability of SnO2-based material or other metal oxide materials. Acknowledgments This work was supported by the National Natural Science Foundation of China (51672021). References:  V. Etacheri, R. Marom, R. Elazari, G. Salitra, D. Aurbach, Challenges in the development of advanced Li-ion batteries: a review, Energy & Environmental Science 4 (2011) 3243-3262. 12 ·
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SnO2/TiO2 nanocomposites embedded in porous carbon were in situ fabricated. This composite shows a stable micro-/nano-scale combined configuration. This composite exhibits a superior performance when used as anode for LIBs.