Improved electrochemical performance of SnO2-mesoporous carbon hybrid as a negative electrode for lithium ion battery applications.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 6630

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Improved electrochemical performance of SnO2–mesoporous carbon hybrid as a negative electrode for lithium ion battery applications† N. R. Srinivasan,a Sagar Mitra*bc and Rajdip Bandyopadhyaya*ac To utilize the high specific capacity of SnO2 as an anode material in lithium-ion batteries, one has to overcome its poor cycling performance and rate capability, which result from large volume expansion (B300%) of SnO2 during charging–discharging cycles. Hence, to accommodate the volume change during cycling, SnO2 nanoparticles of 6 nm diameter were synthesized specifically only on the outer surface of the mesopores, present within mesoporous carbon (CMK-5) particles, resulting in an effective buffering layer. To that end, the synthesis process first involves the formation of 3.5 nm SnO2 nanoparticles inside the mesopores of mesoporous silica (SBA-15), the latter being used as a template subsequently to obtain SnO2–CMK-5 hybrid particles. SnO2–CMK-5 exhibits superior rate capabilities, e.g. after 30 cycles, a specific discharge capacity of 598 mA h g1, at a current density of 178 mA g1. Electrochemical impedance

Received 24th October 2013, Accepted 5th February 2014

spectroscopy reveals that the SnO2–CMK-5 electrode undergoes a significant reduction in solid–electrolyte

DOI: 10.1039/c3cp54492c

lithium ions, all these in comparison to an electrode made of only SnO2 nanoparticles. This enhances the potential of using the SnO2–CMK-5 hybrid as a negative electrode, in terms of improved discharge capacity

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and cycling stability, compared to other electrodes, such as only SnO2 or only CMK-5.

interfacial and charge transfer resistances, with a simultaneous increase in the diffusion coefficient of

Introduction Storage of electrical energy is of utmost importance as an alternative to counter the increased demand for fossil fuels. Lithium-ion batteries (LIBs) are considered amongst the most promising electrochemical storage devices due to their high energy and power densities. This makes them suitable for applications in portable electronic devices, electric vehicles, etc.1,2 Energy density (W h kg1) is the product of reversible specific capacity (A h kg1) and operating voltage (V), where the former depends on the particular redox reactions and concentration of lithium ions in the active material. On the other hand, power density is the product of current and operating voltage, which determines the rate of output energy transfer, depending on impedance of the electrode. Currently, graphite is used as an a

Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. E-mail: [email protected]; Fax: +91 22 2572 6895; Tel: +91 22 2576 7209 b Department of Energy science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India. E-mail: [email protected]; Fax: +91 22 2576 4890; Tel: +91 22 2576 7849 c National center for Photovoltaic Research and Education, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cp54492c

6630 | Phys. Chem. Chem. Phys., 2014, 16, 6630--6640

anode material in commercially available LIBs. It has a specific capacity of 372 mA h g1, due to intercalation of one lithium atom per six carbon atoms (LiC6). However, it has a poor rate performance due to the low diffusion coefficient of Li+ ions in graphite.3 Hence, graphite is suitable for portable electronic devices, but is restricted in its use for electric and hybrid vehicles, where a high discharge rate is required. Consequently, current research is targeted to replace the conventional graphite electrode with non-graphitic carbon, for substantially enhancing power density and cycling stability. In this regard, non-graphitic materials like carbon nanotubes, amorphous carbon, ordered porous carbon, etc. have been investigated.1,4,5 Among all non-graphitic materials, the family of ordered mesoporous carbon materials (CMK-3 and CMK-5) has many advantages, including high reversible specific capacity, a shorter Li+ ion diffusion path and faster electron transport.6 Of these two, CMK-5 consists of a hexagonal arrangement of interconnected mesopores with a thin pore wall (thickness B3 nm), whereas CMK-3 possesses a hexagonal arrangement of interconnected solid carbon rods (diameter B7 nm). This ordered nature of the porous structure in the CMK family of materials can buffer against the large volume change of the electrode, which occurs during charging and discharging steps. An effective buffering leads to enhanced cycling stability. On the other hand, the CMK family has a high specific surface area,

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leading to the formation of a higher surface area solid–electrolyte interface (SEI) layer at the reaction sites. This results in undesirable, irreversible consumption of Li+ ions.7 However, carbon based materials with both high reversible capacity (4800 mA h g1) and high initial columbic efficiency (460%) have not been reported so far. As a result, it cannot be used alone as an anode material for high power LIB applications. To overcome these limitations of pure carbon based materials, metals like Sn and Si and metal oxides like SnO2 and V2O5 have been incorporated with carbon supports and tested for potentially better electrochemical performance.4,8–18 SnO2 is one of the promising anode materials, due to its high specific capacity (780 mA h g1). It reversibly transforms into a lithium based alloy during lithiation, and gives a high specific capacity due to the release of 4.4 electrons per Sn atom, during the redox reaction. However, in spite of its high specific capacity, SnO2 undergoes a large volume expansion (B300%) during Li+ ion insertion–extraction steps. The resulting stress induced by this volume expansion causes cracking and crumbling of the electrode, which leads to loss of electrical contact and poor cycling stability.19 Considering the above, embedding of SnO2 nanoparticles inside the porous CMK particles is an effective way to accommodate the strain arising during cycling. This approach not only integrates the advantages of the carbon support and SnO2 nanoparticles, but also minimizes the disadvantages of both these materials. Since SnO2 undergoes a large volume expansion during charging–discharging cycles, the location and size of the SnO2 nanoparticles in the pores of the CMK particles are of special importance in enhancing the specific capacity and cycling stability. At the same time, due to the hydrophobic nature of CMK, effective embedding of SnO2 nanoparticles inside the CMK particles is a challenging task. For the SnO2–CMK family of electrodes reported so far,4,20–23 different synthesis approaches have led to the formation of SnO2 nanoparticles in different regions of the CMK particle, such as: (i) both inside and on the outer surface of mesopores, present within a CMK-5 particle (Fig. S1a, ESI†), or (ii) even as aggregates of particles, in isolation, in addition to both inside and on the outer surface of mesopores of CMK-5 particles (Fig. S1b, ESI†), or (iii) on the outer surface of the carbon rod present within a CMK-3 particle (not shown in Fig. S1, ESI†). In previous reports, since SnO2 nanoparticles were synthesized on pre-formed CMK particles, the nanoparticles also formed on the external surface of CMK, resulting in their aggregation, with uncontrolled primary particle size distribution. This could have affected the overall electrochemical performance of the electrodes. To overcome this difficulty associated with the reported SnO2–CMK family of electrodes, the new strategy pursued in the present work comprises of first synthesizing SnO2 nanoparticles within mesopores of SBA-15, and then using the SnO2– SBA-15 hybrid as a template to form SnO2–CMK-5. This results in pre-formed SnO2 nanoparticles, finally getting located on the outer surface of mesopores present within CMK-5 (scheme shown in Fig. 1 or Fig. S1c, ESI†). To date, the approach


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followed in this work to form nanoparticles only on the outer surface of mesopores has not been previously demonstrated. Within the CMK family, CMK-5 was selected ahead of CMK-3, because the former has a higher specific surface area (B1800 m2 g1) and pore volume (2 cm3 g1), compared to CMK-3 (B1000 m2 g1, 1.2 cm3 g1). The current work also aims to understand alteration of Li+ ion transport, due to the addition of porous carbon in an electrode containing SnO2 nanoparticles.

Experimental Material preparation SnO2–CMK-5 was synthesized according to the scheme given in Fig. 1. Three stages are involved in its formation. The first stage is SBA-15 synthesis, the second stage is impregnation of SnO2 particles inside the SBA-15, and the third stage is carbon layer formation on the internal and external surfaces of SnO2–SBA-15, with subsequent removal of silica. SBA-15 and SnO2–SBA-15 were synthesized according to our previous work, except with an increase in SnCl4 precursor concentration (0.1 M).24 Typically, SBA-15 was made using Pluronic 123, a triblock copolymer, as the template, and tetraethylorthosilicate (TEOS) as the silica source.25 The molar ratio of reactants used was P123 : HCl : H2O : TEOS = 1 : 278 : 11519 : 60. The next stage was to impregnate SnCl4 inside the pore with subsequent reduction by aqueous NaBH4. 100 ml of 0.1 M aqueous SnCl4 solution was added to 0.5 g of calcined SBA-15 and stirred for 12 h. After impregnation of SnCl4, 100 ml of 0.2 M aqueous solution of NaBH4 was added drop by drop to the solution containing SnCl4 and stirred for 30 min. The resultant solid product was filtered, washed with water, and kept at 100 1C for oxidation of tin, in the presence of air. The final stage was preparation of a carbon layer, both inside and outside of SnO2–SBA-15, by using furfuryl alcohol as a carbon precursor and oxalic acid as a catalyst. In this step, 0.5 ml of furfuryl alcohol and 2.5 mg of oxalic acid were dissolved in 1.5 ml of trimethyl benzene. This solution was incorporated into SnO2–SBA-15 by the impregnation technique. Afterwards, it was heated at 60 1C for 4 h to induce polymerization of furfuryl alcohol and later heated at 150 1C for 6 h to remove unreacted furfuryl alcohol from the hybrid. This mixture was heated at 850 1C for 4 h under a nitrogen atmosphere to carbonize the polyfurfuryl alcohol. The resulting hybrid was treated with hydrofluoric acid solution (20%) for 30 min to remove silica. An aqueous HF solution is capable of dissolving SBA-15 as well as SnO2. However, even though the SnO2–CMK-5 hybrid sample contains nanostructured SnO2, the percentage removal of SnO2 is very less in comparison to silica (for details, see ESI†). Finally, the SnO2–CMK-5 hybrid was washed with water and dried in the presence of air at 100 1C. Electrode preparation The working electrode slurry was made by a mixture of active material (SnO2–CMK-5) with acetylene black, and polyvinylidene

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Fig. 1 Schematic of the synthesis steps for the SnO2–CMK-5 hybrid. (a) SnO2–SBA-15 particle. (b) An enlarged view of the cross section of hexagonally arranged pores in (a). (c) Polyfurfuryl alcohol coated SBA-15. (d) SnO2–CMK-5 particle. (e) An enlarged view of a single micropore, as it appears connected between two mesopores in (d) and (f).

fluoride (in the ratio 8 : 1 : 1) in N-methyl pyrrolidone solvent. The slurry was coated on the copper foil collector and dried at 100 1C for 12 h under vacuum. The coin cell (2032) was made of lithium metal as the reference and counter electrode, with 1 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (LP-30, Merck-Germany) as the electrolyte, and a glass microfiber filter (Whatman GD/F) as the separator. The whole cell assembly was performed in an argon filled glove box (Labstar, mBraun, Germany) under 1 ppm O2 atmosphere. The amount of active material used in the electrode is between 1.5 and 2.5 mg. Characterization Small-angle X-ray scattering (SAXS) measurements were carried out using the SAXSess Anton Paar. Particle morphology and composition were analyzed using a field emission scanning electron microscope (FE-SEM) (Model: JSM-7600F), attached with an energy dispersive X-ray (EDX) spectrometer, and also using a field emission gun-transmission electron microscope (FEG-TEM) (Model: JEM-2100F). X-ray diffraction (XRD) was done using a Philips X’Pert Pro diffractometer, with CuKa radiation. Thermogravimetric analysis (TGA) was carried out using a TG-DTA-DSC apparatus (Netzsch) with a heating rate of 10 K min1, in the presence of air. Nitrogen and hydrogen sorption studies were performed using a Micromeritics ASAP 2020 system. To determine electrical conductivity,


samples were made in the form of pellets and their conductivities were measured at room temperature (23 1C), using a four point probe technique. Electrochemical studies were carried out using the ARBIN battery testing system and a Bio-Logic VMP3 battery tester. For specific capacity measurement, the cell was charged and discharged galvanostatically between 0.002 and 3.0 V vs. Li/Li+, at different current densities at 20 1C. Cyclic voltammetry (CV) was performed between 0.002 and 3.0 V vs. Li/Li+, recorded at a scan rate of 0.1 mV s1. For electrochemical impedance spectroscopy (EIS) studies, a small sinusoidal voltage perturbation (5 mV) is applied between working and counter electrodes over the frequency range of 1 MHz–0.1 mHz at 20 1C.

Results and discussion Structure and morphology of the SnO2–CMK-5 hybrid In the SnO2–SBA-15 hybrid, SBA-15 was modified in order to form Si–O–Sn throughout the surface, which eventually helps in achieving dispersed SnO2 particles. The uniformly dispersed SnO2 particles on the surface of SBA-15 were used as a template to form SnO2–CMK-5. Therefore, the present approach avoids acid treatment to make the CMK-5 surface hydrophilic, and effective dispersion of SnO2 nanoparticles is also achieved. The pore arrangement in CMK-5 is reverse of that of the hexagonal ordered structure of SBA-15. To identify this, SAXS was


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Fig. 2 (a) SAXS of SnO2–SBA-15 and SnO2–CMK-5 and (b) wide angle XRD of SBA-15, CMK-5, SnO2–SBA-15 and SnO2–CMK-5.

performed for both SnO2–SBA-15 and SnO2–CMK-5 hybrids. Fig. 2a shows three peaks in both samples, which can be indexed as (100), (110) and (200) reflections, confirming long range hexagonal order of pores.25 This clearly indicates that SnO2– CMK-5 remains porous even after removal of silica. The scattering pattern of SnO2–CMK-5 confirms that SnO2–SBA-15 is coated with a carbon layer and forms tube like structures in SnO2–CMK-5. For SnO2–SBA-15, the intensity of the (100) peak is higher than the (110) peak, whereas for SnO2–CMK-5, the relative intensities of these two peaks are reversed. This change is due to the scattering interference between hollow cylindrical channels and the spacers interconnecting adjacent solid cylindrical rods.26,27 SAXS (Fig. 2a) and pore-size distribution (bimodal distribution in Fig. 3b) of SnO2–CMK-5 provide the evidence for the formation of CMK-5 (i.e. hollow cylindrical channels), and preclude the existence of CMK-3 (i.e. solid cylindrical rods) in the sample.26 Wide angle XRD patterns of SBA-15, CMK-5, SnO2–SBA-15 and SnO2–CMK-5 hybrids are shown in Fig. 2b. Wide angle XRD


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Fig. 3 (a) Nitrogen adsorption–desorption isotherms of SnO2–SBA-15 and SnO2–CMK-5; (b) pore size distribution of SnO2–SBA-15 and SnO2–CMK-5.

patterns of CMK-5 and SBA-15 show a broad peak between 221 and 241, which relates to the amorphous nature of carbon and silica matrices. The well-resolved diffraction peaks shown in Fig. 2b can be indexed to the tetragonal structure of crystalline SnO2 (JCPDS No. 41-1445). The presence of broad peaks in the XRD pattern indicates the presence of only small crystallites. The mean size of the SnO2 crystallites based on the (110) peak is 3.1 nm and 6.5 nm for SnO2–SBA-15 and SnO2–CMK-5, respectively, as calculated using Scherrer’s formula. The increase in the particle size of SnO2 in SnO2–CMK-5 is due to the growth of SnO2 nanoparticles in the empty space between the ordered mesopores during the carbonization process. The porous nature of the hybrids was identified using nitrogen (N2) adsorption and desorption measurements at 77 K. N2 adsorption–desorption isotherms and pore size distribution of SnO2–SBA-15 and SnO2–CMK-5 hybrids are shown in Fig. 3a and b, respectively. Both are type IV isotherms with


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Table 1

Paper Structural properties of SnO2–SBA-15 and SnO2–CMK-5

Sample

Specific surface area (m2 g1)

Pore diameter (nm)

Pore volume (cm3 g1)

SnO2 particle diameter from XRD (nm)

SnO2–SBA-15 SnO2–CMK-5

218 1358

6.3 2.5, 5.0

0.30 1.56

3.1 6.5

hysteresis loops, which confirm the presence of mesopores. Hence, capillary condensation is observed in the relative pressure range of 0.45–0.8, indicating that the mesoporous structure has been retained in all the samples. The measured SnO2 particle size (3.1 nm) from XRD and pore diameter (6.3 nm) from N2 adsorption–desorption isotherms of SnO2–SBA-15 indicate that furfuryl alcohol can easily diffuse into the pores of SnO2–SBA-15 and undergo polymerization reaction. In the case of SnO2–CMK-5, two pore-spaces are created. The first is a cylindrical mesopore (mean diameter 2.5 nm by BET-BJH), which arises due to partial formation of a carbon layer inside the pore of SBA-15. The second is empty space between the ordered mesopores (mean diameter B5 nm by BET-BJH), which arises due to removal of the pore wall of SBA-15. The bimodal pore system indicates the presence of tubular structure as well as thin carbon walls.25 The pore diameter and specific surface area values are summarized in Table 1, showing an increase in the latter from 218 m2 g1 for SnO2–SBA-15, to 1358 m2 g1 for SnO2–CMK-5. This increase is attributed mainly to the presence of micropores in the carbon matrix. As a result of high temperature treatment, the SnO2 particle size increases from 3.1 nm to 6.5 nm in SnO2– CMK-5 (based on XRD data). Since SnO2 nanoparticles are partially embedded in the carbon wall, SnO2 nanoparticles of 6.5 nm can be accommodated in the empty space between the ordered mesopores in CMK-5. From the TGA of SnO2–CMK-5 (Fig. S2, ESI†), the amount of SnO2 in the hybrid was found to be 41 wt%. Fig. 4a and b show SEM images of SnO2–SBA-15 and SnO2– CMK-5, respectively. It is observed that SnO2–CMK-5 has the same fiber like morphology (fiber being made of many axially aligned, cylindrical SnO2–CMK-5 particles), as in the case of SnO2–SBA-15. This confirms the formation of mesoporous carbon from furfuryl alcohol throughout the templating SnO2–SBA-15 particle. The EDX spectrum of SnO2–SBA-15 (Fig. 4a, inset, and Fig. S3b, ESI†) shows the presence of Si, Sn and O, while the EDX spectrum of SnO2–CMK-5 (Fig. 4b, inset and Fig. S3d, ESI†) shows the presence of C, Sn and O, indicating the absence of silicon in the SnO2–CMK-5 hybrid. Fig. 4c and d show bright field and dark field TEM images of SnO2–SBA-15, respectively. These images reconfirm the presence of well ordered cylindrical channels. The dark and white spots on the surface of the inner walls of the SBA-15 particles, in Fig. 4c and d, respectively, show the presence of SnO2 nanoparticles (Fig. S3a and b, ESI†). The mean diameter of SnO2 particles in the SnO2–SBA-15 hybrid (Fig. 4c and similar images) is 3.5 nm, which is close to 3.1 nm estimated from XRD measurement (Fig. 2b). Fig. 4e shows a dark field TEM image of SnO2–CMK-5, indicating the presence of cylindrical channels and also suggesting that SnO2 particles are attached only on the


external surface of mesopores of CMK-5. The schematic illustration (Fig. 1) is drawn based on the dark field TEM image (Fig. 4e) and logical steps followed in the synthesis protocol. The measured mean diameter of SnO2 particles from Fig. 4e is 6.0 nm, which is close to 6.5 nm estimated from XRD measurement (Fig. 2b). To compare the performance of the SnO2–CMK-5 electrode with a control-electrode sample made of only SnO2 nanoparticles, the latter were separately synthesized (Fig. 4f) inside the pores of SBA-15, so as to obtain similar SnO2 particles, as in SnO2–CMK-5 samples. Particles shown in Fig. 4f were obtained by removing silica from SBA-15, using a hydrofluoric acid solution. The mean particle diameter of SnO2 obtained from SnO2–SBA-15 is 6.8 nm (Fig. 4f), which is close to the diameter (6.0 nm) obtained for SnO2–CMK-5 (Fig. 4e). Electrochemical performance The as-prepared materials have been tested as negative electrodes against Li+ ions for their electrochemical performance using CV, charge–discharge processes and electrochemical impedance spectroscopy (EIS). Cyclic voltammetry CV has been performed to evaluate the potential at which charging and discharging reactions take place. Fig. 5a shows the cyclic voltammograms of SnO2–CMK-5 in the potential range of 0.002–3 V, recorded at a scan rate of 0.1 mV s1. The CV curves almost overlap from the second cycle onwards, indicating cycling stability. In the cathodic scan, the peak at 1.36 V is related to the reduction of SnO2 to Sn and Li2O formation. The peak at 0.66 V is ascribed to the decomposition of solvent, leading to the formation of the SEI layer.28 The peak at around 0.1 V can be attributed to alloying of LixSn.19 There is a decrease of cathodic current between the first and second cycles, which can be attributed to the formation of the SEI layer and the irreversible reaction.22 Two peaks are observed during the anodic scan, one at 0.6 V, which indicates dealloying of LixSn. The other peak at about 1.2 V is related to oxidation of Sn to SnO2. The latter oxidation peak is rarely observed in the literature.29,30 The oxidation of Sn contributes to higher specific capacity than theoretically predicted (780 mA h g1). However, there is no significant change in the CV profile after the first cycle, indicating the reversible nature of the process, which persists in subsequent cycles. Cycling performance The voltage–specific capacity profiles of control electrodes (only SnO2 and only CMK-5) and the SnO2–CMK-5 hybrid electrode for the 1st, 2nd, 10th, and 30th cycles at a current density of 178 mA g1 are shown in Fig. 5b–d, respectively. These show the importance of combining SnO2 with CMK-5. For this purpose, a bare SnO2 electrode is made up of SnO2 nanoparticles of 6.8 nm diameter,31 which is almost the same as the size obtained in SnO2–CMK-5. The voltage plateau of the SnO2–CMK-5 electrode (Fig. 5d) is shorter in comparison to the bare SnO2 electrode (Fig. 5b). This may be due to the presence of carbon in the former, which causes a change in the rate of the LixSn alloy formation.32 During the first discharge cycle, a plateau appears at around


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Fig. 4 (a) and (b) SEM images of SnO2–SBA-15 and SnO2–CMK-5 hybrids, respectively (inset shows respective EDX spectra). (c) Bright field TEM image of the SnO2–SBA-15 hybrid. (d) and (e) dark field TEM images of SnO2–SBA-15 and SnO2–CMK-5 hybrids, respectively. (f) TEM image of SnO2 nanoparticle.

1.0 V in all electrodes, which is due to the conversion reaction between SnO2 and Li, leading to the formation of Li2O and the SEI layer. The plateau region in the first discharge curve disappears in the second cycle, suggesting that there could be an irreversible formation of Li2O and the SEI layer, or a small quantity of Li2O and the SEI layer. The specific capacity obtained for SnO2–CMK-5, pure SnO2, and CMK-5 electrodes in the first discharge is 1574 mA h g1, 1364 mA h g1 and 1311 mA h g1,


respectively, which is higher than the expected capacity for an electrode with only SnO2 (1138 mA h g1, 6.4 mol of Li); the latter value is calculated based on the following equations.33,34 1/2SnO2 + 2Li+ + 2e - 1/2Sn + Li2O Sn + xLi+ + xe 2 LixSn (0 r x r 4.4) C + xLi+ + 2e 2 LixC6 (0.5 r x r 3)


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Fig. 5

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(a) Cyclic voltammogram of SnO2–CMK-5. (b–d) Voltage–specific capacity profiles of SnO2, CMK-5 and SnO2–CMK-5, respectively.

The excess capacity of 173 mA h g1 obtained for the SnO2 electrode is due to SEI formation in the first discharge.34 In contrast, the SnO2–CMK-5 electrode has an excess capacity of 436 mA h g1, resulting from the combined effects of (i) specific capacity contribution from the CMK-5 support, (ii) SEI formation and (iii) reduced aggregation of SnO2 nanoparticles inside the CMK-5 particle. In the case of SnO2–CMK-5 electrode, however, aggregation of SnO2 nanoparticles is probably less due to the presence of CMK-5, resulting in a higher specific capacity (1576 mA h g1) than that of the bare SnO2 electrode (1311 mA h g1). The initial columbic efficiency of SnO2, CMK-5 and SnO2–CMK-5 electrodes is 40.1%, 29.0% and 55.8%, respectively (Fig. S4, ESI†). The reversible discharge capacity after 30 cycles at a current density of 178 mA g1 for SnO2, CMK-5 and SnO2–CMK-5 electrodes is 42, 218 and 598 mA h g1, respectively. The discharge capacity rapidly decreases after the first cycle onwards for SnO2, CMK-5 and SnO2–CMK-5 electrodes. In the case of SnO2, specific capacity after the first cycle decreases rapidly due to its large volume expansion (B300%). The CMK-5 electrode shows cycling stability,


but the specific capacity of CMK-5 also drastically reduces, due to two reasons – firstly, SEI formation on the active sites of CMK-5, and secondly, volume expansion during cycling, causing formation of more pores,18 leading to high irreversibility. During the cycling process, the CMK-5 structure is slightly altered due to volume expansion, whereby existing micropores in the CMK-5 matrix become bigger, or some new micropores may also form. The newly formed pores in the CMK-5 matrix could increase the surface area, which leads to higher irreversible consumption of Li+ ions in the first cycle. In contrast, the SnO2–CMK-5 electrode shows higher specific capacity with better cycling stability (598 mA h g1 after 30 cycles), compared to other electrodes in the present study. In the SnO2–CMK-5 electrode, volume expansion of SnO2 during cycling is accommodated inside the mesochannels of CMK-5, and the active sites in CMK-5 are reduced by the embedded SnO2 nanoparticles. This helps in reducing its irreversible capacity. Fig. 6a shows the specific discharge capacity versus cycle number at different current density values in the voltage range of 0.02–3 V. The initial specific discharge capacities at current


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Fig. 6 (a) Cycling performance of SnO2–CMK-5; (b) rate capability of SnO2–CMK-5.

densities of 35.6, 89 and 178 mA g1 are 1951, 1657 and 1547 mA h g1, respectively. It is observed that there is a reduction in specific discharge capacity with increasing current density. Two possible explanations for this could be either a higher ohmic drop or a limited Li+ ion diffusion, or both.35,36 On the other hand, it has excellent reversible retention capacities of 83.5%, 81.7% and 66.5%, at current densities of 35.6, 89 and 178 mA g1, respectively. The higher reversible retention capacity may be attributed to the presence of SnO2 nanoparticles between the pores of CMK-5, which acts as a buffering matrix during charging–discharging cycles. As the current density increases from 35.6 to 178 mA g1, the specific discharge capacity reduced to 598 mA h g1, which is still 20 times higher than the specific discharge capacity of commercial graphite (30 mA h g1), when cycled at a similar current.37 The resulting higher specific discharge capacity clearly indicates that SnO2–CMK-5 can be a potential material for high energy and high power LIBs. In addition, the SnO2–CMK-5 electrode was evaluated for rate capability as shown in Fig. 6b. This electrode has a reversible capacity of 826 mA h g1, when it is cycled at a current density of 89 mA g1. The following reversible capacities were obtained at


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different current densities: 692 mA h g1 at 178 mA g1, 418 mA h1 at 890 mA g1, 294 mA h g1 at 1780 mA g1 and back to 771 mA h g1 at 89 mA g1 again. It is clear from Fig. 6b that specific capacities can be recovered when the current density changed from high to low, showing good reversibility of the electrode. The reasons for excellent rate capability and cycling stability are, firstly, the small size of SnO2 nanoparticles which is favourable for Li+ ion diffusion. Secondly, the conductive nature of the support, porosity and good contact between SnO2 nanoparticles and CMK-5 could increase the electronic conductivity, facilitating the electrolyte diffusion. The electrical conductivities of CMK-5, SnO2 and SnO2–CMK-5 are 14.9 102, 1.2 102 and 8.1 102 S m1, respectively. The conductivity of CMK-5 is in agreement with the range of values reported in the literature for pure carbon (12.5 102 to 20 102 S m1).38 Since SnO2 is a semiconducting material, its conductivity is less than that of conducting carbon. When both CMK-5 and SnO2 are combined in the hybrid, the CMK-5 matrix helps in increasing electrical conductivity with respect to only SnO2. More importantly, the synthesis approach used in this work comprises of SnO2 nanoparticle intrusion within CMK-5, as SnO2–SBA-15 is used as a template for the formation of the SnO2–CMK-5 hybrid. Therefore, the synthesized SnO2–CMK-5 completely eliminates the formation of SnO2 nanoparticles on the external surface of CMK-5 particles, which will help in accommodating volume expansion (B300%) and prevent capacity fading during charging–discharging cycles (Fig. S5, ESI†). Therefore, CMK-5 is a suitable host for nanoparticles and electrode preparation, so as to reduce ohmic drop. Table S1 (ESI†) shows a comparison of the electrochemical performance of SnO2–CMK-5 with different forms of carbon based SnO2 electrodes, used in recent studies, with a view to identify important parameters, such as current density, reversible specific capacity, initial efficiency and the voltage range. Among all carbon based SnO2 electrodes mentioned in the table, the performance of the present SnO2–CMK-5 electrode is reasonably good in terms of reversible capacity and current density. In order to understand the effect of hybrid on the electrochemical performance, the theoretical specific capacity of SnO2–CMK-5 is also calculated (see ESI†). Electrochemical impedance spectroscopy (EIS) For further insight into the electrode reaction, EIS is performed with SnO2 and SnO2–CMK-5 electrodes. EIS was carried out at different potentials, at a current density of 17.8 mA g1 during the first discharge. The different potentials are chosen based on cathodic peaks in the CV. The obtained impedance spectra are fitted to an appropriate equivalent circuit using ZsimpWin software. In general, the EIS spectrum consists of low (LF), medium (MF) and high frequency (HF) regions, which relate to Li+ ion diffusion in the bulk material, charge transfer reaction, and SEI layer resistance, respectively. The small diameter of the first semicircle at the HF region in the Nyquist plot (Fig. 7a) is a measure of the SEI layer resistance, while the diameter of the second semicircle at the MF region is a measure of the charge transfer resistance. The monotonic line at the LF region is a


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Paper Table 2 Electrode resistance of bare SnO2, CMK-5 and SnO2–CMK-5 at different potentials during the first discharge

SnO2

CMK-5

SnO2–CMK-5

Potential (V) vs. RS Rct RS Rct RS Rct Condition Li/Li+ (ohm) (ohm) (ohm) (ohm) (ohm) (ohm)


First 1.36 discharge 0.66 0.02

13.05 38.16 3.12 20.67 74.52 3.73 25.23 81.68 4.69

19.69 6.39 20.73 7.00 26.03 8.21

21.62 22.78 41.91

The possible reason for increase in Rs and Rct values, when the potential decreases from 1.36 to 0.66 V, could be due to electrolyte decomposition and formation of Sn from SnO2, the latter due to insertion of a large amount of Li+ ions into SnO2 in both electrodes. A further decrease in potential from 0.66 to 0.02 V causes even more increase in Rs and Rct values. The increase in Rs is due to further decomposition of the electrolyte, while an increase in Rct may be due to one or either of the following two reasons: (1) LixSn formation during alloying reaction and (2) breaking of interparticle contact caused by volume expansion. A significant decrease in Rs is observed for SnO2–CMK-5 in comparison with bare SnO2, which may be due to the thin SEI layer formation in the former electrode, as it has a higher surface area than the latter. The surface resistance (Rs) associated with SnO2 electrodes (25.23 ohm) is higher in comparison with the SnO2–CMK-5 electrode (8.21 ohm) and CMK-5 (4.69 ohm). It is also observed that Rct decreases for SnO2–CMK-5, in comparison with bare SnO2, which indicates improved electronic conductivity of the former electrode, due to the addition of CMK-5. Therefore, the impedance results confirm that the addition of CMK-5 enhances both ionic and electronic conductivity. To understand the Li+ ion transport inside all the electrodes, the diffusion coefficient (DLi) of the Li+ ion has been calculated for the first discharge state at 0.02 V, taken from the LF region in the Nyquist plot, according to the following equation.42,43 Fig. 7 (a) Equivalent circuit and Nyquist plot of electrodes (SnO2, CMK-5 and SnO2–CMK-5) at different potentials, showing high, medium and low frequency regions. (b) Real parts of complex impedance versus o1/2 for the first discharge at 0.02 V.

measure of Li+ ion diffusion in the bulk material.39 Nyquist plots of the first discharge of SnO2, CMK-5 and SnO2–CMK-5 electrodes at different potentials are shown in Fig. 7a. As shown in the equivalent circuit in Fig. 7a, Rb corresponds to electrolyte resistance. The RsCs element corresponds to surface resistance coupled with its capacitance, the RctCctW element relates to charge transfer resistance with double layer capacitance and the Warburg component associated with Li+ ion diffusion in the sample, and the Cin element corresponds to intercalation capacitance.40 The fitted electrochemical parameters for bare SnO2, CMK-5 and SnO2–CMK-5 are shown in Table 2. It is observed that Rs and Rct values gradually increase for all electrodes upon discharging, which is in good agreement with a previous report.41


DLi ¼

R2 T 2 2A2 n4 F 4 C 2 s2

(1)

where R is the gas constant (8.314 J mol1 K1), T is the room temperature (293.151 K), n is the number of electrons per oxidized molecule, A is the surface area (1.76 cm2), F is Faraday’s constant (98 486 C mol1), C is the concentration of Li+ ions (mol cm3) calculated based on the ratio between tapping density44 of active material (4.9 g cc1 for SnO2, 0.1 g cc1 for CMK-5 and 1.6 g cc1 for SnO2–CMK-5) and molecular weight, s is the Warburg factor calculated (Table 3) based on the relation between Zre and o0.5, as shown in Fig. 7b. This is the most widely used equation to calculate the diffusion coefficient where intercalation and deintercalation take place in the carbon electrode.38 In the case of SnO2 and SnO2–CMK-5 electrodes, conversion of SnO2 into Sn and Li2O occurs in its first discharge. After completing the conversion reaction (irreversible reaction), Li+ alloying reaction takes place predominantly in both electrodes, between 0.1 and 0.6 V, during its first cycle.45 Hence, the diffusion coefficient is calculated at the end of the first discharge (0.02 V),


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Table 3 Electrode kinetic parameters of bare SnO2, CMK-5 and SnO2– CMK-5 for the first discharge state at 0.02 V

Sample

s (ohm s1/2)

DLi (cm2 s1)

SnO2 CMK-5 SnO2–CMK-5

7.9 2.1 3.2

8.5 1015 3.6 1011 1.1 1013

where lithium intercalation occurs. It can be seen from Table 3 that s is 3.2 ohm s1/2 for the SnO2–CMK-5 electrode, which is 2.5 times lower than that of the bare SnO2 electrode (7.9 ohm s1/2). The diffusion coefficient of the Li+ ion inside CMK-5 is 3.6 1011 cm2 s1, which is higher than that of the Li+ ion diffusion coefficient in both SnO2–CMK-5 (1.1 1013 cm2 s1) and bare SnO2 electrodes (8.5 1015 cm2 s1). This is because, CMK-5 has only mesopores, and hence there will not be much restriction in the Li+ ion diffusion during the cycling process. In the case of SnO2–CMK-5, there will be some restriction in Li+ ion diffusion due to the presence of SnO2 nanoparticles, which results in a lower diffusion coefficient than that in the bare CMK-5 electrode. Therefore, the presence of CMK-5 with embedded SnO2 nanoparticles not only helps in exhibiting superior cycling performance, but also reduces the electrode resistances (Rs, Rct) and improves the diffusion coefficient of Li+ ions during the cycling process.

Conclusions This work demonstrates a simple synthesis method to position the SnO2 nanoparticles only on the outer surface of mesopores of CMK-5. The SnO2–CMK-5 electrode synthesized in the present work addresses the main challenges such as poor cycling stability and low rate capability of the SnO2 electrode. By keeping the SnO2 nanoparticles in the empty space between the mesopores present in the CMK-5, we have achieved a SnO2 particle size of less than 10 nm, resulting in faster diffusion of Li+ ions. In addition, SnO2 particles can expand freely in the empty space during Li+ ion insertion and extraction. These capabilities enhanced the electrochemical behaviour of SnO2– CMK-5 (specific capacity of 598 mA h g1 at a current density of 178 mA g1). Thus, the porous nature of CMK-5 enables it to act effectively as a buffering layer for the volume changes during charging–discharging cycles. It is clear that specific capacities can be recovered when the current density varies from high to low, showing good reversibility of the electrode. The electrochemical performance is improved due to faster Li+ ion diffusion coefficient (1.1 1013) and smaller resistance (50.12 ohm) for the SnO2–CMK-5 electrode, as compared to the bare SnO2 electrode (8.5 1015 cm2 s1, 106.91 ohm). Based on the electrochemical performance, the SnO2–CMK-5 synthesized in this work is a promising anode material for high energy and high power LIBs. The present synthesis approach can be adapted for formation of any metal or metal oxide only on the outer surface of mesopores, without their aggregation on the external surface of a porous particle.


Acknowledgements The financial support received by RB from IRCC, IIT Bombay (grant no. 07IR018), and by SM from the National Centre For Photovoltaics Research And Education (NCPRE), Ministry of New and Renewable energy, India, for this work is gratefully acknowledged.

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