C Nanocomposite with Enhanced Lithium Storage

Jan 23, 2015 - straightforward approach. Both MSNs and DMSNs/C showed uniform pore size distribution, high specific surface area, and large pore volum...
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Dual-Porosity SiO2/C Nanocomposite with Enhanced Lithium Storage Performance Huan-Huan Li,† Xing-Long Wu,† Hai-Zhu Sun,*,† Kang Wang,† Chao-Ying Fan,† Lin-Lin Zhang,† Feng-Mei Yang,† and Jing-Ping Zhang*,† †

Faculty of Chemistry, National & Local United Engineering Laboratory for Power Batteries, Northeast Normal University, Changchun 130024, China S Supporting Information *

ABSTRACT: Mesoporous SiO2 nanospheres (MSNs) and carbon nanocomposite with dual-porosity structure (DMSNs/C) were synthesized via a straightforward approach. Both MSNs and DMSNs/C showed uniform pore size distribution, high specific surface area, and large pore volume. When evaluated as an anode material for lithium ion batteries (LIBs), the DMSNs/ C nanocomposite not only delivered an impressive reversible capacity of 635.7 mAh g−1 (based on the weight of MSNs in the electrode material) over 200 cycles at 100 mA g−1 with Coulombic efficiency (CE) above 99% but also exhibited excellent rate capability. The significant improvement of the electrochemical performance was attributed to synergetic effects of the dualmesoporous structure and carbon coating layer: (i) the dual-porosity structure could increase the contact area and facilitate Li+ diffusion at the interface between the electrolyte and active materials, as well as buffer the volume change of MSNs, and (ii) the homogeneous carbon coating represented an excellent conductive layer, thus significantly speeding the lithiation process of the MSNs significantly, while further restraining the volume expansion. Considering the facile preparation and good lithium storage abilities, the DMSNs/C nanocomposite holds promise in applications in practical LIBs.

1. INTRODUCTION Li ion rechargeable batteries (LIBs) have been widely considered as one of the best power sources for the next generation of portable electronic devices and electric vehicles (EVs).1,2 Graphite has long been a commercially successful anode material because of its low cost and high stability through lithium intercalation.3 However, the theoretical capacity of graphite is relatively low (372 mAh g−1), which cannot meet the increasing requirement for LIBs with better electrochemical performance. Thus, it is urgent to develop new electrode materials with greater power and energy density.4,5 In the past decade, Si-based materials have emerged as a leading anode candidate in view of their highest theoretical capacity and safer operation voltage above lithium.6−8 So far, many Si-based anodes with novel structures and good electrochemical performance, such as nanoparticles,9 nanowires,10−13 nanotubes,14,15 and yolk−shell nanostructured materials,16−19 have been prepared. Unfortunately, the nano-silicon is generally prepared by multisteps and complicated fabrication processes, which severely hinders its pratical application. Therefore, it remains challenging to develop alternative materials that are obtained via a simple process and simultaneously possess comparable electrochemical performance with silicon. In recent years, silica (SiO2) has attracted lots of interest due to its advantages of storing a large quantity of lithium and a low discharge potential similar to that of Si. Moreover, compared © XXXX American Chemical Society

with the transition-metal oxides (such as CoOx, NiOx, SnOx, FeOx, MoOx, MnOx), SiO2 is one of the most abundant materials on earth. Therefore, it is more cost-effective and ecofriendly when used as an energy material. Since Gao et al.20 claimed that commercial SiO2 nanoparticles with diameter size of ∼7 nm could react with lithium in the range of 0.0−1.0 V (vs Li/Li+), various SiO2-based anodes with diverse geometric shapes and morphologies have been successfully prepared for applications in LIBs, such as thin nanofilms,21 hollow porous nanocubes,22 hollow nanospheres,23 nanotubes,24 and SiO2−C nanocomposites.25−30 Recently, Chang et al. have prepared a SiO2-based anode material by high-energy mechanical milling, using quartz (SiO2) as a precursor.31 This anode material exhibited a high reversible capacity of ∼800 mAh g−1 over 100 cycles at 100 mA g−1. Despite these encouraging improvements, the rate performances of SiO2 have been rarely reported.29,30 Considering that the theoretical capacity of SiO2 is as high as 1965 mAh g−1 and the importance of a high rate performance of LIBs in our daily life, it is essential to further enhance the cyclability and rate performance of SiO2. As known, the solid-state chemical diffusion of ions and electrons from the surface into the interior of an active particle Received: November 14, 2014 Revised: January 22, 2015

A

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The Journal of Physical Chemistry C is the slowest step in the charge/discharge kinetics.32 Therefore, a rational design to shorten the diffusion path and to enlarge the specific surface area is the key to enhance the overall kinetics and hence the rate performance of the LIBs.33 In view of this, we designed and prepared mesoporous SiO2 nanospheres (MSNs) and a carbon nanocomposite with dual porosity (DMSNs/C), which combined the nanosize, extra free expansion space, and high specific surface area together to improve the electrochemical performance. Moreover, the mesopores could provide more efficient ionic transportation paths, thus significantly facilitating the diffusion of Li+ between the electrolyte and MSNs. As a result, the DMSNs/C nanocomposite exhibited good cyclability and excellent rate performance, possessing great potential as an anode material for lithium storage.

3. RESULTS AND DISSCUSSION 3.1. Dual-Porosity Structure. The dual-porosity structure, presented in Figure 1, is designed in order to improve the

Figure 1. Dual-porosity structure, which provides more efficient electroyte (and Li+/e−) penetration (transportation) paths.

2. EXPERIMENTAL SECTION 2.1. Synthesis of MSNs and DMSNs/C Nanocomposite. The MSNs was prepared in accordance with our previous work.34 First, cetyltrimethylammonium bromide (CTAB) and polyoxyethylene (10) cetyl ether (Brij-56) were mixed in buffer solution by ultrasonication. Then tetraethoxysilane (TEOS) was added drop-by-drop, and the solution was kept at 95 °C for 8 h in an oil bath. After that, the synthesized inorganic−organic composite was calcined to remove the organic surfactants, thus obtaining the MSNs. Afterword, citric acid (CA) and MSNs were mixed with the molar ratio of 1:1 in anhydrous alcohol to form a uniform solution by ultrasonication. When ethanol was nearly evaporated, the mixture was calcined at 650 °C for 4 h under N2 atmosphere to obtain DMSNs/C nanocomposite. 2.2. Material Characterization. The structures of the obtained MSNs and DMSNs/C were characterized by X-ray diffraction (XRD, Rigaku P/max 2200VPC) using Cu Kα radiation. X-ray photoelectron spectra (XPS) were generated with Al Kα radiation and an energy step size of 0.1 eV. The carbon content was determined by thermogravimetric analysis (TGA) with a heating rate of 10 °C min−1 carried out in air. To confirm the existence of mesopores on the surface of MSNs, nitrogen adsorption isotherms at −196 °C for MSNs were adopted. A transmission electron microscope (TEM, JEM2010F) and scanning electron microscope (SEM, XL 30 ESEM-FEG, FEI Co.) were used to study the morphology of the products. The electrochemical impedance spectroscopy (EIS) measurements were carried out in two-electrode cells on a Par 2273 potentiostat electrochemistry workstation with a ±5 mV ac signal amplitude, and the frequency ranged from 10 kHz to 0.1 Hz. Cyclic voltammetry (CV) measurements were conducted at a scan rate of 0.1 mV s−1. 2.3. Electrochemical Testing. The anodes were prepared by mixing the DMSNs/C (70 wt %), acetylene black (15 wt %), and polyvinylidene fluoride (PVDF) binder (15 wt %) in Nmethyl-2-pyrrolidione (NMP) to produce a homogeneous slurry using a mortar and pestle and then pasting the slurry onto copper foils that were dried in a vacuum oven at 120 °C for 12 h. Pure lithium foil was used as the counter electrode and 1.0 M LiPF6 in 1:1 v/v ethylene carbonate (EC)/dimethyl carbonate (DMC) as the electrolyte. The 2032 coin cells were assembled in an Ar-filled glovebox. Galvanostatic cycling measurements were made using a Land battery test system from 0.01 to 2.5 V.

electrochemical properties, especially the rate performance of the materials. The formation process of the dual-porosity structure in this work is easy and straightforward. To certify the existence of mesopores on the surface of MSNs and the dualporosity property of the DMSNs/C nanocomposite, nitrogen adsorption isotherms at −196 °C for MSNs and DMSNs/C were performed. As shown in Figure 2, both of the nitrogen

Figure 2. Nitrogen adsorption isotherms at −196 °C for (a) MSNs and (b) DMSNs/C; the insets shows the BJH pore size distributions.

adsorption isotherms of MSNs and DMSNs/C have the typical characteristic of materials with small mesopores (type IV).35,36 The pore size distribution (PSD) (inset of Figure 2a) of MSNs is centered at 2.86 nm. It is clearly shown that, after calcinations, the DMSNs/C nanocomposite exhibits a dualporosity property with PSDs centered at 2.17 and 2.85 nm, separately (inset of Figure 2b). Moreover, both MSNs and DMSNs/C show high specific surface areas of 427.9 and 124.1 m2 g−1, respectively (Table S1, Supporting Information). The B

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Figure 3. (a) XRD patterns of MSNs and DMSNs/C nanocomposite. (b) Raman spectrum of DMSNs/C.

Figure 4. (a) SEM image and (b) TG curve measured under an air atmosphere of DMSNs/C nanocomposites. FE-TEM images of (c) MSNs and (d) DMSNs/C nanocomposites.

of the MSNs and the carbon layer, while most of them exist in a separated manner. It is known that the mesopores, as well as the high specific surface area, could provide a more efficient ionic transportation path and faciliate the diffusion of Li+ between the electrolyte and MSNs, thus significantly improving the rate performance of DMSNs/C nanocomposites. 3.2. XRD and Raman Spectra Analysis. The crystallographic structure and phase purity of MSNs and DMSNs/C nanocomposite were examined by X-ray powder diffraction (XRD). As shown in Figure 3, the black line reveals the XRD pattern of MSNs, while the red line represents that of the DMSNs/C nanocomposite. It is observed that both of them show a wide peak at ∼23°, indicating the amorphous nature of MSNs before and after carbonization. Moreover, no diffraction peaks from impurities have been detected, implying the high purity of the product. The graphitization degree of carbon in the composite is analyzed by the Raman spectrum of the DMSNs/C sample (Figure 3b), in which characteristic peaks at

mesopores of the MSNs and the carbon layer could be interconnected or may exist in a separated manner. In order to investigate the relations of mesopores (∼2.85 nm) on the surface of MSNs and pores (2.17 nm) originating from carbon shell, a nitrogen adsorption isotherm for pure carbon from citric acid (being calcined at 650 °C for 4 h under N2 atmosphere) was collected (Figure S1, Supporting Information). It shows that the size of the pores on the carbon layer (2.17 nm) in the DMSNs/C nanocomposite is larger than that of pure carbon from citric acid (mainly centered at 1.95 nm). This difference is probably caused by the following reasons: when heated slowly during the calcination process, the gas adsorbed by the mesopores on the surface of MSNs expanded toward outside the citric acid layer,32 thus forming a number of larger mesopores on the citric acid layer. After carbonization, the mesopores on the surface of the citric acid layer were retained on the carbon layer, resulting in the formation of a partially interconnected dual-porosity structure at the interface C

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Figure 5. (a) Cyclic voltammetry of DMSNs/C nanocomposite from 0.01 to 2.5 V vs Li/Li+ at a scan rate of 0.1 mV s−1. (b) The differential capacity vs voltage (dQ/dV) curves of DMSNs/C nanocomposite for further cycles.

Figure 6. (a) The charge−discharge profiles for different cycles and (b) cycling performance and CE of DMSNs/C electrode under 100 mA g−1 within a voltage of 0.01−2.5 V. (c) Rate performance of DMSNs/C and MSNs electrode. (d) Capacity retention of citric acid (calcined at 650 °C for 4 h under N2 atmosphere) and pristine MSNs under 100 mA g−1.

1350 and 1580 cm−1 are ascribed to the typical D- and G-bands of amorphous carbon, respectively. The ID/IG ratio of the DMSN/C nanocomposite is estimated to be 1.08, indicating the amorphous nature of the carbon in the composite, which is consistent with the analysis result of the XRD. Possessing lots of vacancies and defects, which not only enhance the diffusion Li+ but also offer reversible sites for Li+ storage, the amorphous carbon is thought to be benefical to the improvement of the electrochemical performance.37 3.3. Morphologies of MSNs and DMSNs/C Nanocomposite. TEM and SEM were employed to observe the microstructures of MSNs and DMSNs/C nanocomposite. The SEM image shown in Figure 4a demonstrates that the DMSNs/ C nanocomposites are spherical with a uniform size distribution. The TG curve of the DMSNs/C carried out under an air atmosphere (Figure 4b) shows that the MSNs content in the nanocomposite is as high as ∼82 wt % and that

of carbon is only ∼18 wt %. Figure 4c,d clearly reveals that the diameter of MSNs is ∼130 nm, with a very uniform pore distribution on the surface of MSNs before and after being coated with carbon. In Figure 4d, the distribution of the carbon coating layer among the interspaces of MSNs is clearly observed, which is beneficial for improving the conductivity and the rate performance of the DMSNs/C nanocomposite. Moreover, Figure 4d also exhibits that the mesoporous carbon layer on the surface of the DMSNs is homogeneous. 3.4. Electrochemical Analysis. First, the electrochemical performance of DMSNs/C nanocomposite was evaluated by cyclic voltammetry (CV) in the 0.01−2.5 V voltage window at a scan rate of 0.1 mV s−1 (Figure 5a). In the first cycle, there is an obvious reduction peak located at ∼0.75 V, which becomes undiscernible in the subsequent cycles. In general, it results from the electrolyte decomposition and the formation of the solid electrolyte interface (SEI) layer. In the inset of Figure 5a, D

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Figure 7. (a) Nyquist plots of MSNs and DMSNs/C; the inset is the equivalent circuit. (b) Dependence of Zre on the reciprocal square root of the frequency in the low-frequency region of MSNs and DMSNs/C nanocomposite for the 30th cycle.

100 mA g−1 in the voltage range of 0.01−2.5 V versus Li/Li+. It is found that the curves are generally consistent with the above CV profiles shown in Figure 5a. The large initial irreversible capacity and low CE are mainly caused by the formation of the SEI layer and the irreversible electrochemical reactions between Li+ and MSNs in the DMSNs/C nanocomposite. It has been confirmed that the amorphous nano-SiO2 would be gradually reduced to Si and amorphous Li2O or crystalline Li4SiO4 during repeated discharged processes, which is helpful for the enhancement of Li+ diffusion kinetics. The formation of Li2O and Li4SiO4 is generally thought to be irreversible.28 As a result, the capacity of DMSNs/C largely increases during the following cycles.27 The long-term cycle performance of the DMSNs/C anode was tested at 100 mA g−1, as shown in Figure 6b. Though the DMSNs/C anode shows a low initial charge capacity, with the enhancement of the Li+ diffusion kinetics and the further formation of Si, the reversible capacity increases in the following cycles and remains 635.7 mAh g−1, even after 200 cycles with high CE above 99% (based on the weight of MSNs in the electrode material). This result is much higher than that of pristine MSNs (Figure 6d). This indicates that, except the mesopores, the carbon coating layer also plays a key role in improving the elecreochemical performance of DMSNs/C nanocomposites. In addition, Figure 6c shows that the DMSNs/C nanocomposite exhibits excellent rate performance after being fully activated. It shows highly stable, reversible capacities of 695.2, 659.8, 623.9, 561.3, 430.6, and 340.5 mAh g−1 at the current densities of 100, 200, 300, 500, 1000, and 1500 mA g−1, respectively, which are much higher than those of bare MSNs (190.7, 180.1, 163.2, 132, and 109.7 mA g−1, respectively). Remarkably, when the current density returns to 100 mA g−1 after more than 150 cycles, a highly stable reversible capacity of 620.8 mAh g−1 is recovered, while that of bare MSNs is only 165 mAh g−1. Figure 7d displays the capacities of the pure carbon layer (citric acid, calcined at 650 °C for 4 h under N2 atmosphere) and pristine MSNs for comparison. As the weight ratio of carbon in the nanocomposite is 17.9% (shown in Figure 4b), it is calculated that the capacity contributed by the carbon is only ∼54 mAh g−1 (300 mAh g−1 × 17.9%). As shown in Figure S3 (Supporting Information), most of the spherical structure of the DMSNs/C nanocomposites was kept quite well, even after 50 cycles. Such stable cyclability and excellent rate performance of the DMSNs/C nanocomposite might be explained by multiple factors. First, the existence of large numbers of mesopores on the surface of MSNs and the

it is clearly observed that a peak around 0.33 V, corresponding to the delithiation process of formed Li−Si alloy, occurs during the initial anodic scanning, and in subsequent cycles, this peak becomes more and more visible. The growth of this oxidation peak indicates a rate enhancement of the delithiation of DMSNs/C nanocomposite in the kinetic process.24 As the electrochemical reaction of the DMSNs/C for the initial few cycles was not quite obvious, we analysized the differential capacity vs voltage (dQ/dV) curves for further cycles (Figure 5b). On the basis of dQ/dV analysis, the reaction between Li+ and SiO2 can be expressed as follows:28 SiO2 conversion reaction (C and B) SiO2 + 4Li+ + 4e− → 2Li 2O + Si

(1)

2SiO2 + 4Li+ + 4e− → Li4SiO4 + Si

(2)

alloying process (A) Si + x Li+ + x e− → LixSi

(3)

dealloying process (D and E) LixSi → Si + x Li+ + x e−

(4)

On the basis of eqs 1 and 3 or eqs 2 and 3, the theoretical specific capacity of SiO2 is calculated to be 1965 or 983 mAh g−1, respectively. X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition of DMSNs/C nanocomposites under different charge/discharge stations. Figure S4a (Supporting Information) shows the spectrum of Si 2p under different conditions. It clearly indicates that the peak centered at 103.37 eV assigned to the amorphous SiO2 shifts to 103.05 eV after discharging to 0.01 V. It is observed that the Si 2p peak is remarkably broad after discharging to 0.01 V, which describes the formation of Li4SiO4 (103.31 eV) and Li−Si alloy (102.41 eV),22,28 indicating that the chemical state of Si is complicated rather than a single one. The spectra of O 1s are shown in Figure S4b (Supporting Information). When discharged to 0.01 V, the peak of as-prepared DMSNs/C at 532.65 eV shifts toward 532.03 eV. Moreover, this peak shifts to 532.41 eV after the composite is recharged to 2.5 V, suggesting that the partially irreversible change due to the formation of Li4SiO4 and Li2O. 3.5. Electrochemical Performances of DMSNs/C Nanocomposite. Figure 6a reveals the discharge−charge profiles of DMSNs/C electrode for different cycles at a current density of E

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mA g−1. It is considered that the decent performance of the DMSNs/C nanocomposite is mainly attributed to the synergetic effects of its unique dual-porosity structure, the uniform carbon coating layer, and the nanosized DMSNs/C composite. Moreover, the formed Li2O and Li4SiO4, performing as the stable matrix, also play a key role in improving the cycle stability of the DMSNs/C electrode.

dual-porosity properties of DMSNs/C nanocomposite give both of them a high specific surface area (427.9 and 124.1 m2 g−1) and large pore volume (0.387 and 0.259 cm3 g−1, respectively). They can not only provide more available lithium storage sites, which is beneficial for enhancing the specific capacity of the battery, but also generate extra free space to accommodate the volume variation and mechanical strains and to make the electrolyte penetrate through the mesopores more easily, thus enhancing the rate performance of the DMSNs/C electrode.38 Second, the carbon coating layer can not only increase the conductivity and improve the surface chemistry of the electrode but also protect the electrode from direct contact with the electrolyte, leading to enhanced cycle performance of the LIBs.39 In order to explore the effect of the carbon coating layer on the interfacial impedance of DMSNs/C nanocomposite, EIS measurements are carried out and the resulting Nyquist plots of MSNs and DMSNs/C nanocomposite are compared before and after different cycles (in the delithiated state). Figure 7a is the modified equivalent circuit used from which the key kinetic parameters are obtained by simulation, and the fitted impedance parameters are listed in Table S2 (Supporting Information), including the electrolyte resistance (Re), Li+ migration through the surface film (Rf), and charge transfer resistance between the electrolyte interface and electrode (Rct). It is interesting that, after 30 cycles, the diameters of the semicircles of MSNs and DMSNs/C nanocomposite become smaller than those before cycling. It is believed that this phenomenon resulted from the enhanced Li+ diffusion kinetics contributed by the formed Li2O and Li4SiO4 during the electrochemical reaction between SiO2 and Li+, as well as the slow permeation of electrolyte into the dual-porosity structure. Moreover, it is clearly shown that the semicircles in DMSNs/C (40.5 Ω), which are associated with the SEI film and chargetransfer resistance of Li+ insertion (presented at high and medium frequency region), are smaller than those in MSNs (65.3 Ω) after 30 cycles. In addition, the straight line in the low-frequency region of the EIS is attributed to the Warburg behavior, which is associated with the diffusion of Li+ in the electrode. The diffusion coefficient value (D) of the electrode materials can be evaluated by the following equation:40 D = (R2T 2)/(2A2 n 4F 4C 2σ 2)

(5)

Zre ∝ σω−1/2

(6)

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S4 and Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*H.-Z.S. e-mail: [email protected]. *J.-P.Z e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from the NSFC (21173037 and 21274017) and the Science Technology Program of Jilin Province (201201064, 20140101087JC) are gratefully acknowledged.

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REFERENCES

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where R, T, n, A, F, C, and σ are the gas constant, absolute temperature, surface area of the electrode, number of electrons per molecule during oxidization, Faraday constant, concentration of lithium ions in the solid, and Warburg factor relative to Zre, respectively.40,41 Combining the dependence of Zre on the reciprocal square root of the frequency in the low-frequency region (Figure 7b) and eqs 5 and 6, it is inferred that the D of DMSNs/C is ∼2.3-fold greater than that of the bare MSNs electrode, indicating the faster diffusion of Li+ in the former.

4. CONCLUSIONS In summary, a SiO2/C nanocomposite with dual-porosity structure has been successfully prepared by a straightforward and cost-effective approach. The obtained DMSNs/C nanocomposites exhibited a high reversible capacity of 635.7 mAh g−1 over 200 cycles at 100 mA g−1 with high CE above 99%. After a series of changes in the current density, it could still go back to 620.8 mAh g−1 when the current density returns to 100 F

DOI: 10.1021/jp511435w J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/jp511435w J. Phys. Chem. C XXXX, XXX, XXX−XXX