High Volumetric Capacity of Hollow Structured SnO2@Si

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High Volumetric Capacity of Hollow Structured SnO2@Si Nanospheres for Lithium-Ion Batteries Tianyi Ma,† Xiangnan Yu,† Huiyu Li,‡ Wenguang Zhang,‡ Xiaolu Cheng,† Wentao Zhu,† and Xinping Qiu*,† †

Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China ‡ Institute of Tsinghua University Hebei, Beijing 100084, China S Supporting Information *

ABSTRACT: A novel design of hollow structured SnO2@Si nanospheres was presented, which not only demonstrates high volumetric capacity as anode of LIBs, but also prevents aggregation of Sn and confines solid electrolyte interphase thickening. An impressive volumetric specific capacity of 1030 mAh cm−3 was maintained after 500 cycles. The electrochemical impedance spectroscopy and differential scanning calorimetry indicated that solid electrolyte interphase can be confined in pores of as-prepared hollow structured SnO2@Si. KEYWORDS: Lithium-ion battery, silicon anode, tin dioxide, volumetric capacity, solid−electrolyte interphase

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less than 99%, indicating the SEI excessive growth cannot be prohibited. Another way to increase the volumetric specific capacity is composite silicon with high volume density materials, such as SnO2,20,21 Fe3O4,22,23 and Co3O4.24 Among these materials, SnO2 has higher volume density of 6.5 g cm−3, and it can provide 790 mAh g−1 of theoretical capacity. There has been some research about Si/SnO2 composites such as Si@ SnO2 nanowire arrays,25 Si@SnO2 core−shell particles,26 and Si particles coating with SnO2 nanowires.27 However, the problem of SEI excessive growth in these composites is still unsolved. Moreover, Sn nanoparticles (SnO2 is reduced to Sn during the first discharge process) are easily aggregated to large particles,28 which lead to fracture and lose contact with electrode during battery cycles. Combining the advantages of SnO2 and hollow structured silicon, we present a novel structure of silicon coated hollow SnO2 nanospheres (denoted as h-SnO2@Si) with high volumetric capacity, in which silicon acts not only as an high capacity Li-storage material, but also as a protective layer to prevent nano Sn particles from aggregation by block of Sn atom diffusion. More significantly, benefiting from the hollow structure, the problem of volume expansion and SEI excessive growth can be settled. As a result, the as-prepared h-SnO2@Si not only exhibited a high reversible volumetric specific capacity of 1615 mAh cm−3, but also demonstrated a capacity retention of 1030 mAh cm−3 over 500 cycles at current density of 300 mA g−1, which is twice that of commercialized graphite anode.

ith the active development of portable electronics, higher requirements are put forward on volumetric energy density and gravimetric energy density of lithium-ion batteries (LIBs).1−5 In the development of LIBs, one of the major challenges is to replace commercialized graphite anode with higher capacity ones. Silicon is one of the most promising candidates because of its high theoretical capacity (3572 mAh g−1).6−10 However, the practical utilization of silicon in LIBs is hindered due to the large volume change (300%) of the lithiation/delithiation reaction, which leads to several critical problems including pulverization of silicon particles, unstable of electrode, and thickening of solid−electrolyte interface (SEI).11−15 Hollow structured silicon can effectively accommodate the large volume expansion, which exhibited great cycling performance in Li-half cells. Our group also prepared hollow structured silicon with a facile scarification template method, and we combined electrochemical impedance spectroscopy (EIS) and differential scanning calorimetry (DSC) to study the evolution of SEI during cycling, the results of which showed that hollow structured silicon can confine the excessive growth of SEI and thus exhibit high Coulombic efficiency. Despite the advantages of hollow structured silicon, the large void space will seriously decrease the volumetric specific capacity of anode.16,17 To alleviate this problem, two major strategies have been proposed to enhance the volumetric specific capacity of silicon anode; one of them is to increase the tap density of electrode. For example, silicon@graphitic carbon nanowire array18 and thick CNT-Si film anode19 both demonstrated volumetric specific capacity of up to 1500 mAh cm−3. However, the Coulombic efficiencies of these anodes are © XXXX American Chemical Society

Received: April 20, 2017 Revised: May 19, 2017 Published: May 24, 2017 A

DOI: 10.1021/acs.nanolett.7b01674 Nano Lett. XXXX, XXX, XXX−XXX

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density of 100 mA g−1, and then the cells remained at opencircuit for 2 h to stabilize their potential. All cells were assembled in an Ar-filled glovebox with moisture and oxygen contents below 1 ppm. The h-SnO2@Si was fabricated by a scarification templateCVD method, as illustrated in Figure 1. SiO2 nanospheres were

SiO2 nanospheres were prepared according to the modified Stöber method29 and used as a template for the preparation of SnO2 hollow nanospheres. Briefly, 8 mL of TEOS was hydrolyzed to SiO2 in a mixed solution of deionized water (55 mL), ethanol (130 mL), and ammonia (14 mL) at room temperature with stirring for 30 min, the SiO2 nanospheres were collected and washed by deionized water for several times. Next, SnO2 hollow nanospheres were prepared by template removal method.30 In detail, 0.8 g of SiO2 nanospheres was dispersed in 50 mL of deionized water by ultrasonication; then 100 mL of Na2SnO3 solution (0.5 mol L−1) was added into the dispersion liquid with stirring at 60 °C for 3 h to deposit SnO2 on SiO2 nanospheres. The as-prepared SiO2@SnO2 nanospheres reacted with NaOH solution (2 mol L−1) at 80 °C to remove SiO2 template; then the SnO2 hollow nanospheres were collected and dried in a vacuum for 24 h. Silicon coated hollow SnO2 nanospheres (denoted as hSnO2@Si) were prepared by a simple chemical vapor deposition (CVD) method. In detail, the deposition reaction was conducted at 480 °C, 5 wt % SiH4 and 95 wt % Ar gas was used as silicon source, and 5 wt % H2 and 95 wt % Ar gas was used as protective gas during heating and cooling processes. The thickness of silicon layer was controlled by the time of CVD reaction, samples denoted as h-SnO2@Si-1 and h-SnO2@ Si-2 corresponding to the deposition time of 1 and 2 h, respectively. Morphological characteristics of samples were observed by transmission electron microscopy (TEM, JEOL model JEM2100) operating at 80 keV and scanning electron microscopy (SEM, Zeiss Merlin) operating at 5 keV. X-ray diffraction (XRD) spectra were recorded on an X-ray diffractometer (Bruker D8 Advance) with a Cu Kα radiation source; values of 2-theta were scanned in the range of 10−90° at a rate of 8° min−1 with a step of 0.02°. X-ray photoelectron spectroscopy (XPS) was performed on a PHI Quantera SXM spectrometer, and the binding energy scale was calibrated using the C 1s peak at 284.8 eV. The specific surface area and pore size distribution of samples were collected by N2 sorption measurements on a Quantachrome NOVA 1000e at 77.3 K. Thermal gravimetric analysis (TGA) was conducted by METTLER TOLEDO TGA instrument. Temperature was increased from 30 to 1000 °C with a rate of 10 °C min−1 and then kept at 1000 °C for 1 h. Differential scanning calorimetry (DSC) was performed on a Mettler-Toledo calorimeter to analyze the evolution of SEI growth. DSC measurements were performed from 30 to 300 °C at a temperature ramp of 3 °C min−1. Electrochemical Measurements. Electrochemical performance measurements were carried out by assembling 2025-type coin cells. The electrodes consist of 60 wt % active material, 20 wt % Super P Li (TIMCAL), and 20 wt % poly(acrylic acid) (PAA); Li foil was used as counter electrode. The loading of active materials were controlled to be 1.7−1.8 mg cm−2. The electrolyte was 1 M LiPF6 in 1:1:1 in volume ratio of ethylene carbonate (EC)/dimethyl carbonate (DMC)/ ethylmethyl carbonate (EMC). Galvanostatic tests were performed between 0.01 and 1.1 V versus Li/Li+. Threeelectrode system (ECC-REF model, EL-CELL) was assembled for electrochemical impedance spectra (EIS) tests using Li foil as reference and counter electrodes. EIS measurements were conducted on PARSTAT2273 electrochemical workstation. AC potential was controlled with the amplitude of 5.0 mV in the frequency range of 105−0.01 Hz. Before tests, the battery was discharged to 1 mV versus Li/Li+ under a constant current

Figure 1. Schematic diagram of the fabrication process of SnO2@Si hollow nanospheres.

prepared as template due to their facile preparation and nontoxic attributes. SnO2 was coated onto SiO2 nanospheres by decomposition of Na2SnO3, and SiO2 template was removed by its reaction with NaOH solution. Silicon was coated onto SnO2 hollow spheres by CVD reaction. Thickness of silicon layer can be tailored by controlling the deposition time. SEM image of SiO2 nanospheres is shown in Figure 2a, with uniform distribution with particle size of 100−150 nm. Morphological features of the SnO2 hollow nanospheres and h-SnO2@Si are shown in Figure 2b−h. The SnO2 hollow nanospheres in Figure 2b−d are nearly spherical shape, with outer radius of about 60−80 nm. The HRTEM image of Figure 2d shows that the wall thickness of SnO2 hollow nanospheres is about 10 nm; broad diffuse rings in the selected-area electron diffraction (SAED) pattern illustrate the polycrystal of SnO2. TEM images of Figure 2e and f and Figure 2g and h show the h-SnO2@Si with CVD time of 1 and 2 h (denoted as h-SnO2@Si-1 and hSnO2@Si-2), respectively. Silicon layer on the surface of SnO2 spheres can be clearly distinguished, which corresponds to thickness of about 10−20 nm, respectively. Broad diffuse rings in the SAED pattern of h-SnO2@Si-1 and h-SnO2@Si-2 reveal the polycrystal silicon phase. XRD patterns of h-SnO2@Si-1 and h-SnO2@Si-2 are shown in Figure 3a. Characteristic peaks of silicon (28°, 47°, and 56°) and SnO2 (27°, 34°, 52°) are all broad peaks, corresponding to the polycrystalline nanostructure of Si and SnO2, consistent with SAED.31−34 It is worth noting that characteristic peaks of Sn at 30°, 32°, 44°, and 45° are clearly distinguishable, which indicates that SnO2 was partially reduced to Sn during the CVD process.35,36 To further investigate the state of Sn in h-SnO2@ Si, XPS measurement of Sn 3d 5/2 orbit is conducted, as shown in Figure 3b. The main peak located at 486.5 eV, corresponding to Sn4+ (93.7% content), indicates that most of the tin elements exist as SnO2.27 The peak located at 483.9 eV corresponds to Sn0, with a content of 6.7%. Combined with the results of XRD and XPS analysis, we can conclude that only a small amount of SnO2 was reduced to Sn0 in h-SnO2@Si. XPS spectra of Si 2p orbit are also fitted, as shown in Figure 3c. The main doublet, located at 99.1 and 99.7 eV, corresponds to Si0 (92.4% content). The peak located at a higher binding energy (100.0 B

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Figure 3. (a) XRD patterns of h-SnO2@Si. (b, c) XPS analysis (Sn 3d 5/2 and Si 2p) of h-SnO2@Si-2. (d) Nitrogen sorption isotherms of SnO2 hollow spheres, h-SnO2@Si-1, and h-SnO2@Si-2.

SnO2@Si-2 are calculated to be about 23.3% and about 32.4%, respectively. The electrochemical performance of h-SnO2@Si is characterized by galvanostatic cycling tests, as shown in Figure 4. The specific capacity is calculated based on the mass of active materials (Si and SnO2), mass loading of electrode is 1.9−2.0 mg cm−2. Figure 4a shows the charge/discharge profiles of hSnO2@Si-2 at current density of 100 mA g−1. A long irreversible capacity in the first discharge−charge curve corresponds to the growth of SEI.38,39 The discharge and charge capacities of the first cycle are 1218 mAh g−1 and 763 mAh g−1, which correspond to Coulombic efficiency of 62.6%. Plots of differential capacity of h-SnO2@Si-2 after first, second, 50th, and 100th cycles are presented in Figure 4b. Two reduction peaks at 0.05 and 0.19 V are attributed to the formation of Li−Si alloy reaction, and formation of Li−Sn alloy also contributes to the peak at 0.19 V.26,40,41 During the first charge curve, two peaks at 0.32 and 0.48 V indicate the dealloying reaction of LixSi, and the peak at 0.61 V is related to the dealloying reaction of LixSn, consistent with the previous report.42 Besides, the nearly identical peak values after the 50th and 100th cycles suggest little capacity loss during cycling. The galvanostatic cyclabilities of h-SnO2@Si-1 and h-SnO2@ Si-2, shown in Figure 4c, were performed at current densities of 100 mA g−1 for the first three cycles and 300 mA g−1 for subsequent cycles. The h-SnO2@Si-2 exhibits an initial gravimetric specific capacity of 1218 mAh g−1 and a reversible gravimetric specific capacity of 778 mAh g−1 over 500 cycles. These values are much higher than the initial gravimetric specific capacity of 1118 mAh g−1 and the reversible gravimetric specific capacity of 579 mAh g−1 after 100 cycles. The better electrochemical performance of h-SnO2@Si-2 is owing to the thicker silicon layer, which not only is helpful to attain a high specific capacity, but also is more effective to prevent Sn from aggregation. Details about calculation of volumetric specific capacity of h-SnO2@Si-2 have been shown in the Supporting Information; thickness of electrode is based on the SEM studies about cross-section of h-SnO2@Si-2 electrode, as shown in Figure 4d. The h-SnO2@Si-2 exhibits an initial reversible volumetric specific capacity of 1615 mAh cm−3. Owing to the excellent capacity retention, the volumetric specific capacity is

Figure 2. SEM images of (a) SiO2 nanospheres, (b) SnO2 hollow spheres. HRTEM images of (c, d) SnO2 hollow spheres, (e, f) hSnO2@Si-1, and (g, h) h-SnO2@Si-2.

eV) corresponds to SiOx (0 < x < 2),37 with a proportion of 7.6%. To investigate the specific surface area and pore volume distribution of as-prepared materials, nitrogen gas sorption measurements are conducted. Nitrogen adsorption and desorption isotherms of SnO2 hollow spheres, h-SnO2@Si-1, and h-SnO2@Si-2 are shown in Figure 3d. The cone shape isotherms at high relative pressures (P/P0 = 0.85−0.99) reveal the existence of mesopores. Results of specific surface area and pore volume are supplied in Table S1. Owing to the integrated silicon layer and enhanced density of material, the specific surface area and pore volume decrease obviously with the increase of silicon thickness. TGA is utilized to estimate the content of Si and SnO2 in h-SnO2@Si, as supplied in Figure S1. According to the mass increase, which is owing to the oxidation of Si to SiO2, the contents of silicon in h-SnO2@Si-1 and hC

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Figure 4. Electrochemical performance of h-SnO2@Si samples. (a) Charge and discharge profiles of h-SnO2@Si-2 at 100 mA g−1 in the first and second cycles. (b) Differential capacity of h-SnO2@Si-2 in different cycles. (c) Gravimetric specific capacity of h-SnO2@Si-1 and h-SnO2@Si-2; volumetric specific capacity of h-SnO2@Si-2 is shown in the right coordinate axis. (d) SEM image of cross-section of h-SnO2@Si-2 electrode. (e) Coulombic efficiency of h-SnO2@Si-2. (f) Volumetric specific capacity of h-SnO2@Si-2 at various charge and discharge rates.

higher than 1030 mAh cm−3 after 500 cycles, which is much better than that of practical graphite anode (560 mAh cm−3). Moreover, h-SnO2@Si-2 also demonstrates excellent Coulombic efficiency, as shown in Figure 4e. The first cycle Coulombic efficiency of h-SnO2@Si-2 is 62.6%, which increased to 95.1% after the second cycle and finally stabilized to 99.95% at the subsequent cycles. Rate capability of h-SnO2@Si-2 is shown in Figure 4f. With increased current density from 0.2 to 2 A g−1, the volumetric specific capacity of h-SnO2@Si-2 maintained at 558 m Ah cm−3. After the current density changed back to 0.2 A g−1, the specific capacity recovered to 1082 m Ah cm−3, which demonstrated excellent stability during high current density charge/discharge. To further study the structural stability of as-prepared material during cycles, the electrode of h-SnO2@Si-2, after 100 cycles, was taken out from cell and removed the residual electrolyte by dimethyl carbonate. The morphologies of electrode after cycles have been shown in Figure 5a and b. The h-SnO2@Si, surrounded by binder and conductive additives, can be observed in Figure 5a. The SnO2 hollow nanospheres and Si layer were maintained without fracture and

crack (Figure 5b), which indicates that the h-SnO2@Si structure can withstand the large volume change during repeated discharge/charge cycles. To investigate the evolution of SEI during cycles, electrochemical impedance spectroscopy (EIS) measurements were conducted after different cycles. Three-electrode cell with hSnO2@Si-2 as work electrode was assembled, and lithium foil was used as reference and counter electrode. Before EIS test, the cell was discharged to 0.01 V and then rested for 1 h to stabilize the potential. Figure 5c shows the EIS profiles of hSnO2@Si-2 for different cycles. The slope in low-frequency regime is attributed to the bulk diffusion of Li+, and the semicircle in midfrequency regime is generally associated with interfacial phenomena, including charge transfer and SEI growth between active particles and liquid electrolyte.43 The experimental curves were fitted by equivalent circuit, as shown in the inset of Figure 5c. RSur here was adopted to evaluate the whole surface resistance, and fitting results of RSur are supplied in Table S2. The value of RSur increases in the first three cycles and maintains at about 315 Ohm in the subsequent cycles, which indicates that the SEI remain stable during cycling. D

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Figure 5. (a, b) TEM images of h-SnO2@Si-2 after 100 cycles, electrolyte was washed away. (c) Nyquist plot of h-SnO2@Si-2 at the end of discharge after different cycles (inset is the equivalent circuit) and (d) DSC curves for h-SnO2@Si-2 after different cycles.

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To evaluate the amount of SEI growth directly, differential scanning calorimetry (DSC) after different cycles was conducted. In detail, batteries after different cycles were discharged to 0.01 V and remained at open-circuit for 2 h. Then cells were disassembled in the Ar-filled glovebox, and the retrieved electrodes were soaked in dimethyl carbonate solvent to remove the electrolyte and dried in vacuum. The electrode was scratched from the current collector and sealed into highpressure stainless steel crucibles for DSC test. Figure 5d shows the DSC curves of lithiated h-SnO2@Si-2 after different cycles. The exothermic peak among 70−75 °C is visible for all the curves, associated with the reaction between ROCOOLi (or ROLi) in SEI and Li from LixSi,44−46 and the heat flux of this exothermic peak can represent the amount of SEI growth. The heat flux among 70−75 °C is 0.081 W g−1 after 10 cycles and increases to 0.082 W g−1, 0.086 W g−1, and 0.086 W g−1 after 20, 50, and 100 cycles, respectively. Compared with DSC tests of silicon nanoparticles in our previous work,47 in which heat flux increases to 0.285 W g−1 after 50 cycles, we can conclude that the h-SnO2@Si structure can effectively confine the excessive growth of SEI. In summary, a novel structure of h-SnO2@Si nanospheres is successfully prepared by a facile template scarification/CVD method. The h-SnO2@Si can not only prevent aggregation of Sn, but also accommodate the volume expansion and confine the excessive growth of SEI. As a result, the as-prepared hSnO2@Si exhibited a high initial volumetric specific capacity of >1600 mAh cm−3 and demonstrated a capacity retention of >1000 mAh cm−3 over 500 cycles, which has great potential to be used in real batteries. Considering the high abundance of Si and SnO2 on the earth, the h-SnO2@Si will be one of the promising anode candidates for lithium-ion batteries with high volumetric energy density.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01674. TGA of h-SnO2@Si-1 and h-SnO2@Si-2, specific surface area and pore volume of SnO2 and h-SnO2@Si samples, resistance of the semicycle in EIS test, calculation of volumetric capacity of h-SnO2@Si-2 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-1062794234. Fax: +86-10-62794234. ORCID

Huiyu Li: 0000-0003-2462-1170 Xinping Qiu: 0000-0001-5291-7943 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate financial support from National Key Project on Basic Research (2015CB251104), Natural Science Foundation of China (U1664256), and China-US Electric Vehicle Project (S2016G9004).

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