Functional Materials Letters Vol. 4, No. 4 (2011) 377381 © World Scientific Publishing Company DOI: 10.1142/S1793604711002251
SnO2 NANOCRYSTALLITE: NOVEL SYNTHETIC ROUTE FROM DEEP EUTECTIC SOLVENT AND LITHIUM STORAGE PERFORMANCE C. D. GU*,†, Y. J. MAI, J. P. ZHOU and J. P. TU State Key Laboratory of Silicon Materials and Department of Materials Science and Engineering Zhejiang University, Hangzhou 310027, P. R. China *
[email protected] †
[email protected] Received 3 June 2011; Revised 11 August 2011
A new synthetic route to 4 nm grain-sized SnO2 was proposed which involved a homogeneous precipitation in a deep eutectic solvent (DES) at room temperature. The white SnO2 precipitate was obtained from SnCl2 2H2 O dissolved DES by injecting slow drop-wise H4 N2 H2 O under stirring. The as-prepared nanocrystalline SnO2 has a BrunauerEmmettTeller surface area of 65:12 m 2 /g with an average BarretlJoynerHalenda pore diameter of 3.5 nm. As anode for lithium ion batteries, the nanocrystalline SnO2 electrode delivered an initial charge capacity as high as 1045 mAh/g and its capacity retention is 58% after 30 cycles. Keywords: Tin oxide; deep eutectic solvent; ionic liquid; homogeneous precipitation; lithium-ion battery.
inactive Sn clusters during cycling.11,16 According to Kim's work, the SnO2 nanoparticles with a grain size below a critical value of 4 nm did not aggregate after cycles, yielding excellent capacity retention due to that the Sn nanoparticles undergo a reversible volume change without aggregation into larger Sn clusters during cycling.11 Therefore, significant effects should be done to develop facile synthetic routes to fabricate SnO2 with the critical grain size. A novel preparation technique to produce uniform nanoparticles is important for the production of battery materials. This thought formed the basis of this work. Room temperature ionic liquids (RTILs) bestow a series of advantages such as non-volatility, low toxicity, high thermal stability, and recyclability, making them a promising reaction media for nonaqueous processes to synthesize metal oxide nanostructures.1,7,17 Unlike the conventional RTILs, deep eutectic solvents18 (DESs) can be easily prepared at low cost and with high purity, which are promising solvents to be used in shape-controlled synthesis of nanomaterials.1921 Although tin oxide nanostructures had been demonstrated to be obtained from RTILs,17,2224 there are very few reports of the use of DESs in the synthesis of tin oxide and other metal oxide nanoparticles. In this letter, we proposed a novel one-pot route to synthesize 4 nm sized SnO2 nanocrystallites from the interesting
Tin oxide is a well-investigated semiconductor that offers a balanced combination of chemical, optical, and electronic properties resulting in an impressive range of technological applications in various fields such as gas sensors, optoelectronic devices, and lithium-ion batteries (LIB).1 Tin oxide is special in the respect that tin possesses a dual valency preferably attaining an oxidation state of 2 þ or 4 þ , which facilitates a variation in the surface oxygen composition. It is well established that the performance of the tin oxide-based devices is largely influenced by the nanostructure and crystallinity of tin oxide, as well as the oxygen deficiency and stoichiometry.2,3 Therefore, various research groups have been devoted to developing controllable synthetic routes to tune the morphology and composition of tin oxide.39 Owing to its high theoretically lithium storage capacity of 781 mAh/g when forming alloy Li4:4 Sn10 and low potential of lithium ion intercalation, SnO2 is regarded as one of the most promising candidates for anode materials in LIBs.8,1014 However, the detrimental pulverization problem as a result of the drastic volume change between Sn and Li4:4 Sn blocks the electrical contact pathways in the electrode and leads to rapid deterioration in capacity.15 Furthermore, small and active Sn particles would aggregate into larger and *Corresponding
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ethylene glycol/choline chloride (EG/ChCl) based DES at room temperature. The nanocrystalline SnO2 with a high BrunauerEmmettTeller (BET) surface area exhibited a satisfactory electrochemical performance in LIB. The 1ChCl:2EG based DES was prepared according to Ref. 18. SnO2 nanoparticles were facilely prepared via homogeneous precipitation at room temperature (253 ○ C). In a typical synthesis procedure, 4 ml of H4 N2 H2 O (SigmaAldrich, 85%) was added slowly dropwise to 100 ml 0.05 M SnCl2 2H2 O 1ChCl:2EG based DES under stirring. The reaction mixture was stirred for about 1 h. White precipitates were collected by centrifugation and sequentially rinsed with methanol and deionized water, then dried in vacuum at 60 ○ C for 24 h. Structure and morphology of the precipitate were analyzed by X-ray diffraction (XRD, Rigaku D/max-2550 with CuKα radiation) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20). X-ray photoelectron spectroscopy (XPS, AXIS UTLTRADLD) was used to characterize the composition and chemical state of the product using Al (monochromatic) Kα radiation with E ¼ 1486:6 eV. All core-level spectra were referenced to the C 1s peak at 284.8 eV assumed to originate from the surface hydrocarbon contamination. BET surface area and pore volume were tested at 77 K using 3H-2000PS (Beishide, Beijing). Electrochemical performances of the as-prepared SnO2 nanoparticles were investigated with two-electrode coin-type cells (CR 2025) and room temperature. The working electrodes were prepared by a slurry coating procedure. The slurry consisted of 85 wt.% as-synthesized powder, 10 wt.% acetylene black and 5 wt.% polyvinylidene fluorides (PVDF) dissolved in N-methyl pyrrolidinone (NMP) were incorporated on copper foils with 12 mm in diameter. Test cells were assembled in an argon-filled glove box with the metallic lithium foil as both the reference and counter electrodes, 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DME) (1:1 in volume) as the electrolyte, and a polypropylene (PP) micro-porous film (Cellgard 2300) as the separator. Chargedischarge tests were conducted on LAND battery program-control test system between 0.05 and 3.0 V at a rate of 50 mA/g. XRD patterns of the as-prepared precipitate are shown in Fig. 1(a). Two broad bands of diffraction are observed, peaking at around 2θ ¼ 2438 ○ and 4766 ○ , respectively, which indicates that the precipitate possesses a nanocrystalline structure. However, peaks centered at 26:6 ○ , 33: 8 ○ and 51:7 ○ can also be distinguished, which could be readily indexed to (110), (101) and (211) planes of the tetragonal rutile-like SnO2 (JCPDS #41-1445). The broad peaks imply the presence of nano-sized crystals, which is confirmed by HRTEM observations [Fig. 1(c)]. TEM images of the as-prepared SnO2 are shown in Figs. 1(b) and 1(c),
where SnO2 agglomerates with the size of about 100 nm can be found. Figure 1(c) shows the SnO2 agglomerates are formed by plenty of uniform spherical SnO2 nanocrystallites with a 4 nm-sized distribution. The lattice fringe of nanocrystalline SnO2 with (110) orientation is also observed. The faintness ring patterns of the selected area diffraction (SAD) also confirm the very small grain sized structure of the SnO2 nanoparticles (Fig. 1(b) inset). The spot array of the fast Fourier transform (FFT) patterns [as shown in Fig. 1(d)] is assigned to f110g, f101g, and f211g Bragg reflections of tetragonal SnO2 verifying the formation of SnO2 nanoparticles. A possible formation mechanism of the nanocrystalline SnO2 is proposed as follows. Single nucleation event of Sn should firstly occur in the DES as Sn 2þ is reduced by H4 N2 H2 O. A possible template effect of the DES would confine the Sn particles within nanoscales and suppress the crystal growth.7,25 Since the DES is not degassed and no attempt is made to remove dissolved oxygen, Sn nanocrystallites are readily to be oxidized to SnO2 nanocrystallites due to the high chemical activity of nanoparticles. With the rinse process to remove the DES, the collected SnO2 precipitate is aggregated to larger clusters as shown in Fig. 1(b). However, the SnO2 nanostructure with nano-scaled grains remains as shown in Fig. 1(c). The final SnO2 precipitate is further characterized by nitrogen adsorption and desorption isotherms at 77 K. The corresponding results are given in Fig. 2, where the pore size distribution calculated using desorption isotherm is also presented as inset. The nanocrystalline SnO2 has a BET surface area of 65:12 m 2 =g calculated by multipoint BET method with an average BarretlJoynerHalenda (BJH) pore diameter of 3.5 nm and a pore volume of 0.20 cm 3 /g. The BET surface area of the sample is similar to the mesoporous SnO2 obtained by non-aqueous synthetic route.26 Moreover, the isotherm with a type IV with H3 hysteresis loop for the sample reveals that the SnO2 precipitate is composed of aggregates (loose assemblages) of nanoparticles forming slit-like pores, which is agreement with the TEM observations [Figs. 1(b) and 1(c)].27 All peaks in the XPS survey spectrum of the as-synthesized SnO2 sample can be ascribed to Sn, C, and O, as shown in Fig. 3(a). The presence of C comes from pump oil in the vacuum system of the XPS equipment, which indicates that the surface layer of the product is only composed of Sn and O. The compositional analysis using the area of the most intense XPS peaks (Sn 3d5=2 and O 1s) and taking into account the atomic sensitivity factor of each peak has been carried out using the CasaXPS software. The analysis reveals the O/Sn atomic ratio to be around 2.75 in the SnO2 sample, which is higher than the value of stoichiometric SnO2 . As shown in Fig. 3(b), the peaks profiles of Sn 3d are symmetric
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and center at 486.5 and 895.0 eV, which can be assigned to Sn 3d5=2 and Sn 3d3=2 corresponding to Sn 4þ , respectively.28 The deconvolution of the asymmetry O 1s peak [Fig. 3(c)] reveals that the main component is located at 530.5 eV corresponds to O-Sn 4þ bonding, and a component of unknown origin is located at 531.7 eV, which can be attributed to the surface chemisorbed oxygen.2,29,30 The slightly higher (>2) value of the estimated O/Sn ratio of the present SnO2 sample could be related to the small contribution from this highenergy O 1s component. Generally, the Sn 3d5=2 peak in SnOx can be built up of the following three components: Sn 4þ (486.4 eV), Sn 2þ shifted with respect to Sn 4þ towards the lower binding energy by 0.7 eV, and Sn 0 shifted with respect to Sn 4þ towards the lower binding energy by 2.2 eV.28 The O 1s peak in SnOx can be built up of the following two components: O-Sn 4þ (530.37 eV) and O-Sn 2þ shifted with respect to O-Sn 4þ towards the lower binding
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energy by 0.6 eV. 3 As the Sn 3d5=2 and O 1s peaks in the present SnO2 sample are built up by only Sn 4þ and O-Sn 4þ components, respectively, and the peaks corresponding to Sn 2þ and O-Sn 2þ are not detected, it can be concluded that the stoichiometric SnO2 has been obtained via the proposed homogeneous precipitation method in a DES-based solution at room temperature. The cycling performance of the SnO2 as an anode in LIB is shown in Fig. 4, which shows that the Coulombic efficiency keeps steadily more than 95%. It is also found that the SnO2 sample delivers a satisfactory initial charge capacity as high as 1045 mAh/g and a large capacity loss (48%) which could be attributed to the irreversible reaction of the conversion of SnO2 to Sn and Li2 O and the SEI formation.31 Although the capacity fades gradually with repeated cycling, a reversible capacity of 620 mAh/g is maintained for more than 30 cycles and its capacity retention is 58%. As shown in Fig. 4, the lithium storage performance of the SnO2 is
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comparable to those of the SnO2 nanosheets [at 78.2 mA/g and potential range ¼ 0:053 V (see Ref. 12)] and the SnO2 /Graphene nanosheets (GNS, at 50 mA/g and potential range ¼ 0:052 V).13 The abundant surface defects and large surface-to-volume ratio of the 4 nm grain sized SnO2 sample would effectively enhance the surface electrochemical reactivity and improve Li ion storage capacity.13,3133 Another possible reason for the high reversible capacity of the 4 nm grain sized SnO2 might be related to the size of SnO2 aggregations and the size of SnO2 grains. Prior work has shown that both parameters play critical roles in the anode performances.8 Moreover, SnO2 with the grain size less than 4 nm could help to limit the aggregation of tin during cycling.11 However, how the SnO2 aggregation affects the anode performance should be investigated to understand the underlying electrochemical mechanism of the novel SnO2 nanocrystallites in LIBs. In summary, we suggested a new chemical route to prepare SnO2 nanocrystallites through facile homogeneous precipitation in a DES-based solution at room temperature. The nanocrystalline SnO2 exhibited a high surface area, which make it to be a promising anode material for LIB. The present investigation is expected to stimulate interest of the development of high performance nanomaterials for the facile fabrications from DESs.
Acknowledgment The work was supported by the National Natural Science Foundation of China (51001089), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20100101120026), the Research Foundation of Education Bureau of Zhejiang Province (Y200906938), the Fundamental Research Funds for the Central Universities (2009QNA4006), and the 2010 Natural Science Basic Research Open Foundation of the Key Lab of Automobile Materials, Ministry of Education, Jilin University (Grant No. 11-450060445349).
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