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polyaniline nanocomposites as negative electrode material for lithium-ion batteries
Applied Surface Science 258 (2012) 9896–9901
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Synthesis and properties of Li2 SnO3 /polyaniline nanocomposites as negative electrode material for lithium-ion batteries Qiufen Wang a,b , Ying Huang a,∗ , Juan Miao b , Yang Zhao a , Yan Wang a a b
School of Science, Northwestern Polytechnical University, Xi’an 710129, PR China School of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, PR China
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Article history: Received 27 March 2012 Received in revised form 10 June 2012 Accepted 14 June 2012 Available online 23 June 2012 Keywords: Nanocomposites Li2 SnO3 /polyaniline Micro emulsion polymerization method Electrochemical properties
a b s t r a c t The nanocomposites Li2 SnO3 /polyaniline (Li2 SnO3 /PANI) have been synthesized by a micro emulsion polymerization method. The structure, morphology and electrochemical properties of the as-prepared materials are characterized by XRD, FTIR, Raman, XPS, TGA, TEM and electrochemical measurements. Results show that Li2 SnO3 /PANI nanocomposites are composed of uniform and blocky nano-sized particles (40–50 nm) with clear lattice fringes. Electrochemical measurement suggests that Li2 SnO3 /PANI exhibits better cycling properties and lower initial irreversible capacities than Li2 SnO3 as negative electrodes materials for lithium-ion batteries. At a current density of 60 mA g−1 in the voltage about 0.05–2.0 V, the initial irreversible capacity of Li2 SnO3 /PANI is 563 mAh g−1 while it is 687.5 mAh g−1 to Li2 SnO3 . The capacity retained of Li2 SnO3 /PANI (569.2 mAh g−1 ) is higher than that of Li2 SnO3 (510.2 mAh g−1 ) after 50 cycles. The PANI in the Li2 SnO3 /PANI nanocomposites can buffer the released stress caused by the drastic volume variation during the alloying/de-alloying process of Li–Sn. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With the development of modern portable electronic devices, communication devices and hybrid electric vehicles, graphite as the negative electrode for lithium ion batteries cannot meet the growing demand of high-energy application fields for its low theoretical capacity (372 mAh g−1 [1,2]). In recent years, tin-based materials such as CdSnO3 [3], CaSnO3 [4], Mg2 SnO4 [5], CoSnO3 [6], LiCo1−x Snx O2 (x = 0–0.1) [7], LiMO2 (M = Co, Ni) [8], SnO2 –xAl2 O3 [9], TiO2 /SnO2 [10], Co/SnO2 [11], Zn/SnO2 [12], SnO2 /MnO [13] have been explored as high-capacity (∼993 mAh g−1 ) negative electrodes for lithium ion batteries. Among these materials, NaCl-type Li2 SnO3 attracts researchers’ more attention for its good properties [14,15]. For example, Li2 SnO3 has been prepared by a solid-state reaction route, a sol–gel route [16] and a ball-milled method [17] and used as the negative electrode materials for lithium-ion batteries. However, the large expansion-contraction volume occurs when Li+ is alloyed and de-alloyed or metal is reduced and oxidized [18–20], which leads to large initial irreversible capacity and poor cycle performance [21]. Thus, some means have been adopted to deal with these problems. By coating or doping some stable materials on the surface
∗ Corresponding author. Tel.: +86 29 88431636; fax: +86 15339205038. E-mail addresses: [email protected], [email protected], [email protected] (Y. Huang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.047
of the tin-based materials, the composites formed can greatly improve the electrochemical properties of the electrodes. Nowadays, Li2 SnO3 /C [22], Zn2 SnO4 /C [23] and SnO2 /CNT [24], F-doped SnO2 [25] have been deeply investigated. Moreover, conductive polymers polyaniline and polypyrrole have also been used as additives to improve the electrode performance. The reasons are that the polymer can provide a conducting backbone for the active materials and its soft structure matrix could buffer the internal stress of electrode suffering from large volume change [26–29]. For example, SnO2 @ polypyrrole [30] and SnO2 –polyaniline [31] composites have been synthesized to improve the electrode performance. To reduce the initial irreversible capacities and improve cycling properties of Li2 SnO3 , in this work, Li2 SnO3 /polyaniline is synthesized by a micro emulsion polymerization process and used as negative electrode materials for lithium-ion batteries.
2. Experimental 2.1. Materials Tin (IV) chloride pentahydrate (SnCl4 ·5H2 O), lithium hydroxide (LiOH·H2 O), absolute ethanol (CH2 OH), acetone, n-butylalcohol (C4 H9 OH) concentrated hydrochloric acid (HCl) and ammonium persulfate (APS) of analytical purity were supplied from Sinopharm Chemical Reagent Co., Ltd. (China) without further purification. Polyethylene glycol (PEG, molecular of 6000) and sodium dodecyl benzene sulfonate (SDBS, C18 H29 SO3 Na) as the surfactants of
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analytical purity were also supplied from Sinopharm Chemical Reagent Co., Ltd. (China). The monomer of aniline (AN) was used with further purification. 2.2. Preparation of nanocomposite Li2 SnO3 Li2 SnO3 was synthesized by a hydrothermal route. SnCl4 ·5H2 O (0.02 mol), LiOH (0.12 mol) and CTAB (0.5 g) were dissolved in a solution of 60 ml alcohol and 15 ml distilled water. The mixture was transferred into a Teflon–lined stainless steel autoclave (100 ml) and then maintained at 180 ◦ C for 12 h. The solid product was centrifuged, rinsed and dried under vacuum at 60 ◦ C for 12 h to obtain the precursor. Then the precursor was sintered at 800 ◦ C for 4 h under argon atmosphere to obtain Li2 SnO3 . 2.3. Preparation of nanocomposite Li2 SnO3 /PANI Li2 SnO3 /PANI was synthesized by a micro emulsion polymerization [31]. Firstly, the monomer pyrrole (0.12 mol) was dropped into the flask with the solution of C18 H29 SO3 Na (0.25 g) and the pH value was adjusted to 1–2. Then Li2 SnO3 (the molar ratio of Li2 SnO3 to pyrrole was 1:2) was added into the solution with stirring for 1 h. After that, (NH4 )2 S2 O8 water solution (the molar ratio of APS to pyrrole was 1:1) was dropped into the liquid and the reaction was maintained under ice–water conditions for 12 h. The emulsion was broken by acetone and the resulting composite was washed several times by HCl (1 mol l−1 ), deionized water and ethanol, and then dried at 60 ◦ C under vacuum. The similar procedures were carried out to prepare the nanocomposites Li2 SnO3 /PANI with different mass contents of Li2 SnO3 as 100%, 49.22%, 16.24%, and 0%. The nanocomposites were labeled as H1, H2, H3 and H4, respectively.
Fig. 1. XRD patterns of H1, H2, H3 and H4.
and 2.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were performed on a Series G 750TM Redefining Electrochemical Measurement (USA GMARY Co.). CV was carried out at a scan rate of 0.2 mVS−1 between 0 and 2.5 V vs Li/Li+ . EIS was carried out by applying an ac voltage of 0.5 V over the frequency range from 0.01 Hz to 100 kHz. All measurements were carried out at room temperature. 3. Results and discussion 3.1. Characterization 3.1.1. XRD analysis The crystal structures of the as-prepared samples are confirmed by XRD. Results are shown in Fig. 1. In the XRD curve of H1, the
2.4. Characterization The structure and surface morphology of the prepared samples were characterized by X-ray diffraction analysis (XRD) (XRD, PANalytical, Holland) and model Tecnai F30 G2 (FEI Co., USA) field emission transmission electron microscope (FETEM). XPS analysis was characterized by Thermal Scientific K Alpha photoelectron spectrometer equipped with monochromatized Al K␣ X-ray source. The fourier transform infrared spectroscopy (FTIR) spectra and Raman spectra of the composite samples were obtained by using Model NIcolETiS10 Fourier transform spectrometer (Thermo SCIENTIFIC Co., USA) with a 2 cm−1 resolution in the range of 400–4000 cm−1 and an inVia Laser-Raman spectrometer (Renishaw Co., England) with a 514 nm radiation. Thermal analysis of the composites was performed by thermal gravimetric analysis (TGA) (Model Q50, TA, USA) under a nitrogen atmosphere, with a heating rate of 20 ◦ C/min, and the temperature range from 20 to 800 ◦ C. 2.5. Electrochemical measurements Electrochemical measurements were carried out by using twoelectrode cells with lithium metal as the counter electrode. The working electrode was prepared by mixing the active materials, conducting carbon black and poly(vinylidene fluoride) (PVDF) binder at a mass ratio of 80:10:10. N-methylpyrrolidone (NMP) was used as a solvent to form homogeneous slurry. Microporous polypropylene membrane of Celgard 2400 was used as separator. The electrolyte was made by 1 M LiPF6 in a mixture of ethylene carbonate (EC)/diethyl carbonate (DMC)/ethylene methyl carbonate (EMC) (volume ratio of 1:1:1). The cells were assembled in an Ar-filled grove box. The charge–discharge measurements were measured on an eight-channel battery test system (Wu Han Land) between 0.01
Fig. 2. FTIR spectra (a) of H1, H2 and H4; Raman spectra (b) of H2.
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reduced diffraction intensity of Li2 SnO3 particles in the composites. The result confirms the formation of Li2 SnO3 /PANI composites.
Fig. 3. XPS of H2. (The insets show XPS spectra of N 1s, C 1s and Sn 3d.)
diffraction peaks of Li2 SnO3 (0 0 2), (0 2 0), (2 0 0), (2 0 2) and (0 6 0) at 2 = 17.97◦ , 19.34◦ , 34.56◦ ,41.91◦ and 60.46 ◦ can be observed which agree well with the standard patterns reported (JCPDS. File No. 31-0761). In the XRD curve of H4 (PANI), two broadened diffraction peaks at around 2 = 20.2◦ and 25.6◦ , which indicates the amorphous behavior of the polymer [32]. In the curve of H2 and H3, the peak intensities of Li2 SnO3 in the composites are weaker than that of the pure Li2 SnO3 and decreases with increasing of PANI content, which reveals that the PANI content has an effect on the peak intensity of Li2 SnO3 .This may indicate that there is a kind of interaction between PANI and Li2 SnO3 particles which resulted in the
3.1.2. Analysis of FTIR and Raman Fig. 2a shows the FTIR spectra of H1, H2 and H4. In the curve of H4, the peaks at 1574 and 1492 cm−1 are attributed to C C stretching vibration of the quinoid and benzenoid rings while the stretching vibration of C N is at 1296 cm−1 . The broad peak at 1132 cm−1 corresponds to the stretching vibration of N Q N (Q refers to the quinonic-type rings). The peak at 1037 cm−1 and 800 cm−1 are assigned to the in-plane vibrations of C H and the out-of-plane deformation of C H in the benzene rings [33–35], respectively. In the curve of H1, the strong vibrations around 519 and 478 cm−1 can be seen, which corresponds to the stretching vibration of Sn O Sn and Sn O groups [30]. The shape of H2 curve is almost identical to that of PANI while the peaks of the Sn O Sn and Sn O groups are slightly shifted to the lower wave number. It indicates that there is interaction between Li2 SnO3 and polymer in the composites, which makes the reductions of the electronic cloud of the polymer chain and the atomic force constant [36,37]. Therefore, FTIR results ascertain the presence of PANI in the as-prepared Li2 SnO3 /PANI nanocomposites. Fig. 2b shows the Raman spectra of H2. The prominent bands can be seen in the spectra of Li2 SnO3 /PANI. In the curve of H2, the band at 1559 cm−1 represents the C C stretching of the quinoid and benzenoid rings. The band at 1325 cm−1 is assigned to semiquinone
Fig. 4. TEM images of (a, b) H1 and (c, d) H2.
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energy range from 484 eV to 498 eV, which correspond to Sn 3d5/2 and 3d3/2 [44]. Thus, the chemical state of Sn in the composites Li2 SnO3 /PANI is Sn4+ . These confirm the formation of Li2 SnO3 /PANI composites.
Fig. 5. TGA curves of H1, H2 and H4.
radical structure in PANI while the band at 1173 cm−1 is attributed to in-plane C H bending of the quinoid ring. The band at 427 cm−1 is related to Sn O torsional mode, and the band at 576–766 cm−1 is ascribed to the Sn O stretching mode [38,39].Therefore, Raman spectra of Li2 SnO3 /PANI are almost agreed well with FTIR spectra. 3.1.3. XPS analysis To further investigate the surface structure of composites, the XPS spectrum of H2 has been provided in Fig. 3. It can be seen that all the composites contain the elements Sn, O, C, N, and Li. The small inset figures in Fig. 3a present the characteristic peaks of C 1s, N 1s and Sn 3d of Li2 SnO3 /PANI, respectively. The N 1s spectra of Li2 SnO3 /PANI show the peaks in the energy range from 398 eV to 401 eV, which corresponds to C N (imine), C N(amine) and protonated amine groups [40–42]. The N 1s spectra can be seen between 284 eV and 289 eV, which are agreement with the peaks of C N, C N, C O and C C [43]. The Sn 3d spectra of Li2 SnO3 /PANI show two peaks in the
3.1.4. Morphology Fig. 4 shows TEM images of H1 and H2, respectively. It can be seen that pure Li2 SnO3 (Fig. 4a and b) is composed of agglomerated nano-crystallites with clear lattice fringes. The particle sizes are in the range of 40–50 nm. The 0.46 nm spacing of lattice (in inset of Fig. 4b) corresponds to the (0 2 0) plane of the monoclinic Li2 SnO3 . Fig. 4c and d show that Li2 SnO3 nanocrystallites are homogeneously distributed in the PANI matrix to form the Li2 SnO3 /PANI composites. Compared with the pure Li2 SnO3 , Li2 SnO3 /PANI composites can exhibit uniform and blocky nano-sized particles (40–50 nm). The HRTEM image in insets of Fig. 4d shows the lattice fringes, which indicates the crystalline of Li2 SnO3 nanoparticles. The composites Li2 SnO3 /PANI are composed of polycrystalline Li2 SnO3 particles and amorphous PANI. The results are in accordance with the results obtained from XRD, FTIR, Raman and XPS. 3.1.5. TGA analysis To investigate the weight loss of the as-synthesized composite samples, thermogravimetric analysis has been carried out in a nitrogen atmosphere. The TGA curves of H1, H2 and H4 are shown in Fig. 5. The pure Li2 SnO3 (H1) almost has not any weight loss in the whole investigated temperature range. In the curves of H2, the first weight loss in the temperature between 20 ◦ C and 120 ◦ C is attributed to the release of the absorbed water, while the second weight loss in the range from 120 ◦ C to 800 ◦ C are ascribed to the oxidation of PANI and the desorption of the doping agents (HCl and SDBS). To the pure PANI (H4), step-wise weight decreases from room temperature to 140 ◦ C, 170–300 ◦ C, and 300–800 ◦ C can be observed. The weight losses in the temperature of 170–300 ◦ C are due to the desorption of doping agents (HCl and SDBS), while the weight losses from 300 ◦ C to 800 ◦ C are ascribed to the degradation of the conjugated chains and macromolecular chains [29,45,42]. 3.2. Electrochemical properties
Fig. 6. The discharge–charge curves in the first cycle (a) and the cycling performance (b) of H1, H2 and H3.The voltage window is set between 0.01 and 2.0 V and the current density is 60 mA g−1 .
3.2.1. Charge/discharge curves and cycling performance Fig. 6 shows the discharge-charge curves in the first cycle and the cycling performance of H1, H2 and H3. The voltage window is set between 0.01 and 2.0 V and the current density is 60 mA g−1 . Results show that the initial discharge capacities of H1, H2 and H3 are 2104.5, 1388.4 and 941.5 mAh g−1 while their initial charge capacities are 1417.0, 825.4 and 628.3 mAh g−1 (Fig. 6a). The first discharge-charge capacities of pure PANI are not detected for the low capacities [29]. Thus, their initial irreversible capacities are 687.5, 563 and 313.2 mAh g−1 , respectively. These irreversible capacities are attributed to the formation of Li2 O and solid-electrolyte interface (SEI) during the first charge/discharge process [21]. After 50 cycles (Fig. 6b), the capacities of H1, H2 and H3 retain 510.2, 569.2 and 532.4 mAh g−1 while H2 exhibits the best cycling performance. The results indicate that the cycling performance of Li2 SnO3 /PANI is better than that of pure Li2 SnO3 . The poor cycling performance of pure Li2 SnO3 is due to the large volume expansion occurring and the collapse of electrode during cycling [18–21]. However, for their high dispersion, good conductivity and soft structure [26–29], the amorphous PANI in the Li2 SnO3 /PANI nanocomposites can provide free space to buffer the release of the stress caused by the drastic volume variation during the Li–Sn alloying/de-alloying process, and further improve the utilization of active Li2 SnO3 .
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the low-frequency region. The impedance plots can be fitted by the equivalent circuit diagram (Fig. 8b). Rs is the electrolyte resistance, Rf is the SEI resistance, W is the Warburg impedance related to the diffusion of lithium ions into the bulk of the electrode materials, CPE1 and CPE2 are two constant phase elements associated with the interfacial resistance and charge-transfer resistance, respectively. Rct is the charge-transfer resistance [23]. Obviously, the Rct of Li2 SnO3 /PANI composite (11.08 ) is significantly smaller than that of Li2 SnO3 (1108 ), which indicates a smaller electrochemical reaction resistance because PANI can improve the electronic contact among the active particles. The EIS spectra fitted from their model is roughly agreed well with the result of the experiment, indicating the equivalent circuit diagram is reasonable. Fig. 7. The cyclic voltammograms of H2. The scan rate is 0.2 mVs−1 between 0 and 2.5 V vs Li/Li+ .
3.2.2. CV analysis The electrochemical characteristics of Li2 SnO3 /PANI composites (H2) have been evaluated by cyclic voltammetry analysis. Fig. 7 displays the cyclic voltammetry curves of H2 electrode at 3 cycles. The scanning rate is 0.2 mVS−1 and the voltage window is set between 0 and 2.5 V vs Li/Li+ in these two-electrode systems. The cathodic peaks are observed at 0.12–0.15 V and 0.70–0.80 V vs Li/Li+ while the anodic peaks are located at 0.5–0.7 V vs Li/Li+ in the first cycle. The reduction peak may be ascribed to the formation of the solid electrolyte interface (SEI) film on the surface of the electrode, the reduction of Li2 SnO3 to Sn and the synchronous formation of Li2 O [46–50]. In the following cycles, the negative electrode peaks can be found to move slightly to the left while the anodic peaks are shifted positively, which is attributed to the alloying and dealloying process between Li and Sn. So, the electrode reactions of the discharge–charge process can be written as [22]:
4. Conclusions Li2 SnO3 /PANI nanocomposites have been prepared by a micro emulsion polymerization method. Results show that it is composed of uniform and blocky nano-sized particles (40–50 nm) with clear lattice fringes. The electrode cycling performance of Li2 SnO3 /PANI is better than that of Li2 SnO3 as negative electrodes materials for lithium-ion batteries. At a current density of 60 mA g−1 in the voltage about 0.05–2.0 V, the retain capacity (569.2 mAh g−1 ) of Li2 SnO3 /PANI is higher than that of Li2 SnO3 (510.2 mAh g−1 ) after 50 cycle. The amorphous PANI in the Li2 SnO3 /PANI nanocomposites can provide free space to buffer the release of the stress caused by the drastic volume variation during the Li–Sn alloying/de-alloying process because of its highly dispersion, high conductivity and soft structure, which further improve the utilization of the active Li2 SnO3 . Acknowledgments
Li2 SnO3 + 4Li → 3Li2 O + Sn
This work was supported by the Spaceflight Foundation of the People’s Republic of China under grant no. N8XW0002.
Sn + xLi ↔ Lix Sn(x ≤ 4.4)
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3.2.3. EIS analysis Fig. 8 shows the EIS analysis of the electrodes of H1 and H2 at 0.5 V over the frequency range from 0.01 Hz to 100 kHz after 50 cycles. It can be observed that the impedance curves consist of one semicircle in the medium-frequency region and an inclined line in
Fig. 8. The EIS (a) of H1 and H2; the corresponding equivalent circuit (b).
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Synthesis and properties of Li2 SnO3 /polyaniline nanocomposites as negative electrode material for lithium-ion batteries Qiufen Wang a,b , Ying Huang a,∗ , Juan Miao b , Yang Zhao a , Yan Wang a a b
School of Science, Northwestern Polytechnical University, Xi’an 710129, PR China School of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2012 Received in revised form 10 June 2012 Accepted 14 June 2012 Available online 23 June 2012 Keywords: Nanocomposites Li2 SnO3 /polyaniline Micro emulsion polymerization method Electrochemical properties
a b s t r a c t The nanocomposites Li2 SnO3 /polyaniline (Li2 SnO3 /PANI) have been synthesized by a micro emulsion polymerization method. The structure, morphology and electrochemical properties of the as-prepared materials are characterized by XRD, FTIR, Raman, XPS, TGA, TEM and electrochemical measurements. Results show that Li2 SnO3 /PANI nanocomposites are composed of uniform and blocky nano-sized particles (40–50 nm) with clear lattice fringes. Electrochemical measurement suggests that Li2 SnO3 /PANI exhibits better cycling properties and lower initial irreversible capacities than Li2 SnO3 as negative electrodes materials for lithium-ion batteries. At a current density of 60 mA g−1 in the voltage about 0.05–2.0 V, the initial irreversible capacity of Li2 SnO3 /PANI is 563 mAh g−1 while it is 687.5 mAh g−1 to Li2 SnO3 . The capacity retained of Li2 SnO3 /PANI (569.2 mAh g−1 ) is higher than that of Li2 SnO3 (510.2 mAh g−1 ) after 50 cycles. The PANI in the Li2 SnO3 /PANI nanocomposites can buffer the released stress caused by the drastic volume variation during the alloying/de-alloying process of Li–Sn. © 2012 Elsevier B.V. All rights reserved.
1. Introduction With the development of modern portable electronic devices, communication devices and hybrid electric vehicles, graphite as the negative electrode for lithium ion batteries cannot meet the growing demand of high-energy application fields for its low theoretical capacity (372 mAh g−1 [1,2]). In recent years, tin-based materials such as CdSnO3 [3], CaSnO3 [4], Mg2 SnO4 [5], CoSnO3 [6], LiCo1−x Snx O2 (x = 0–0.1) [7], LiMO2 (M = Co, Ni) [8], SnO2 –xAl2 O3 [9], TiO2 /SnO2 [10], Co/SnO2 [11], Zn/SnO2 [12], SnO2 /MnO [13] have been explored as high-capacity (∼993 mAh g−1 ) negative electrodes for lithium ion batteries. Among these materials, NaCl-type Li2 SnO3 attracts researchers’ more attention for its good properties [14,15]. For example, Li2 SnO3 has been prepared by a solid-state reaction route, a sol–gel route [16] and a ball-milled method [17] and used as the negative electrode materials for lithium-ion batteries. However, the large expansion-contraction volume occurs when Li+ is alloyed and de-alloyed or metal is reduced and oxidized [18–20], which leads to large initial irreversible capacity and poor cycle performance [21]. Thus, some means have been adopted to deal with these problems. By coating or doping some stable materials on the surface
∗ Corresponding author. Tel.: +86 29 88431636; fax: +86 15339205038. E-mail addresses: [email protected], [email protected], [email protected] (Y. Huang). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.047
of the tin-based materials, the composites formed can greatly improve the electrochemical properties of the electrodes. Nowadays, Li2 SnO3 /C [22], Zn2 SnO4 /C [23] and SnO2 /CNT [24], F-doped SnO2 [25] have been deeply investigated. Moreover, conductive polymers polyaniline and polypyrrole have also been used as additives to improve the electrode performance. The reasons are that the polymer can provide a conducting backbone for the active materials and its soft structure matrix could buffer the internal stress of electrode suffering from large volume change [26–29]. For example, SnO2 @ polypyrrole [30] and SnO2 –polyaniline [31] composites have been synthesized to improve the electrode performance. To reduce the initial irreversible capacities and improve cycling properties of Li2 SnO3 , in this work, Li2 SnO3 /polyaniline is synthesized by a micro emulsion polymerization process and used as negative electrode materials for lithium-ion batteries.
2. Experimental 2.1. Materials Tin (IV) chloride pentahydrate (SnCl4 ·5H2 O), lithium hydroxide (LiOH·H2 O), absolute ethanol (CH2 OH), acetone, n-butylalcohol (C4 H9 OH) concentrated hydrochloric acid (HCl) and ammonium persulfate (APS) of analytical purity were supplied from Sinopharm Chemical Reagent Co., Ltd. (China) without further purification. Polyethylene glycol (PEG, molecular of 6000) and sodium dodecyl benzene sulfonate (SDBS, C18 H29 SO3 Na) as the surfactants of
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analytical purity were also supplied from Sinopharm Chemical Reagent Co., Ltd. (China). The monomer of aniline (AN) was used with further purification. 2.2. Preparation of nanocomposite Li2 SnO3 Li2 SnO3 was synthesized by a hydrothermal route. SnCl4 ·5H2 O (0.02 mol), LiOH (0.12 mol) and CTAB (0.5 g) were dissolved in a solution of 60 ml alcohol and 15 ml distilled water. The mixture was transferred into a Teflon–lined stainless steel autoclave (100 ml) and then maintained at 180 ◦ C for 12 h. The solid product was centrifuged, rinsed and dried under vacuum at 60 ◦ C for 12 h to obtain the precursor. Then the precursor was sintered at 800 ◦ C for 4 h under argon atmosphere to obtain Li2 SnO3 . 2.3. Preparation of nanocomposite Li2 SnO3 /PANI Li2 SnO3 /PANI was synthesized by a micro emulsion polymerization [31]. Firstly, the monomer pyrrole (0.12 mol) was dropped into the flask with the solution of C18 H29 SO3 Na (0.25 g) and the pH value was adjusted to 1–2. Then Li2 SnO3 (the molar ratio of Li2 SnO3 to pyrrole was 1:2) was added into the solution with stirring for 1 h. After that, (NH4 )2 S2 O8 water solution (the molar ratio of APS to pyrrole was 1:1) was dropped into the liquid and the reaction was maintained under ice–water conditions for 12 h. The emulsion was broken by acetone and the resulting composite was washed several times by HCl (1 mol l−1 ), deionized water and ethanol, and then dried at 60 ◦ C under vacuum. The similar procedures were carried out to prepare the nanocomposites Li2 SnO3 /PANI with different mass contents of Li2 SnO3 as 100%, 49.22%, 16.24%, and 0%. The nanocomposites were labeled as H1, H2, H3 and H4, respectively.
Fig. 1. XRD patterns of H1, H2, H3 and H4.
and 2.0 V. Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) were performed on a Series G 750TM Redefining Electrochemical Measurement (USA GMARY Co.). CV was carried out at a scan rate of 0.2 mVS−1 between 0 and 2.5 V vs Li/Li+ . EIS was carried out by applying an ac voltage of 0.5 V over the frequency range from 0.01 Hz to 100 kHz. All measurements were carried out at room temperature. 3. Results and discussion 3.1. Characterization 3.1.1. XRD analysis The crystal structures of the as-prepared samples are confirmed by XRD. Results are shown in Fig. 1. In the XRD curve of H1, the
2.4. Characterization The structure and surface morphology of the prepared samples were characterized by X-ray diffraction analysis (XRD) (XRD, PANalytical, Holland) and model Tecnai F30 G2 (FEI Co., USA) field emission transmission electron microscope (FETEM). XPS analysis was characterized by Thermal Scientific K Alpha photoelectron spectrometer equipped with monochromatized Al K␣ X-ray source. The fourier transform infrared spectroscopy (FTIR) spectra and Raman spectra of the composite samples were obtained by using Model NIcolETiS10 Fourier transform spectrometer (Thermo SCIENTIFIC Co., USA) with a 2 cm−1 resolution in the range of 400–4000 cm−1 and an inVia Laser-Raman spectrometer (Renishaw Co., England) with a 514 nm radiation. Thermal analysis of the composites was performed by thermal gravimetric analysis (TGA) (Model Q50, TA, USA) under a nitrogen atmosphere, with a heating rate of 20 ◦ C/min, and the temperature range from 20 to 800 ◦ C. 2.5. Electrochemical measurements Electrochemical measurements were carried out by using twoelectrode cells with lithium metal as the counter electrode. The working electrode was prepared by mixing the active materials, conducting carbon black and poly(vinylidene fluoride) (PVDF) binder at a mass ratio of 80:10:10. N-methylpyrrolidone (NMP) was used as a solvent to form homogeneous slurry. Microporous polypropylene membrane of Celgard 2400 was used as separator. The electrolyte was made by 1 M LiPF6 in a mixture of ethylene carbonate (EC)/diethyl carbonate (DMC)/ethylene methyl carbonate (EMC) (volume ratio of 1:1:1). The cells were assembled in an Ar-filled grove box. The charge–discharge measurements were measured on an eight-channel battery test system (Wu Han Land) between 0.01
Fig. 2. FTIR spectra (a) of H1, H2 and H4; Raman spectra (b) of H2.
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reduced diffraction intensity of Li2 SnO3 particles in the composites. The result confirms the formation of Li2 SnO3 /PANI composites.
Fig. 3. XPS of H2. (The insets show XPS spectra of N 1s, C 1s and Sn 3d.)
diffraction peaks of Li2 SnO3 (0 0 2), (0 2 0), (2 0 0), (2 0 2) and (0 6 0) at 2 = 17.97◦ , 19.34◦ , 34.56◦ ,41.91◦ and 60.46 ◦ can be observed which agree well with the standard patterns reported (JCPDS. File No. 31-0761). In the XRD curve of H4 (PANI), two broadened diffraction peaks at around 2 = 20.2◦ and 25.6◦ , which indicates the amorphous behavior of the polymer [32]. In the curve of H2 and H3, the peak intensities of Li2 SnO3 in the composites are weaker than that of the pure Li2 SnO3 and decreases with increasing of PANI content, which reveals that the PANI content has an effect on the peak intensity of Li2 SnO3 .This may indicate that there is a kind of interaction between PANI and Li2 SnO3 particles which resulted in the
3.1.2. Analysis of FTIR and Raman Fig. 2a shows the FTIR spectra of H1, H2 and H4. In the curve of H4, the peaks at 1574 and 1492 cm−1 are attributed to C C stretching vibration of the quinoid and benzenoid rings while the stretching vibration of C N is at 1296 cm−1 . The broad peak at 1132 cm−1 corresponds to the stretching vibration of N Q N (Q refers to the quinonic-type rings). The peak at 1037 cm−1 and 800 cm−1 are assigned to the in-plane vibrations of C H and the out-of-plane deformation of C H in the benzene rings [33–35], respectively. In the curve of H1, the strong vibrations around 519 and 478 cm−1 can be seen, which corresponds to the stretching vibration of Sn O Sn and Sn O groups [30]. The shape of H2 curve is almost identical to that of PANI while the peaks of the Sn O Sn and Sn O groups are slightly shifted to the lower wave number. It indicates that there is interaction between Li2 SnO3 and polymer in the composites, which makes the reductions of the electronic cloud of the polymer chain and the atomic force constant [36,37]. Therefore, FTIR results ascertain the presence of PANI in the as-prepared Li2 SnO3 /PANI nanocomposites. Fig. 2b shows the Raman spectra of H2. The prominent bands can be seen in the spectra of Li2 SnO3 /PANI. In the curve of H2, the band at 1559 cm−1 represents the C C stretching of the quinoid and benzenoid rings. The band at 1325 cm−1 is assigned to semiquinone
Fig. 4. TEM images of (a, b) H1 and (c, d) H2.
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energy range from 484 eV to 498 eV, which correspond to Sn 3d5/2 and 3d3/2 [44]. Thus, the chemical state of Sn in the composites Li2 SnO3 /PANI is Sn4+ . These confirm the formation of Li2 SnO3 /PANI composites.
Fig. 5. TGA curves of H1, H2 and H4.
radical structure in PANI while the band at 1173 cm−1 is attributed to in-plane C H bending of the quinoid ring. The band at 427 cm−1 is related to Sn O torsional mode, and the band at 576–766 cm−1 is ascribed to the Sn O stretching mode [38,39].Therefore, Raman spectra of Li2 SnO3 /PANI are almost agreed well with FTIR spectra. 3.1.3. XPS analysis To further investigate the surface structure of composites, the XPS spectrum of H2 has been provided in Fig. 3. It can be seen that all the composites contain the elements Sn, O, C, N, and Li. The small inset figures in Fig. 3a present the characteristic peaks of C 1s, N 1s and Sn 3d of Li2 SnO3 /PANI, respectively. The N 1s spectra of Li2 SnO3 /PANI show the peaks in the energy range from 398 eV to 401 eV, which corresponds to C N (imine), C N(amine) and protonated amine groups [40–42]. The N 1s spectra can be seen between 284 eV and 289 eV, which are agreement with the peaks of C N, C N, C O and C C [43]. The Sn 3d spectra of Li2 SnO3 /PANI show two peaks in the
3.1.4. Morphology Fig. 4 shows TEM images of H1 and H2, respectively. It can be seen that pure Li2 SnO3 (Fig. 4a and b) is composed of agglomerated nano-crystallites with clear lattice fringes. The particle sizes are in the range of 40–50 nm. The 0.46 nm spacing of lattice (in inset of Fig. 4b) corresponds to the (0 2 0) plane of the monoclinic Li2 SnO3 . Fig. 4c and d show that Li2 SnO3 nanocrystallites are homogeneously distributed in the PANI matrix to form the Li2 SnO3 /PANI composites. Compared with the pure Li2 SnO3 , Li2 SnO3 /PANI composites can exhibit uniform and blocky nano-sized particles (40–50 nm). The HRTEM image in insets of Fig. 4d shows the lattice fringes, which indicates the crystalline of Li2 SnO3 nanoparticles. The composites Li2 SnO3 /PANI are composed of polycrystalline Li2 SnO3 particles and amorphous PANI. The results are in accordance with the results obtained from XRD, FTIR, Raman and XPS. 3.1.5. TGA analysis To investigate the weight loss of the as-synthesized composite samples, thermogravimetric analysis has been carried out in a nitrogen atmosphere. The TGA curves of H1, H2 and H4 are shown in Fig. 5. The pure Li2 SnO3 (H1) almost has not any weight loss in the whole investigated temperature range. In the curves of H2, the first weight loss in the temperature between 20 ◦ C and 120 ◦ C is attributed to the release of the absorbed water, while the second weight loss in the range from 120 ◦ C to 800 ◦ C are ascribed to the oxidation of PANI and the desorption of the doping agents (HCl and SDBS). To the pure PANI (H4), step-wise weight decreases from room temperature to 140 ◦ C, 170–300 ◦ C, and 300–800 ◦ C can be observed. The weight losses in the temperature of 170–300 ◦ C are due to the desorption of doping agents (HCl and SDBS), while the weight losses from 300 ◦ C to 800 ◦ C are ascribed to the degradation of the conjugated chains and macromolecular chains [29,45,42]. 3.2. Electrochemical properties
Fig. 6. The discharge–charge curves in the first cycle (a) and the cycling performance (b) of H1, H2 and H3.The voltage window is set between 0.01 and 2.0 V and the current density is 60 mA g−1 .
3.2.1. Charge/discharge curves and cycling performance Fig. 6 shows the discharge-charge curves in the first cycle and the cycling performance of H1, H2 and H3. The voltage window is set between 0.01 and 2.0 V and the current density is 60 mA g−1 . Results show that the initial discharge capacities of H1, H2 and H3 are 2104.5, 1388.4 and 941.5 mAh g−1 while their initial charge capacities are 1417.0, 825.4 and 628.3 mAh g−1 (Fig. 6a). The first discharge-charge capacities of pure PANI are not detected for the low capacities [29]. Thus, their initial irreversible capacities are 687.5, 563 and 313.2 mAh g−1 , respectively. These irreversible capacities are attributed to the formation of Li2 O and solid-electrolyte interface (SEI) during the first charge/discharge process [21]. After 50 cycles (Fig. 6b), the capacities of H1, H2 and H3 retain 510.2, 569.2 and 532.4 mAh g−1 while H2 exhibits the best cycling performance. The results indicate that the cycling performance of Li2 SnO3 /PANI is better than that of pure Li2 SnO3 . The poor cycling performance of pure Li2 SnO3 is due to the large volume expansion occurring and the collapse of electrode during cycling [18–21]. However, for their high dispersion, good conductivity and soft structure [26–29], the amorphous PANI in the Li2 SnO3 /PANI nanocomposites can provide free space to buffer the release of the stress caused by the drastic volume variation during the Li–Sn alloying/de-alloying process, and further improve the utilization of active Li2 SnO3 .
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the low-frequency region. The impedance plots can be fitted by the equivalent circuit diagram (Fig. 8b). Rs is the electrolyte resistance, Rf is the SEI resistance, W is the Warburg impedance related to the diffusion of lithium ions into the bulk of the electrode materials, CPE1 and CPE2 are two constant phase elements associated with the interfacial resistance and charge-transfer resistance, respectively. Rct is the charge-transfer resistance [23]. Obviously, the Rct of Li2 SnO3 /PANI composite (11.08 ) is significantly smaller than that of Li2 SnO3 (1108 ), which indicates a smaller electrochemical reaction resistance because PANI can improve the electronic contact among the active particles. The EIS spectra fitted from their model is roughly agreed well with the result of the experiment, indicating the equivalent circuit diagram is reasonable. Fig. 7. The cyclic voltammograms of H2. The scan rate is 0.2 mVs−1 between 0 and 2.5 V vs Li/Li+ .
3.2.2. CV analysis The electrochemical characteristics of Li2 SnO3 /PANI composites (H2) have been evaluated by cyclic voltammetry analysis. Fig. 7 displays the cyclic voltammetry curves of H2 electrode at 3 cycles. The scanning rate is 0.2 mVS−1 and the voltage window is set between 0 and 2.5 V vs Li/Li+ in these two-electrode systems. The cathodic peaks are observed at 0.12–0.15 V and 0.70–0.80 V vs Li/Li+ while the anodic peaks are located at 0.5–0.7 V vs Li/Li+ in the first cycle. The reduction peak may be ascribed to the formation of the solid electrolyte interface (SEI) film on the surface of the electrode, the reduction of Li2 SnO3 to Sn and the synchronous formation of Li2 O [46–50]. In the following cycles, the negative electrode peaks can be found to move slightly to the left while the anodic peaks are shifted positively, which is attributed to the alloying and dealloying process between Li and Sn. So, the electrode reactions of the discharge–charge process can be written as [22]:
4. Conclusions Li2 SnO3 /PANI nanocomposites have been prepared by a micro emulsion polymerization method. Results show that it is composed of uniform and blocky nano-sized particles (40–50 nm) with clear lattice fringes. The electrode cycling performance of Li2 SnO3 /PANI is better than that of Li2 SnO3 as negative electrodes materials for lithium-ion batteries. At a current density of 60 mA g−1 in the voltage about 0.05–2.0 V, the retain capacity (569.2 mAh g−1 ) of Li2 SnO3 /PANI is higher than that of Li2 SnO3 (510.2 mAh g−1 ) after 50 cycle. The amorphous PANI in the Li2 SnO3 /PANI nanocomposites can provide free space to buffer the release of the stress caused by the drastic volume variation during the Li–Sn alloying/de-alloying process because of its highly dispersion, high conductivity and soft structure, which further improve the utilization of the active Li2 SnO3 . Acknowledgments
Li2 SnO3 + 4Li → 3Li2 O + Sn
This work was supported by the Spaceflight Foundation of the People’s Republic of China under grant no. N8XW0002.
Sn + xLi ↔ Lix Sn(x ≤ 4.4)
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3.2.3. EIS analysis Fig. 8 shows the EIS analysis of the electrodes of H1 and H2 at 0.5 V over the frequency range from 0.01 Hz to 100 kHz after 50 cycles. It can be observed that the impedance curves consist of one semicircle in the medium-frequency region and an inclined line in
Fig. 8. The EIS (a) of H1 and H2; the corresponding equivalent circuit (b).
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