Synthesis and electrochemical characterizations of Ce doped SnS2 anode materials for rechargeable lithium ion batteries

Synthesis and electrochemical characterizations of Ce doped SnS2 anode materials for rechargeable lithium ion batteries

Electrochimica Acta 93 (2013) 120–130 Contents lists available at SciVerse ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/loca...

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Electrochimica Acta 93 (2013) 120–130

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Synthesis and electrochemical characterizations of Ce doped SnS2 anode materials for rechargeable lithium ion batteries Qiufen Wang a,b , Ying Huang a,∗ , Juan Miao b , Yang Zhao a , Yan Wang a a The Key Laboratory of Space Applied Physics and Chemistry, Ministry of Education, Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710129, People’s Republic of China b School of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454000, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 6 October 2012 Received in revised form 11 January 2013 Accepted 15 January 2013 Available online 23 January 2013 Keywords: Nanocomposites Ce SnS2 Hydrothermal route Electrochemical properties

a b s t r a c t The nanocomposites Ce doped SnS2 (Ce SnS2 ) have been synthesized by a hydrothermal route. The Ce SnS2 composites exhibit 3D flowerlike structures. The particle sizes of each petal are in the range from 100 to 200 nm with clear lattice fringes. The electrode cycling performance and rate retention ability of Ce SnS2 are better than those of SnS2 as anode electrodes materials for lithium ion batteries. The Ce SnS2 compound (Ce of 5 mol%) shows the best reversible capacities and cycling performance among the synthesized Ce SnS2 compounds. The reason is that the part of large-radius cerium ions (much larger than that of Sn4+ ) can be the substitutes for Sn4+ in the SnS2 lattice. The expansion of the crystal lattice can provide more lattice space for lithium intercalation and de-intercalation, and further improves the cycling performance of Ce SnS2 . © 2013 Elsevier Ltd. All rights reserved.

1. Introduction With the development of modern portable electronic devices, communication devices and hybrid electric vehicles, graphite as the anode electrode for lithium ion batteries cannot meet the growing demand of high-energy application fields because of its low theoretical capacity (372 mAh g−1 [1,2]). In recent years, some tin-based materials [3–13] have been explored as high-capacity (>600 mAh g−1 ) anode electrodes for lithium ion batteries. Among these materials, SnS2 attracts researchers’ more attention for the higher reversible capacities than that of the traditional graphite [14–17]. The reaction mechanism has been proposed for SnS2 with lithium as follows [18]:

formed can improve the electrochemical properties of the electrodes greatly [18,22–24]. Moreover, rare earth doped electrode materials, such as Ce LiMn2 O4 [25], In SnS2 [26], LiRx Mn2−x O4 (R = La3+ , Ce3+ , Pr3+ ) [27] and Ce TiO2 [28] have been synthesized to improve the cycle life and structural stability because some largeradius rare earth ions could be the substitutes for Sn+ in the crystal lattice. The expansion of the crystal lattice provides more lattice space for lithium intercalation and de-intercalation. To improve the reversible capacities and cycling properties of SnS2 electrode materials, in this work, Ce doped SnS2 composites are synthesized by a hydrothermal route and are compared with SnS2 in terms of structure, morphology and the electrochemical properties as negative electrode materials for lithium ion batteries.

SnS2 + 4Li+ + 4e− → Sn + 2Li2 S

2. Experimental

+



Sn + xLi + xe ↔ Lix Sn(0 ≤ x ≤ 4.4)

(1) (2)

According to the mechanism, the large expansion-contraction volume occurs when Li+ is alloyed and de-alloyed or the metal is reduced and oxidized, which leads to large initial irreversible capacity and poor cycle performance [19–21]. Thus, some means have been adopted to deal with the problem. By reducing the particle size to nanoscale or coating or doping the active particles with the tin sulfur compounds, the composites

2.1. Materials Tin(IV) chloride pentahydrate (SnCl4 ·5H2 O), thioacetamide (CH3 CSNH2 ), cetyl trimethyl ammonium bromide (CTAB) and cerium nitrate (Ce(NO3 )3 ·6H2 O) of analytical purity were supplied from Sinopharm Chemical Reagent Co., Ltd. (China) without further purification. 2.2. Preparation of Ce SnS2

∗ Corresponding author. Tel.: +86 29 88431636. E-mail addresses: [email protected], [email protected] (Y. Huang). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.01.072

The composites Ce SnS2 have been synthesized by a hydrothermal route. SnCl4 ·5H2 O (0.02 mol), Ce(NO3 )3 ·6H2 O (different

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mole ratio of Ce), CH3 CSNH2 (0.05 mol) and CTAB were dissolved in a mixture of alcohol and distilled water. The mixture was transferred into a Teflon-lined stainless steel autoclave (200 ml). The autoclave was maintained at 180 ◦ C for 12 h, and cooled naturally to room temperature. The solid product was centrifuged, rinsed and dried under vacuum at 45 ◦ C for 24 h to obtain the composites. The similar procedures were carried out to prepare the nanocomposites Ce SnS2 with different mole ratio of Ce as 0%, 1%, 2%, 5% and 10%. The nanocomposites were labeled as Ce0, Ce001, Ce002, Ce005 and Ce010, respectively. 2.3. Characterization The structure and surface morphology of the prepared samples were characterized by X-ray diffraction analysis (XRD, PANalytical, Holland), scanning electron microscope (SEM, VEGA 3 LMH), LS Particle Size Analyer (LS 13 320, Beckman Coulter), inductively coupled plasma-atomic emission spectroscopy (ICP-AES, ACTIVAS, Horiba Jobin Yvon) 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. Raman spectra of the composite samples were obtained by using a Via Laser-Raman spectrometer (Renishaw Co., England) with a 514 nm radiation. 2.4. Electrochemical measurements Electrochemical measurements were carried out by using two electrode 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. The Microporous polypropylene membrane of Celgard 2400 was used as a separator. The electrolyte was made by 1 mol l−1 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 V 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 mV s−1 between 0.01 V and 2.5 V vs. Li+ /Li. EIS was carried out by applying an ac cell potential of 0.5 V from 0.01 Hz to 100 kHz. All measurements were carried out at room temperature. 3. Results and discussion 3.1. Microstructural characterization of materials The crystal structures of the as-prepared samples (Ce0, Ce001, Ce002, Ce005 and Ce010) are confirmed by XRD. Results are shown in Fig. 1a. The diffraction peaks of these materials (0 0 1), (1 0 0), (1 0 1), (1 0 2), (1 1 0) and (1 1 1) at 2 = 15.03◦ , 28.20◦ , 32.12◦ , 41.89◦ , 49.96◦ and 52.45◦ can be observed which agrees well with the standard patterns of hexagonal SnS2 (JCPDS file card no. 230677), without other phases formed. This indicates that the cerium doped has not destroyed the crystal structure of SnS2 . Moreover, the diffraction peaks intensities of Ce SnS2 are weaken with the increasing of cerium contents, indicating the change of the crystal size and crystallization. The lattice parameters and the volume of the Ce SnS2 unit cell are calculated and summarized. The result is shown in Table 1. The lattice parameters and the volume of the unit cell increase with the doping degree. These indicate that the

Fig. 1. (a) XRD patterns of Ce0, Ce001, Ce002, Ce005 and Ce010; (b) Raman spectra of Ce0 and Ce005.

large-radius cerium ions (much larger than that of Sn4+ ) can be the substitutes for Sn4+ and cause lattice distortion [28]. The expansion of the lattice crystal could provide more lattice space for lithium intercalation and de-intercalation [25]. Fig. 1b shows the Raman spectra of Ce0 and Ce005. A strong Raman peak at around 311 cm−1 can be seen, corresponding to the A1g mode of SnS2 hexagonal phase [29,30]. The Raman peak of Ce SnS2 has not any change after the doping of cerium. However, the band at about 210 cm−1 for Eg mode is not observed in the Raman spectra, which could be attribute to the nanosize effect of Ce SnS2 [15,31,32]. To further investigate the surface structure of the composites, the XPS spectra of Ce005 has been provided in Fig. 2. It can be seen that the composites contain the elements Sn, S, and Ce. The Sn 3d spectra shows two peaks from 484 eV to 498 eV, corresponding to Sn 3d5/2 and Sn 3d3/2 [33] in Fig. 2a. Thus, the chemical state of Sn in the composite is Sn4+ . In addition, the binding energies of S 2p3/2 and S 2p1/2 can be observed at about 162.38 eV and 163.48 eV (Fig. 2b), respectively, which agrees with the reference values of SnS2 crystal [34]. For the Ce 3d5/2 and Ce 3d3/2 (Fig. 2c), the stronger peak at the higher binding energy side (886.38 eV) corresponds to Table 1 The lattice parameters and the volume of the Ce SnS2 unit cell. Composites

Lattice parameter (nm)

Unit cell volume (nm)3

Ce0 Ce001 Ce002 Ce005 Ce010

0.364085 × 0.591775 0.365086 × 0.593077 0.365223 × 0.595622 0.367047 × 0.598014 0.367803 × 0.600003

67.94 × 10−3 68.46 × 10−3 68.8 × 10−3 69.77 × 10−3 70.29 × 10−3

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Fig. 2. XPS of Ce005: (a) Sn 3d; (b) S 2p; (c) Ce 3d and (d) Survey spectra.

the final state 3d9 4f2 . The peak position is the sign of the compound Ce(III) The weak peaks at the lower binding energy (905.4 eV and 901 eV) are u0 and u1 peak of spin orbital separation, corresponding to the two peaks of Ce 3d3/2 [35]. The interval energy from 886.38 eV to 905.4 eV is 19.02 eV. On the basis of the data, we can easily rule out the existence of Ce3+ in the cerium doped SnS2 . The atomic ratio of [Sn]:[S]:[Ce] is estimated to be 1:2:0.05 in Ce005 (Fig. 2d). The morphology and structure are also important factors for cycling performance. The morphology and structure of the samples are examined by SEM and TEM. Fig. 3 shows the SEM images of (a) Ce0, (b) Ce001, (c) Ce002, (d) Ce005 and (e) Ce010, respectively. The images (Fig. 3a–e) confirm the formation of SnS2 flowerlike structures with an average diameter of 2 ␮m. Each flowerlike structure is composed of several dozen nanopetals with smooth surfaces. The nanopetals are connected to each other to form porous 3D flowerlike structures. This indicates that the cerium doped has not destroyed the crystal surface morphology of SnS2 . Compared with these micrographs of the cerium doped and undoped SnS2 , some obvious differences of the particle morphology could be observed. With the low cerium doped contents, the crystallizations of Ce SnS2 become complete and the grains become regular (Fig. 3b–d). The regular particles can shorten the diffusion path of the lithium ion as well as increase the area for electrode reaction, which is a benefit to improve the electrochemical performance. But for the higher contents of the cerium doped (Fig. 3e), the crystallizations become low, which suggests that higher cerium doped SnS2 has slightly changed the original phase structure. In addition, Fig. 3f is corresponding figure of the Ce SnS2 particle size distribution. It can be seen that the particle sizes of the as-prepared materials are distributed mainly into micrometer and nanometer grades. The particle sizes from Ce0, Ce001, Ce002 and Ce005 are reduced with the increasing of cerium doped contents, while those of the nanometer grades are put on with increasing cerium doping, indicating that the Ce005 particles with uniform shape and size distribution. But the opposite result can be obtained for the higher

content of the cerium doped (Ce010). So doped by cerium can influence the particle size and distribution of SnS2 , and further improves the electrochemical performance of SnS2 anode materials. Table 2 shows the ICP analysis of the composite. The molar ratio of [Sn]:[S] is 1:1.987 in Ce0, which is roughly consistent with the molar ratio of SnS2 . Along with the cerium doped, the molar ratios of [Sn]:[S]:[Ce] for Ce001, Ce002, Ce005, Ce010 are 1:1.959:0.0099, 1:1.957: 0.0184, 1:1.999:0.0501 and 1:1.958:0.0929, respectively. The ICP results are approximately in agreement with the composition as designed one. Figs. 4a and c are the TEM images of Ce0 and Ce005 samples. The TEM images are in agreement well with the morphology as presented in the SEM pictures. Many nanopetals are connected to each other to form porous flowerlike spheres. The particle sizes of each petal are in the ranges from 100 nm to 200 nm. Figs. 4b and d represent the individual petal of Ce0 and Ce005 with a high magnification. The nanopetals exhibit the fringes with a spacing of 0.589 nm (Fig. 4b) and 0.595 nm (Fig. 4d), which can be assigned to the (0 0 1) plane of SnS2 . This indicates the spacing lattice SnS2 increases after the cerium doping, which is a benefit for lithium intercalation and de-intercalation [25]. 3.2. Electrochemical properties Figs. 5a and b show the discharge–charge curves in the first cycle and the cycling performance of the electrodes made from Ce0, Ce001, Ce002, Ce005 and Ce010, respectively. The cell potential window is set between 0.01 V and 2.0 V vs. Li+ /Li at the rate of 0.1 C. Results show that the discharge plateau emerges at 1.0–1.45 V and 0.01–0.5 V while the charge plateau emerges at 1.0–1.45 V and 0.4–0.6 V. The initial discharge capacities of Ce0, Ce001, Ce002, Ce005 and Ce010 are 1647.7 mAh g−1 , 1303.7 mAh g−1 , 1097.7 mAh g−1 , 998.2 mAh g−1 and 984.4 mAh g−1 , while their initial charge capacities are 669.98 mAh g−1 , 655.9 mAh g−1 , 607.6 mAh g−1 , 569.7 mAh g−1 and

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Fig. 3. SEM images of (a) Ce0; (b) Ce001; (c) Ce002; (d) Ce005 and (e) Ce010; (f) particle size distribution.

547.7 mAh g−1 , respectively (Fig. 5a). According to the reactions (1) and (2), the theoretical initial discharge capacity of SnS2 should be 1232.2 mAh g−1 , which is the irreversible capacity (586.8 mAh g−1 ) and reversible capacity (645.4 mAh g−1 ) [14]. These irreversible

capacities are attributed to the formation of Li2 O and solidelectrolyte interface (SEI) during the first charge–discharge process [21]. The initial irreversible capacities of Ce0 and Ce001 are larger than the theoretical value. The flowerlike nanostructures with a

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Table 2 The chemical composition results of ICP analysis of Ce SnS2 . Samples

Ce0 Ce001 Ce002 Ce005 Ce010

Wt.% (mol%) S

Sn

Ce

[Sn]:[S]:[Ce] (mol)

36.087 (66.525) 34.019 (65.985) 33.618 (65.471) 33.684 (65.564) 31.763 (64.182)

63.913 (33.475) 65.263 (33.688) 64.897 (33.905) 62.591 (32.791) 61.784 (32.771)

0 (0) 0.718 (0.327) 1.485 (0.624) 3.725 (1.645) 6.453 (3.047)

1:1.987:0 1:1.959:0.0099 1:1.957:0.0184 1:1.999:0.0501 1:1.958:0.0929

large surface area can accelerate the side-reactions of the electrode with electrolytes, leading to a large amount of irreversible trapped lithium [14,36,37]. Moreover, the initial irreversible capacities of the electrodes made from Ce0, Ce001, Ce002 and Ce005 are reduced with the increasing of cerium doped contents. The Ce005 compound has the lowest initial irreversible capacities among the synthesized Ce SnS2 compounds. This suggests that the largeradius cerium ions could be the substitutes for Sn4+ and cause the expansion of the lattice crystal, which could provide more lattice space for lithium intercalation and de-intercalation. At the same time it has promoted side-reactions with the electrolytes and the lithium intercalation of SnS2 . But compared to Ce005, Ce010 has lower initial capacity and higher irreversible capacity during cycling. The higher cerium doped SnS2 has low Sn ions in lattice structures because only the amount of Sn ions contributes to charge–discharge capacities during the electrochemical reaction

[25]. Thus, the Ce005 compound has the lowest initial irreversible capacities among the synthesized Ce SnS2 compounds. After 50 cycles (Fig. 5b), the capacities of Ce0, Ce001, Ce002, Ce005 and Ce010 retain 160.5 mAh g−1 , 229.8 mAh g−1 , 358.3 mAh g−1 , 450.7 mAh g−1 and 265 mAh g−1 . The Ce SnS2 compound (Ce005) shows the best reversible capacities and cycling performance among the synthesized Ce doped SnS2 compounds while the SnS2 compound (Ce0) exhibits the poorest one. The fast capacity fading for the electrode of Ce0 (undoped SnS2 ) is ascribed to the large volume expansion occurring and the collapse of electrode during the cycling process [11,21,25]. But the cycling performance of Ce doped SnS2 is better than that of pure SnS2 , which can be described that the part of large-radius cerium ions (much larger than that of Sn4+ ) could be the substitutes for Sn4+ into lattice. The expansion of the lattice crystal could provide much lattice space for lithium intercalation and de-intercalation, and further improves

Fig. 4. TEM images of (a, b) Ce0 and (c, d) Ce005.

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Fig. 6. The cyclic voltammograms of the electrode from (a) Ce0 and (b) Ce005. The scan rate is 0.2 mV s−1 between 0.01 V and 2.5 V.

Fig. 5. (a) The discharge–charge curves in the first cycle and (b) the cycling performance of the electrodes made from Ce0, Ce001, Ce002, Ce005 and Ce010. The cell potential window is set between 0.01 V and 2.0 V vs. Li+ /Li at the rate of 0.1 C. (c) The cycling performance of Ce0 and Ce005 electrode at different rates (0.1 C, 0.3 C, 0.5 C, 1.0 C and 0.1 C).

the cycling performance of Ce SnS2 . However, the cycling performance of the higher cerium doped SnS2 (Ce010) becomes poorer because the high cerium doped has slightly changed the original phase structure and provide excess lattice space, which may assist the co-insertion of solvent molecule during the lithium insertion. The co-inserted solvent molecules could increase the capacity fading [25]. To further investigate the rate capability of the electrode, the electrochemical properties of the Ce0 and Ce005 electrodes are measured by charge–discharge at different C-rates ranging from 0.1 to 1 C (0.1 C = 90 mA g−1 ) after 50 cycles, as shown in Fig. 5c. For the Ce005 electrode, it can be seen that the discharge capacity decreases from 447 mAh g−1 , 401 mAh g−1 , 370 mAh g−1 to

321.9 mAh g−1 with increasing C-rate ranging from 0.1 C, 0.3 C, 0.5 C to 1 C. The discharge capacity is reversibly back to 438 mAh g−1 once the charge–discharge rate is set back to 0.1 C again, revealing that almost 97% of the discharge capacity has been recovered. It may suggest that the capacity is stable at each rate. However, the Ce0 electrode delivers the reversible capacity of 158 mAh g−1 at 0.1 C. When the increasing rate is set back to 0.1 C again, the discharge capacity is reversibly back to 138 mAh g−1 , suggesting that only about 87% of the discharge capacity has been recovered. The results demonstrate that Ce–SnS2 possess high rate retention ability for lithium ion battery. The electrochemical characteristics of Ce SnS2 composites have been evaluated by cyclic voltammetry analysis. Fig. 6 displays the cyclic voltammetry curves of Ce0 and Ce005 electrode at 5 cycles. The scanning rate is 0.2 mV s−1 and the cell potential window is set between 0.01 V and 2.5 V vs. Li+ /Li in these two electrode systems. For the curves of Ce0 (Fig. 6a), two cathodic peaks are present at 1.2–1.3 V and 0.01–0.25 V, while two anode peaks are at about 1.5 V and 0.4–0.6 V in the first potential sweeping process. The reduction peak at 1.2–1.3 V may be ascribed to the formation of the solid electrolyte interface (SEI) film on the surface of the electrode, the reduction of SnS2 to Sn and the synchronous formation of Li2 S (Eq. (1)) [38,39,17,40–42], which should be responsible for the first irreversible capacity loss. The cathodic and anode peaks at 0.1–0.25 V and 0.4–0.6 V are attributed to the alloying and de-alloying process between Li and Sn (Eq. (2)) [40–42]. The weak anode peak at 1.5 V may be the subsidiary redox reaction between Sn and Li2 S [43,44]. During the subsequent cycles, the intercalation of lithium ions from Sn occurs at 0.01–0.25 V and the de-intercalation occurs at 0.6 V. Another weak cathodic and anode peaks of the subsidiary redox reaction between Sn and Li2 S are at 0.7–1.1 V and 1.4–1.7 V. In contrast, the CV curves for Ce005 cell shows almost the same feature as Ce0 cell (Fig. 6b). The broader

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Fig. 7. SEM images of the electrodes before (the insets) and after 50 cycles of (a) Ce0, (b) Ce001, (c) Ce002, (d) Ce005 and (e) Ce010.

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Fig. 8. Cross-sectional SEM images of the electrodes before (the insets) and after 50 cycles of (a) Ce0, (b) Ce001, (c) Ce002, (d) Ce005 and (e) Ce010.

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peaks of cathodic are observed at 1.3 V and 0.01–0.25 V while the corresponding anodic peaks are located at 1.3–1.7 V and about 0.6 V in the first potential sweeping process. During the subsequent cycles, the intensities of cathodic and anode peaks become stable. These features indicate that the curves of Ce005 have good reversibility and high intensity of peaks current, which improves the reversibility and electrochemical performances. To further understand the electrochemical performance of the as-prepared samples, the morphology variation of the Ce SnS2 electrodes after 50 discharge–charge cycles has been examined by SEM. Fig. 7 shows the SEM images of the Ce SnS2 electrodes before (the insets) and after 50 cycles. Fig. 8 is the cross-sectional SEM images of the electrodes of Ce SnS2 films on Cu substrates before (the insets) and after 50 cycles. It is obviously demonstrated that the Ce SnS2 particles are like flower and well-dispersed in the initial state (the insets of Fig. 7a–e). The cross-sectional morphologies of Ce SnS2 films on Cu substrates (the insets of Fig. 8a–e) are agreed with those of Ce SnS2 powers in the initial states, confirming the formation of the flowerlike structures. However, after 50 cycles, the flowerlike particles become withered, and the particles become smaller and agglomerated (Fig. 7a–e and Fig. 8a–e), indicating the pulverization of the particles during cycling. The most serious pulverization of pure SnS2 (Ce0) leads to the poor cycling stability. In contrast, the pulverization from the cycle electrodes of cerium doped SnS2 is reduced with the increasing of cerium doped contents (Fig. 7b–e and Fig. 8a–e). It indicates that cerium play an important role and can expand the lattice crystal structure and provide more lattice space in SnS2 . The lattice space can accommodate the strain and stress of volume change and obstacle the detachment and agglomeration of pulverized SnS2 during cycling, which consequently enhances the cyclic stability and rate capability [45]. The pulverization is the minimum for the Ce005 electrode. So, the Ce005 compound shows the best reversible capacities and cycle stability among the synthesized Ce SnS2 compounds. To further evaluate the diffusion of lithium ion in the samples, Fig. 9a shows EIS analysis of the electrodes of the samples at 0.5 V from 0.01 Hz to 100 kHz after 50 cycles. The equivalent circuit model is shown in Fig. 9b. 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 [11,17,40,41]. It is observed that the high frequency semicircle is corresponded to the resistance Rf and CPE1 of SEI film, the medium frequency region is assigned to the charge-transfer resistance Rct and CPE2 of electrode/electrolyte interface, and the 45◦ inclined line is assigned to the lithium-ion diffusion process inside the electrode materials. Obviously, the charge-transfer resistances of Ce0, Ce001, Ce002, Ce005 and Ce010 are 652.02, 263.14, 182.43, 145.25 and 154.78 , respectively. Both of the high frequency semicircle and the medium frequency region are smaller with the increasing of the cerium doped contents. The Ce005 compound has the lowest charge-transfer resistance among the synthesized Ce SnS2 compounds. Moreover, the EIS spectra fitted from their models agree with those of the experiment, indicating the equivalent circuit diagram is reasonable. The facts confirm that the cerium doped can improve the electron transport during lithium insertion and extraction process, and further enhance reaction kinetics. The diffusion coefficient of lithium ion can be obtained from the plots in the low–frequency region according to the following equation [46]:

D=



R2 T 2

 2

2A2 n4 F 4 C 2 бw

(3)

where R is the gas constant, T is the temperature, n is the number of electron per molecule oxidized, A is the area of the electrode surface, F is Faraday’s constant, C is the molar concentration of Li+ , D is the diffusion coefficient, and бw is the Warburg coefficient which has the relationship with Zre as follows: Zre = Rct + Rs + бw ω−1/2

(4)

where Rs is the resistance of the electrolyte, Rct is the chargetransfer resistance and ω is the angular frequency in the low frequency region. The relationship between Zre and ω−1/2 for Ce0, Ce001, Ce002, Ce005 and Ce010 samples in the low frequency region is shown in Fig. 9c. The slope of the fitted line is the Warburg coefficient бw . It is observed that the Warburg coefficients бw of Ce0, Ce001, Ce002, Ce005 and Ce010 are 105.23, 22.02, 19.96, 14.97 and 17.00  cm2 s−1/2 , respectively. The relevant diffusion coefficients of lithium ion are calculated to be 1.81 × 10−14 , 4.10 × 10−11 , 4.99 × 10−11 , 8.88 × 10−11 and 6.89 × 10−11 cm2 s−1 , respectively. As a consequence, the diffusion coefficient of lithium

Fig. 9. EIS of the electrodes at 0.5 V in the frequency range from 0.01 Hz to 100 kHz after 50 cycles; (b) the equivalent circuit model of the studied system and (c) the relationship between Zre and ω−1/2 at low frequencies for Ce0, Ce001, Ce002, Ce005 and Ce010 samples.

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ion is increased by the cerium doping, and Ce005 shows the highest lithium ion diffusion coefficient compared with the others. The enhancement of the diffusion coefficient is mainly attributed to the large radius of cerium ions which can cause lattice expansion, and the expansion of the crystal lattice can provide more lattice space for lithium intercalation and de-intercalation. Therefore, combining the above results, the reversible capacities and cycle stability of the Ce SnS2 sample can be attributed to two factors. On one hand, the improved performance is associated with the structural stabilization effect induced by cerium doping. Because cerium is not only involved in the electrochemical reaction, its existence in the crystal lattice can lessen the distortion of the crystal lattice during charge–discharge. Therefore, cerium doping can stabilize the crystal lattice during charge–discharge and thus improve the cycle stability. On the other hand, cerium doping can modify the electronic structure because the electronic state of Ce3+ is different from that of Sn4+ . The presence of the dopant may change the local electronic environment of the lithium ions and finally influence the diffusion behavior [45]. The diffusion behavior of the lithium ion may have an impact on the reversible capacities and cycle stability, which contribute to the better reversible capacities and cycle stability of the Ce SnS2 (Ce005) sample compared with the Ce0 (undoped SnS2 ) sample. Based on the above results, cerium doped SnS2 has improved the reversibility and electrochemical performance. 4. Conclusions The nanocomposites Ce doped SnS2 have been synthesized by a hydrothermal route. The structure, morphology and electrochemical properties of the as-prepared materials are characterized by XRD, Raman, XPS, SEM, LS Particle Size Analyer, ICP-AES, TEM and electrochemical measurements. Results show that Ce SnS2 composites exhibit 3D flowerlike structures. The particle sizes of each petals are from 100 to 200 nm with clear lattice fringes. The chemical state of Ce in Ce SnS2 is Ce3+ . The electrode cycling performance and rate retention ability of Ce SnS2 are better than those of SnS2 as anode electrodes materials for lithium ion batteries. The Ce SnS2 compound (Ce of 5 mol%) shows the best reversible capacities and cycling performance among the synthesized Ce SnS2 compounds. The reason is that some large-radius cerium ions (much larger than that of Sn4+ ) could be the substitutes for Sn4+ into lattice. The expansion of the lattice could provide much lattice space for lithium intercalation and de-intercalation, and enhance the diffusion of lithium ion, and further improves the reversible capacities and cycling performance of Ce SnS2 . Acknowledgments This work was supported by the Spaceflight Foundation of the People’s Republic of China under Grant no. NBXT0002 and the Spaceflight Innovation Foundation of China under Grant no. NBXW0001. References [1] Y.P. Wu, C. Jiang, C. Wu, R. Holze, Anode materials for lithium ion batteries by oxidative treatment of common natural graphite, Solid State Ionics 156 (2003) 283. [2] W.J. Zhang, A review of the electrochemical performance of alloy anodes for lithium-ion batteries, Journal of Power Sources 196 (2011) 13. [3] Y. Sharma, N. Sharma, G.V. Subba Rao, B.V.R. Chowdari, Lithium-storage and cycleability of nano-CdSnO3 as an anode material for lithium-ion batteries, Journal of Power Sources 192 (2009) 627. [4] N. Sharma, K.M. Shaju, G.V.S. Rao, B.V.R. Chowdari, Sol-gel derived nanocrystalline CaSnO3 as high capacity anode material for li-ion batteries, Electrochemistry Communications 4 (2002) 947. [5] P.A. Connor, J.T.S. Irvine, Novel tin oxide spinel-based anodes for Li-ion batteries, Journal of Power Sources 97–98 (2001) 223.

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