Synthesis and properties of carbon-doped Li2SnO3 nanocomposite as cathode material for lithium-ion batteries

Synthesis and properties of carbon-doped Li2SnO3 nanocomposite as cathode material for lithium-ion batteries

Materials Letters 71 (2012) 66–69 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mat...

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Materials Letters 71 (2012) 66–69

Contents lists available at SciVerse ScienceDirect

Materials Letters journal homepage: www.elsevier.com/locate/matlet

Synthesis and properties of carbon-doped Li2SnO3 nanocomposite as cathode material for lithium-ion batteries Qiufen Wang a, 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 September 2011 Accepted 2 December 2011 Available online 8 December 2011 Keywords: Nanocomposite Carbon-doped Li2SnO3 Sol–gel preparation Carbothermic reduction process Electrochemical properties

a b s t r a c t Tin-based oxides Li2SnO3 precursor and carbon-doped Li2SnO3 (Li2SnO3/C) nanocomposite with good cycle performance for lithium-ion batteries have been synthesized by sol–gel preparation and carbothermic reduction process. Results show that Li2SnO3/C is composed of regular rod nanoparticles (20–30 nm) with clear lattice fringes, which exhibit better electrochemical properties than Li2SnO3. At a current density of 60 mA g− 1 in the voltage about 0.05–2.0 V, the first discharge–charge capacities of the composite Li2SnO3/ C are 1671.1 mAh g− 1 and 1558.6 mAh g − 1 while they are 1909.4 mAh g− 1 and 1745.8 mAh g− 1 to Li2SnO3. The retain capacity (576.9 mAh g− 1) of Li2SnO3/C is higher than that of Li2SnO3 after 50 cycles. © 2011 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

With the development of modern electronic devices and electric vehicles, the low theoretical capacity (372 mAh g − 1 [1]) and security of graphite for lithium-ion batteries cannot meet the growing demand of high-energy application fields. Tin-based materials have received considerable interests as high-capacity (~993 mAh g − 1) cathodes for lithium-ion batteries [2–5]. However, the large expansion–contraction volume occurs when Li + is alloyed and de-alloyed or metal is reduced and oxidized [6–8]. Some means have been adopted to deal with these problems. By coating or doping some carbon materials into tin-based materials, the composites formed can greatly improve the electrochemical properties of the electrodes, such as, Zn2SnO4/C [9] and SnO2/CNT [10], F-doped SnO2 [11], etc. In recent years, Li2MO3 [12] has attracted researchers’ attention because of its good electrochemical properties. Li2SnO3 has been successfully prepared by various methods such as a solid-state reaction route, a sol–gel route [13] and a ball-milled method [14]. But the first-cycle irreversible capacity of Li2SnO3 is large [15]. To reduce the first-cycle irreversible capacities and improve cycling properties of it, Li2SnO3/C is synthesized by carbothermic reduction process in our experiment to be used as cathode material for lithium-ion batteries.

A sol–gel method [13, 16] was employed to synthesize Li2SnO3. Firstly, 0.05 mol of SnCl4·5H2O, 2 mol of ethylene glycol, 0.5 mol of anhydrous citric acid and 0.05 mol of Li2CO3 were mixed and stirred at 90 °C. After the solution became completely clear, it was dried and then sintered in an YFX5/160-YC muffle furnace at 400 °C for 4 h and 700 °C for 5 h. The final product was a white powder. Li2SnO3/C was prepared through carbothermic reduction process. The as-prepared Li2SnO3 and glucose (C6H12O6, AR) were mixed together with a mass ratio of Li2SnO3:C6H12O6 = 6:2. The mixture was put into distilled water under magnetic stirring at 70 °C for 8 h and cooled naturally to room temperature. After that, the upper clear liquid was poured out and the solution was dried at 120 °C for 12 h to obtain the precursor. The dried mixture was sintered at 600 °C for 2.5 h under argon atmosphere to obtain the final composite Li2SnO3/C. The microstructures, surface morphology, specific surface area and porosity of Li2SnO3 and Li2SnO3/C were characterized by X-ray diffraction (XRD, PANalytical, Holland), scanning electron microscope (SEM, SuPRA 55, German ZEISS) equipped with an energy dispersive spectroscopy (EDS) system, transmission electron microscopy (TEM, Tecnai F30 G 2, American FEI), X-ray photoelectron spectroscopy (XPS, Thermal Scientific K Alpha) and Surface Area and Porosity Analyzer (Micromeritics, ASAP 2020), respectively. 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 65:15:20. N-methylpyrrolidone (NMP) was used as a solvent to form homogeneous slurry. Microporous polypropylene membrane of

⁎ Corresponding author. Tel.: + 86 29 88431636, 15339205038. E-mail addresses: [email protected], [email protected] (Y. Huang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.12.005

Q. Wang et al. / Materials Letters 71 (2012) 66–69

Celgard 2400 was used as a 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 electrochemical performances of the samples were measured on a battery test system (Wu Han Land) between 0.05 V and 2.0 V at room temperature. Cyclic voltammetry (CV) was also performed on a Series G 750™ Redefining Electrochemical Measurement (USA GMARY Co.) at a scan rate of 0.2 mV S− 1 between 0.0 V and 3.0 V versus Li/Li+.

b = 9.181 Å, and c = 10.027 Å (JCPDS No. 31-0761). A small amount of impurity phase SnO2 can also be detected in these two products. The highest intensity diffraction peaks are the plane (002) and the best growth direction of the crystal particle is along the c axis. The intensities of all main diffraction peaks of Li2SnO3/C are stronger than those of Li2SnO3 and the stability of Li2SnO3/C is better than

3. Results and discussion The crystal structures of Li2SnO3 and Li2SnO3/C are confirmed by XRD. Results are shown in Fig. 1a. The diffraction peaks of the two samples are corresponding well with the standard data of Li2SnO3 monoclinic crystal structure with lattice constants a = 5.301 Å,

Fig. 1. XRD patterns of Li2SnO3 and Li2SnO3/C (a); EDS (b) and XPS (c) of Li2SnO3/C (the inset shows Li1s XPS spectrums).

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Fig. 2. TEM of Li2SnO3 (a) and Li2SnO3/C (b, c, d).

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Q. Wang et al. / Materials Letters 71 (2012) 66–69

Li2SnO3. The diffraction peaks of carbon cannot be detected in Fig. 1(a), which suggests that carbon exists in amorphous form in Li2SnO3/C. The peaks of oxygen, tin and carbon are observed in the EDS graph of Li2SnO3/C (Fig. 1b). The atomic ratio of [O]:[Sn]:[C] is estimated to be about 5:1:2. The XPS analysis shows that the atomic contents of Sn, O and C are 10.51%, 61.01% and 28.48%, respectively (Seen in Fig. 1c). All these prove that element carbon exists in Li2SnO3/C. The practical O content is higher than theoretical data, which attributes to the functional group of oxygen. Moreover, there is a larger amount of lithium

Fig. 3. XRD patterns before and after the charge–discharge (a); the initial five CV curves (b) and nitrogen adsorption–desorption isotherm (the inset shows the pore diameter distribution) of Li2SnO3/C (c).

on the surface of the particles by the high resolution XPS spectrum for Li1s (Fig. 1d). Fig. 2 shows TEM images of Li2SnO3 and Li2SnO3/C, respectively. From Fig. 2a, it can be seen that Li2SnO3 is composed of uniform nano-crystallites (200–300 nm) with a narrow particle size distribution, while Li2SnO3/C is composed of more dense, uniform and regular nano-crystallites (20–30 nm) (Fig. 2b) with clear lattice fringes in

Fig. 4. The charge–discharge (a, b) and the cycle performance (c) of Li2SnO3 and Li2SnO3/C.

Q. Wang et al. / Materials Letters 71 (2012) 66–69

Fig. 2c. The 0.49 nm spacing of lattice (seen in Fig. 2d) corresponds to the (002) plane of monoclinic Li2SnO3. Fig. 3a shows XRD patterns of Li2SnO3/C electrode before, after the first cycle and after 50 cycles at a current density of 60 mA g − 1 between 0.05 V and 2.0 V. It can be seen that Li2SnO3 is reduced to Sn in the discharge process. Most of the peaks are corresponding well with the standard data of the crystal structure of Sn (JCPDS No. 040673). There are no diffraction peaks in Li2SnO3 after 50 cycles. Fig. 3b shows the cyclic voltammetry curves of Li2SnO3/C electrode at 5 cycles. The scanning rate is 0.2 mV S− 1 between 0.0 V and 3.0 V. The first cycle, the anodic peaks are located at 1.3 V and 0.6 V while the cathode peak is observed at the potential of 1.0 V. 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 Li2SnO3 to Sn and the synchronous formation of Li2O [15–20]. In the following cycles, the cathode peaks move to the left between 0.7 and 0.9 V. The peak at 1.0 V disappeared, indicating that the irreversible reaction has taken place at the first cycles. Another two increasing anodic peaks are found at 0.65 V and 1.4 V, which can be attributed to the alloying and de-alloying process [21] between Li and Sn. Fig. 3c shows the nitrogen adsorption–desorption isotherm and pore diameter distribution of Li2SnO3/C. Results show that the BET surface area before and after cycling is 583.8385 m 2 g − 1 and 4131.8048 m 2 g − 1. The average pore diameter before cycling is 28.04 nm while it is 5.87 nm after cycling. Li2SnO3/C has a porous system, which proves that the doped carbon can provide a high conductive medium for electron transfer. So, the electrode reactions of the first discharge–charge process can be written as [13, 15]: Li2 SnO3 þ 4Li→3Li2 O þ Sn

ð1Þ

Sn þ xLi↔Lix Snðx ≤ 4:4Þ

ð2Þ

Fig. 4 is the cycling curves of Li2SnO3 and Li2SnO3/C at a current density of 60 mA g− 1 between 0.05 V and 2.0 V (Li/Li+). Results show that the first discharge–charge capacities of the composite Li2SnO3/C are 1671.1 mAh g− 1 and 1558.6 mAh g− 1 (Fig. 4b) while they are 1909.4 mAh g− 1 and 1745.8 mAh g − 1 (Fig. 4a) to Li2SnO3. The capacities retain 576.9 mAh g− 1 for Li2SnO3/C and 330.6 mAh g− 1 for Li2SnO3 after 50 times cycles (Fig. 4c).The discharge platform at 0.9– 0.6 V and 0.5–0.38 V is observed in the first-second discharge/charge. In the 20 times and 40 times cycle, its capacities are 820.0 and 617.2 mAh g− 1, respectively (Fig. 4b). Li2SnO3/C electrode has lower initial capacity and exhibits better cycling performance than Li2SnO3 electrode because the carbon doped can buffer the expansion volume of Li–Sn and provide a highly conductive medium for electron transfer.

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The generation of Li2O can improve the cycle ability of the Li–Sn alloy by Eq. (1) [13, 15]. 4. Conclusions Li2SnO3/C is prepared by a carbothermic reduction route. Results show that it is composed of dense, uniform and regular nanocrystallites (20–30 nm) with clear lattice fringes. The electrochemical performance of Li2SnO3/C is better than that of Li2SnO3 because it is a porous system. The capacity of Li2SnO3/C is higher (576.9 mAh g − 1) than Li2SnO3 after 50 cycles. The doped carbon can buffer the expansion volume of Li–Sn and provide a highly conductive medium for electron transfer. Thus, the cycle capability of cathode material can be greatly improved. Acknowledgements This work was supported by the Spaceflight Foundation of the People's Republic of China under Grant no. N8XW0002.This work was supported by graduate starting seed fund of Northwestern Polytechnical University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]

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