J. Cent. South Univ. Technol. (2008) 15: 214-217

DOI: 10.1007/s11771-008-0041-8

Synthesis and electrochemical properties of SnO₂-polyaniline composite

HE Ze-qiang(何则强)^{1, 2}, XIONG Li-zhi(熊利芝)^1,2, LIU Wen-ping(刘文萍)¹,

WU Xian-ming(吴显明)¹, CHEN Shang(陈 上)¹, HUANG Ke-long(黄可龙)²

(1. College of Chemistry and Chemical Engineering, Jishou University, Jishou 416000, China;

2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)

Key words:

lithium ion battery; synthesis; electrochemical properties; microemulsion polymerization method; SnO2; polyaniline；

1 Introduction

Recently, tin oxide and tin-based composite oxide electrodes have been investigated as possible anode electrodes for the fourth generation lithium-ion batteries due to the desirable material properties of high capacity on both a gravimetric and a volumetric basis and the low potential of Li ion intercalation^[1-5]. However, there exist large volume expansion and contraction problems associated with the Li⁺ insertion and removal reactions, respectively. Thus, mechanical stresses, related to the volume charges, induce a rapid decay in mechanical stability. The electrode suffers from cracking and crumbling (pulverization), as well as from consequent loss of electrode interparticle contact, resulting in loss of capacity^[6-8]. Tin or tin oxide was combined with graphite and carbon in an attempt to reduce capacity fade. The approach has been to prevent the tin from becoming electrically disconnected through the use of a conductive matrix^[9-13]. Recently, the conducting polymer has been studied as an additive to improve the performances of cathode and anode materials in lithium ion batteries^[14-16]. The conducting polymer composites exhibit good charge-discharge properties, indicating clearly that the conducting polymer can work as a conducting matrix for the particles of electrode. ZHANG et al^[17] reported the nano-tin-polyaniline lithium ion composite anode and SCHNITZLER et al^[18] reported the synthesis and characterization of new organic/inorganic hybrid materials formed from the mixed oxide (Ti,Sn)O₂ nanoparticles and polyaniline (PAn). However, using polyaniline as an additive for a tin-oxide based anode material in lithium-ion batteries has not been explored. In this work, SnO₂-polyaniline (SnO₂-PAn) composite was synthesized using a microemulsion polymerization method. The electrochemical properties of SnO₂-PAn composite as an anode material for lithium-ion batteries were examined.

2 Experimental

0.5 g sodium dodecyl benzene sulfonate (C₁₈H₂₉- SO₃Na) was dissolved in 100 mL deionized water, in which pH value was adjusted to about 1-2 by dropping 0.1 mol/L HCl. 22.5 g aniline was slowly added dropwise into the solution under strong stirring. The solution was ultrasonically treated for 30 min, and then a specific mass of SnO₂ powder (the molar ratio of SnO₂ to aniline (n(SnO₂)?n(aniline)) was 2?5) and 10 mL n-butyl alcohol (C₄H₉OH) was added under strong stirring^[19]. The stirring was lasted for 45 min to obtain a micro emulsion liquid, and the solution was cooled with ice-water. A certain amount of ammonium peroxydisulfate (APS, (NH₄)₂S₂O₈) was dissolved in 100

mL deionized water in terms of n(aniline)?n(APS)=1?1

and added dropwise to the micro emulsion in 1 h to form green polyaniline. The reaction temperature was kept at 10 ℃. After 6 h, the resulting green dispersion was centrifuged. The resulting SnO₂-PAn composite was washed thoroughly by deionized water, ethanol and acetone for several times, and then dried at 60 ℃ under vacuum.

Phase identification and surface morphology studies of the samples were carried out by an X-ray diffractometer (XRD; Rigaku D/MAX-gA) with Cu K_α radiation and scanning electron microscope (SEM, JSM 5600LV).

A slurry containing 80% SnO₂-PAn composite, 10% PVDF (polyvinylidene fluoride) and 10% acetylene black (mass fraction) was made using N-methylprrolidinone(NMP) as the solvent. The electrodes with an area of 1 cm² were prepared by coating the slurry (about 100 μm in thickness) on copper foils, followed by drying in vacuum at 60 ℃ for 12 h. Electrochemical tests were performed using a conventional coin-type cell, employing lithium foil as a counter electrode and 1.0 mol/L LiPF₆ in ethylene carbonate/dimethyl carbonate (EC/DMC) (with volume ratio of EC to DMC was 1?1) as the electrolyte. The assembly was carried out in an Ar-filled glove box. The electrochemical tests were carried out with an electrochemical work station(CHI660B, Shanghai, China).

3 Results and discussion

Fig.1 shows the XRD patterns of SnO₂, PAn and SnO₂-PAn composite.

Fig.1 XRD patterns of samples: (a) SnO₂; (b) SnO₂-PAn;    (c) PAn

The XRD pattern of SnO₂ agrees well with JCPDS 41-1445, suggesting that the prepared powder is pure SnO₂ with single crystalline of tetragonal (p42/mnm) structure. The XRD diffraction peaks of SnO₂ are somewhat wide, indicating that the particle size of SnO₂ is small. The XRD pattern of SnO₂-PAn composite assembles well with that of SnO₂, in which the XRD peaks of PAn cannot be found, suggesting that the PAn in the composites is amorphous. The PAn formed in the reaction is deposited preferentially on the SnO₂ particles, giving a SnO₂-PAn composite, in which SnO₂ is coated with the PAn^[20].

Fig.2 shows the SEM images of SnO₂ and SnO₂-PAn composite. It can be seen from Fig.2(a) that the SnO₂ powder is composed of spherical particles with size of 15-20 nm. Due to the high activity of nano- particles, there are some agglomerates. After coating, the porous nanosized PAn particles are uniformly coated on the surface of the SnO₂ particles, as seen in Fig.2(b). The layer of conductive PAn particles may improve the contact of Sn particles in the electrode reaction process and enhance the cycling performance of nano-SnO₂^[21]. In order to verify the improvement of PAn coating layer, the conductivities of nano-SnO₂, PAn and SnO₂-PAn composite were tested. The results show that the conductivity of nano-SnO₂, PAn and SnO₂-PAn composite are 2.28×10^-4, 0.11 and 0.24 S/cm, respectively, suggesting that the conductivity of SnO₂ is improved by coating with PAn.

Fig.2 SEM images of samples: (a) SnO₂; (b) SnO₂-PAn composite

The first discharge-charge curves of nano-SnO₂ and SnO₂-PAn composite are shown in Fig.3. It can be seen that the dischrge-charge curves of nano-SnO₂ and SnO₂-PAn composite are very similar, indicating that they have the same mechanism of lithium storage^[22-24]. Two obvious plateaus appear on the first discharge curve. The first one is at 0.9 V, which corresponds to the reaction between SnO₂ and Li, leading to the dissociation of SnO₂ and the formation of solid electrolyte interface (SEI) film on the surface of electrode. The second appears at 0-0.5 V, which is associated with the alloying reactions between Sn and Li. The only plateau at 0-0.5 V during charging corresponds to the de-alloying reactions of Li_xSn (0≤x≤4.4), i.e, the deinsertion process of lithium ion.

Fig.3 The first discharge-charge curves of different anodes

By comparing the discharge-charge curves of SnO₂ and SnO₂-PAn composite, it is found that SnO₂-PAn composite has lower capacity than nano-SnO₂. This may be caused by the decrease of active material in the SnO₂-PAn electrode  due to the introduction of PAn.

The cycling performance of nano-SnO₂ and SnO₂-PAn composite is shown in Fig.4. Cycling at 0.1C (70 mA/g), the initial specific capacity of nano-SnO₂ is 702.1 mA?h/g, 577.5 mA?h/g of that is retained after 80

Fig.4 Cyclability of different anodes

cycles, and the capacity loss per cycle is 0.22%. Under the same conditions, the specific capacity of SnO₂-PAn composite is 657.6 mA?h/g, 609.3 mA?h/g of that can be retained after 80 cycles, the capacity loss per cycle is only 0.092%. It is obvious that SnO₂-PAn composite shows better cycling performance than nano-SnO₂, which is due to the improvement of electrical contact between SnO₂ particles after being coated with PAn^[21].

Fig.5 shows the AC impedance diagrams of nano-SnO₂ and SnO₂-PAn anodes after the 80th cycle operated in the voltage range from 0 to 1.0 V. The frequency range is changed from 1×10^-3 to 1×10⁶ Hz and current rate is 0.1C (70 mA/g). A typical semicircle and an inclined line are seen in both the figures of nano-SnO₂ and SnO₂-PAn. The high frequency arc is attributed to the charge transfer reaction at the interface of electrolyte and electrode^[25]. The inclined line corresponds to Warburg impedance related to the diffusion of lithium ion in the anode^[26]. By comparing the AC impedance diagram of SnO₂ and SnO₂-PAn composite, it is easily found that SnO₂-PAn composite shows lower charge transfer resistance, indicating that the conductive PAn coated on the surface of SnO₂ may improve the electrochemical kinetics of SnO₂ and accelerate the diffusion rate of lithium ion.

Fig.5 AC impedance diagrams of different anodes

4 Conclusions

1) The PAn formed in the reaction is deposited preferentially on the SnO₂ particles to obtain a SnO₂-PAn composite, in which SnO₂ is coated with amorphous PAn.

2) The conductive PAn can improve the electro- chemical kinetics of SnO₂ and accelerate the diffusion rate of lithium ion and thus improve the electrochemical properties of SnO₂.

3) SnO₂-PAn composite may be a promising anode material for lithium ion batteries with high specific capacity and good cycling stability.

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(Edited by CHEN Wei-ping)

Foundation item: Project(20376086) supported by the National Natural Science Foundation of China; Project(2005037700) supported by the Postdoctoral Science Foundation of China; Project(07A058) supported by the Scientific Research Fund of Hunan Provincial Education Department; Project(07JJ3014) supported by Hunan Provincial Natural Science Foundation of China

Received date: 2007-06-28; Accepted date: 2007-09-13

Corresponding author: HE Ze-qiang, PhD; Tel: +86-13787930478; E-mail: csuhzq@163.com