Synthesis and electrochemical properties of

SnO₂-CuO nanocomposite powders

MA Ming-you(麻明友)¹, HE Ze-qiang(何则强)^{1, 2}, XIAO Zhuo-bing(肖卓炳)¹,

             HUANG Ke-long(黄可龙)², XIONG Li-zhi(熊利芝)¹, WU Xian-ming(吴显明)¹

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

Received 10 April 2006; accepted 25 April 2006

Abstract: SnO₂-CuO nanocomposite powders were prepared by chemical coprecipitation method using SnCl₄·5H₂O, NH₃·H₂O and Cu(NO₃)₂·3H₂O as raw materials. The powders were characterized by thermogravimertric(TG) analysis and differential thermal analysis(DTA), X-ray diffraction(XRD), and scanning electron microscope(SEM). The electrochemical properties of SnO₂-CuO and undoped SnO₂ powders as anode materials of lithium ion batteries were investigated comparatively by galvanostatic charge-discharge experiments and AC impedance. The results show that SnO₂-CuO nanocomposite powders with the average particle size of 87 nm can be obtained by this method. The structure of SnO₂ does not change with the introduction of CuO, but the average particle size of nano-SnO₂ decreases. SnO₂-CuO nanocomposite powders show a reversible capacity of 752 mA·h/g and better cycleability compared with nano-SnO₂. The capacity retention rates after 60 cycles of nano-SnO₂-CuO and SnO₂ are 93.6% and 92.0% at the charge- discharge rate of 0.1 C, respectively, which suggests that the introduction of CuO into SnO₂ can improve the cycleability of nano- SnO₂.

Key words: SnO₂; CuO; chemical coprecipitation method; lithium ion batteries; electrochemical properties

1 Introduction

Since YOSHIO et al[1] announced the commercia- lization of tin oxide as negative electrodes of 1ithium-ion batteries, the tin oxide anode has attracted much attention due to its high specific capacity, which is about twice that of graphite, and has been considered the best candidate for lithium-ion battery anode material[2-7]. When SnO₂ was used as the anode material of 1ithium- ion batteries, tin works as the virtual part, and its reversible capacity is based on the formation and decomposition of 1ithium tin alloys, LiSn, Li₇Sn₃, Li₅Sn₂, Li₃Sn₅, Li₇Sn₂ or Li₂₂Sn₅[8]. The oxygen in SnO₂ can combine with lithium to form Li₂O in the first cycle, which acts as the buffer for tin aggregation. Lithium intercalating in tin dioxide in the first cycle can be described by the following reactions[2 ,9, 10]:

              (1)

                (2)

Different synthetic techniques can make SnO₂ with different particle sizes and morphologies, which can further affect its electrochemical performance. In this works, SnO₂-CuO was synthesized by chemical copreci- pitation method and its electrochemical properties were investigated in detail.

2 Experimental

2.1 Preparation of SnO₂-CuO

A certain amount of SnCl₄·5H₂O and Cu(NO₃)₂·3H₂O at a mass ratio of w(CuO):w(SnO₂) =1:10 was dissolved in de-ionized water to get a mixed aqueous solution. Then NH₃·H₂O was added into the solution dropwise under strong stirring to keep the pH value of the solution in the range of 8-9. When the reaction completed, an azury precipitation was formed. After aged at room temperature for 2 h, the precipitation was washed with water and ethanol until no Cl^- can be detected by AgNO₃ in the washing liquid. After the precipitation was dried at 105 ℃ for 4 h, it was calcined at 600 ℃ for 3 h in air to obtain the product of SnO₂-CuO.

2.2 Material characterization

The thermal behavior of the precursor was investigated by thermogravimetric analysis(TGA) and differential thermal analysis(DTA) on a TGA-SDTA851^e thermal analysis system(Mettler Toledo Corp) from 25℃ to 800 ℃ in Ar atmosphere at a heating rate of 10 ℃/min. The phase of the samples was characterized by X-ray diffraction(XRD, Rigaku D/MAX-gA) with a mono-chromatic Cu K_α radiation (0.1540 5 nm). The surface morphology, composition of elements of the samples were analyzed by means of scanning electron microscope (SEM, JEOL JSM 5600LV). The grain size was calculated using the Scherrer formula of D=0.89λ/ βsin θ, in which D is the crystalline size, λ is the wavelength of X-ray, and θ is the diffraction peak angle.

2.3 Electrode preparation and electrochemical chara- cterization

The SnO₂-CuO was made into slurry with 80% SnO₂ powder, 10% acetylene black, and 10% polyvinylidene difluoride(PVDF) in N-methyl pyrolidinone(NMP). The mixture was then coated on copper foils followed by a drying in vacuum at 120 ℃ for 10 h to obtain the SnO₂-CuO electrode.

The electrochemical tests were conducted using a conventional coin-type(2025) cell, employing SnO₂-CuO as positive electrode, a polypropylene microporous separator, utilizing 1.0 mol/L LiPF₆ in ethylene carbonate/dimethyl carbonate (EC/DMC) (with an EC to DMC volume ratio of 1:1) as the electrolyte and lithium foil as negative electrode. The assembly was carried out in an Ar-filled glove box. The discharge-charge tests were done with a PCBT-110-8D battery tester under a constant current rate of 0.1C and a constant temperature of (25±0.01) ℃ in the voltage range of 0-1.0 V. The AC impedance measurement was carried out on CHI660B electrochemical station. Here, the discharge process means the process of Li⁺ insertion into SnO₂-CuO electrode and the charge process means the process of Li⁺ de-insertion out of SnO₂-CuO electrode.

3 Results and discussion

Fig.1 shows the TG-DTA curves of the precursor dried at 105 ℃. From DTA curve, a large endothermic peak appears at 123 ℃. The endothermic peak corresponds to a clear mass loss on the TG curve, which is caused by the evaporation of physically absorbed water on the surface of the precursor. Due to the decomposition of little NH₄Cl impurity in the precursor, an unconspicuous endothermic peak appears at 234 ℃ and a tardigrade mass loss can be found on the TG curve. A great exothermic peak near 275 ℃ on DTA curve and obvious mass loss on TG curve are found, which may correspond to the decomposition of precursor to form SnO₂, CuO and CuSnO₃. Because CuSnO₃ is unstable and easy to decompose and form stable SnO₂-CuO, a small exothermic peak at 450 ℃ is seen in the DTA curve [11]. From the analysis of TG-DTA curves, it can be concluded that the crystallization of SnO₂-CuO takes place at 250-500 ℃.

Fig.1 TG-DTA curves of precursor

The XRD patterns of SnO₂-CuO and undoped SnO₂ calcined at 500 ℃ for 3 h are presented in Fig.2. In the scan range from 10? to 90?, both the samples show the four major peaks (100), (101) , (200) and (211), which correspond well with that of cassiterite phase (JCPDS file 41-1445). No diffraction peaks of CuO can be seen in Fig.2, which indicates that CuO has entered the crystal lattice and SnO₂-CuO solid solution is formed[12]. However, the peak half width of SnO₂-CuO is wider than that of SnO₂. According to Scherrer formula, we can calculate the crystal grain size of SnO₂-CuO to be 87 nm,

Fig.2 XRD patterns of nano SnO₂(a) and nano CuO-SnO₂ powders(b) calcined at 500 ℃ for 3 h

which is smaller than that of SnO₂, 92 nm. This shows the grain size of SnO₂ decreases with the doping of CuO, suggesting that the growth of the crystal of SnO₂ is depressed by the doping of CuO[13].

Fig.3 shows the morphologies of SnO₂-CuO and undoped SnO₂ calcined at 500 ℃ for 3 h. From Fig.3 we can see there is no essential difference between SnO₂-CuO and undoped SnO₂ and the average particle size is 80-90 nm. However, the particle size of SnO₂- CuO seems to be smaller than that of the undoped SnO₂, which agrees with the result of XRD analyses.

Fig.3 SEM images of nano SnO₂(a) and nano SnO₂-CuO(b) powders calcined at 500 ℃ for 3 h

Fig.4 shows the EDS results of nano SnO₂-CuO calcined at 500 ℃ for 3 h. As can be seen in the figure, except for Sn and Cu, no other metal element can be

Fig.4 EDS results of nano SnO₂-CuO powders calcined at 500℃ for 1 h

detected, which indicates that the Cu element is doped into SnO₂. According to the EDS results, it can be calculated that the mass ratio of CuO to SnO₂ is 1:10.1, which agrees well with the mass ratio of 1:10.0, suggesting that the obtained sample is CuO-doped SnO₂.

The first discharge-charge curves of nano SnO₂ and nano SnO₂-CuO powders calcined at 500℃ for 3 h are shown in Fig.5. The first discharge-charge curves of nano SnO₂ and nano SnO₂-CuO are similar to each other, indicating that nano SnO₂-CuO has a similar mechanism of Li storage as nano SnO₂[2, 4]. The plateau between 1.0 V and 1.5 V in the discharging curves disappears in the latter cycle, corresponding well to the irreversible formation of Li₂O and solid-electrolyte interface(SEI) in the first discharging process[14]. They can give a total theoretical irreversible capacity of about 933 and 899 mA·h/g for nano SnO₂ and nano SnO₂-CuO, respectively. The plateau at 0.5-0.7 V in the discharge-charge curves is due to the reversible alloying process of lithium and tin, and it is the source of reversible capacity, which is about 760 and 752 mA·h/g for nano SnO₂ and nano SnO₂-CuO respectively.

Fig.5 First discharge-charge curves of nano SnO₂(a) and SnO₂- CuO(b) (0.1C)

Fig.6 shows the variation of reversible capacity as a function of cycle number. For nano SnO₂, after 60 cycles the reversible capacity remains 92.0% that of the first cycle, the average capacity fade rate is 0.13% per cycle; while for nano SnO₂-CuO, the values are 93.6% and 0.11%, respectively. Compared with nano SnO₂, nano SnO₂-CuO anode has a smaller reversible capacity and better cycleability, which may result from the decreased mass of active material and particle size due to the doping of CuO.

Fig.7 shows the AC impedance diagram of nano SnO₂-CuO anode. Two semicircles and an inclined line can be seen in the figure. The high frequency arc is attributed to the charge-transfer reaction at the interface of electrolyte and electrode. The medium frequency arc implies the formation of a new phase during the discharge and charge process, which may be caused by the formation of solid electrolyte interface(SEI) film on the surface of nano SnO₂-CuO[15]. The inclined line corresponds to Warburg impedance related to the diffusion of lithium ion in nano SnO₂-CuO anode [16].

Fig.6 Cycleability of nano SnO₂-CuO and nano SnO₂ calcined at 500 ℃ for 3 h (0.1C)

Fig.7 AC impedance diagram of nano SnO₂-CuO calcined at 600 ℃ for 3 h

4 Conclusions

Nano SnO₂-CuO were prepared by chemical co- precipitation method using SnCl₄·5H₂O, NH₃·H₂O and Cu(NO₃)₂·3H₂O as raw materials. The average particle size of synthesized SnO₂-CuO is 87 nm. The structure of SnO₂ does not change with the introduction of CuO, but the average particle size of nano-SnO₂ decreases. The nano SnO₂-CuO shows a reversible capacity of 752 mA·h/g and better cycleability compared with the undoped nano-SnO₂. The capacity retention rate after 60 cycles of nano SnO₂-CuO and SnO₂ is 93.6% and 92.0% at the charge-discharge rate of 0.1 C, respectively, which suggests that the introduction of CuO in SnO₂ can improve the cycleability of nano-SnO₂.

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(Edited by YUAN Sai-qian)

Foundation item: Project(05C140) supported by the Scientific Research Fund of Hunan Provincial Education Department

Corresponding author: MA Ming-you; Tel: +86-743-8564088; E-mail: mamy_9589@163.com