Materials Letters 63 (2009) 1788–1790

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Materials Letters

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Effects of MoS2 doping on the electrochemical performance of FeF3 cathode materialsfor lithium-ion batteries

Wen Wu, Xianyou Wang ⁎, Xin Wang, Shunyi Yang, Xiuming Liu, Quanqi ChenSchool of Chemistry, Xiangtan University, Hunan, Xiangtan 411105, China

⁎ Corresponding author. Tel.: +86 732 8293043; fax:E-mail address: wxianyou@yahoo.com (X. Wang).

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a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 February 2009Accepted 19 May 2009Available online 22 May 2009

Keywords:Lithium-ion batteriesCathode materialFeF3/MoS2Composite materialsElectrochemical performance

Orthorhombic structure FeF3 was synthesized by a liquid-phase method. The FeF3/MoS2 for the application ofcathode material of lithium-ion battery was prepared through mechanical milling with molybdenumbisulfide. The structure and morphology of the FeF3/MoS2 were characterized by X-ray diffraction (XRD) andscanning electron microscopy (SEM). The electrochemical behavior of FeF3/MoS2 was studied by charge/discharge, cyclic voltammetry and electrochemical impedance spectra measurements. The results show thatthe prepared FeF3/MoS2 was typical orthorhombic structure, uniform surface morphology, better particle-size distribution and excellent electrochemical performances. The initial discharge capacity of FeF3/MoS2 was169.6 mAh·g−1 in the voltage range of 2.0–4.5 V, at room temperature and 0.1 C charge–discharge rate. After30 cycles, the capacity retention is still 83.1%.

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1. Introduction

Because of their high operating voltage, large energy density, lowself-discharge rate and long cycle life, lithium-ion batteries are widelyused in power portable electronic devices [1]. The first commerciallithium-ion batteries introduced in 1990 by SONY, used the layeredLiCoO2 as a cathode, but cobalt is expensive and relatively toxic [2].Many attempts are being made for preparing cheap and effectivematerials to replace LiCoO2 [3–5]. Metal fluoride compounds havebeen widely investigated for their high theoretical energy densitiesand are considered as next-generation cathode material for lithium-ion battery. However, because of their electrically insulative nature,metal fluoride compounds are poor conductor as electrode materials.Iron trifluoride (FeF3) is themost typical transitionmetal fluoride, andthe electrochemical performance has been first studied by Arai et al.[6]. However as cathode material of lithium-ion battery, FeF3 behavespoor specific capacity and inferior cycle performance. Recently,Amatucci's group reported that carbon could be used as conductorto improve the electrochemical activity of metal fluorides such as FeF3and BiF3 [7–9]. Moreover, to improve the electrochemical perfor-mance of CuF2, Badway et al. used MoS2, V2O5 or MoO3 as theconductor to prepare the mixed conducting matrices (MCM) [10]. TheMCM approach focuses on effective transport of both ions andelectrons to the metal fluoride nanodomain. Mixed conductorsutilized as the matrix can maintain excellent electronic and ionicconduction.

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In our previous works [11], FeF3 for the application of lithium-ionbattery has been prepared through hydrothermal method, and itsstructural and electrochemical properties were investigated in detail.In this paper, to prepare the FeF3/MoS2 composite for the applicationof lithium-ion battery, the conductive MoS2 was firstly added to theelectroactive FeF3 by high-energy mill. The effects of MoS2 doping onthe structural, morphological and electrochemical performance ofFeF3 were studied.

2. Experimental

The precursor was prepared as follows: firstly, a certain amount ofFeCl3 solution was added to the 10% excess mass of stoichiometricNaOH solution violently stirred for 20 min and the resulted Fe(OH)3precipitation was aged for 12 h, then the precipitation was washedand separated by filtration. Secondly, excessive HF solutionwas addedto the above precipitation in a sealed PTFE bottlewith continued stirringfor 12 h at 70 °C, then the unreacted HF and water were eliminated byheating in air and the residue was dried at 70 °C for 12 h in a vacuumoven to obtain the precursor FeF3. Themixture of 85 wt.% precursor and15 wt.% MoS2 was milled in the ball mill (ND2-2L) in the air for 3 h toobtain the FeF3/MoS2 composite material.

Structural and crystallographic analyses of FeF3/MoS2 compositematerial were taken from powder X-ray diffraction data obtainedusing diffractometer (D/MAX-3C) with Cu Kα radiation. The surfacemorphology of the samples was observed using the SM-5600LVscanning electron microscopy (SEM).

The electrochemical tests of FeF3/MoS2 samples were carried outusing coin cells assembled in an argon-filled glove box (MIKROUNA1220/750). The cathode electrodes were made by mixing 15%

Fig. 2. SEM photographs of (a) FeF3 and (b) FeF3/MoS2.

1789W. Wu et al. / Materials Letters 63 (2009) 1788–1790

acetylene black, 8% polyvinylidene fluoride binder and 77% activematerial. Lithium was used as counter and reference electrodes, Cel-gard2400 as separators, and 1 mol·L−1 LiPF6 dissolved in ethylenecarbonate (EC)-diethylene carbonate (DEC) (1:1 in volume) as anelectrolyte. Galvanostatic discharge/charge measurements werecarried out in Neware battery test system (BTS-51, Shenzhen, China)at 0.1 C (23.7 mA·g−1) between 2.0 and 4.5 V versus Li+/Li at roomtemperature, and the capacity of samples was evaluated on the activematerials. The cyclic voltammetry (CV) tests were conducted using aCHI 660a electrochemical analyzer (CH Instrument Inc., USA) between2.0 and 4.5 V versus Li+/Li at a scan rate of 0.1 mV·s−1 at roomtemperature.

3. Results and discussion

The XRD patterns of FeF3, FeF3/MoS2 and standard FeF3 (PDF:76-1262) are shown in Fig. 1. It is evident that all samples can be indexedbased on the orthorhombic structure type (space group Cmcm), andMoS2 phase exists in the composites. Except the diffraction peaks ofFeF3, some parasitic diffraction peaks (marked by asterisk in Fig. 1b)can be found from Fig. 1b, which are considered to be MoS2. Besides,the diffraction peaks of FeF3 are quite narrow, indicating its high cry-stallinity. After doping MoS2, the diffraction peaks are apparentlybroadening, suggesting that FeF3/MoS2 could reduce the crystallite size.

The morphologies of the synthesized FeF3 and FeF3/MoS2 areshown in Fig. 2. In Fig. 2(a), the pure FeF3 shows hexahedral and cubicmorphologies. It can be seen from Fig. 2(b) that FeF3/MoS2 showsrelatively small and uniform particle size, and some small grainsabsorb on the hexahedral and cubic face of the FeF3, the absorbedparticles are probably MoS2 [10].

Fig. 3 shows the cell voltage plotted versus gravimetric specificcapacity for the first charge–discharge at a constant current density of23.7 mA·g−1 (0.1 C) in the voltage range of 2.0–4.5 V at roomtemperature. All the curves are smooth and in monotonous profile.The capacities are determined based on themass of the activematerialFeF3. The curves show very flat voltage plateaus at 3.2–2.6 V versus Li/Li+ on discharging process and at 3.3–2.7 V versus Li/Li+ on chargingprocess. The flat voltage plateaus indicate the two-phase natures ofthe lithium extraction/insertion reactions, and corresponding to thetwo-phase charge–discharge reactions of the Fe3+/Fe2+ redox couple.The pure FeF3 electrode delivers an initial discharge capacity of115.7 mAh·g−1. Discharge and charge capacities of FeF3/MoS2 are169.6 mAh·g−1 and 169.0 mAh·g−1, respectively. Therefore, it can beconcluded that the addition of MoS2 can significantly improve theelectrochemical performance of FeF3.

The cyclic performance of FeF3 and FeF3/MoS2 at 0.1 C in thevoltage range of 2.0–4.5 V at room temperature is shown in Fig. 4. For

Fig. 1. XRD patterns of (a) pure FeF3, (b) FeF3/MoS2 and (c) standard FeF3 (PDF:76-1262).

the pure FeF3, the capacity is only 58.5% after 30 cycles, indicating itspoor cycling stability, which may be associated with its lowconductivity; while for FeF3/MoS2, the initial capacity of FeF3/MoS2was 169.6 mAh·g−1, and after 30 cycles the discharge capacity is still141.0 mAh·g−1, the capacity retention ratio is 83.1%, which ascribed tothe MoS2 doping and improvement of FeF3 electronic conductivity.

4. Conclusions

Orthorhombic structured FeF3 was synthesized via a simple liquid-phase method and the high conductive composite FeF3/MoS2 wasprepared by milling the mixture of FeF3 and conductive MoS2. Theaddition of MoS2 can significantly improve the electrochemicalperformance of FeF3, and the FeF3/MoS2 exhibits a discharge capacity

Fig. 3. The initial discharge/charge curves of (a) FeF3 and (b) FeF3/MoS2 at 0.1 C in thevoltage range of 2.0–4.5 V at room temperature.

Fig. 4. Discharge capacity versus cycle number for (a) FeF3 and (b) FeF3/MoS2 in thevoltage range of 2.0–4.5 V with a discharge rate 0.1 C at room temperature.

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of 169.6 mAh·g−1 between 2.0 and 4.5 V at a current density of23.7 mAh·g−1, and the capacity retention of FeF3/MoS2 is 83.1% after30 cycles. Consequently, the outstanding electrochemical properties of

FeF3/MoS2 make it a promising next-generation cathode material forlithium-ion batteries.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (Grant No. 20871101) and Key Project ofEducation Department of Hunan Province Government (GrantNo.08A067).

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