J. Cent. South Univ. (2012) 19: 1791−1795 DOI: 10.1007/s11771­012­1209­9

Synthesis and electrochemical performance of Li2Mg0.15Mn0.4Co0.45SiO4/C cathode material for lithium ion batteries

HU Chuan­yue(胡传跃), GUO Jun(郭军), LI Si­jun(李四军), PENGYang­xi(彭秧锡), WEN Jin(文瑾)

Department of Chemistry and Material Science, Hunan Institute of Humanities, Science and Technology, Loudi 417000, China

© Central South University Press and Springer­Verlag Berlin Heidelberg 2012

Abstract: The synthesis, structure and performance of Li2Mg0.15Mn0.4Co0.45SiO4/C cathode material were studied. The Li2Mg0.15Mn0.4Co0.45SiO4/C solid solution with orthorhombic unit cell (space group Pmn21) was synthesized successfully by combination of wet process and solid­state reaction at high temperature, and its electrochemical performance was investigated primarily. Li2Mg0.15Mn0.4Co0.45SiO4/C composite materials deliver a charge capacity of 302 mA∙h/g and a discharge capacity of 171 mA∙h/g in the first cycle. The discharge capacity is stabilized at about 100 mA∙h/g after 10 cycles at a current density of 10 mA/g in the voltage of 1.5−4.8 V vs Li/Li + . The results show that Mg­substitution for the Co ions in Li2Mn0.4Co0.6SiO4 improves the stabilization of initial structure and the electrochemical performance.

Key words: lithium ion battery; Li2Mg0.15Mn0.4Co0.45SiO4/C; cathode material; synthesis

1 Introduction

Rechargeable lithium ion batteries are considered to be one of the most advanced energy storage systems. Now, commercial lithium­ion batteries mostly rely on lithium transition metal oxide, such as LiCoO2 and LiMn2O4 [1−3]. However, the toxicity and high cost of cobalt represent some of the problems of LiCoO2

material. The cycling performance of LiMn2O4 is poor in high temperature conditions. These defects of materials inhibit their further use in price sensitive and large­scale applications, such as hybrid electric vehicles. Therefore, much effort has been made to find alternative cathode materials for lithium­ion batteries. In recent years, poly anionic cathodes, such as olivine­type LiMPO4, have attracted much attention as next­generation cathodes with high voltage [4−6]. The poly anion­type cathode materials have better safety characteristics compared with the lithium metal oxide materials, which may act as strong oxidizers at a highly charged state when contacting with an organic electrolyte [7]. The theoretical capacity of Li2MSiO4 as poly anionic cathodes can reach as high as 330 mA∙h/g (e.g. 333 mA∙h/g for M=Mn; 325 mA∙h/g for M=Co; and 325.5 mA∙h/g for M=Ni) [8]. This is significantly superior to the corresponding LiMPO4 where usually only one electron exchange is

available with the theoretical capacity of only 170 mA∙h/g, and is also much higher than that of the commercialized LiCoO2 with theoretical capacity of 274 mA∙h/g [8].

NYTEN et al [9] first reported Li2FeSiO4 as a new cathode material. The Li2FeSiO4 material showed a reversible capacity of around 130 mA∙h/g in the first cycle at a rate of C/16. As Li2FeSiO4 operates on only one­electron redox couples, Fe 3+ /Fe 2+ , its theoretical and practical capacity is limited. The theoretical capacity of Li2FeSiO4 material is 166 mA∙h/g.

Recently, DOMINKO et al [10] reported some results about Li2MnSiO4 as a cathode material. About 0.6 Li was reversibly exchanged in the first cycle, but the reversible capacity faded considerably to only about 0.3 Li in the fifth cycle at a rate of C/30 in their work. This poor cycling performance resulted from the change of initial structure of Li2MnSiO4 during the first charge−discharge process. However, the initial structure of Li2MnSiO4 can be stabilized by doping Fe 2+ to prepare Li2MnxFe1−xSiO4 [11]. In fact, the Li2MnxFe1−xSiO4 has been successfully synthesized and the Li2Mn0.5Fe0.5SiO4

was found to be able to possess excellent electrochemical performance [11−12]. Some researchers reported that the Li2CoSiO4 was successfully synthesized with different synthesis routes [13−14]. The reversible electrochemical extraction was limited to less than 0.5 lithium per unit

Foundation item: Project(10B054) supported by Scientific Research Fund of Hunan Provincial Education Department, China; Projects(2011GK2002, 2011FJ3160) supported by the Planned Science and Technology Program of Hunan Province, China

Received date: 2011−04−25; Accepted date: 2011−09−05 Corresponding author: HU Chuan­yue, Associate Professor, PhD; Tel: +86−738−8325065; E­mail: huchuanyue@vip.sina.com

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formula for the Li2CoSiO4/C composite material, with a charge capacity of more than 234 mA∙h/g and a discharge capacity of less than 100 mA∙h/g. The poor electrochemical performance was due to the change of initial structural, the poor electronic conduction, and the poor lithium ion conduction. But, the Na­substitution on the Li ion in Li2CoSiO4 was found to be beneficial to improve the electronic conductivity and the Li ion diffusion according to the results of the density functional theory calculations within GGA+U framework [15].

A new Li2Mg0.15Mn0.4Co0.45SiO4/C was successfully synthesized in this work. The synthesis method of Li2Mg0.15Mn0.4Co0.45SiO4/C sample was described and the cathode performance was studied.

2 Experimental

Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) precursor was prepared by mixing LiAc∙2H2O, Mn(Ac)2∙2H2O, MgC2O4∙2H2O, Co(Ac)2∙2H2O and Si(OC2H5)4 in ethanol solvent. The mixture was stirred at 60 °C for 24 h in a reflux system and then dried at 110 °C. After cooling to room temperature, the mixture was milled with sucrose in acetone solvent. After the evaporation of acetone, the mixture was pressed into pellets and heated to 700 °C for 7 h in a flow of Ar. The ICP was used to determine the Li, Mg, Mn and Co contents. Chemical analysis was used to determine the Si content. Element analysis results showed that the molar ratio of Li:Mg:Mn:Co:Si was approximate to the designed molar ratio. The carbon content in Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) samples was determined to be 4.4% (mass fraction) by the VarioEL III (Elementar, Germany). X­ray diffraction (XRD) was used to characterize the structure of Li2MgxMn0.4Co0.6−xSiO4/C powder with a Y­2000 (Dandong Aolong Readation Instrument Co. Ltd., China) using a Cu Kα radiation source (λ=1.540 6 Å). FEG XL­30 SEM was used to observe the particle size and morphology of the samples.

The electrochemical performance of the Li2MgxMn0.4Co0.6−xSiO4/C as the positive electrode was evaluated using a coin­type cell (R2025) with a lithium metal anode. The working electrode was produced by dispersing 90% active materials, 3% carbon black, and 7% polyvinylidene fluoride (PVDF) binder (mass fraction) in N­methylpyrrolidone (NMP) solvent to form a homogeneous slurry. The slurry was then spread on Al foil. The coated electrode was dried in vacuum at 110 °C for 12 h. The working electrode comprised approximately 5 mg of the active mass. The electrolyte was 1 mol/L LiPF6 in a mixture of ethylene carbonate

(EC)­dimethylene carbonate (DMC)­ethylmethyl carbonate (EMC) (1:1:1 in mass ratio). Cyclic voltammogram (CV) experiment was performed using a CHI 660C potentiostat/galvanostat system (Shanghai, China) in the potential range of 1.5−5.5 V at a scanning rate of 0.2 mV/s. Galvanostatic charge−discharge cycling was carried out with a multichannel battery tester (BK6061 testing system, Guangzhou Lanqi Electronic Co. Ltd., China). A constant current­constant voltage (CC­CV) protocol was used in the potential range of 1.5−4.8 V versus Li/Li + at a current density of 10 mA/g at 25 °C. The constant voltage was applied until the charging current dropped to values corresponding to C/50 ratio (1 C =200 mA∙h/g)

3 Results and discussion

The XRD patterns of the Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) powder are shown in Fig. 1. The impurities of Co3O4 and MnO2 are detected in both samples. The results of X­ray diffraction show that the crystal system of Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) solid solution can be indexed on the basis of orthorhombic unit cell with space group Pmn21, which is similar to the iso­structure of low­temperature Li3PO4 [9, 16]. The most intense diffraction peak of Li2MgxMn0.4Co0.6−xSiO4/C samples is observed at 45°. However, the most intense diffraction peak of LiMnSiO4/C and Li2CoSiO4/C occurs at 24° [10, 14]. This result indicates that the crystal structure of Li2MgxMn0.4Co0.6−xSiO4/C is a little different from that of the LiMnSiO4/C and Li2CoSiO4/C materials. The lattice constants (a, b and c) calculated for Li2Mg0.15Mn0.4Co0.45SiO4/C and Li2Mn0.4Co0.6SiO4/C samples are: a=9.679 1(5), b=9.728 4(6), c=6.190 9(0) Å, and a=9.668 1(4), b=9.684 1(3), c=6.184 6(7) Å, respectively.

Fig. 1 XRD patterns of Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15)

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The electrochemical performances of Li2MgxMn0.4­ Co0.6−xSiO4/C (x=0, 0.15) were investigated and the influence of Mg substitution on the electrochemical performance was tested. Figure 2 shows the initial voltage profiles of Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) samples in a voltage range of 1.5−4.8 V vs Li/Li + at a current density of 10 mA/g. The initial voltage profile shows two voltage plateaus at 4.2 and 4.4 V during the charge process and two voltage plateaus at 3.3 and 2.9 V during the sequent discharge process. An initial charge capacity is 252 mA∙h/g for Li2Mn0.4Co0.6SiO4/C sample (about 75.8% of the theoretical capacity), and 302 mA∙h/g for Li2Mg0.15Mn0.4Co0.45SiO4/C sample (about 91.5% of the theoretical capacity). An initial discharge capacity is 109 mA∙h/g for Li2Mn0.4Co0.6SiO4/C (about 33.03% of the theoretical capacity, and 0.66 electrons per unit formula exchanged), and 171 mA∙h/g for Li2Mg0.15­ Mn0.4Co0.45SiO4/C (about 51.8% of the theoretical capacity of the material). That is to say, about 1.83 Li +

per unit formula can be extracted in the charge process and about 1.04 Li + per unit formula can be inserted back over the following discharge process. These results indicate that the second lithium ion in per unit formula of Li2Mg0.15Mn0.4Co0.45SiO4/C can be reversibly extracted and inserted in the voltage range of 1.5−4.8 V vs Li/Li + .

Fig. 2 Charge−discharge curves of Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) samples in first cycle at current density of 10 mA/g

Figure 3 shows the cycling performances of Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) samples at a current density of 10 mA/g. Clearly, the discharge capacity of both samples fades rapidly from the first cycle to the 5th cycle. The Li2Mg0.15Mn0.4Co0.45SiO4/C cathode shows the better cycling performance, and the discharge capacity is stabilized at about 100 mA∙h/g after 10 cycles.

The cyclic voltammogram profiles for Li2Mg0.15Mn0.4Co0.45SiO4/C sample are shown in Fig. 4. The results show that the material exhibits three

Fig. 3 Cycling performances of Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) composites at a current density of 10 mA/g between 1.5 and 4.8 V

Fig. 4 Cyclic voltammograms for Li2Mg0.15Mn0.4Co0.45SiO4/C sample at 0.2 mV/s between 1.5 and 5.5 V vs Li/Li +

oxidation peaks at 2.2, 2.6 and 4.3 V, and two reduction peaks at 3.3 and 2.9 V during the first cycle. The oxidation peak of 4.3 V is resulted from the oxidation reaction of Mn 2+ and Co 2+ in Li2Mg0.15Mn0.4Co0.45SiO4/C solid solution, which is much lower than that of the theoretical extraction potential of lithium ions in Li2MnSiO4 and Li2CoSiO4 [17]. The oxidation peaks at 2.2 and 2.6 V are possibly resulted from the reaction of forming the solid electrolyte film on the positive electrode surface. The reduction peaks at 2.9 and 3.3 V are resulted from the intercalation lithium reaction into Li2Mg0.15Mn0.4Co0.45SiO4/C electrode.

The differential capacity (dx/dV) plots for the Li2Mg0.15Mn0.4Co0.45SiO4/C sample are shown in Fig. 5 during the first and the second cycle. Two oxidation peaks are observed on the differential capacity plots. The oxidation peak occurs at the potential of 4.2 and 4.4 V, indicating that the oxidation potential of Mn 2+ is different from that of Co 2+ during the first cycle. In

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Fig. 5 Differential capacity (dx/dV) plots for Li2Mg0.15Mn0.4­ Co0.45SiO4/C sample in the first and the second cycle

addition, the oxidation peaks of Mn 2+ and Co 2+ occur at 4.55 and at 4.75 V, respectively, during the second cycle, indicating that the rearrangement of initial structure for Li2Mg0.15Mn0.4Co0.45SiO4/C materials occurs during the first cycle.

The particle morphology of the Li2Mg0.15Mn0.4­ Co0.45SiO4/C sample is observed by scanning electron microscopy (SEM), and the image is shown in Fig. 6. SEM analysis indicates that the average size of Li2Mg0.15Mn0.4Co0.45SiO4/C particles is about 1−3 μm and the powders consist of agglomerates of primary particles. The particle shape is approximately spherical. The spherical particles are resulted from the addition of carbon, which inhibits the particle growth and induces the formation of spherical particle during sintering process.

Fig. 6 SEM images of Li2Mg0.15Mn0.4Co0.45SiO4/C powder

4 Conclusions

1) The Li2MgxMn0.4Co0.6−xSiO4/C (x=0, 0.15) solid solutions with orthorhombic unit cell (space group Pmn21) are prepared by combination of wet process and

solid­state reaction. The average size of Li2Mg0.15Mn0.4Co0.45SiO4/C particles is 1−3 μm and the particle shape is approximately spherical.

2) The Li2Mg0.15Mn0.4Co0.45SiO4/C composite material has good electrochemical performance, which delivers a charge capacity of 302 mA∙h/g and a discharge capacity of 171 mA∙h/g in the first cycle.

3) The discharge capacity is stabilized at about 100 mA∙h/g after 10 cycles at a current density of 10 mA/g in the voltage range of 1.5−4.8 V vs Li/Li + .

4) Mg­substitution for Co ions in Li2Mn0.4Co0.6SiO4

improves the stabilization of initial structure and the electrochemical performance.

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(Edited by YANG Bing)