J. Cent. South Univ. (2014) 21: 1285-1289

DOI: 10.1007/s11771-014-2064-7

Preparation and electrochemical performance of Li₂Mn_0.5Fe_0.5SiO₄ cathode material with sol-gel method for lithium ion batteries

HU Chuan-yue(胡传跃), GUO Jun(郭军), WEN Jin(文瑾), PENG Yang-xi(彭秧锡)

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

Central South University Press and Springer-Verlag Berlin Heidelberg 2014

Abstract: Li₂Fe_0.5Mn_0.5SiO₄ material was synthesized by a citric acid-assisted sol-gel method. The influence of the stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) on the electrochemical properties of Li₂Fe_0.5Mn_0.5SiO₄ was studied. The final sample was identified as Li₂Fe_0.5Mn_0.5SiO₄ with a Pmn2₁ monoclinic structure by X-ray diffraction analysis. The crystal phases components and crystal phase structure of the Li₂Fe_0.5Mn_0.4SiO₄ material were improved as the increase of the stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺). Field-emission scanning electron microscopy verified that the Li₂Fe_0.5Mn_0.5SiO₄ particles are agglomerates of Li₂Fe_0.5Mn_0.5SiO₄ primary particles with a geometric mean diameter of 220 nm. The Li₂Fe_0.5Mn_0.5SiO₄ sample was used as an electrode material for rechargeable lithium ion batteries, and the electrochemical measurements were carried out at room temperature. The Li₂Fe_0.5Mn_0.5SiO₄ electrode delivered a first discharge capacity of 230.1 mAh/g at the current density of 10 mA/g in first cycle and about 162 mAh/g after 20 cycles at the current density of 20 mA/g.

Key words: lithium ion battery; Li₂Fe_0.5Mn_0.5SiO₄; citric acid assisted sol-gel method; cathode

1 Introduction

Rechargeable lithium ion batteries are considered to be one of the most advanced energy storage systems used by today’s information-rich and mobile society. Now, commercial lithium-ion batteries mostly rely on conventional lithium transition metal oxide as cathode materials [1-6]. However, the toxicity, safety problem and high cost of cobalt represent some of the problems of the transition metal oxidation. Recently, poly anion compounds with (XO₄)^n- group (X=P, Si, Ge, and etc.) have been extensively investigated owing to their safety and lower cost, in spite of their intrinsic low electronic conductivity and slow lithium ion diffusion [7-9]. The poly anion-type cathode materials have better safety characteristics compared with the lithium metal oxide materials, which may act as strong oxidizers at a highly charged state while contacting with an organic electrolyte [7]. The theoretical capacity of Li₂MSiO₄ as poly anionic cathodes can reach as high as 330 mAh/g (e.g. 333 mAh/g for M=Mn; 325 mAh/g for M=Co; and 325.5 mAh/g for M=Ni) [11]. The theoretical capacity of LiMPO₄ is 170 mAh/g, where usually only one electron exchange is available.

NYTEN et al [12] first reported Li₂FeSiO₄ as a new cathode material. The Li₂FeSiO₄ material showed a reversible capacity of around 130 mAh/g in the first cycle at a rate of C/16. As Li₂FeSiO₄ operates on only one-electron redox couples, Fe³⁺/Fe²⁺, its theoretical and practical capacity is limited. The theoretical capacity of Li₂FeSiO₄ material is 166 mAh/g.

Recently, DOMINKO et al [13] 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 ratio in their work. This poor cycling performance resulted from the change of initial structure of Li₂MnSiO₄ during the first charge-discharge process. Compared to Li₂MnSiO₄ [13-14] and Li₂CoSiO₄ [15-17], Li₂FeSiO₄ has been attracted much more attention due to the high cycling stability [18]. Coating carbon on the surface of Li₂FeSiO₄ particles was an effected method to improve the poor electron conductivity [19-21]. However, the poor electron conductivity of Li₂MSiO₄ can be improved by using cation doping which has positive effect on the structural and electrochemical properties of Li₂FeSiO₄. According to the similar radius, the Fe²⁺ in Li₂FeSiO₄ material can be substituted with Cu²⁺, Ni²⁺ and Zn²⁺ [22]. The initial structure of Li₂FeSiO₄ can be stabilized by doping Mn²⁺ to prepare Li₂Mn_xFe_1-xSiO₄ [24]. In fact,the Li₂Mn_xFe_1-xSiO₄ has been successfully synthesized and the Li₂Mn_0.5Fe_0.5SiO₄ was found to be able to deliver excellent electrochemical performance [20-21]. A reversible capacity of 214 mAh/g was achieved for Li₂Fe_0.5Mn_0.5SiO₄ material with sucrose as carbon resources [20]. On the other hand, the poor electron counductivity was a key element to result poor electrochemical performance of silicate. In order to impove the electrochemical properties of the LiFePO₄ material, two methods were used to prepare the nano-particle to shorten the transfer distance of lithium ion and coating carbon on the LiFePO₄ particle [25].

In this work, we investigated the impact of citric acid on the crystal structural and electrochemical characteristics of Li₂Fe_0.5Mn_0.5SiO₄. All the samples were prepared by a citric acid-assisted sol-gel method.

2 Experimental

The Li₂Fe_0.5Mn_0.4SiO₄ precursor was prepared by a sol-gel method. Firstly, the stoichiometric amount of LiAC·2H₂O, Mn(AC)₂·4H₂O and citric acid was dissolved into water. Then, a designed amount of (C₂H₅)₄SiO₄ ethonal solution and FeC₂O₄·2H₂O were added into the mixture solution. The mixture was stirred at 60 °C for 24 h in a reflux system and then dried at 110 °C in vacuum after ethonal-water solvents were evaporated by reduced pressure distillation. Lastly, the mixture was pressed into pellets after milling for 10 h. The prepared precursors were heated to 700 °C for 10 h in a flow of Ar.

Morphology of the fabricated sample was characterized by field emission scanning electron microscopy (SEM, FEI Quanta-200) operated at an acceleration voltage of 5 kV. Powder X-ray diffraction (XRD, Philips X’Pert MPD diffractometer, Cu K_α radiation) was used for phase identification. Scan rate of 4(°)/min and step size of 0.04° were applied to record the pattern. The operation voltage and current were 40 kV and 45 mA, respectively.

The electrochemical performance of the Li₂Fe_0.5Mn_0.4SiO₄/C as the positive electrode was evaluated using a coin-type cell (size 2025) with a lithium metal anode. The working electrode was produced by dispersing 90% (mass fraction) active materials, 3% (mass fraction) carbon black, and 7% (mass fraction) polyvinylidene fluoride (PVDF) binder in N-methylpyrrolidone (NMP) solvent to form a homogeneous slurry. The slurry was then spread on Al foil. The coated electrode was dried in vacuum at 120 °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 volume). Galvanostatic charge-discharge cycling was carried out with a multichannel battery tester (BK6061 testing system, Guangzhou Lanqi Electronic Co.Ltd., China). A constant current-constant voltage (CC-CV) protocol was used with the potential range of 1.5-4.8 V versus Li/Li⁺ at a current density of 10 mA/g at 25°C in the first cycle and at a current density of 20 mA/g for the cycling life test. The constant voltage was applied until the charging current dropped to values corresponding to a 0.02 mA.

3 Results and discussion

The XRD patterns of the Li₂Fe_0.5Mn_0.5SiO₄ powder are shown in Fig. 1. The results of X-ray diffraction show that the crystal system of Li₂Fe_0.5Mn_0.4SiO₄/C solid solution is identified with a Pmn2₁ monoclinic structure, which is similar to the iso-structural of low-temperature Li₃PO₄ [12, 26]. The most intense diffraction peak of Li₂Fe_0.5Mn_0.4SiO₄/C samples is observed at 24°, which is similar with the most intense diffraction peak of LiMnSiO₄/C and Li₂CoSiO₄/C [13,16]. The impurities of Fe₂O₃ and FeO are detected in the samples prepared when the stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) was from 0.2 to 0.4. The peak intensity of impurities decreased with the increase of stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺). The Fe₂O₃ purity resulted from the oxidation of FeC₂O₄ was reduced to Fe²⁺ by the carbon thermal reduction reaction with the increase of stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺). The good crystal structure of Li₂Fe_0.5Mn_0.4SiO₄ material with high (022) peak intensity was obtained when the stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) is 0.4 and 0.5 . Therefore, the crystal phases components and crystal phase structure for the synthesized Li₂Fe_0.5Mn_0.4SiO₄ material were strongly influenced by the stoichiometric ratio value of n (citric acid) to n (Fe²⁺-Mn²⁺).

The particle morphology of the Li₂Fe_0.5Mn_0.5SiO₄

Fig. 1 XRD patterns of Li₂Fe_0.5Mn_0.5SiO₄ with different stoichiometric ratio values of n(citric acid) to n(Fe²⁺-Mn²⁺):

sample prepared with stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺)=0.5 was observed by scanning electron microscopy (SEM), and the image is shown in Fig. 2. SEM analysis indicates that Li₂Fe_0.5Mn_0.5SiO₄ powder consists of agglomerates of primary particles. The average size of primary particles is about 221 nm, which means the shorter diffusion distance of lithium ion and improves the electrochemical performance of Li₂Fe_0.5Mn_0.5SiO₄ materials with poor electron conductivity. The particle shape is approximately spherical. The spherical particles are resulted from the addition of carbon, which inhibit the particle growth and induce the formation of spherical particle during sintering process. On the other hand, the carbon coming from the citric acid coated on the surface of Li₂Fe_0.5Mn_0.5SiO₄ particle and improved the electronic conductivity.

Fig. 2 SEM image of Li₂Fe_0.5Mn_0.5SiO₄ powder (Stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) is 0.5)

The electrochemical performances of Li₂Fe_0.5Mn_0.5SiO₄ were investigated and the influence of citric acid content on the electrochemical performance was tested. Figure 3 shows the initial voltage profiles of Li₂Fe_0.5Mn_0.5SiO₄ samples in a voltage range of 1.5-4.8 V vs Li/Li⁺ at a current density of 10 mA/g. The discharge capacity and coulombic efficiency are listed in Table 1. The initial voltage profile shows two voltage plateaus at 4.2 V and 4.4 V during the charge process and at 3.3 V and 2.9 V during the sequent discharge process. When the stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) is 0.5, the Li₂Fe_0.5Mn_0.5SiO₄ shows the excellent electrochemical performance. The initial reversible capacity is 220.4 mAh/g, which is the 88.3% of the theoretic capacity in the first cycle. The coulombic efficiency is 88.3%. The theoretic capacity of Li₂Fe_0.5Mn_0.5SiO₄ is 249.1 mAh/g according to the following equation:

                           (1)

In addition, the reversible capacity of Li₂Fe_0.5Mn_0.5SiO₄ increases with the increase of stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) from 0.2 to 0.5, which indicates that the poor electron conductivity of Li₂Fe_0.5Mn_0.5SiO₄ is improved by the coated carbon on the particle surface.

Figure 4 shows the differential capacity (dQ/dV)

Fig. 3 Initial charge-discharge curves of Li₂Fe_0.5Mn_0.5SiO₄ material with different stoichiometric ratio values of n(citric acid) to n(Fe²⁺-Mn²⁺)

Table 1 Charge-discharge data of Li₂Fe_0.5Mn_0.5SiO₄ in initial charge-discharge cycle with different stoichiometric ratio values of n(citric acid) to n(Fe²⁺-Mn²⁺)

Fig. 4 Differential capacity plots of Li₂Fe_0.5Mn_0.5SiO₄ sample during first cycle (Stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) is 0.5)

plots for the Li₂Fe_0.5Mn_0.5SiO₄/C sample during the first cycle. Four oxidation peaks are observed at the potentials of 3.5, 4.10, 4.25 and 4.48 V, corresponding to the oxidation reactions of SEI film formation, Fe²⁺→Fe³⁺, Mn²⁺→Mn³⁺ and Mn³⁺→Mn⁴⁺. The four reduction peaks are also observed at the potentials of 3.70, 2.70, 2.35 and 1.90 V, which correspond to the reduction reactions of Fe³⁺→Fe²⁺, Mn⁴⁺→Mn³⁺, Mn³⁺→Mn²⁺ and SEI film formation. These results indicate that the second lithium ion in per unit formula of Li₂Fe_0.5Mn_0.5SiO₄/C is reversible extracted and inserted in the voltage range of 1.5-4.8 V vs Li/Li⁺.

EIS measurements are conducted from 0.01 Hz to 100 kHz. The Nyquist plots of the Li₂Fe_0.5Mn_0.5SiO₄ electrodes after the first cycle are shown in Fig. 5. The Nyquist plots exhibit the similar curves consisting of a depressed semicircle in high-medium frequency and a 45° line in low frequency. It is supposed that the high-medium frequency semicircle corresponds to the charge-transfer resistance rather than the intrinsic resistance and contact resistance. As can be seen from Fig. 5, the internal resistances (Rs) of the electrode materials are 1605, 1423, 798, 580 and 1600 W when the stoichiometric ratio value n(citric acid) to n(Fe²⁺-Mn²⁺) is 0.2, 0.3, 0.4, 0.5 and 0.8. Rs includes the ionic resistance of electrolyte (Re), the intrinsic resistance of the active material (Ra) and the contact resistance (Ri) at the interface active material/ current collector. The results showed that the AC impedance of Li₂Fe_0.5Mn_0.5SiO₄ was obviously influenced by the stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺), which suggested that the electronic conduction of Li₂Fe_0.5Mn_0.5SiO₄ was strongly influenced. When the stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) is 0.5, the Li₂Fe_0.5Mn_0.5SiO₄ electrodes show the lower internal resistances, which indicates that the lithium ions and electrons can transfer easily across the Li₂Fe_0.5Mn_0.5SiO₄/ electrolyte interfaces, leading to the enhanced electrode reaction kinetics and the

Fig. 5 Nyquist plots of Li₂Fe_0.5Mn_0.5SiO₄ electrodes with different stoichiometric ratio values of n(citric acid) to n(Fe²⁺-Mn²⁺) after first cycle

improved electrochemical performance.

Figure 6 shows the cycling performance of the Li₂Fe_0.5Mn_0.5SiO₄ samples at a current density of 20 mA/g. Clearly, the discharge capacity of all samples fades rapidly from the first cycle to the 5th cycle and gradually stabilizes after 10 cycles. When the stoichiometric ratio value of n(citric acid) : n(Fe²⁺-Mn²⁺) is 0.5, Li₂Fe_0.5Mn_0.5SiO₄ cathode shows good cycling performance, and discharge capacity is stabilized at 162 mAh/g after 20 cycles. The electrochemcial performance of Li₂Fe_0.5Mn_0.5SiO₄ was influenced by the stoichiometric ratio value of n (citric acid) : n (Fe²⁺-Mn²⁺) due to the different complex effect between citric acid and transition metal ions. The Fe₂O₃ purity could be produced by the oxidation of FeC₂O₄, which can decrease the electrochemical performance of Li₂Fe_0.5Mn_0.5SiO₄ materials. However, the Fe³⁺ can be reduced to the Fe²⁺ by the carbon thermal reduction reaction when the increase of stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) increased from 0.2 to 0.5. The electrochemical perfomance of Li₂Fe_0.5Mn_0.5SiO₄ can be improved by increasing the n value.

Fig. 6 Cycling performance of Li₂Fe_0.5Mn_0.5SiO₄ electrodes with different stoichiometric ratio values of n(citric acid) to n(Fe²⁺-Mn²⁺) at current density of 20 mA/g

4 Conclusions

1) Li₂Fe_0.5Mn_0.5SiO₄ positive-electrode material for lithium-ion batteries is synthesized by using a citric acid assisted sol-gel method. The influence of stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺) on the electrochemical properties of Li₂Fe_0.5Mn_0.5SiO₄ materitals is investigated. Li₂Fe_0.5Mn_0.5SiO₄ solid solution is identified with a Pmn2₁ monoclinic structure by XRD analysis.

2) The electrochemical performance of Li₂Fe_0.5Mn_0.5SiO₄ samples are greatly influenced by the stoichiometric ratio value of n(citric acid) to n(Fe²⁺-Mn²⁺). When the stoichiometric ratio of n(citric acid) to n(Fe²⁺-Mn²⁺) is 0.5, the Li₂Fe_0.5Mn_0.5SiO₄ material shows the higher reversible capacity and good cycling performance. The discharge capacity is 230.1 mAh/g in the first cycle at the current densities of 10 mA/g and 162 mAh/g after 20 cycles at the current density of 20 mA/g. Three oxidation-reduction reactions occur between the Fe²⁺/Fe³⁺, Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ redox couples according to the the differential capacity (dQ/dV) plots.

3) Field-emission scanning electron microscopy verified that the Li₂Fe_0.5Mn_0.5SiO₄ composites are agglomerates of Li₂Fe_0.5Mn_0.5SiO₄ primary particles with a geometric mean diameter of 221 nm, indicating the shorter diffusion distance of lithium ion.

4) The electrochemical performance of Li₂Fe_0.5Mn_0.5SiO₄ will be improved by doping transination metal ions to substitute to the Fe²⁺ and Mn²⁺ and by doping Al³⁺ to substitute the Si⁴⁺.

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(Edited by DENG Lü-xiang)

Foundation item: Projects(13A047, 10B054) supported by the Scientific Research Fund of Hunan Provincial Education Department, China; Projects (2011GK2002, 2011FJ3160) supported by the Planned Science and Technology Project of Hunan Province, China

Received date: 2012-11-23; Accepted date: 2013-12-12

Corresponding author: HU Chuan-yue, PhD; Tel: +86-738-8325065; E-mail: huchuanyue@vip.sina.com.cn