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Synthesis and electrochemical properties of LiMn2O4 by microwave-assisted sol–gel method
Materials Letters 59 (2005) 3761 – 3765 www.elsevier.com/locate/matlet
Synthesis and electrochemical properties of LiMn2O4 by microwave-assisted sol–gel method Shu-Juan Bao, Yan-Yu Liang, Hu-Lin Li * College of Material Science and Engineering of Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China College of Chemistry and Chemical Engineering of Lanzhou University, Lanzhou 730000, PR China Received 15 March 2005; accepted 6 July 2005 Available online 8 August 2005
Abstract In this work, a timesaving and efficient method, which is microwave-assisted sol – gel method, has been used for synthesis of LiMn2O4 spinel as a cathode material for lithium ion batteries. The influence of synthesis conditions on the structural and electrochemical properties of LiMn2O4 was investigated by thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and charge – discharge experiments. The powders resulting from the microwave-assisted sol – gel method are pure, spinel-structure LiMn2O4 particles of regular shapes that exhibit promising electrochemical behavior for battery. In addition, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to analyze the reason of capacity fading of LiMn2O4 electrodes. D 2005 Elsevier B.V. All rights reserved. Keywords: Lithium ion batteries; Spinel-type; LiMn2O4; Microwave
1. Introduction Rechargeable lithium ion batteries become a commercial reality in recent years. They are key components of the portable, entertainment, computing and telecommunication equipment required by today’s information-rich, mobile society. The global projections for the marketing of portable electronic devices with extraordinary capabilities create a very strong driving force for R&D of light, efficient, environmental-friendly, and cheap rechargeable lithium ion batteries [1,2]. Currently the choice for the cathode material is LiCoO2, which, although having good capacity and recharge-ability, suffers from the high cost and environmental toxicity of cobalt [3]. Consequently, much effort has been put into developing alternatives. At present, the spinel LiMn2O4 has been extensively studied as promising positive materials due * Corresponding author. College of Chemistry and Chemical Engineering of Lanzhou University, Lanzhou 730000, PR China. Tel.: +86 931 891 2517; fax: +86 931 891 2582. E-mail address: [email protected] (H.-L. Li). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.012
to its low cost, high environmental acceptability and excellent voltage profile characteristics [4,5]. The traditional method to synthesize spinel LiMn2O4 is solid-state reaction, which is tedious and time-consuming, often requires extensive mechanical mixing, high sintering temperature and extended grinding process that are detrimental to the quality of the final product [6,7]. In order to overcome the shortcomings of the solid-state reaction and improve the performance of the spinel LiMn2O4 in batteries, sol –gel method has been introduced [8,9]. Yet, it does not appear to be greatly advantageous over the solid-state reaction in terms of long reaction time. Recently, a novel method, known as the microwave synthesis method, was developed to prepare cathode materials for lithium ion battery [10 – 12]. In the microwave irradiation field, since the microwave energy is absorbed directly by the bulk of the heated object, uniform and rapid heating can be achieved within several minutes. Therefore, the reaction time can be significantly decreased and this synthesis becomes very economical and clean [13,14]. With these considerations, an improved sol – gel method, which is microwave-
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assisted sol-gel method, has been used to synthesize positive electrode materials in this work. In this study, an attempt was made to stabilize the spinel structure by improving the synthesis method. The LiMn2O4 with regular morphology was successfully prepared by microwave-assisted sol – gel method. The physical characteristics and electrochemical properties of the synthesized products were investigated in detail. The results have indicated that the obtained LiMn2O4 shows good performance as the lithium ion batteries cathode material.
2. Experimental 2.1. Materials preparation All the chemical reagents used in the experiments were analytical grade without further purification. LiMn2O4 powders were synthesized by microwave-assisted sol – gel method using citric acid as a chelating agent. A stoichiometric amount of lithium acetate [Li(CH3COO)I2H2O] and manganese acetate [Mn(CH3COO)2I4H2O] were dissolved in distilled water and mixed with aqueous solution of citric acid. The resulting solution was mixed with continuous magnetic stirring at 90 -C until a clear viscous gel occurs. The obtained precursor was preserved under vacuum at 100 -C for 12 h to eliminate water adequately and then was placed in microwave oven. The microwave power operated at 50% (650 W) for 10 min, and then at 100% for 10 min. After the microwave treatment, the samples were sintered at 750 -C for different times, followed by cooling to room temperature slowly. In addition, LiMn2O4 was also prepared following the same procedure without microwave treatment for comparison, which was calcined at 750 -C for 6 h. (This is the traditional sol – gel method.) 2.2. Structure and morphology characterization Thermogravimetric analysis (TGA) was performed on Setarm TGDTA92A with a-Al2O3 as the reference substance at a heating rate of 10 -C/min. The structure of products was characterized by X-ray diffractometer (D/max˚) 2400 Rigaku, Japan) with Cu Ka radiation (k = 1.54178 A operating at 40.0 kV and 60.0 mA. SEM (JSM-5600LV, Japan) was used to observe the morphology of the product.
polyvinylidene fluoride (PVDF) binder (5 wt.%) homogeneously mixed in N-methyl pyrrolidone (NMP) solvent and then coated uniformly on an aluminum foil. Finally, the electrode was dried under vacuum at 100 -C for 12 h. The assembly of the cells was conducted in an Ar-filled glove box. The cells were cycled under constant current conditions (current density of 40 mA/g, cutoff voltage 3.0 – 4.4 V) on Land CT2001A (China) at room temperature. A three-electrode cell was employed for the cyclic voltammetry and impedance measurements in which Li metal disk served as both counter and reference electrodes. Cyclic voltammetry scans were recorded from 3.0 to 4.5 V at a scan rate of 0.1 mV/s. The electrochemical impedance measurements were carried out by applying 100 kHz to 0.01 Hz frequency ranges with acoscillation amplitude of 5 mV. In this investigation, the cells were discharged to a designated potential, then were kept at the open-circuit condition for 1 h before performing impedance tests such that the equilibrium of the cells could be ensured. Both of the electrochemical measurements were done using a CHI760 model Electrochemical Workstation (CH Instruments).
3. Results and discussion 3.1. TGA and XRD studies The thermal properties of the obtained LiMn2O4 precursor and the sample prepared by microwave treatment for 20 min were studied using TG analysis. Fig. 1a displays the TG curve for the LiMn2O4 precursor. It shows a first weight loss of ¨ 4.8% around 200 -C, which is attributed to the evaporation of residual water and the removal of chemically bound water in the sample. The second step weight loss of 56.7% around 300 -C corresponds to the decomposition of organics and the formation of LiMn2O4 phase. The weight loss between 350 and 800 -C is very little. This behavior implies that the formation of LiMn2O4 phase is completed at this stage. Fig. 1b shows the TG curve for the LiMn2O4 obtained by microwave treatment for 20 min, which indicates that the
2.3. Electrochemical measurements The charge and discharge capacities were measured with two-electrode Swagelock-type cells in which a lithium metal foil was used as the counter electrode. The electrolyte employed was 1 M LiPF6 in a 1:1 (volume ratio) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Celgard 2400 membrane was used as the cell separator. The composite electrodes were made of the as-prepared cathode materials (80 wt.%), acetylene black (15 wt.%) and
Fig. 1. The thermogravimetric analysis curves for (a) the LiMn2O4 precursor and (b) the LiMn2O4 obtained by microwave treatment for 20 min.
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weight loss is very little during the whole heating process. It is ascribed that most of the LiMn2O4 spinel phase has been formed during the microwave treatment process. Fig. 2 illustrates the XRD patterns of the LiMn 2O 4 synthesized by different methods. The powder obtained by microwave treatment for 20 min has transformed into cubic spinel structure (this is in agreement with the TG analysis above); however, the impurity peaks of Mn2O3 (marked by *) and other unidentified phase also retained in the spinel LiMn2O4 phase. Probably, a large amount of carbon contents in the precursor (lithium acetate, manganese acetate and citric acid) tends to reduce manganese ions during microwave treatment process and favors the formation of Mn2O3 impurities [15,16]. After further sintering for 6 h, the impurity Mn2O3 phase completely disappeared and the powder crystallized into pure cubic spinel structure as indicated by Fig. 2b. Fig. 2c displays the XRD patterns of the LiMn2O4 obtained by traditional sol – gel method. Evidently, it exhibits striking similarity to those of the LiMn2O4 prepared by microwave-assisted sol – gel method. All of the diffraction peaks were assigned to the spinel compound. The result is in good accordance with the standard spectrum (JCPDS, card no. 35-0782). The lattice constant of LiMn2O4 obtained by microwaveassisted sol – gel method, which is on the basis of a least square ˚ . The value is very refinement program, is calculated to be 8.245 A ˚ ). close to the standard spinel LiMn2O4 (8.247 A 3.2. SEM studies Fig. 3 shows the scanning electron microscope (SEM) images for the LiMn2O4 obtained by microwave-assisted sol – gel method sintering at 750 -C for 6 h. The particles of LiMn2O4 powders obtained by microwave-assisted sol – gel method are distributed uniformly and have regular shapes. The well-dispersed particles are the result of the treatment of microwave due to the microwave heated not from the outside but from the inside of the precursor and thus provided a uniform heating environment which shortened the synthesizing time and overcame the agglomeration of particles. Such kind of morphology is very important to both the high specific capacity and good cyclability of the materials [17,18].
Fig. 2. The XRD patterns of LiMn2O4 obtained by different heat treatment methods and processes. (a) The sample obtained by microwave treatment for 20 min; (b) the sample obtained by microwave-assisted sol – gel method; (c) the sample prepared by traditional sol – gel method. Key: (*) Mn2O3.
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Fig. 3. SEM images of the LiMn2O4 obtained by microwave-assisted sol – gel method sintering at 750 -C for 6 h.
3.3. Electrochemical properties In order to study the influence of different synthesis conditions on the cycle behavior, the cells were tested at a charge – discharge current density of 40 mA/g between 3.0 and 4.4 V. The variations of the discharge capacity with the cycle number for LiMn2O4 powders prepared under different synthesis methods are shown in Fig. 4. For the sample obtained by microwave treatment for 20 min, the discharge capacity is relatively low and fades very fast. However, when it was calcined at 750 -C for 6 h, the discharge capacity increased significantly and the cycling behavior of the powders became much better. This is probably explained by the fact that the microwave treatment delivers an impure Li – Mn – O spinel with Mn2O3 phase (this can be confirmed by XRD analysis); during cycling, the unit cell of the incomplete grown LiMn2O4 crystallites will distort, so its capacity decreased rapidly. After following sintering for 6 h, the LiMn2O4 has a very well ordered Li – Mn – O spinel without any other impure phases, hence, its capacity increases significantly. Comparing Fig. 4a with Fig. 4b, the LiMn2O4 prepared by microwave-assisted sol – gel method
Fig. 4. The typical discharge capacity versus the cycle number for the LiMn2O4 obtained by different synthesis conditions. (a) The sample obtained by microwave-assisted sol – gel method; (b) the sample prepared by traditional sol – gel method; (c) the sample obtained by microwave treatment for 20 min.
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presented a higher discharge capacity and better capacity retention than that of the LiMn2O4 obtained by traditional sol – gel method. This may be due to the fact that samples obtained by microwaveassisted sol – gel have a better crystallinity and regular morphology. In order to evaluate the influence of sintering time on the electrochemical characteristics of the spinel LiMn2O4, the samples were microwave treated for 20 min, and then heated continuously at 750 -C for 1, 3, 6, 12 and 24 h, respectively. Fig. 5 shows the initial discharge capacities at a discharge current density of 40 mA/ g in a potential range from 3.0 to 4.4 V of the LiMn2O4 sintered at 750 -C for different times. As shown in Fig. 5, the capacity of the material sintered for 1 h is lowest, which is due to incomplete crystallization and some structure defects of the materials. With increasing sintering time, the capacity becomes greater, and a welldefined maximum between 6 and 12 h is observed; further heating for 24 h reduces the specific capacity and this may be ascribed to the decrease of surface area as reported by Xia et al. [19]. Therefore, the results obtained above confirm that microwaveassisted sol – gel method appears to be a better alternative to the traditional sol – gel method for preparing lithium ion battery cathode materials, and after sintering for 6 h, the samples have a better battery performance. Hence, in this study, further tests were focused on the LiMn2O4 prepared by microwave-assisted sol – gel method sintered at 750 -C for 6 h. Fig. 6 compares impedance spectra of the pristine and cycled electrodes, measured at potentials close to the CV peak potential, in which the charge – transfer resistance of the electrode is minimal. As could be seen from the plots, the EIS of the LiMn2O4 electrode is composed of two partially overlapped semicircles and a straight sloping line, and they were fitted with the equivalent circuit depicted in the inset of Fig. 6. In the equivalent circuit, the R b corresponds to the solution resistance between the reference electrode and the cathode; R sei and C sei are resistance and capacitance of the solid-state interface layer formed on the surface of the electrode, which correspond to the semicircle at high frequencies; R ct and C dl are Faradic charge– transfer resistance and its relative double-layer capacitance, corresponding to the semicircle at medium frequencies; W is the Warburg impedance related to a combination of the diffusional effects of Li ion on the interface between the active material particles and electrolyte, which is generally indicated by a straight sloping line at low frequency end.
Fig. 5. The initial discharge capacity of the LiMn2O4 obtained by different heating times.
Fig. 6. AC impedance spectra, presented as Nyquist plots, measured at E = 4.1 V (Li/Li+) with a fresh LiMn2O4 cathode and with the same cathode after 100 cycles.
The combination of R ct and W is called Faradic impedance, which reflects kinetics of the cell reactions. It can be seen from Fig. 6 that the R sei value for the fresh electrode (calculated from the first semicircle’s diameter) is 134 V; after 100 cycles, it increases to 178 V, which means that an SEI film has been performed upon the contact of LiMn2O4 electrode and electrolyte, and with cycling, it only grows slightly. During cycling, the R ct of the electrode (calculated from the second semicircle’s diameter) increases from ca. 161 to 234 V. This can be explained by the fact that manganese dissolution reduces the three phase boundary sites (the contact points between oxide, acetylene black and electrolyte solution), where the electrode reaction for Li+ intercalation/deintercalation takes place [20]. The increase in R sei and R ct will cause a high cell polarization, leading to an incomplete charging and therefore apparent capacity losses. Fig. 7 compares the cyclic voltammetric behavior of a fresh LiMn2O4 electrode with that of the same electrode after 100 consecutive galvanostatic charge – discharge cycles (at C/3 rate) within the same potential range. The anodic and cathodic peaks observed in the CV of the fresh electrode exhibit reversible oxidation and reduction reactions. The split of the redox peaks into two couples indicates that the electrochemical reactions of the
Fig. 7. Cyclic voltammograms of spinel LiMn2O4 electrodes: fresh (top) and after 100 cycles (bottom) at 0.1 mV/s.
S.-J. Bao et al. / Materials Letters 59 (2005) 3761 – 3765
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insertion and extraction of Li ion proceed in two stages [9,21], which can be written as the following two reactions:
Acknowledgements
ð1=2ÞLiþ þ ð1=2Þe þ 2k MnO2 S Li0:5 Mn2 O4
ð1Þ
ð1=2ÞLiþ þ ð1=2Þe þ Li0:5 Mn2 O4 S LiMn2 O4 :
ð2Þ
The authors are thankful for the support provided by National Nature Science Foundation of China (no. 60471014).
As can be seen clearly from Fig. 7, with cycling, the CV peaks became much less sharp and the peak separation became larger than that of the fresh electrode. Moreover, the peaks current for the cycled electrode is much lower than that for the fresh ones. It means that further cycling leads to a pronounced capacity fading of the electrode. During charge– discharge, the capacity fading of the LiMn2O4 electrode is caused by the following reasons [22 – 25]: (1) slow dissolution of the LiMn2O4 electrode into the electrolyte according to the dissolution of Mn3+ by the disproportionation reaction; (2) transformation of two-phase structure in the high voltage region to a more stable single structure via the loss of MnO. These two chemical reactions can be depicted by the following reaction [26,27]: 2LiMn2 O4 Y 3k MnO2 ðsolidÞ þ MnOðsolutionÞ þLi2 OðsolutionÞ
ð3Þ
From the experiments’ datum and analysis above, the capacity losses are implied to arise from two major causes: one is deterioration of the active material itself and another is the increase of R sei and R ct.
4. Conclusions This work demonstrated that the spinel-phased, wellcrystallized LiMn2O4 powders could be effectively synthesized via the microwave-assisted sol – gel process. In comparison with the conventional sol – gel method, microwave-assisted sol – gel process is superior in terms of shorter reaction time. TGA and XRD studies of the material obtained by microwave treatment for 20 min have confirmed that spinel phase formed during microwave treatment process. Electrochemical test of the as-prepared samples as cathode for Li ion batteries has shown that further sintering improved significantly the quality of LiMn2O4, which displays excellent capability retention. The results of experiments indicated that the capacity losses arise from two major causes: one is deterioration of the active material itself and another is increase of resistance. Future research to optimize the composition and sintering temperature, particularly to further increase the cyclability, is currently in progress.
References [1] J.M. Tarascon, M. Armand, Nature 414 (2001) 359. [2] V. Ganesh Kumar, J.S. Ganaraj, S. Ben-David, D.M. Pickup, Ernst R.H. Van-Eck, A. Gedanken, D. Aurbach, Chem. Mater. 15 (2003) 4211. [3] Y.K. Sun, C.S. Yoon, C.K. Kim, S.G. Youn, Y.S. Lee, M. Yoshio, I.H. Oh, J. Mater. Chem. 11 (2001) 2519. [4] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc. 143 (1996) 2204. [5] Y. Xia, Y. Zhou, M. Yoshio, J. Electrochem. Soc. 144 (1997) 2593. [6] E. Rossen, J.R. Reimers, J.R. Dahn, Solid State Ionics 62 (1993) 53. [7] R. Alcantara, P. Lavela, P.L. Relano, J.L. Tirado, E. Zhecheva, R. Stoyanova, Inorg. Chem. 37 (1998) 264. [8] J.H. Choy, D.H. Kim, C.W. Kwon, S.J. Hwang, Y.I. Kim, J. Power Sources 77 (1999) 1. [9] X.M. Wu, X.H. Li, Z.B. Xiao, J.B. Liu, W.B. Yan, M.Y. Ma, Mater. Chem. Phys. 84 (2004). [10] P.S. Whitfield, I.J. Davidson, J. Electrochem. Soc. 147 (2000) 4476. [11] H. Yan, X. Huang, H. Li, L. Chen, J. Power Sources 81 – 82 (1999) 647. [12] H. Yan, X. Huang, H. Li, L. Chen, Solid State Ionics 11 (1998) 113. [13] K.J. Rao, P.D. Ramesh, Bull. Mater. Sci. 18 (1995) 447. [14] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem. Mater. 11 (1999) 882. [15] S. Choi, A. Manthiram, J. Electrochem. Soc. 147 (2000) 1623. [16] N. Santander, S.R. Das, S.B. Majumder, R.S. Katiyar, Surf. Coat. Technol. 177 – 178 (2004) 60. [17] S.T. Yang, Y.F. Zhang, Q.Z. Lv, J. Inorg. Mater. 15 (2000) 312. [18] F.K. Shokoohi, J.M. Tarascon, B.J. Wilkens, D. Guyomard, C.C. Chang, J. Electrochem. Soc. 139 (1992) 1847. [19] Y. Xia, H. Takesshige, H. Noguchi, M. Yoshio, J. Power Sources 56 (1995) 61. [20] D.H. Jang, Y.J. Shin, S.M. Oh, J. Electrochem. Soc. 143 (1996) 2204. [21] M. Armand, F. Dalard, D. Reroo, C. Mouliom, Solid State Ionics 15 (1985) 205. [22] G.G. Amatucci, A. Dupasquier, A. Blyr, T. Zheng, J.M. Tarascon, J. Electrochem. Soc. 146 (1999) 255. [23] G.G. Amatucci, N. Pereira, T. Zheng, J.M. Tarascon, J. Electrochem. Soc. 148 (2001) A171. [24] Y. Xia, T. Sakai, T. Fujieda, X.Q. Yang, X. Sun, Z.F. Ma, J. McBree, M. Yoshio, J. Electrochem. Soc. 148 (2001) 2593. [25] K.W. Kim, M.R. Kim, S.W. Lee, K.S. Han, S.I. Woo, Chem. Vap. Depos. 9 (2003) 187. [26] R. Premanand, A. Durairajan, B. Haran, R. White, B. Popov, J. Electrochem. Soc. 149 (2002) A54. [27] J.C. Hunter, J. Solid State Chem. 39 (1981) 142.
Synthesis and electrochemical properties of LiMn2O4 by microwave-assisted sol–gel method Shu-Juan Bao, Yan-Yu Liang, Hu-Lin Li * College of Material Science and Engineering of Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China College of Chemistry and Chemical Engineering of Lanzhou University, Lanzhou 730000, PR China Received 15 March 2005; accepted 6 July 2005 Available online 8 August 2005
Abstract In this work, a timesaving and efficient method, which is microwave-assisted sol – gel method, has been used for synthesis of LiMn2O4 spinel as a cathode material for lithium ion batteries. The influence of synthesis conditions on the structural and electrochemical properties of LiMn2O4 was investigated by thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and charge – discharge experiments. The powders resulting from the microwave-assisted sol – gel method are pure, spinel-structure LiMn2O4 particles of regular shapes that exhibit promising electrochemical behavior for battery. In addition, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to analyze the reason of capacity fading of LiMn2O4 electrodes. D 2005 Elsevier B.V. All rights reserved. Keywords: Lithium ion batteries; Spinel-type; LiMn2O4; Microwave
1. Introduction Rechargeable lithium ion batteries become a commercial reality in recent years. They are key components of the portable, entertainment, computing and telecommunication equipment required by today’s information-rich, mobile society. The global projections for the marketing of portable electronic devices with extraordinary capabilities create a very strong driving force for R&D of light, efficient, environmental-friendly, and cheap rechargeable lithium ion batteries [1,2]. Currently the choice for the cathode material is LiCoO2, which, although having good capacity and recharge-ability, suffers from the high cost and environmental toxicity of cobalt [3]. Consequently, much effort has been put into developing alternatives. At present, the spinel LiMn2O4 has been extensively studied as promising positive materials due * Corresponding author. College of Chemistry and Chemical Engineering of Lanzhou University, Lanzhou 730000, PR China. Tel.: +86 931 891 2517; fax: +86 931 891 2582. E-mail address: [email protected] (H.-L. Li). 0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.07.012
to its low cost, high environmental acceptability and excellent voltage profile characteristics [4,5]. The traditional method to synthesize spinel LiMn2O4 is solid-state reaction, which is tedious and time-consuming, often requires extensive mechanical mixing, high sintering temperature and extended grinding process that are detrimental to the quality of the final product [6,7]. In order to overcome the shortcomings of the solid-state reaction and improve the performance of the spinel LiMn2O4 in batteries, sol –gel method has been introduced [8,9]. Yet, it does not appear to be greatly advantageous over the solid-state reaction in terms of long reaction time. Recently, a novel method, known as the microwave synthesis method, was developed to prepare cathode materials for lithium ion battery [10 – 12]. In the microwave irradiation field, since the microwave energy is absorbed directly by the bulk of the heated object, uniform and rapid heating can be achieved within several minutes. Therefore, the reaction time can be significantly decreased and this synthesis becomes very economical and clean [13,14]. With these considerations, an improved sol – gel method, which is microwave-
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assisted sol-gel method, has been used to synthesize positive electrode materials in this work. In this study, an attempt was made to stabilize the spinel structure by improving the synthesis method. The LiMn2O4 with regular morphology was successfully prepared by microwave-assisted sol – gel method. The physical characteristics and electrochemical properties of the synthesized products were investigated in detail. The results have indicated that the obtained LiMn2O4 shows good performance as the lithium ion batteries cathode material.
2. Experimental 2.1. Materials preparation All the chemical reagents used in the experiments were analytical grade without further purification. LiMn2O4 powders were synthesized by microwave-assisted sol – gel method using citric acid as a chelating agent. A stoichiometric amount of lithium acetate [Li(CH3COO)I2H2O] and manganese acetate [Mn(CH3COO)2I4H2O] were dissolved in distilled water and mixed with aqueous solution of citric acid. The resulting solution was mixed with continuous magnetic stirring at 90 -C until a clear viscous gel occurs. The obtained precursor was preserved under vacuum at 100 -C for 12 h to eliminate water adequately and then was placed in microwave oven. The microwave power operated at 50% (650 W) for 10 min, and then at 100% for 10 min. After the microwave treatment, the samples were sintered at 750 -C for different times, followed by cooling to room temperature slowly. In addition, LiMn2O4 was also prepared following the same procedure without microwave treatment for comparison, which was calcined at 750 -C for 6 h. (This is the traditional sol – gel method.) 2.2. Structure and morphology characterization Thermogravimetric analysis (TGA) was performed on Setarm TGDTA92A with a-Al2O3 as the reference substance at a heating rate of 10 -C/min. The structure of products was characterized by X-ray diffractometer (D/max˚) 2400 Rigaku, Japan) with Cu Ka radiation (k = 1.54178 A operating at 40.0 kV and 60.0 mA. SEM (JSM-5600LV, Japan) was used to observe the morphology of the product.
polyvinylidene fluoride (PVDF) binder (5 wt.%) homogeneously mixed in N-methyl pyrrolidone (NMP) solvent and then coated uniformly on an aluminum foil. Finally, the electrode was dried under vacuum at 100 -C for 12 h. The assembly of the cells was conducted in an Ar-filled glove box. The cells were cycled under constant current conditions (current density of 40 mA/g, cutoff voltage 3.0 – 4.4 V) on Land CT2001A (China) at room temperature. A three-electrode cell was employed for the cyclic voltammetry and impedance measurements in which Li metal disk served as both counter and reference electrodes. Cyclic voltammetry scans were recorded from 3.0 to 4.5 V at a scan rate of 0.1 mV/s. The electrochemical impedance measurements were carried out by applying 100 kHz to 0.01 Hz frequency ranges with acoscillation amplitude of 5 mV. In this investigation, the cells were discharged to a designated potential, then were kept at the open-circuit condition for 1 h before performing impedance tests such that the equilibrium of the cells could be ensured. Both of the electrochemical measurements were done using a CHI760 model Electrochemical Workstation (CH Instruments).
3. Results and discussion 3.1. TGA and XRD studies The thermal properties of the obtained LiMn2O4 precursor and the sample prepared by microwave treatment for 20 min were studied using TG analysis. Fig. 1a displays the TG curve for the LiMn2O4 precursor. It shows a first weight loss of ¨ 4.8% around 200 -C, which is attributed to the evaporation of residual water and the removal of chemically bound water in the sample. The second step weight loss of 56.7% around 300 -C corresponds to the decomposition of organics and the formation of LiMn2O4 phase. The weight loss between 350 and 800 -C is very little. This behavior implies that the formation of LiMn2O4 phase is completed at this stage. Fig. 1b shows the TG curve for the LiMn2O4 obtained by microwave treatment for 20 min, which indicates that the
2.3. Electrochemical measurements The charge and discharge capacities were measured with two-electrode Swagelock-type cells in which a lithium metal foil was used as the counter electrode. The electrolyte employed was 1 M LiPF6 in a 1:1 (volume ratio) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Celgard 2400 membrane was used as the cell separator. The composite electrodes were made of the as-prepared cathode materials (80 wt.%), acetylene black (15 wt.%) and
Fig. 1. The thermogravimetric analysis curves for (a) the LiMn2O4 precursor and (b) the LiMn2O4 obtained by microwave treatment for 20 min.
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weight loss is very little during the whole heating process. It is ascribed that most of the LiMn2O4 spinel phase has been formed during the microwave treatment process. Fig. 2 illustrates the XRD patterns of the LiMn 2O 4 synthesized by different methods. The powder obtained by microwave treatment for 20 min has transformed into cubic spinel structure (this is in agreement with the TG analysis above); however, the impurity peaks of Mn2O3 (marked by *) and other unidentified phase also retained in the spinel LiMn2O4 phase. Probably, a large amount of carbon contents in the precursor (lithium acetate, manganese acetate and citric acid) tends to reduce manganese ions during microwave treatment process and favors the formation of Mn2O3 impurities [15,16]. After further sintering for 6 h, the impurity Mn2O3 phase completely disappeared and the powder crystallized into pure cubic spinel structure as indicated by Fig. 2b. Fig. 2c displays the XRD patterns of the LiMn2O4 obtained by traditional sol – gel method. Evidently, it exhibits striking similarity to those of the LiMn2O4 prepared by microwave-assisted sol – gel method. All of the diffraction peaks were assigned to the spinel compound. The result is in good accordance with the standard spectrum (JCPDS, card no. 35-0782). The lattice constant of LiMn2O4 obtained by microwaveassisted sol – gel method, which is on the basis of a least square ˚ . The value is very refinement program, is calculated to be 8.245 A ˚ ). close to the standard spinel LiMn2O4 (8.247 A 3.2. SEM studies Fig. 3 shows the scanning electron microscope (SEM) images for the LiMn2O4 obtained by microwave-assisted sol – gel method sintering at 750 -C for 6 h. The particles of LiMn2O4 powders obtained by microwave-assisted sol – gel method are distributed uniformly and have regular shapes. The well-dispersed particles are the result of the treatment of microwave due to the microwave heated not from the outside but from the inside of the precursor and thus provided a uniform heating environment which shortened the synthesizing time and overcame the agglomeration of particles. Such kind of morphology is very important to both the high specific capacity and good cyclability of the materials [17,18].
Fig. 2. The XRD patterns of LiMn2O4 obtained by different heat treatment methods and processes. (a) The sample obtained by microwave treatment for 20 min; (b) the sample obtained by microwave-assisted sol – gel method; (c) the sample prepared by traditional sol – gel method. Key: (*) Mn2O3.
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Fig. 3. SEM images of the LiMn2O4 obtained by microwave-assisted sol – gel method sintering at 750 -C for 6 h.
3.3. Electrochemical properties In order to study the influence of different synthesis conditions on the cycle behavior, the cells were tested at a charge – discharge current density of 40 mA/g between 3.0 and 4.4 V. The variations of the discharge capacity with the cycle number for LiMn2O4 powders prepared under different synthesis methods are shown in Fig. 4. For the sample obtained by microwave treatment for 20 min, the discharge capacity is relatively low and fades very fast. However, when it was calcined at 750 -C for 6 h, the discharge capacity increased significantly and the cycling behavior of the powders became much better. This is probably explained by the fact that the microwave treatment delivers an impure Li – Mn – O spinel with Mn2O3 phase (this can be confirmed by XRD analysis); during cycling, the unit cell of the incomplete grown LiMn2O4 crystallites will distort, so its capacity decreased rapidly. After following sintering for 6 h, the LiMn2O4 has a very well ordered Li – Mn – O spinel without any other impure phases, hence, its capacity increases significantly. Comparing Fig. 4a with Fig. 4b, the LiMn2O4 prepared by microwave-assisted sol – gel method
Fig. 4. The typical discharge capacity versus the cycle number for the LiMn2O4 obtained by different synthesis conditions. (a) The sample obtained by microwave-assisted sol – gel method; (b) the sample prepared by traditional sol – gel method; (c) the sample obtained by microwave treatment for 20 min.
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presented a higher discharge capacity and better capacity retention than that of the LiMn2O4 obtained by traditional sol – gel method. This may be due to the fact that samples obtained by microwaveassisted sol – gel have a better crystallinity and regular morphology. In order to evaluate the influence of sintering time on the electrochemical characteristics of the spinel LiMn2O4, the samples were microwave treated for 20 min, and then heated continuously at 750 -C for 1, 3, 6, 12 and 24 h, respectively. Fig. 5 shows the initial discharge capacities at a discharge current density of 40 mA/ g in a potential range from 3.0 to 4.4 V of the LiMn2O4 sintered at 750 -C for different times. As shown in Fig. 5, the capacity of the material sintered for 1 h is lowest, which is due to incomplete crystallization and some structure defects of the materials. With increasing sintering time, the capacity becomes greater, and a welldefined maximum between 6 and 12 h is observed; further heating for 24 h reduces the specific capacity and this may be ascribed to the decrease of surface area as reported by Xia et al. [19]. Therefore, the results obtained above confirm that microwaveassisted sol – gel method appears to be a better alternative to the traditional sol – gel method for preparing lithium ion battery cathode materials, and after sintering for 6 h, the samples have a better battery performance. Hence, in this study, further tests were focused on the LiMn2O4 prepared by microwave-assisted sol – gel method sintered at 750 -C for 6 h. Fig. 6 compares impedance spectra of the pristine and cycled electrodes, measured at potentials close to the CV peak potential, in which the charge – transfer resistance of the electrode is minimal. As could be seen from the plots, the EIS of the LiMn2O4 electrode is composed of two partially overlapped semicircles and a straight sloping line, and they were fitted with the equivalent circuit depicted in the inset of Fig. 6. In the equivalent circuit, the R b corresponds to the solution resistance between the reference electrode and the cathode; R sei and C sei are resistance and capacitance of the solid-state interface layer formed on the surface of the electrode, which correspond to the semicircle at high frequencies; R ct and C dl are Faradic charge– transfer resistance and its relative double-layer capacitance, corresponding to the semicircle at medium frequencies; W is the Warburg impedance related to a combination of the diffusional effects of Li ion on the interface between the active material particles and electrolyte, which is generally indicated by a straight sloping line at low frequency end.
Fig. 5. The initial discharge capacity of the LiMn2O4 obtained by different heating times.
Fig. 6. AC impedance spectra, presented as Nyquist plots, measured at E = 4.1 V (Li/Li+) with a fresh LiMn2O4 cathode and with the same cathode after 100 cycles.
The combination of R ct and W is called Faradic impedance, which reflects kinetics of the cell reactions. It can be seen from Fig. 6 that the R sei value for the fresh electrode (calculated from the first semicircle’s diameter) is 134 V; after 100 cycles, it increases to 178 V, which means that an SEI film has been performed upon the contact of LiMn2O4 electrode and electrolyte, and with cycling, it only grows slightly. During cycling, the R ct of the electrode (calculated from the second semicircle’s diameter) increases from ca. 161 to 234 V. This can be explained by the fact that manganese dissolution reduces the three phase boundary sites (the contact points between oxide, acetylene black and electrolyte solution), where the electrode reaction for Li+ intercalation/deintercalation takes place [20]. The increase in R sei and R ct will cause a high cell polarization, leading to an incomplete charging and therefore apparent capacity losses. Fig. 7 compares the cyclic voltammetric behavior of a fresh LiMn2O4 electrode with that of the same electrode after 100 consecutive galvanostatic charge – discharge cycles (at C/3 rate) within the same potential range. The anodic and cathodic peaks observed in the CV of the fresh electrode exhibit reversible oxidation and reduction reactions. The split of the redox peaks into two couples indicates that the electrochemical reactions of the
Fig. 7. Cyclic voltammograms of spinel LiMn2O4 electrodes: fresh (top) and after 100 cycles (bottom) at 0.1 mV/s.
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insertion and extraction of Li ion proceed in two stages [9,21], which can be written as the following two reactions:
Acknowledgements
ð1=2ÞLiþ þ ð1=2Þe þ 2k MnO2 S Li0:5 Mn2 O4
ð1Þ
ð1=2ÞLiþ þ ð1=2Þe þ Li0:5 Mn2 O4 S LiMn2 O4 :
ð2Þ
The authors are thankful for the support provided by National Nature Science Foundation of China (no. 60471014).
As can be seen clearly from Fig. 7, with cycling, the CV peaks became much less sharp and the peak separation became larger than that of the fresh electrode. Moreover, the peaks current for the cycled electrode is much lower than that for the fresh ones. It means that further cycling leads to a pronounced capacity fading of the electrode. During charge– discharge, the capacity fading of the LiMn2O4 electrode is caused by the following reasons [22 – 25]: (1) slow dissolution of the LiMn2O4 electrode into the electrolyte according to the dissolution of Mn3+ by the disproportionation reaction; (2) transformation of two-phase structure in the high voltage region to a more stable single structure via the loss of MnO. These two chemical reactions can be depicted by the following reaction [26,27]: 2LiMn2 O4 Y 3k MnO2 ðsolidÞ þ MnOðsolutionÞ þLi2 OðsolutionÞ
ð3Þ
From the experiments’ datum and analysis above, the capacity losses are implied to arise from two major causes: one is deterioration of the active material itself and another is the increase of R sei and R ct.
4. Conclusions This work demonstrated that the spinel-phased, wellcrystallized LiMn2O4 powders could be effectively synthesized via the microwave-assisted sol – gel process. In comparison with the conventional sol – gel method, microwave-assisted sol – gel process is superior in terms of shorter reaction time. TGA and XRD studies of the material obtained by microwave treatment for 20 min have confirmed that spinel phase formed during microwave treatment process. Electrochemical test of the as-prepared samples as cathode for Li ion batteries has shown that further sintering improved significantly the quality of LiMn2O4, which displays excellent capability retention. The results of experiments indicated that the capacity losses arise from two major causes: one is deterioration of the active material itself and another is increase of resistance. Future research to optimize the composition and sintering temperature, particularly to further increase the cyclability, is currently in progress.
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