Electrochemical Behavior of Li2FeSiO4 with Ionic ...

Journal of The Electrochemical Society, 156 共7兲 A619-A626 共2009兲

A619

0013-4651/2009/156共7兲/A619/8/$25.00 © The Electrochemical Society

Electrochemical Behavior of Li2FeSiO4 with Ionic Liquids at Elevated Temperature Martina Nadherna,a,b Robert Dominko,c,z Darko Hanzel,d Jakub Reiter,a and Miran Gaberscekc,e,* a

Institute of Inorganic Chemistry of the Academy of Sciences of the Czech Republic, v.v.i., 250 68 Řež near Prague, Czech Republic Department of Analytical Chemistry, Faculty of Science, Charles University in Prague, 128 40 Prague 2, Czech Republic c National Institute of Chemistry, 1000 Ljubljana, Slovenia d Jozef Stefan Institute, 1000 Ljubljana, Slovenia e Faculty for Chemistry and Chemical Technology, 1000 Ljubljana, Slovenia b

Ionic liquids, 1-butyl-2,3-dimethylimidazolium bis共trifluoromethanesulfonyl兲imide and 1-butyl-1-methylpyrrolidinium bis共trifluoromethanesulfonyl兲imide with 0.7 M lithium bis共trifluoromethanesulfonyl兲imide 共LiTFSI兲 or 0.5 M LiPF6, were successfully tested as electrolytes for Li2FeSiO4 cathodes operating at elevated temperatures of 60°C. The electrolytes based on ionic liquids show good ionic conductivity 共from 3.3 ⫻ 10−3 to 4.5 ⫻ 10−3 S cm−1兲 together with good electrochemical stability up to 5 V vs Li/Li+. The electrochemical stability of electrolytes based on ionic liquids against an aluminum current collector was comparable to a diethyl carbonate:ethylene carbonate 共DEC:EC兲 1 M LiPF6 electrolyte and even better when the electrolytes were tested with blank electrodes 共only aluminum current collector, carbon black, and binder兲. In the first cycle the electrochemical testing of Li2FeSiO4 showed a slightly lower reversibility in ionic liquids when compared to a DEC:EC 1 M LiPF6 electrolyte; at higher cycles the reversibility and the obtained capacity were comparable to the one obtained in a DEC:EC 1 M LiPF6 electrolyte. All electrochemical results show that LiTFSI can be used as a salt in ionic liquid-based electrolytes. These properties allow their potential application in large-scale lithium-ion batteries with improved safety. © 2009 The Electrochemical Society. 关DOI: 10.1149/1.3133183兴 All rights reserved. Manuscript submitted March 5, 2009; revised manuscript received March 31, 2009. Published May 20, 2009.

For more than a decade, rechargeable lithium-ion batteries have been the main source of power in portable applications and communication devices. Climate changes and world energy crises are pushing them to more demanding applications, for instance, for use in the automotive industry or as large-scale batteries for electricity storage. For large batteries safety concerns are one of the most important parameters which will determine their successful use. One of the possibilities to achieve safety regulations at the present step of the research is the use of ionic liquids 共ILs兲 in combination with a polyanionic type of cathode materials. Polyanionic types of cathode materials were proposed by Goodenough’s group1 by showing the electrochemical behavior of triphylite; since then, several other polyanionic types of cathode materials have been proposed.2-8 The common denominator of these cathode materials is covalently bonded oxygen atoms which make polyanionic cathode materials thermally more stable than, for example, layered transition-metal oxides. Several most recent works on Li2FeSiO4 demonstrate the possibility of using Li2FeSiO4 as an active cathode material for large-scale batteries.9-11 Li2FeSiO4 shows high thermal stability because the first exothermic evolution of energy in the differential scanning thermogravimetry is at about 400°C when heating is performed in the presence of air.10 The major difficulty is to prepare phase pure and small enough particles of Li2FeSiO4 for the electrochemical storage of energy. The safety issues are closely related also to the use of electrolytes. In this context, due to their physical properties, roomtemperature ionic liquids 共RTILs兲 are a possible choice to improve safety. The introduction of a generation of RTILs based on weakly coordinating and hydrolytically stable anions12 by Wilkes and Zaworotko has promoted research on this field. The ILs attract the interest of electrochemists because of several unique properties important for their application in lithium-ion batteries,13 supercapacitors,14 fuel cells,15 solar cells,16 and electrochromics.17 The key parameters allowing their application as electrolytes18 in large-scale lithium-ion batteries are as follows: Higher safety due to their nonflammability, high thermal and chemical stability, espe-

* Electrochemical Society Active Member. z

E-mail: [email protected]

cially toward water and oxygen, negligible volatility, and low impact on the environment and human health. ILs, based mainly on bis共fluorosulfonyl兲imide 共FSI−兲 or bis共trifluoromethanesulfonyl兲imide 共TFSI−兲 anion, have been tested with various cathode materials, such as LiCoO2,19-23 LixTiyMn1−yO2,24 and LiMn2O4.25,26 LiFePO4 has been tested with FSI- or TFSI-based ILs either in neat form27 or entrapped in the polymer network.28-31 It was reported that 1-butyl-2,3-dimethylimidazolium bis共trifluoromethanesulfonyl兲imide 共BMMITFSI兲 was much safer than conventional carbonate-based electrolytes.32 This work proves that TFSI-based ILs can be considered as potential solvents for electrolytes in lithium-ion batteries, although lithium bis共trifluoromethanesulfonyl兲imide 共LiTFSI兲 salt in organic electrolytes is corrosive against the conventional aluminum current collector at potentials higher than 3.5 V vs metallic lithium due to the formation of Al–TFSIbased complex.33 The so-called corrosion of the aluminum current collector renders the practical use of LiTFSI salt in the 4 V lithiumion batteries difficult.34 It was shown recently that LiTFSI could be used in combination with ILs at potentials above 3.5 V vs Li/Li+; however, the voltage stability range of the aluminum current collector corrosion also depends on temperature.35,36 The two limiting factors for using ILs are the relatively high viscosity both of neat ILs and lithium salt solutions in RTILs37 and the high cost. The lower transference number of lithium cation also has to be considered when designing the electrolyte composition.38 The high viscosity can be overcome by impregnating electrodes at elevated temperatures or in vacuum,27 while the cost of ILs can be reduced by using some cheaper anion, like TFSI. In this work, we evaluate the compatibility and the electrochemical performance of four different electrolytes based on two different ILs 关1-butyl-2,3-dimethylimidazolium 共BMMI+兲 and 1-butyl-1methylpyrrolidinium 共PYR+14兲兴 with TFSI− anion and two different lithium salts 共LiPF6 and LiTFSI兲. We compare the present results with those obtained using conventional electrolytes 关1 M LiPF6 in diethyl carbonate:ethylene carbonate 共DEC:EC兲 and 1 M LiTFSI in DEC:EC兴. All electrochemical measurements 关i.e., 共i兲 cells with sole aluminum current collector, 共ii兲 cells with aluminum collector covered with carbon black 共CB兲 and binder, and 共iii兲 cells with aluminum collector covered with Li2FeSiO4, CB, and binder兴 were performed at an elevated temperature of 60°C. As a suitable cathode

Downloaded 16 Jun 2009 to 147.231.133.115. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp

A620


material, we chose Li2FeSiO4, which was prepared by an improved hydrothermal synthesis followed by firing to prepare appropriate carbon-coated particles. The stability window of used ILs at 60°C toward an aluminum current collector and an aluminum current collector with CB and binder is discussed. Capacity retention of Li2FeSiO4 in combination with ILs is compared to that obtained in conventional electrolytes. Experimental IL synthesis.— Considering the environmental requirements, the method of IL preparation was based on a direct synthesis eliminating use of either a large excess of alkylbromide or halogenated solvents in both synthetic steps. Two ILs were prepared using modified synthetic procedures:38,39 BMMITFSI and PYR14TFSI. The general method of preparation was based on a two-step synthesis when bromide 共BMMI Br or PYR14Br兲 was prepared by direct alkylation of 1,2dimethylimidazole or 1-methylpyrrolidine and then the anion was substituted by bis共trifluoromethanesulfonyl兲imide in aqueous solution, where both ILs were insoluble. 1-Bromobutane 共⬎96%兲 was distilled before the IL synthesis. 1-Methylimidazole 共99%兲, 1-methylpyrrolidine 共⬎99%兲, and LiTFSI 共⬎99%兲 were purchased from Sigma-Aldrich and used as received. A total of 68.5 g 共0.5 mol兲 of 1-bromobutane 共freshly distilled兲 was mixed with 48.1 g 共0.5 mol兲 of 1,2-dimethylimidazole in a three-neck flask with a reflux condenser. The mixture was stirred at 80°C for 2 h to give a brownish solid. This solid was twice recrystallized from hot acetonitrile, washed twice with 100 mL hexane, and dried at 80°C under vacuum to give white crystals 共99.1 g; 85% yield兲. The reaction temperature in both cases should not exceed 90°C; otherwise the reactants are partially decomposed. The substitution of bromide anion by TFSI− was performed in water, where BMMITFSI was not soluble and formed hydrophobic globules at the bottom of the flask. LiTFSI of 36.93 g 共0.129 mol兲 in 120 mL water was added into a solution of 30.00 g 共0.129 mol兲 BMMI Br in 90 mL distilled water and stirred for 12 h at room temperature. The arising phase of BMMITFSI was removed and washed four times with 100 mL portions of distilled water and then two times with 100 mL portions of redistilled water to remove LiBr. The absence of bromide anions was confirmed using the test with AgNO3. The remaining water was distilled off under vacuum at 60°C, and the remaining viscous liquid was dried at 80°C under vacuum for 10 h with a yield of 49.5 g of slightly yellow liquid 共88%兲. PYR14Br was synthesized in a similar way 共34% yield of white solid兲 as well as PYR14TFSI 共86% of colorless liquid兲. The ILs were bubbled by argon for 30 min and dried in vacuum at 80°C for 24 h before storing in a dry argon-filled glove box 共关O2兴 and 关H2O兴 ⬍ 1 ppm. The purity of dry ILs was confirmed by NMR measured on Varian Mercury 400 high resolution NMR spectrometer: BMMITFSI–␦H 共400 MHz, CDCl3, ppm兲: 0.91 共3H, t, CH2CH2CH2CH3兲, 1.35 共2H, m, CH2CH2CH2CH3兲, 1.75 共2H, m, CH2CH2CH2CH3兲, 2.59 共3H, s, CH3兲, 3.79 共3H, s, NCH3兲, 4.03 共2H, t, CH2CH2CH2CH3兲, 7.17 共1H, d, ring兲, and 7.26 共1H, d, ring兲; PYR14TFSI-␦H 共400 MHz, CDCl3, ppm兲: 0.99 共3H, s, CH2CH2CH2CH3兲, 1.41 共2H, sext, CH2CH2CH2CH3兲, 1.74 共2H, m, CH2CH2CH2CH3兲, 2.26 共4H, br, ring兲, 3.05 共3H, s, CH3兲, 3.31 共2H, t, CH2CH2CH2CH3兲, and 3.47 共4H, br, ring兲. Solutions of lithium salts LiTFSI 共battery grade; Ferro, Cleveland, OH兲 and LiPF6 共battery grade; Sigma-Aldrich兲 were prepared by dissolving, in particular, IL at 110°C in a glove box. Conventional electrolytes, 1 M LiPF6 or 1 M LiTFSI, were prepared by dissolving in DEC:EC 共50:50 vol %; both Sigma-Aldrich兲 at room temperature in a glove box. Cathode materials.— Li2FeSiO4 cathode material was prepared by an optimized hydrothermal synthesis, very similar to the one described recently.40 This time we used a Teflon-lined stainless steel

autoclave with a magnetic stirrer inside 共Bergof 200兲. The slurry was sealed into an autoclave with an overpressure of argon of 3 bar and left for 6 h at 150°C. After the hydrothermal treatment was completed, the resulting grayish-green powder was rinsed several times with an alkaline solution of distilled water. Before use, the distilled water was boiled with a continuous flow of argon through the whole volume. To improve the overall electrochemical performance of Li2FeSiO4, we prepared a carbon-coated Li2FeSiO4 sample. This was done by simply grinding a mixture of pure Li2FeSiO4 sample and citric acid 共1:1.3 in weight ratio兲. After thorough grinding with a mortar and pestle, the obtained mixture was heat-treated in a gastight quartz tube with a constant flow of CO/CO2 共approximately 100 mL min−1兲. The initial heating rate was 5°C min−1. After reaching the preselected temperature, the samples were maintained at that temperature for 5 h and then left to cool down slowly to room temperature. Electrode composites of Li2FeSiO4 /C samples were prepared by soft milling of the active material with 10 wt % of CB and 10 wt % of ethylene propylene diene terpolymer 共EPDM兲 as a binder for 20 min. The obtained slurry was cast onto a circular Al foil 共Hohsen兲 with a diameter of 16 mm 共2 cm2兲. Additional electrodes were prepared without any active material 共CB and EPDM were ground together and cast onto Al foil兲. Before use, the electrodes with a loading between 6 and 8 mg were dried in vacuum at 90°C for at least 12 h. Characterization methods.— The simultaneous thermogravimetric analysis 共TGA兲 coupled with differential thermal analysis 共DTA兲 measurement was taken in argon at the heating rate of 5°C min−1 with a simultaneous thermal analysis 共Netzsch STA 409 Germany兲. The differential scanning calorimetry 共DSC兲 analysis was performed in the temperature range from ⫺160 to 100°C at the heating rate of 10°C min−1. The potentiogalvanostats PGSTAT 30 共Eco Chemie, The Netherlands兲 and VMP3 potentiostat/galvanostat 共Bio-Logic, France兲 were used for electrochemical measurements including the impedance measurements. Conductivities of prepared ILs were measured at 25°C using a conductivity cell 共Jenway, platinum electrodes, cell constant S = 1.00 ⫾ 0.01兲. Temperature-dependent conductivity measurements were performed in the temperature range from 0 to 100°C using a circulating bath Ministat 125 cm2 共with the precision of the temperature ⫾0.1°C, Huber, Germany兲. The initial electrochemical investigation of prepared ILs and electrolytes was performed in a three-electrode arrangement with a gold or platinum 共both from BASi, 1.6 mm in diameter兲 working electrode and lithium counter and reference electrodes. The electrochemical characteristics of half batteries were measured in vacuumsealed triplex foil 共coffee bag foil兲 cells by using a Celgard 2300 separator and a lithium foil as counter and reference electrodes. Six different electrolyte solutions were used as explained above. The electrochemical measurements were performed using a VMP3 potentiostat/galvanostat at a constant temperature of 60°C with a current density corresponding to C/10 or C/2. The compatibility of prepared electrolytes with aluminum current collector or with additives 共CB and EPDM binder兲 was tested by cyclovoltammetric measurements at 60°C with a scan rate of 0.5 mV s−1 in the potential range from 5 to 1 V vs metallic lithium reference. The surfaces of samples were observed and analyzed with a field-emission scanning electron microscope 共Supra 35 VP, Carl Zeiss, Germany兲 at an accelerating voltage of 1 kV. 57Fe Mössbauer experiments were performed at room temperature using a constant acceleration spectrometer. The source was 57Co in rhodium matrix. Velocity calibration and isomer shifts were quoted relative to an absorber of metallic iron at room temperature. The experiments were performed in transmission geometry. Parameter fits were performed using a standard least-squares fitting routine with Lorentzian lines.



Figure 1. TGA curves for neat ILs and LiTFSI or LiPF6 solutions in ILs 共5°C min−1 heating rate, temperature range 30–530°C; argon atmosphere; platinum crucible兲.

Results and Discussion In view of the potential application of ILs in large-scale batteries, thermal stability and nonflammability are the principal parameters to be considered. Figure 1 presents the decomposition curves for studied IL-based electrolytes, as measured by TGA. In all cases, a single-step decomposition reaction occurs at rather high temperatures, especially when compared to common, organic carbonatebased systems. We can define the start decomposition temperature 共Tdec兲 as the temperature at which the weight loss exceeds 1 wt % because the ILs used in our study have no distinguishable vapor pressure. For neat ILs and their mixtures with LiTFSI, Tdec varies from 385 to 405°C, as shown in Table I. In the presence of LiPF6, the thermal stability was lowered to 315 and 255°C, respectively. A lower stability of PF−6 anion was already observed by Fredlake et al.41 and Awad et al.42 Generally, the ILs with TFSI− anion exhibit the highest thermal stability and are much safer than the conventional carbonate-based electrolytes, as confirmed by the accelerating rate calorimetry.32 We need to emphasize that thermal stability tests in our study measured by the TGA method do not reflect the real thermal stability of ILs when they are stored at elevated temperatures because it was reported that some ILs lose their mass even at 150°C if they are stored at elevated temperature for a longer period.43 The DSC measurements showed a simple behavior of BMMITFSI with a Tg of −81°C. The presence of lithium salt increases Tg by ca. 10–15°C. The phase behavior of PYR14TFSI is more complex. Besides the Tg at −90°C we found two solid–solid phase

A621

Figure 2. Arrhenius plot for neat ILs and LiTFSI or LiPF6 solutions in ILs 共temperature range 0–100°C; for the electrolyte composition see Table I兲.

transitions at ⫺55 and −26°C. The presence of lithium salt does influence Tg, but also strongly supports the crystallization processes at temperatures mentioned above. However, the crystallization process is not complete and some amorphous phase remains even below −60°C. Contrary to the pure PYR14TFSI 共Tm − 9°C兲, two separate melting points were found for PYR14TFSI − LiTFSI at ⫺13 and 12°C. The complex behavior of PYR1nTFSI and PYR1nTFSI − LiTFSI systems was described by Henderson and Passerini when they suggested the formation of metastable phases.44 The temperature dependence of the ionic conductivity was measured in the region from 0 to 100°C and is shown in Fig. 2. As expected, the bulk conductivity of the lithium salt solutions in the ILs is lower than the conductivity of neat ILs. The equilibria 共BMMI+兲dissoc + 共TFSI−兲dissoc ↔ 共BMMI+ ¯ TFSI−兲assoc

关1兴

共PYR+14兲dissoc + 共TFSI−兲dissoc ↔ 共PYR+14 ¯ TFSI−兲assoc

关2兴

are shifted to the right by the addition of lithium salt, which leads to a decrease in conductivity. Unlike in carbonate-based electrolytes, where a conductivity maximum appears,45 the RTIL-based systems show a gradual decrease in conductivity in the whole range of salt concentration.23,45 If we use LiPF6 instead of LiTFSI, the conductivity decrease is not so deep. For PYR14TFSI − LiTFSI we observed a reproducible and reversible drop of conductivity at ca. 15°C, which is close to the temperature of a crystallization process 共formation of a new phase兲 observed by DSC. Generally, the ILs and their lithium salt solutions show a lower conductivity than the conventional carbonate-based

Table I. Specific conductivities (at 20 and 60°C), parameters A, EA, and T0 from the fit by VTF equation, the glass-transition temperatures determined by DSC „Tg…, and the start decomposition temperatures „Tdec… determined by TGA of studied ILs and their lithium salt electrolytes. Electrolyte BMMITFSI BMMITFSI–0.7 M LiTFSI BMMITFSI–0.5 M LiPF6 PYR14TFSI PYR14TFSI–0.7 M LiTFSI PYR14TFSI–0.5 M LiPF6

␴共20°C兲共S cm−1兲 1.6 6.1 6.7 2.1 4.6 7.5

⫻ ⫻ ⫻ ⫻ ⫻ ⫻

10−3 10−4 10−4 10−3 10−4 10−4

␴共60°C兲共S cm−1兲

A 共S cm−1 K0.5兲

EA 共kJ mol−1兲

T0 共°C兲

Tg 共°C兲

Tdec 共°C兲

⫻ ⫻ ⫻ ⫻ ⫻ ⫻

14.9 13.1 19.5 18.4 14.5 55.7

6.2 6.4 7.0 6.9 6.7 10.0

⫺98 ⫺88 ⫺93 ⫺113 ⫺88 ⫺123

⫺81 ⫺71 ⫺70 ⫺90 ⫺90 ⫺88

405 395 315 385 390 255

7.4 4.0 4.4 8.3 3.3 4.5

10−3 10−3 10−3 10−3 10−3 10−3


A622


Figure 4. 共a兲 Mössbauer spectra of prepared Li2FeSiO4 sample. 共b兲 Mössbauer spectra of carbon-coated Li2FeSiO4 /C sample. Figure 3. 共a兲 Cyclic voltammograms 共first cycle兲 of 共a兲 neat BMMI TFSI, 共b兲 BMMI TFSI–LiTFSI, and 共c兲 BMMI TFSI–LiPF6 on gold electrode at 10 mV s−1. Inserted: Linear sweep voltammograms of the same electrolytes on platinum electrode at 10 mV s−1. Arrows indicate initial directions of scans. 共b兲 Cyclic voltammograms 共first cycle兲 of 共a兲 neat PYR14TFSI, 共b兲 PYR14TFSI–LiTFSI, and 共c兲 PYR14TFSI–LiPF6 on gold electrode at 10 mV s−1. Inserted: Linear sweep voltammograms of the same electrolytes on platinum electrode at 10 mV s−1. Arrows indicate initial directions of scans.

electrolytes, especially at low temperatures. This disadvantage can be overcome by the addition of organic solvents that shift the equilibrium described above to the left22,46,47 or by elevation of the operation temperature. Both techniques also lower the electrolyte viscosity and improve the Li+ transference number. The conductivities at 60°C vary from 3.3 ⫻ 10−3 to 4.5 ⫻ 10−3 S cm−1, which is a sufficient value for appropriate electrochemical performance both for cathode and anode materials.

The obtained data were fitted with the Vogel–Tamman–Fulcher 共VTF兲 equation 共in the logarithmic form兲 ␴T1/2 = A exp关− EA /R共T − T0兲兴

关3兴

where A is a parameter related to the number of charge carriers, EA is the activation energy for conduction, R is the universal gas constant, and T0 is the ideal glass-transition temperature indicating the temperature at which the free volume extrapolates to zero. The analysis of the experimental conductivity data in terms of the VTF relationship leads to the determination of three empirical parameters: A, EA, and T0, where T0 is determined by fitting the experimental data with relationship 3. The obtained equation parameters A, EA, and T0 for the electrolytes are summarized in Table I. The electrochemical stability window of the electrolytes was measured by linear sweep and cyclic voltammetries on platinum and gold electrodes. The latter was chosen because we expected that it would exhibit higher catalytic activity than other electrode types, such as glassy carbon, etc. Both measurements showed high anodic stability up to ca. 5 V vs Li/Li+, where the limiting factor for both



A623

Figure 5. 关共a兲 and 共b兲兴 SEM micrographs of as-prepared Li2FeSiO4 sample and 关共c兲 and 共d兲兴 SEM micrographs of as-prepared Li2FeSiO4 /C sample at lower and higher magnifications.

BMMITFSI and PYR14TFSI becomes the stability of the anion. The results are presented in Fig. 3. In both cases, the addition of LiTFSI to neat IL enhances the stability of the IL against oxidation, consistent with previous observations by Saint et al.24 for lithium salts in various RTILs. This effect is remarkable especially for LiTFSI, while the presence of LiPF6 suppresses currents, but also induces an irreversible cathodic wave at ca. 2 V vs Li/Li+ at the reverse part of the cyclic voltammetry 共CV兲. This peak does not appear during the measurements down to low potentials. Because the electrolytes with LiPF6 are functional with Li2FeSiO4, one can attribute this peak to re-reduction in species generated during the cycling at potentials close to 5 V vs Li/Li+. A model-active cathode material in this work was Li2FeSiO4. With optimized synthesis conditions we were able to prepare fully lithiated Li2FeSiO4 samples with all iron in Fe共II兲 oxidation state 共Fig. 4a兲. The major contribution of Fe共II兲 as observed from Mössbauer spectra can be ascribed to only one local environment of iron in the Li2FeSiO4 sample. The minor Fe共II兲 phase in the sample is most probably another Li2FeSiO4 compound that crystallized into a slightly different crystal structure because silicates are known to crystallize in several different polymorphic crystal structures8,48 共Xray diffraction pattern, not shown, did not show any impurities兲. For preparation of carbon-coated Li2FeSiO4 particles we used citric acid as a source for amorphous carbon coating. Admixed citric acid and Li2FeSiO4 particles were fired in one step at two different temperatures. The first was slightly above the melting point of citric acid 共153°C兲 to ensure homogeneous distribution of liquefied citric acid around the particles. The second selected temperature of 700°C promotes the formation of an amorphous carbon surface layer with good electronic conductivity.49 This is realized via decomposition of citric acid, which is accompanied with an evolution of large quantities of gases. The latter made the formed composite very porous. In such an architecture the carbon coating ensures fast paths for electrons while the pores enable good ionic wiring 共electrolyte distribution兲. The latter is particularly important for ILs because they possess a higher viscosity than conventional electrolytes. After the formation of a carbon coating around the active particles, the Mössbauer spectra 共Fig. 4兲 showed the presence of Fe共III兲 species in the prepared composites. The content of formed Fe共III兲

was 4 atom %; the rest of the iron was in Fe共II兲 form. Fe共II兲 was detected in three different local environments, which probably means that the heating procedure created an additional polymorph in the sample 共again no impurities were detected after carbonization兲. Li2FeSiO4 particles obtained by the hydrothermal synthesis were homogeneous in size 共Fig. 5a兲. The particle size of around 80 nm 共Fig. 5b兲 was in good agreement with the value calculated from the Scherrer equation; the Brunauer, Emmett, and Teller surface area was close to 35 m2 g−1. The amount of carbon in the composite was 7 wt %, as determined by TGA. The prepared composite showed a certain porosity 共Fig. 5c兲. The carbon coating was not optimized because amorphous carbon can be observed from scanning electron microscopy 共SEM兲 micrographs 共Fig. 5d兲. The stability of the Al current collector was tested at an elevated temperature of 60°C in four different electrolytes based on the present ILs. As reference electrolytes, we used two conventional electrolytes with organic solvents. To evaluate the practical use of ILs with Li2FeSiO4, we first determined the electrochemical stability of aluminum current collector 共Fig. 6a兲 and the stability of blank electrodes 共Al current collector covered with CB/EPDM composite兲共Fig. 6b兲 in all six electrolytes. The electrochemical window was from 5 to 1 V vs metallic lithium reference. Figure 6a shows the CV behavior in the first cycle for all six electrolyte solutions. For the sake of clarity, the CV curve for DEC:EC in 1 M LiTFSI solution is shown in the inset because in this case the corrosion current was ca. 2 orders of magnitude higher than in the other electrolyte solutions. In all cases, the corrosion of aluminum current collector started at potentials close to 3.5 V vs lithium reference. The lowest corrosion current density was measured for DEC:EC in 1 M LiPF6 electrolyte solution, while the highest corrosion current density was measured for DEC:EC in 1 M LiTFSI 共inset of Fig. 6b兲, probably due to the formation of an Al–TFSI-based complex.33 The measured corrosion currents in the IL electrolytes were very similar for all four electrolyte solutions; no real difference has been observed between the two different ILs used and also between the two different electrolyte salts used in this study. These results confirm a good compatibility of TFSI-based electrolytes with the Al current collector32 at potentials higher than 4 V vs lithium metallic reference, even though the electrolyte solutions contained the LiTFSI salt. Testing of blank


A624


Figure 6. 共a兲 Cyclic voltammograms 共first cycle兲 of the aluminum current collector in the six different electrolytes: 共a兲 PYR14TFSI–LiTFSI, 共b兲 PYR14TFSI–LiPF6, 共c兲 BMMITFSI–LiTFSI, 共d兲 BMMITFSI–LiPF6, 共e兲 1 M LiPF6 in EC–DEC, and 共f兲 1 M LiTFSI in EC–DEC 共inserted plot兲 at 0.5 mV s−1 scan rate. The current densities are normalized per surface area of Al current collector. 共b兲 Cyclic voltammogram 共first cycle兲 of the aluminum current collector covered with CB/EPDM composite in the six different electrolytes: 共a兲 PYR14TFSI–LiTFSI, 共b兲 PYR14TFSI–LiPF6, 共c兲 BMMITFSI–LiTFSI, 共d兲 BMMITFSI–LiPF6, 共e兲 1 M LiPF6 in EC–DEC, and 共f兲 1 M LiTFSI in EC–DEC at 0.5 mV s−1 scan rate. The current densities are normalized per mass of CB/EPDM composite.

electrodes 共electrodes without active material兲 in IL-based electrolytes is not common in research reports. With the aim to evaluate possible oxidation reactions occurring on the surface of additives 共binder and CB兲, we decided to evaluate separately the compatibility of IL-based electrolyte solutions with electrodes that were prepared only from inactive additives 共CB and EPDM binder in our case兲 pressed on the surface of the Al current collector. Figure 6b shows

CV behavior in the first cycle for all six electrolyte solutions used in this study. For the sake of clarity, the CV curve for the Al current collector in DEC:EC 1 M LiTFSI electrolyte is shown in the inset of Fig. 6b because in this case the current density was almost 2 orders of magnitude higher than in the other electrolytes. The stability of CB/EPDM 共1:1 w/w兲 composite in 1 M LiTFSI in DEC:EC electrolyte cannot be compared with the stability in other electrolytes due to Al corrosion. Comparison of stability between IL electrolytes and 1 M LiPF6 in DEC:EC electrolyte with CB/EPDM composite shows that, at elevated temperatures, the ILs are much more stable in the presence of CB/EPDM composite at potentials above 4 V vs metallic lithium. At potentials up to 4 V vs metallic lithium, we did not detect any increase in oxidation currents in CVs with electrolyte solutions based on ILs and DEC:EC 1M LiPF6 electrolyte. And exactly stable voltage range from 2 up to 3.9 V is the electrochemical window for electrochemical exploration of Li2FeSiO4 cathode material. In the galvanostatic measurements we used two-electrode cells with metallic lithium as a counter electrode. The batteries were cycled in the potential range from 3.9 to 2 V vs metallic lithium reference with current densities corresponding to rates between C/10 and C/2. The graphs in Fig. 7 show discharge/charge curves in the first three cycles for the same batch of Li2FeSiO4 /C cathode material tested in the different electrolyte solutions prepared in this study. All tested batteries showed the typical shift of potential voltage plateau from 3.1 to 2.8 V due to structural rearrangements.11 The electrolyte solution had a significant influence on the capacity difference between oxidation and reduction process共es兲 in the first cycle, on irreversible capacity at higher cycles, and on achieved reversible capacity. The best electrochemical results in terms of these influences have been achieved with the DEC:EC 1 M LiPF6 electrolyte. The achieved capacity of more than 130 mAh g−1 is almost fully reversible and, that is, the cycling stability in the first three cycles shows a negligible irreversible capacity. At the other extreme, the worst electrochemical results were obtained with the DEC:EC 1 M LiTFSI electrolyte. Here the capacity loss in the first cycle due to Al corrosion was the largest; however, the capacity retention in the first cycle was comparable to the one observed in the organic electrolyte with the LiPF6 salt. The capacity stability was very poor during the first three cycles; we observed a capacity drop of 5 mAh/g per cycle, so we decided to abandon further testing. Three general trends can be observed with the use of ILs in various electrolyte formulations. The first trend, somehow already expected from the CVs shown in Fig. 6, is that the LiTFSI salt can be used quite successfully in combination with ILs. The second trend is a smaller reversible capacity when IL electrolytes are used instead of DEC:EC 1 M LiPF6 electrolyte. Finally, the capacity retention in the first cycle is much lower for liquid electrolytes compared to the DEC:EC 1 M LiPF6 electrolyte. We can explain the smaller reversible capacity by the high viscosity of the electrolytes based on ILs. The high viscosity probably prevents uniform distribution of the electrolyte within the pores of electrode composite. Partly, the decrease in reversible capacity could also be ascribed to the fact that we did not optimize the electrolytes with respect to their conductivity 共transference number兲, etc. The reason for the high capacity loss in the first cycle is not understood well; it could be connected with the well-known phenomenon of surface film formation.50 There is an additional feature that can be extracted from the present electrochemical testing: The reversible capacity in the first cycle is very similar for all four ILbased electrolytes. However, in the cases where LiPF6 was used as the electrolyte salt, we see an increase in capacity in the first three cycles 共Fig. 7c and e兲, while in the cases when LiTFSI salt was used, the reversible capacity remained constant 共Fig. 7d and f兲. An increase in capacity was also observed in the DEC:EC 1 M LiPF6 electrolyte, while in DEC:EC 1 M LiTFSI we observed a capacity drop. This was not the case in LiTFSI-based IL electrolytes although the oxidation capacity was much higher than the reversible capacity in the first cycles. Cycling stability of the Li2FeSiO4 /C cathode material in the DEC:EC 1 M LiPF6 electrolyte and in the four elec-



A625

Figure 7. Discharge/charge curves of the Li2FeSiO4 /C during the first three cycles in the six different electrolyte solutions. Electrochemical tests were performed at 60°C with current density corresponding to C/10.

trolyte solutions based on ILs is shown in Fig. 8. The cycling stability that was obtained at a cycling rate of C/10 proves once again the slightly higher reversible capacity in the organic electrolyte and the increased reversible capacity in the cases when the LiPF6 salt was used. The difference in the reversible capacity is even more pronounced when the batteries were cycled with the current density corresponding to the C/2 rate 共Fig. 8b兲. Let us compare the capacity retention and the obtained reversible capacity in the DEC:EC 1 M LiPF6 electrolyte and PYR14TFSI with 0.7 M LiTFSI or 0.5 M LiPF6 electrolytes. We can find that the comparison follows the general trend observed at the C/10 rate with slightly higher differences in the obtained reversible capacity. Figure 8b clearly shows that, using Li2FeSiO4 /C cathode material, stable cycling can be achieved in both organic and IL-based electrolytes. Further decrease in particle size 共shorter paths for lithium diffusion兲 and improvement, or both, in electrode architecture 共better wetting with electrolyte兲 are necessary to obtain a full capacity at current densities used in our study.

3.3 ⫻ 10−3 to 4.5 ⫻ 10−3 S cm−1. The electrochemical tests against aluminum current collector showed slightly higher corrosion of Al when the ILs were used compared to DEC:EC–1 M LiPF6 electrolyte solution; however, the corrosion currents in DEC:EC–1 M LiTFSI were 100 times higher. The electrolytes based on ILs were more compatible with electrode additives 共a combination of Al, CB, and EPDM binder兲 than the conventional electrolytes. Furthermore, we showed that at 60°C the Li2FeSiO4 cathode material gives comparable capacities regardless of the electrolyte solution. The stability was excellent in all tested electrolyte solutions, except in DEC:EC–1 M LiTFSI where substantial Al corrosion occurred. Cycling stability at C/10 and C/2 rates was excellent, given the fact that the system was not optimized. Capacities of 110–130 mAh g−1 were achieved at a C/10 cycling rate. The kinetic limitations resulted in a slightly higher difference when the C/2 cycling rate was used. The stability of LiTFSI salt in IL electrolytes above the potential 3.5 V vs lithium reference suggests that one can use it with silicates.

Conclusions

M.N. and J.R. acknowledge the support by the Academy of Sciences 共research plan no. AV0Z40320502兲, by the Grant Agency of the Academy of Sciences 共grant no. KJB400320701兲, and by the Ministry of Education, Youth and Sports, Czech Republic 共project no. MSMT LC523, no. MEB090806, and no. MSM0021620857兲. R.D. acknowledges the support by the Slovenian Research Agency,

We compared quite extensively the electrochemical properties of Li2FeSiO4 in conventional organic electrolytes with the properties obtained in electrolytes based on ILs. The typical temperature for electrochemical testing of Li2FeSiO4 was 60°C. At this temperature the conductivity of electrolytes based on ILs was in the range from

Acknowledgment


A626


Figure 8. Cycling stability of the Li2FeSiO4 /C sample in different electrolytes at the current density corresponding to 共a兲 C/10 and 共b兲 C/2.

by the Ministry of Education, Science and Sport of Slovenia 共MNT ERA-net project兲, and by the European Network of Excellence “ALISTORE.” National Institute of Chemistry assisted in meeting the publication costs of this article.

References 1. A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, J. Electrochem. Soc., 144, 1188 共1997兲. 2. A. K. Padhi, K. S. Nanjundaswamy, C. Masquelier, S. Okada, and J. B. Goodenough, J. Electrochem. Soc., 144, 1609 共1997兲. 3. V. Legagneur, Y. An, A. Mosbah, R. Portal, A. Le Gal La Salle, A. Verbaere, D. Guyomard, and Y. Piffard, Solid State Ionics, 139, 37 共2001兲. 4. J. Gaubicher, C. Wurm, G. Goward, C. Masquelier, and L. Nazar, Chem. Mater., 12, 3240 共2000兲. 5. A. Nyten, A. Abouimrane, M. Armand, T. Gustafsson, and J. O. Thomas, Electrochem. Commun., 7, 156 共2005兲. 6. R. Dominko, M. Bele, M. Gaberscek, A. Meden, M. Remskar, and J. Jamnik, Electrochem. Commun., 8, 217 共2006兲. 7. A. S. Prakash, P. Rozier, L. Dupont, H. Vezin, F. Sauvage, and J.-M. Tarascon, Chem. Mater., 18, 407 共2006兲. 8. C. Lyness, B. Delobel, A. R. Armstrong, and P. G. Bruce, Chem. Commun. (Cam-

bridge), 46, 4890 共2007兲. 9. K. Zaghib, A. Ait Salah, N. Ravet, A. Mauger, F. Gendron, and C. M. Julien, J. Power Sources, 160, 1381 共2006兲. 10. R. Dominko, J. Power Sources, 184, 462 共2008兲. 11. A. Nytén, S. Kamali, L. Hänggstrom, T. Gustafsson, and J. O. Thomas, J. Mater. Chem., 16, 2266 共2006兲. 12. J. S. Wilkes and M. J. Zaworotko, J. Chem. Soc., Chem. Commun., 13, 965 共1992兲. 13. B. García, S. Lavallée, G. Perron, C. Michot, and M. Armand, Electrochim. Acta, 49, 4583 共2004兲. 14. C. Arbizzani, S. Beninati, M. Lazzari, F. Soavi, and M. Mastragostino, J. Power Sources, 174, 648 共2007兲. 15. R. F. de Souza, J. C. Padilha, R. S. Goncalves, and J. Dupont, Electrochem. Commun., 5, 728 共2003兲. 16. P. Wang, S. M. Zakeeruddin, P. Comte, I. Exnar, and M. Grätzel, J. Am. Chem. Soc., 125, 1166 共2003兲. 17. D. W. Steuerman, H. R. Tseng, A. J. Peters, A. H. Flood, J. O. Jeppesen, K. A. Nielsen, J. F. Stoddart, and J. R. Heath, Angew. Chem., Int. Ed., 43, 6486 共2004兲. 18. M. Galinski, A. Lewandowski, and I. Stepniak, Electrochim. Acta, 51, 5567 共2006兲. 19. T. Sato, T. Maruo, S. Marukane, and K. Takagi, J. Power Sources, 138, 253 共2004兲. 20. H. Matsumoto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, and M. Kono, J. Power Sources, 160, 1308 共2006兲. 21. H. Sakaebe, H. Matsumoto, and K. Tatsumi, J. Power Sources, 146, 693 共2005兲. 22. H. Nakagawa, Y. Fujino, S. Kozono, Y. Katayama, T. Nukuda, H. Sakaebe, H. Matsumoto, and K. Tatsumi, J. Power Sources, 174, 1021 共2007兲. 23. S. Seki, Y. Ohno, Y. Kobayashi, H. Miyashiro, A. Usami, Y. Mita, H. Tokuda, M. Watanabe, K. Hayamizu, S. Tsuzuki, et al., J. Electrochem. Soc., 154, A173 共2007兲. 24. J. Saint, A. S. Best, A. F. Hollenkamp, J. Kerr, J.-H. Shin, and M. M. Doeff, J. Electrochem. Soc., 155, A172 共2008兲. 25. S. C. Luo, Z. X. Zhang, and L. Yang, Chin. Sci. Bull., 53, 1337 共2008兲. 26. Z. X. Zhang, H. Y. Zhou, L. Yang, K. Tachibana, K. Kamijima, and J. Xu, Electrochim. Acta, 53, 4833 共2008兲. 27. A. Guerfi, S. Duchesne, Y. Kobayashi, A. Vijh, and K. Zaghib, J. Power Sources, 175, 866 共2008兲. 28. J.-H. Shin, W. A. Henderson, C. Tizzani, S. Passerini, S.-S. Jeong, and K. W. Kim, J. Electrochem. Soc., 153, A1649 共2006兲. 29. Y. Kobayashi, Y. Mita, S. Seki, Y. Ohno, H. Miyashiro, and N. Terada, J. Electrochem. Soc., 154, A677 共2007兲. 30. J.-H. Shin, W. A. Henderson, S. Scaccia, P. P. Prosini, and S. Passerini, J. Power Sources, 156, 560 共2006兲. 31. G.-T. Kim, G. B. Appetecchi, F. Alessandrini, and S. Passerini, J. Power Sources, 171, 861 共2007兲. 32. Y. Wang, K. Zaghib, A. Guerfi, F. F. C. Bazito, R. M. Torresi, and J. R. Dahn, Electrochim. Acta, 52, 6346 共2007兲. 33. X. Wang, E. Yasukawa, and S. Mori, Electrochim. Acta, 45, 2677 共2000兲. 34. W. K. Behl and E. J. Plichta, J. Power Sources, 72, 132 共1998兲. 35. C. X. Peng, L. Yang, Z. X. Zhang, K. H. Tachibana, and Y. Yang, J. Power Sources, 173, 510 共2007兲. 36. B. Garcia and M. Armand, J. Power Sources, 132, 206 共2004兲. 37. M. Abu Bin Hasan Susan, A. Noda, and M. Watanabe, in Electrochemical Aspects of Ionic Liquids, H. Ohno, Editor, Chap. 5, Wiley-Interscience, New York 共2005兲. 38. M. J. Monteiro, F. F. C. Bazito, L. J. A. Siqueira, M. C. C. Ribeiro, and R. M. Torresi, J. Phys. Chem. B, 112, 2102 共2008兲. 39. F. F. C. Bazito, Y. Kawano, and R. M. Torresi, Electrochim. Acta, 52, 6427 共2007兲. 40. R. Dominko, D. E. Conte, D. Hanzel, M. Gaberscek, and J. Jamnik, J. Power Sources, 178, 842 共2008兲. 41. C. P. Fredlake, J. M. Crosthwaite, D. G. Hert, S. N. V. K. Aki, and J. P. Brennecke, J. Chem. Eng. Data, 49, 954 共2004兲. 42. W. H. Awad, J. W. Gilman, M. Nyden, R. H. Harris, T. E. Sutto, J. Callahan, P. C. Trulove, H. C. Delong, and D. M. Fox, Thermochim. Acta, 409, 3 共2004兲. 43. Q. Zhou, W. A. Henderson, G. B. Appetecchi, M. Montanino, and S. Passerini, J. Phys. Chem. B, 112, 13577 共2008兲. 44. W. A. Henderson and S. Passerini, Chem. Mater., 16, 2881 共2004兲. 45. M. S. Ding and T. R. Jow, J. Electrochem. Soc., 151, A2007 共2004兲. 46. M. Diaw, A. Chagnes, B. Carré, P. Willmann, and D. Lemordant, J. Power Sources, 146, 682 共2005兲. 47. J. Reiter, J. Vondrák, J. Michálek, and Z. Mička, Electrochim. Acta, 52, 1398 共2006兲. 48. M. M. E. Arroyo–DeDompablo, R. Dominko, J. M. Gallardo–Amores, L. Dupont, G. Mali, H. Ehrenberg, J. Jamnik, and E. Moran, Chem. Mater., 20, 5574 共2008兲. 49. J. Moskon, R. Dominko, M. Gaberscek, R. Cerc–Korosec, and J. Jamnik, J. Electrochem. Soc., 153, A1805 共2006兲. 50. D. Ensling, M. Stjerndahl, A. Nytén, H. Rensmo, T. Gustafsson, and J. O. Thomas, J. Mater. Chem., 19, 82 共2009兲.
