PEO-Based Polymer Electrolytes with Ionic Liquids ...

Journal of The Electrochemical Society, 152 共5兲 A978-A983 共2005兲

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0013-4651/2005/152共5兲/A978/6/$7.00 © The Electrochemical Society, Inc.

PEO-Based Polymer Electrolytes with Ionic Liquids and Their Use in Lithium Metal-Polymer Electrolyte Batteries Joon-Ho Shin, Wesley A. Henderson,* and Stefano Passerini*,z ENEA (Italian National Agency for New Technologies, Energy and the Environment), IDROCOMB, Casaccia Research Center, 00060 Rome, Italy The influence of adding the room-temperature ionic liquid 共RTIL兲 N-methyl-N-propylpyrrolidinium bis共trifluoromethanesulfonyl兲imide 共PYR13TFSI兲 to P共EO兲20LiTFSI polymer electrolytes and the use of these electrolytes in solid-state Li/V2O5 batteries has been investigated. P共EO兲20LiTFSI + xPYR13TFSI polymer electrolytes with various PYR+13 /Li+ mole fractions 共x = 0.66, 1.08, 1.73, 1.94, 2.15, and 3.24兲 were prepared. The addition of up to a 3.24 mole fraction of the RTIL to P共EO兲20LiTFSI electrolytes, corresponding to a RTIL/PEO weight fraction of up to 1.5, resulted in freestanding and highly conductive electrolyte films reaching 10−3 S/cm at 40°C. The electrochemical stability of PYR13TFSI was significantly improved by the addition of LiTFSI. Li/V2O5 cells using the polymer electrolyte with PYR13TFSI showed excellent reversible cyclability with a capacity fading of 0.04% per cycle over several hundreds cycles at 60°C. The incorporation of the RTIL into lithium metal-polymer electrolyte batteries has resulted in a promising improvement in performance at moderate to low temperatures. © 2005 The Electrochemical Society. 关DOI: 10.1149/1.1890701兴 All rights reserved. Manuscript submitted July 12, 2004; revised manuscript received September 23, 2004. Available electronically April 1, 2005.

Since the first report by Walden in 1914,1 large efforts have been devoted to the investigation of room temperature ionic liquids 共RTILs兲 as green chemistry materials, especially over the last six years.2 RTILs that consist of organic cations and inorganic anions are attracting wide interest for applications in catalysis, fuel cells, electrochemical capacitors, and batteries3-8 due to their nonvolatility, nonflammability, high thermal stability, and high conductivity.9,10 Recently, a few reports have demonstrated the enhanced performance of electrochemical devices using electrolytes composed of pure RTILs11 or RTILs doped with a suitable lithium salt 共to supply the Li+ cations required in the electrochemical reactions兲 and combined with an appropriate polymer.12-16 Rechargeable lithium metal polymer electrolyte batteries 共LMPBs兲 are now considered to be the most probable next generation of power sources for portable electronic devices and electric vehicles. The performance of LMPBs, however, is still limited by the ionic conductivity of the polymer electrolyte. Most of the polymer electrolytes reported to date do not have ionic transport properties suitable for state-of-the-art lithium batteries. Nevertheless, dry 共molecular solvent-free兲 polymer electrolytes have been extensively investigated. Poly共ethylene oxide兲共PEO兲 based electrolytes are one of the most promising materials due to their good thermal properties and interfacial stability with the Li electrode. PEO-LiX electrolytes are hindered, however, by a low-room-temperature ionic conductivity. The addition of molecular solvents able to compete with the polymer ether oxygens for Li+ cation coordination has been demonstrated as a means of attaining a high ionic conductivity at room temperature.17,18 Unfortunately, the reactivity of such solvents in these gelled electrolytes results in a poor interfacial stability with Li metal. Additionally, the volatile nature of these solvents may cause a battery to explode if short circuits create localized heating. In contrast, the addition of RTILs, which are able to dissolve lithium 共LiX兲 salts, to PEO-LiX electrolytes may improve the ionic conductivity of the polymer electrolytes without the detrimental effects noted for molecular solvents. The selection of an adequate RTIL, however, is required for its use into LMPB electrolytes. For example, RTILs based upon the 1-ethyl-3-methylimidazolium 共EMI兲 cation have many desirable properties such as a low viscosity, wide electrochemical stability window, and high ionic conductivity,5,7,19 but they have poor stability toward Li metal.20,21 Other RTILs, consisting of N-alkyl-N-methylpyrrolidinium cations and bis共trifluoromethanesulfonyl兲imide anions 共PYR1RTFSI兲, also have a high ionic conductivity and wide electrochemical stability window.15,22 In

* Electrochemical Society Active Member. z

E-mail: [email protected]

a previous paper,15 we demonstrated that the combination of PYR13TFSI and P共EO兲20LiTFSI resulted in freestanding, easy to handle membranes which have a high conductivity at moderate temperatures. These are true homogeneous “dry” solid polymer electrolytes 共consisting solely of commercial PEO and two salts兲 rather than gel electrolytes. In this paper we report on the electrochemical and interfacial properties of P共EO兲20LiTFSI + PYR13TFSI electrolytes and their performance in solvent-free, solid-state Li/V2O5 batteries. Experimental Materials.—PEO 共Mw = 4 ⫻ 106, Union Carbide兲 and LiTFSI 关LiN共SO2CF3兲2, 3 M兴 were dried under vacuum for 48 h at 50 and 150°C, respectively. Vanadium oxide 共V2O5, Pechiney兲 and carbon 共KJB, Akzo Nobel兲 were dried under vacuum at 150°C and sieved 共400 mesh sieve兲. 1-Methylpyrrolidine 共97%兲 and 1-iodopropane 共99%兲 were purchased from Aldrich and used as received. PYR13TFSI RTIL preparation.—PYR13I was prepared by the reaction of 1-methylpyrrolidine with a stoichiometric amount of 1-iodopropane in ethyl acetate. The resulting PYR13I salt was washed several times with ethyl acetate and then recrystallized by dissolving/melting the salt in hot acetone and adding ethyl acetate to obtain a pure white salt. Combining PYR13I and LiTFSI 共1:1 mole ratio兲 in deionized H2O gave PYR13TFSI 共the aqueous layer and RTIL phase separated兲. The aqueous phase was removed and the salt was washed five times with deionized H2O to remove residual LiI. The final aqueous layer was removed and the PYR13TFSI was heated to high temperature on a hot plate to remove trace solvents. Activated carbon 共Darco-G60, Aldrich兲 was added and the hot mixture was stirred on a hot plate overnight. Acetone was added and the mixture was filtered through a short activated alumina 共acidic, Brockmann I, Aldrich兲 column. The acetone was removed by heating and the PYR13TFSI was dried under high vacuum at 100°C overnight and then at 120°C for 6 h. The resulting PYR13TFSI is a clear colorless liquid at room temperature. The RTIL was stored and handled in a dry room 共⬍0.2% relative humidity, 20°C兲. Polymer electrolyte preparation.—P共EO兲20LiTFSI + xPYR13TFSI 共x = PYR+13 /Li+ mole fraction兲 polymer electrolytes were prepared as follows. The materials were first mixed in a mortar and then vacuum sealed in aluminum laminate bags and annealed at 90°C overnight to homogenize. Freestanding polymer electrolyte films, with x up to 3.24, were obtained by hot-pressing the resulting mixtures at 110°C for about 20 min. No solvent was used during the electrolyte preparation. All operations were performed in a dry room 共⬍0.2% relative humidity, 20°C兲.

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Composite cathode preparation.—Composite cathodes were made with the following composition: V2O5 共43 wt %兲, carbon 共7 wt %兲, PEO 共Mw = 4 ⫻ 106, 17.5 wt %兲, LiTFSI 共5 wt %兲兲, and PYR13TFSI 共27.5 wt %兲. The EO/Li+ ratio was kept at 20 while the PYR+13 /Li+ ratio was fixed at 3.86. Lower amounts of ionic liquid in the composite cathodes resulted in their poor electrochemical performance. This issue is presently under investigation; however, our preliminary observations support the entrapment of a large fraction of the ionic liquid onto the active material and the carbon particles. The cathodes were prepared in a manner similar to the polymer electrolytes. The materials were mixed in a mortar and then vacuum sealed in aluminum laminate bags and annealed at 90°C for 24 h. Composite cathode films were made by calendering the resulting mixtures at room temperature. Thin cathode films were hot-pressed at 100°C for a few minutes. No solvent was used during the composite cathode preparation. All operations were performed in a dry room 共⬍0.2% relative humidity, 20°C兲. Cell and battery assembly and instrumentation.—Ionic conductivity and Li+ diffusion coefficient 共DLi+兲 measurements were performed on cells in which the polymer electrolyte films were sandwiched between Cu and Li electrodes 共4 and 1 cm2兲, respectively. The cells were vacuum sealed in aluminum laminate bags and then laminated by hot-rolling at 100°C. The batteries were prepared in a similar manner by sandwiching the polymer electrolyte films between a Li metal anode 共50 ␮m thick兲 and a composite cathode tape. Cu and Al foils were used as current collectors for the anode and cathode, respectively. The electrochemical stability windows 共ESW兲 were measured by sandwiching the samples between two stainless steel 共SS兲 electrodes 共0.8 cm2兲共the pure PYR13TFSI was absorbed in a polyethersulfone membrane兲. All cells were assembled and sealed in a dry room 共⬍0.2% relative humidity, 20°C兲. The test temperature was controlled by using a ethylene glycol cooling bath or ventilated oven. All electrochemical and impedance measurements were performed using a Solartron electrochemical interface 共ECI 1287兲 and a Solartron frequency response analyzer 共FRA 1260兲. All cells were thermally equilibrated for at least 1 h at the selected temperature prior to measurements. The battery cycle tests were performed using a Maccor 共S4000兲 battery tester. Differential scanning calorimetry 共DSC兲 measurements were performed on a TA model 2910 differential scanning calorimeter. Samples were hermetically sealed in Al pans and stored in a refrigerator for several weeks prior to the measurements in a dry room 共 ⬍0.2% relative humidity, 20°C兲. The pans were cooled to −140°C at 20°C/min, heated to −10°C and held at this temperature for 15 min, cooled to −140°C at 20°C/min, and then heated to 200°C at 10°C/min. Results and Discussion Thermal behavior of P共EO兲20LiTFSI + xPYR13TFSI electrolytes.—DSC heating traces for the P共EO兲20LiTFSI + xPYR13TFSI polymer electrolytes are shown in Fig. 1. These data differ from those previously reported,15 because the history of thermal treatment before the DSC scans in this paper were not the same manner as that in the previous report; in this paper, the sample was first stored in a refrigerator and then cooled in the DSC instrument to −140°C and annealed at −10°C for 15 min prior to the measurements to aid in the crystallization of the PYR13TFSI phase. For the polymer electrolyte without PYR13TFSI 共x = 0兲, two endothermic peaks are present due to crystalline P共EO兲6:LiTFSI and PEO phases. No significant glass transition, Tg, is observed due to an amorphous phase, indicating that the sample is predominantly crystalline. In contrast, for the P共EO兲20LiTFSI + xPYR13TFSI electrolyte with x = 1.08, a clear Tg is observed and no endothermic peak for the crystalline P共EO兲6:LiTFSI phase is present. A new small endothermic peak appears at a lower temperature for the crystalline PYR13TFSI phase for the x = 1.73 electrolyte. This peak increasing with increasing PYR13TFSI mole fraction is attributed to the presence of “excess”

Figure 1. DSC heating traces of P共EO兲20LiTFSI + xPYR13TFSI 共x = PYR+13 /Li+兲 polymer electrolytes. The traces were normalized to the weight of PEO in each sample.

RTIL, which does not strongly interact with either the PEO or LiTFSI. For these mixtures, this excess PYR13TFSI is present for x 艌 1.7. The RTIL evidently forms an amorphous phase 共with x 艋 1.7兲 with the LiTFSI and some of the PEO. The remainder of the PEO crystallizes. Because all the electrolytes have approximately the same amount of crystalline PEO, the DSC results indicate that the presence of the excess RTIL does not affect the remained PEO significantly. Despite the presence of amorphous and crystalline PEO phases, the electrolytes are homogenous, slightly tacky but easily handled membranes at ambient temperature. Ionic conductivity of P共EO兲20LiTFSI + PYR13TFSI electrolytes.—Figure 2 shows the temperature dependence of the ionic conductivity of P共EO兲20LiTFSI polymer electrolytes containing different amounts of PYR13TFSI. The ionic conductivity of pure PYR13TFSI is also shown for comparison. The ionic conductivity values reported were obtained during the first heating scan from 20 to 90°C after storage at 20°C for several weeks. The ionic conductivity of the pure PYR13TFSI at 20°C is ca. 2.8 ⫻ 10−3 S/cm; this value is slightly higher than that previously reported by MacFarlane et al.22 The incorporation of PYR13TFSI into the P共EO兲20LiTFSI electrolytes improves the ionic conductivity over the entire temperature range investigated, but the greatest enhancement is at lower temperatures. The ionic conductivity of the polymer electrolyte at 20°C with a PYR+13 /Li+ mole fraction of 0.66 showed an increase of about one order of magnitude reaching 6.2 ⫻ 10−5 S/cm. The conductivity rose to 2.8 ⫻ 10−4 S/cm at 20°C for a mole fraction of 2.15. Although the room temperature conductivity of these electrolytes is still below 10−3 S/cm, which is required for many practical applications of Li batteries, this is the highest room-temperature conductivity reported to date for a solvent-free, PEO-based electrolyte. The above-mentioned conductivity limit is reached at moderate temperatures as shown in Table I. Note that these ionic conductivity values are due to contributions from both the LiTFSI and PYR13TFSI salts. Polarization measurements, however, have demonstrated that approximately one quarter of the conductivity is due to Li+ cation transport.15 The ionic conductivity of selected RTIL-containing samples was measured after storage 共aging兲 at 20°C to verify that the conductivity enhancement was a true thermodynamic effect and not simply related to slow crystallization kinetics. Figure 3 shows that the conductivity of



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Figure 3. Ionic conductivity of P共EO兲20LiTFSI + xPYR13TFSI 共x = 1.08 and 2.15兲 polymer electrolytes upon storing 共aging兲 at 20°C. Two samples were checked for each electrolyte.

Figure 2. Temperature dependence of the ionic conductivity for PYR13TFSI and P共EO兲20LiTFSI + xPYR13TFSI 共x = PYR+13 /Li+兲. All measurements were performed while heating the samples. The samples were stored at room temperature prior to the measurements.

P共EO兲20LiTFSI + x PYR13TFSI 共x = 1.08 and 2.15兲 polymer electrolytes remains stable over an extended period of time exceeding 1 year. The trend of the ionic conductivity vs. temperature of these electrolytes shows the typical two-region behavior of PEO-based electrolytes 共due to PEO melting兲, indicating that the Li+ cations are still interacting with the PEO chains. The activation energies 共Ea兲 obtained from the Arrhenius curves of Fig. 2 listed in Table I are classified into low-and high-temperature regions. The Ea values significantly decrease with the addition of increasing amounts of PYR13TFSI. The low-temperature activation energy values were similar to that of poly共vinyl difluoride兲-1-ethyl-3methylimidazolium polymer electrolytes.12 + Li+ diffusion coefficient 共DLi 兲 of P共EO兲20LiTFSI + xPYR13TFSI electrolytes.—The restricted diffusion method 共RDM兲23 was used to determine D+Li values of the P共EO兲20LiTFSI + xPYR13TFSI electrolytes. A constant potential of 50 mV was applied to the Li/polymer electrolyte/Li cells until the current reached a steady state. The current was then interrupted and the cell potential was recorded as a function of time. D+Li values were obtained from the slope of the log ⌬⌽ vs. time plots. Figure 4 shows the influence of the addition of

PYR13TFSI on the D+Li values of the polymer electrolytes. The D+Li value of P共EO兲20LiTFSI 共x = 0兲 is close to that reported recently by Edman et al.24 共obtained using the same method兲. Only slight differences in the D+Li values where observed over the mole fraction range investigated with the exception of x = 3.24 where a substantial reduction of D+Li was observed. At this time, it is not known why this is so. Electrochemical stability window (ESW) of P共EO兲20LiTFSI + xPYR13TFSI electrolytes.—For practical battery applications, it is important to demonstrate the electrolyte’s electrochemical stability within the operation voltage of the battery system. Linear sweep voltammetry 共LSV兲 experiments were performed to investigate the ESW of P共EO兲20LiTFSI + xPYR13TFSI polymer electrolytes. The results are shown in Fig. 5 共the ESW curve of pure PYR13TFSI is included for comparison兲. The curves are the results of two different experiments performed to investigate the anodic and the cathodic electrochemical stabilities. The potential was swept either positive or negative from open circuit with new samples. The anodic and cathodic current may correspond to the oxidation of the TFSI− anion

Table I. Activation energies „Ea… calculated from the Arrhenius plots of the ionic conductivities „␴… of pure PYR13TFSI and P„EO…20LiTFSI + x PYR13TFSI „x = PYR+13 /Li+… polymer electrolytes. Ea 共kJ/mol兲 PYR+13 /Li+ PYR13TFSI 0 0.66 1.08 1.73 2.15 3.24

␴40°C 共S/cm兲

Low-temp. region

High-temp. region

⫻ ⫻ ⫻ ⫻ ⫻ ⫻ ⫻

9.6 36.2 25.3 21.9 22.0 16.6 14.6

6.4 12.5 9.1 6.9 7.9 5.6 4.7

5.2 6.1 2.7 4.2 5.9 7.9 1.2

10−3 10−5 10−4 10−4 10−4 10−4 10−3

Figure 4. Li+ cation diffusion coefficient 共DLi+兲 obtained using the restricted diffusion method on Li/P共EO兲20LiTFSI + xPYR13TFSI/Li cells at 60°C. Li electrode area 1 cm2. Applied potential 50 mV. The solid lines are intended as a guide for the eye. Three samples were tested for each electrolyte.



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Figure 6. Interfacial resistance 共Ri兲 of nonblocking cells, Li/P共EO兲20LiTFSI + xPYR13TFSI/Li, as a function of storage time at opencircuit condition at 90°C. Li electrode area 1 cm2.

Figure 5. LSV curves of PYR13TFSI and P共EO兲20LiTFSI + xPYR13TFSI 共x = PYR+13 /Li+兲 at scan rate of 10 mV/s at 20°C. SS electrode area 0.8 cm2. Reference electrode Ag/Ag+. The LSV of P共EO兲20LiTFSI 共x = 0兲 was carried out at 60°C to ensure a comparable ionic conductivity 共see Fig. 2兲.

and the reduction of PYR+13 cation, respectively. The pure RTIL, PYR13TFSI, is seen to be electrochemically stable between −3.01 and 3.02 V 共vs. Ag/Ag+兲, with an overall ESW of about 6 V, i.e., a slightly higher ESW than reported for the PYR14TFSI salt.22 The cathodic stability limit, however, is higher than the Li plating potential 共the Ag/Ag+ couple is about 3.2 V above the Li+ /Li couple兲. This suggests that the pure PYR13TFSI should not be stable in contact with Li metal. When the PYR13TFSI is added to the PEOLiTFSI electrolytes, however, the cathodic stability limit is improved dramatically, extending below the Li plating potential.25 Other authors have observed a similar effect in related RTIL-LiX salt systems.26,27 The reason for this improvement is due to the formation of a stable, Li+ cation-conducting passivation layer on the Li metal surface, which prevents further reaction of the RTIL as reported by Howlett et al.16 These experiments suggest that the P共EO兲20LiTFSI + xPYR13TFSI membranes should be suitable polymer electrolytes for rechargeable lithium batteries if there is no excess ionic liquid in the system. Interfacial resistance 共Ri兲 of P共EO兲20LiTFSI + xPYR13TFSI electrolytes.—The interfacial resistance 共Ri兲, always present when Li electrodes are involved, is determined by the properties of the passivation layer spontaneously formed on the surface of the Li electrode by reactions with the electrolyte. The ac impedance technique was used to evaluate the interfacial resistance. The effect of PYR13TFSI on the resistance of the polymer electrolyte/Li metal interface at 90°C is shown in Fig. 6. The RTIL-free, P共EO兲20LiTFSI polymer electrolyte had an interfacial resistance of 20 ⍀ cm2, which increased to about 40 ⍀ cm2 after 60 days of storage. Such a low value for the resistance confirms the excellent compatibility with the Li electrode at high temperature with the polymer electrolytes prepared through solvent-free procedures, in good agreement with the results previously reported for a P共EO兲20LiBETI electrolyte.28,29 The electrolytes with a lower mole fraction of PYR13TFSI 共x 艋 1.08兲 have Ri values 共46 ⍀ cm2 after 60 days兲 comparable to the RTIL-free electrolyte. As expected from the previous discussion re-

garding the ESW of the RTIL, the addition of a higher mole fraction of RTIL 共x 艌 2.15兲 in which there is excess RTIL present resulted in a steep increase in the interfacial resistance over the entire storage period. The Ri value after 60 days was 76 ⍀ cm2, i.e., about twice that for the other electrolytes 共x = 0 and 1.08兲. This clearly indicates that the presence of free RTIL in the system causes a high interfacial resistance due to the reaction, forming a resistive passivation layer with Li metal. Quenching the cells to room temperature and reheating after 25 days of storage does not substantially change the trend of the interfacial resistance vs. time, though a decrease of Ri was observed. Battery performance.—The performance of Li/V2O5 cells incorporating P共EO兲20LiTFSI + xPYR13TFSI electrolytes have been characterized at moderate temperatures. The choice of the polymer electrolyte was restricted to PYR+13 /Li+ mole fractions of 1.73 and 1.94 on the basis of the ionic conductivity, ESW, and Ri measurements. Crystalline V2O5 was used as the active cathode material because of its ready availability and high Li intercalation capability. Typically, 1 M of V2O5 can intercalate up to 2 equiv of Li. However, after the intercalation of more than 1 equiv of Li, V2O5 shows a capacity fading typically of at least 0.28%/cycle.29 Figure 7 compares the capacity delivered by two Li/P共EO兲20LiTFSI + xPYR13TFSI/V2O5 cells with different PYR+13 /Li+ mole fractions 共x = 0 and 1.94兲 cycled at 60°C and a 0.5 C charge/discharge rate. Initially, the addition of the RTIL doubles the cell performance at moderate temperature. After an initial increase 共perhaps due to the optimization of the electrode/ electrolyte interfaces兲, the cell incorporating the RTIL delivered a maximum capacity of about 184 mAh/g 共corresponding to 62% of the theoretical capacity兲 even at this relatively high rate 共0.5 C兲. In addition, the cells were cycled for several hundred cycles with a capacity fading of approximately 0.04%/cycle 关a much lower value than previously reported by Villano et al.30 for a Li/P共EO兲20LiBETI/V2O5 cell tested at 90°C, but with similar current density 共0.28%/cycle兲兴. The RTIL-free cell showed a much lower delivered capacity and a slightly higher 共0.05%/cycle兲 capacity fading. The voltage profiles of the RTIL-containing cell during cycling are shown in Fig. 8. The observed behavior is typical of deeply charged and discharged Li/V2O5 cells. The ohmic drop 共⌽兲 of the cell on charge and discharge was found to increase from about 150 mV 共at the 50th cycle兲 to about 300 mV 共at the 280th cycle兲, in good agreement with that of a Li/P共EO兲20LiBETI/V2O5 cell tested at 90°C.30 The performance of a Li/P共EO兲20LiTFSI + xPYR13TFSI/V2O5


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Figure 7. 共쎲, 䊏兲 Charge and 共䊊, 䊐兲 discharge capacities during cycling of two Li/P共EO兲20LiTFSI + xPYR13TFSI/V2O5 共x = 0 and 1.94兲 cells at 60°C. Charge/discharge rate 0.5 C. Current density 共a兲 0.23 and 共b兲 0.20 mA/cm2. Loading of V2O5: 共a兲 3.8 and 共b兲 3.4 mg. Cathode area 1 cm2. The first discharge and charge were performed at a 0.05 C rate.

cell with a PYR+13 /Li+ mole fraction of 1.94 has been also tested at 40°C and different discharge rates. The charge rate was always fixed at 0.05 C 共0.033 mA/cm2兲. As shown in Fig. 9, the cell displayed continuous charge and discharge curves without a substantial change

Figure 8. Voltage profile of the Li/P共EO兲20LiTFSI + xPYR13TFSI/V2O5 共x = 1.94兲 cell after prolonged cycling test 共see Fig. 7兲. ⌽ indicates the ohmic drop of the cell on charge and discharge.

Figure 9. Discharge and charge 共see inset兲 profiles of the Li/P共EO兲20LiTFSI + xPYR13TFSI/V2O5 共x = 1.94兲 cell tested at 40°C and different rates. Loading of V2O5 2.3 mg. Cathode area 0.8 cm2. Charge current 0.033 mA/cm2 共0.05 C兲. Discharge current: from 0.033 mA/cm2 共0.05 C兲 to 0.66 mA/cm2 共1 C兲.

of shape over the entire range of current rate tested extending from 0.033 mA/cm2 共0.05 C兲 to 0.66 mA/cm2 共1 C兲. The delivered capacity decreased almost linearly with increasing current density. The active material utilization changed from about 88.3% at 0.05 C to 11.2% for a 1 C rate. Nevertheless, to our knowledge, this is the best low-temperature performance of a solvent-free, PEObased, LMB yet reported. The PEO-LiTFSI + PYR13TFSI polymer electrolyte system thus appears to be a promising candidate for achieving solid-state rechargeable LMBs operating at room to moderate temperatures. Figure 10 illustrates the cycle performance of Li/P共EO兲20LiTFSI + xPYR13TFSI 共x = 1.73兲/V2O5 cells tested at 40°C and a 0.05 C rate. After an initial decay in the performance, the cell was able to deliver a capacity higher than 200 mAh/g 共corresponding to 68% of the cathode theoretical capacity兲 with a charge/discharge efficiency of about 99.5%.

Figure 10. 共䊐兲 Charge and 共䊏兲 discharge capacities upon cycling of a Li/P共EO兲20LiTFSI + xPYR13TFSI/V2O5 cell 共x = 1.73兲 at 40°C. Charge/ discharge current 0.037 mA/cm2 共0.05 C兲. Loading of V2O5 2.0 mg. Cathode area 0.8 cm2. The first discharge and charge were performed at a 0.02 C rate.


Journal of The Electrochemical Society, 152 共5兲 A978-A983 共2005兲 Conclusions The incorporation of the PYR13TFSI 共RTIL兲 into P共EO兲20LiTFSI, up to a PYR+13 /Li+ mole fraction of 3.24, results in freestanding, highly ionically conductive polymer electrolyte films. The room-temperature ionic conductivity approaches 10−4 S/cm with increasing PYR+13 /Li+ mole fraction. A value of 10−3 S/cm, often considered to be the lower limit for practical applications in batteries, was reached at moderate temperatures. The electrochemical stability of the RTIL was significantly improved when combined with the PEO-LiTFSI. It is possible to plate Li metal from these electrolytes, but the presence of excess RTIL in the electrolytes was also found to cause a substantial increase in the interfacial resistance. Li/V2O5 batteries with appropriate amounts of PYR13TFSI, however, showed good capacity performance and excellent cyclability with a capacity fading of 0.04%/cycle over several hundreds of cycles at moderate temperature. The incorporation of the RTIL into LMPBs may therefore be a suitable methodology for achieving safe LMPBs operating at room temperature. Acknowledgments The financial support of MIUR is gratefully acknowledged. J.-H.S. and W.A.H. also thank the Italian Foreign Ministry and NSF International Research Fellowship Program 共IRFP 0202620兲, respectively, for research fellowships. ENEA assisted in meeting the publication costs of this article.

References 1. 2. 3. 4. 5.

P. Walden, Bull. Acad. Imp. Sci. St. Petersbourg , 1914, 405. K. R. Seddon, Nat. Mater., 2, 363 共2003兲. M. Doyle, S. K. Choi, and G. Proulx, J. Electrochem. Soc., 147, 34 共2000兲. A. Lewandowski and A. Swiderska, Solid State Ionics, 161, 243 共2003兲. N. Papageorgiou, Y. Athanassov, M. Armand, P. Bonhôte, H. Pettersson, A. Azam, and M. Gratzel, J. Electrochem. Soc., 143, 3009 共1996兲.

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6. W. Lu, A. D. Fadeev, B. Qi, E. Smela, B. R. Mattes, J. Ding, G. M. Spinks, J. Mazurkiewicz, D. Zhou, G. G. Wallace, D. R. MacFarlane, S. A. Forsyth, and M. Forsyth, Science, 297, 983 共2002兲. 7. Y. S. Fung and R. Q. Zhou, J. Power Sources, 81, 891 共1999兲. 8. Y. Hu, H. Li, X. Huang, and L. Chen, Electrochem. Commun., 6, 28 共2004兲. 9. C. L. Hussey, Pure Appl. Chem., 60, 1763 共1998兲. 10. A. A. Fannin, L. A. King, J. A. Levisky, and J. S. Wilkes, J. Phys. Chem., 88, 2609 共1984兲. 11. A. Balducci, U. Bardi, S. Caporali, M. Mastragostino, and F. Soavi, Electrochem. Commun. 6, 566 共2004兲. 12. J. Fuller, A. C. Breda, and R. T. Carlin, J. Electroanal. Chem., 459, 29 共1998兲. 13. H. Nakagawa, S. Izuchi, K. Kuwana, T. Nukuda, and Y. Aihara, J. Electrochem. Soc., 150, A695 共2003兲. 14. H. Sakaebe and H. Matsumoto, Electrochem. Commun., 5, 594 共2003兲. 15. J. H. Shin, W. A. Henderson, and S. Passerini, Electrochem. Commun., 5, 1016 共2003兲. 16. P. C. Howlett, D. R. MacFarlane, and A. F. Hollenkamp, Electrochem. Solid-State Lett., 7, A97 共2004兲. 17. S. Chintapalli and R. Frech, Solid State Ionics, 86-88, 341 共1996兲. 18. L. R. A. K. Bandara, M. A. K. L. Dissanayake, and B.-E. Mellander, Electrochim. Acta, 43, 1447 共1998兲. 19. V. R. Koch, L. A. Dominey, C. Nanjundiah, and M. J. Ondrechen, J. Electrochem. Soc., 143, 798 共1996兲. 20. H. Matsumoto and Y. Miyazaki, Chem. Lett., 2000, 922. 21. P. Bonhote, A. P. Dias, N. Papageorgiou, K. Kalyanasundaram, and M. Grätzel, Inorg. Chem., 35, 1168 共1996兲. 22. D. R. MacFarlane, P. Meakin, J. Sun, N. Amini, and M. Forsyth, J. Phys. Chem. B, 103, 4164 共1999兲. 23. Y. Ma, M. Doyle, T. F. Fuller, M. M. Doeff, L. C. De Jonghe, and J. Newman, J. Electrochem. Soc., 142, 1859 共1995兲. 24. L. Edman, A. Ferry, and G. Oradd, Phys. Rev. E, 65, 042803 共2002兲. 25. J. H. Shin, W. A. Henderson, and S. Passerini, Electrochem. Solid-State Lett., 8, A125 共2005兲. 26. Y. Katayama, T. Morita, M. Yamagata, and T. Miura, Electrochemistry (Tokyo, Jpn.), 71, 1033 共2003兲. 27. H. Matsumoto, H. Kageyama, and Y. Miazaki, Electrochemistry (Tokyo, Jpn.), 71, 1058 共2003兲. 28. G. B. Appetecchi and S. Passerini, J. Electrochem. Soc., 149, A891 共2002兲. 29. J. H. Shin and S. Passerini, Electrochim. Acta, 49, 1605 共2004兲. 30. P. Villano, M. Carewska, G. B. Appetecchi, and S. Passerini, J. Electrochem. Soc., 149, A1282 共2002兲.
