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A lithium battery electrolyte based on gelled polyethylene oxide
Solid State Ionics 146 (2002) 65 – 72 www.elsevier.com/locate/ssi
A lithium battery electrolyte based on gelled polyethylene oxide Pier Paolo Prosini *, Stefano Passerini ENEA, ERG-TEA-ECHI, C.R.E. Casaccia, S.P. 89, Via Anguillarese 301, 00060-Rome, Italy Received 18 May 2001; received in revised form 3 October 2001; accepted 18 October 2001
Abstract Gel polymer electrolytes (GPEs) were prepared by dipping a polyethylene oxide (PEO)-based solid polymer electrolyte in lithium triflate/propylene carbonate (PC) liquid electrolyte solutions. The quantity of the liquid electrolyte gelled in the polymer was monitored as a function of dipping time in several liquid electrolytic solutions characterized by a different salt concentration. The GPE conductivity was measured as a function of the salt concentration and the liquid fraction content. The Li/GPE interface properties were evaluated by monitoring the charge transfer resistance at open circuit voltage and the lithium cycling efficiency under dynamic conditions. Chronopotentiometry measurements were used to study the variations of the lithium ion concentration in the electrolyte near the electrode surface. The transition time and the lithium diffusion coefficient were calculated as a function of the salt concentration of the liquid electrolyte used to swell the polymer electrolyte. The GPE electrochemical stability was measured by slow scan voltammetry sweep. Battery cells were assembled by sandwiching a GPE between a lithium disk and a gelled PEO-based composite cathode. The battery performance was evaluated at various discharge rates, while the rechargeability was tested under galvanostatic conditions at the C/10 rate. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Gel polymer electrolyte; Lithium battery; Polyethylene oxide
1. Introduction In the past years, many efforts have been done to enhance the transport properties of polymer electrolytes (PE) in order to make them feasible for power applications [1]. Although a large number of polymer electrolytes have been described and characterized, it is possible to group all the polymers within of two classes, namely true solid polymer electrolytes (TSPEs) and gel polymer electrolytes (GPEs).
*
Corresponding author. Tel.: +39-6-3848-6768; fax: +39-63048-6357. E-mail address: [email protected] (P.P. Prosini).
The most common TSPE is a complex between polyethylene oxide (PEO) and a lithium salt. The major drawback of PEO-based TSPE is its low ionic conductivity at room temperature. The specific ionic conductivity of about 1.0 104 S cm1 is only attainable at around 100 C. The low ionic conductivity is related in part to the high degree of crystallinity, and in part, to the low solubility of the salt in the amorphous phase [2]. GPEs can be obtained by dissolving low molecular weight organic molecules with a high dielectric constant in a PEO matrix. Propylene carbonate (PC) has a dielectric constant of 64.4 and its addition to PEO leads to a significant increase of the ionic charge carriers along with a gain in ionic mobility. Kelly et al.
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 1 ) 0 1 0 1 2 - 8
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[3] prepared PEO-based electrolytes with PC as the plasticizer, but the conductivities of the resulting materials were not reported. Munshi and Owens [4] plasticized a (PEO)8 –LiCF3SO3 TSPE by adding PC during the cell assembly. The electrolyte conductivity was not measured and there was little information regarding the dimensional stability of the electrolyte. GPEs were also prepared by casting from acetonitrile solutions containing the lithium salt, PC and PEO [5]. North [6] prepared a series of electrolytes of the formula [PEO: (2PC:2EC)]20 – LiClO4 (EC is ethylene carbonate) with a room conductivity of about 1 103 S cm1 but with poor dimensional stability. To summarize, PEO-based GPEs are characterized by higher ionic conductivity with respect to the corresponding host matrix, but their mechanical properties are poor when compared with TSPE. However, we have found that the salt-containing polymer can swell large amounts of liquid electrolyte without loosing their mechanical properties. In the light of these evidences, the aim of this work was (a) to develop a method to obtain a stable freestanding GPE starting from a TSPE, (b) to investigate the effect of the GPE composition on the ionic conductivity, lithium cycling efficiency and Li/GPE interface resistance and (c) to evaluate the ability of GPE to operate in a battery working at room temperature. Later, it will be shown that the mechanical properties of the PEO-based gelled electrolytes can be largely improved by gelling a TSPE, which already contained lithium salt.
2. Experimental The composite polymer cathode (CPC) and the TSPE were prepared by following a dry, solvent-free procedure. The TSPE was prepared following the procedure described earlier [7,8]. LiCF3SO3 (LTF, Fluka 99%) was used as received. PEO (Union Carbide, MW = 4,000,000) and g-LiAlO2 (Cyprus Foote Mineral, HSA10) were dried in dry argon atmosphere (H2O < 5 ppm), respectively, at 55 and 300 C for 24 h. The powders were carefully sieved trough 200 and 400 mesh sieves and gently mixed by ball milling. The mixture was placed in an aluminium dye covered with releasing Mylar films and hot-pressed at about
1000 N cm 2 and 120 C for 30 min. During the hot pressing, the mixture melted, and after cooling, a 1mm-thick ribbon was obtained. The thinner tape was obtained by cold calendering the ribbon. The electrolyte tape was heated at 110 C under pressure to relax the stress induced by the cold calendering. A mechanically stable membrane with an average thickness of 150 mm was produced. The weight fraction of the polymer electrolyte was 70.6% PEO, 12.8% lithium salt and 16.6% g-LiAlO2. The EO/Li (EO = ethylene oxide unit) ratio in the polymer electrolyte was 20. The CPC was prepared by a modification of the procedure described to prepare the TSPE [8]. PEO and LiCF3SO3 were weighted to give an EO/Li ratio of 20 and mixed in a mortar. The active material (LiMn3O6, Seimi) and the carbon (Super-P, MMM Carbon) were dried at 100 C. The active material and carbon were gently mixed in the proportion of 11:1. The blend of powders was added to the polymer – salt mixture and homogenized in the mortar. The electrodes were obtained by a direct cold calendering of the powder. The weight fraction of the composite cathode was 55% active material, 10% carbon, 5% lithium salt and 30% PEO. The TSPE and the CPC were dried in an oven under a reduced pressure at 60 C for 24 h and stored in an atmosphere-controlled room before use. The GPE was obtained by dipping the TSPE in solutions of LTF in PC at various concentrations for a suitable period of time. After dipping, the GPE’s weight, thickness and conductivity were measured. The GPE conductivity and the lithium/GPE charge transfer resistance were evaluated by impedance spectroscopy (IS) using a frequency response analyzer (Solartron 1260) in the range 10 mHz – 10 kHz, sandwiching the GPE between two lithium disks. The cell was pressed between two stainless steel current collectors. To evaluate the lithium cycling efficiency, the GPE was sandwiched between a stainless steel electrode and a lithium electrode. Lithium was first plated on the stainless steel electrode, and 10% of the total charge deposited was then cycled between the two electrodes. The test was stopped when the cell voltage was larger than 2 V. To study the variations of the lithium concentration into the GPE near the electrodes due to the reduction (and oxidation) of the lithium ions on the electrode surfaces, the voltage of the Li/GPE/Li symmetrical
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cells was recorded with respect to the time at various current densities. The electrochemical stability was tested by sandwiching the GPE between a lithium electrode and a stainless steel electrode and by slowly increasing the cell voltage until the onset of the current flow was recorded. The voltage was swept from the OVC towards a more anodic potential with a scan rate of 0.1 mV s1. Battery cells were assembled by sandwiching the GPE between a lithium electrode and a gelled CPC electrode. The cathode diameter was 1.0 cm. The electrochemical system was contained in stainless steel cells. The cells were galvanostatically cycled between fixed voltage values by using a battery cycler (Maccor 2000). GPE preparation and electrochemical test were conducted in a dry room (RH < 0.2% at 20 C).
3. Results and discussion 3.1. GPE preparation To evaluate the quantity of the liquid electrolyte swelled in the TSPE as a function of the dipping time in LTF/PC solutions (at various concentrations), the weight increase was measured and the results are plotted in Fig. 1. It is important to point out that a similar experiment performed on a pure polymer film (prepared in the same way as that of the TSPE) led to
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Fig. 2. Trend of thickness increase versus the amount of electrolytic solution (0.2 M) gelled in (PEO)20 – LiCF3SO3 films.
the dissolution of the film itself in the liquid electrolyte solution. It is the presence of the lithium salt in the TSPE that forces the liquid electrolyte into the TSPE itself and reduces the tendency of the polymer to dissolve into the liquid electrolyte solution. As a matter of fact, the amount of electrolytic solution incorporated into the GPE is seen to increase upon decreasing the salt concentration. The time required to swell the TSPE with a reasonable amount of liquid electrolyte is in the order of 1 h or more. Such a long time is certainly due to the absence of any porosity in the TSPE film prepared through the solvent-free procedure [8] that requires the liquid electrolyte to diffuse into the TSPE. As a matter of fact, the initial weight increase as a function of time followed a square root law for any electrolyte solution as expected from a diffusion process. However, such a correlation is lost for longer dipping times, most likely as a result of the modification of the material itself. The sample thickness also increased with the liquid fraction content. In Fig. 2, the percentage of the thickness increase with respect to the weight increase for a sample dipped in the 0.2 M solution is plotted. The thickness increased linearly with weight. 3.2. GPE conductivity
Fig. 1. Trend of weight increase versus dipping time in different molarity electrolytic solutions of (PEO)20 – LiCF3SO3 films.
Fig. 3 shows the evolution of the GPE bulk ionic resistance and the percentage of the liquid electrolyte
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liquid fraction content, reduced the number of free ions that are able to transport the charge. Thus, a maximum in the conductivity is found for the polymer electrolyte gelled with the 0.1 M solution. 3.3. Lithium/GPE interface properties
Fig. 3. Specific resistance (circles) and weight increase (squares) of gelled (PEO)20 – LiCF3SO3 films as a function of dipping time in 0.1 M electrolytic solution.
content with respect to the dipping time in the 0.1 M solution. The resistance decreased very quickly for a low liquid fraction content, then reached a constant value for a dipping time longer than 50 min, which is equivalent to a liquid electrolyte content larger than 48 wt.%. The conductivity of the gelled samples for a period of 2 h in several liquid electrolyte solutions at different salt concentrations is reported in Fig. 4. Initially, the conductivity increased with the increasing salt content due to the increase of the charge carriers. However, for a larger salt concentration, the formation of the ion pairs, together with a reduced
Fig. 4. Conductivity of several GPE obtained by dipping (PEO)20 – LiCF3SO3 for a fixed period of time (2 h) in different electrolytic concentration solutions.
To evaluate the interface properties of the GPE in contact with the lithium metal, the charge transfer resistance in the open circuit condition and the lithium cycling efficiency under dynamic conditions were monitored. The passive layer intrinsically present on the lithium foil or formed when contacted with the GPE reduces the electron transfer rate between the GPE and the lithium itself. The charge transfer resistance at the Li/GPE interface increases with the thickness of the passive layer. In Fig. 5, the interface resistance as a function of the contact time for a gelled TSPE in the 0.2 M solution, which liquid content was about 100%, is plotted. From an initial value of about 200 V cm2, the charge transfer resistance increased rapidly during the first 2 days, reaching a value of about 600 V cm2. At a longer time, the resistance was seen to slowly increase, and after 16 days, it reached an almost steady value of 1.5 kV cm2. Fig. 6 shows the impedance spectra of a Li/GPE/Li cell recorded immediately after preparation (curve a), after 4 days at OCV (curve b) and after a current (0.5 mA cm2) was flowed through the cell for a period of 15 s (curve c). The data clearly show that the passivation film grown for 4 days at OCV was easily destroyed by applying a constant current between the
Fig. 5. Specific resistance of gelled (PEO)20 – LiCF3SO3 versus time. The liquid fraction content (solution 0.2 M) was 100% in weight.
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respectively. By substituting Eqs. (2) and (3) in Eq. (1), it reduces to: e ¼ ðntp Þ=ðntp þ td Þ:
ð4Þ
Fig. 7 shows the voltage profiles as a function of time. The cell was able to cycle for 58 times before the stripping voltage passed the 2.0-V limit. By substituting this value in Eq. (4), a coulombic efficiency of 85.3% was calculated. Such a value is certainly too low to allow the use of the GPE in lithium metal batteries. 3.5. GPE chronopotentiometry Fig. 6. Impedance spectra of gelled (PEO)20 – LiCF3SO3 sandwiched between two lithium electrode: as-made (a), after 4 days at OCV (b) and after a current (0.5 mA cm2) was flowed through the cell for 15 s (c). The liquid fraction content (0.2 M solution) was 100 % in weight.
two lithium electrodes. After the current flowed, a new fresh lithium surface was formed, and the interface resistance went back to a value very close to the initial one as shown in Fig. 6. 3.4. Lithium cycling efficiency The lithium cycling efficiency (e) was calculated by using the following equation: e ¼ Qc =ðQc þ Qt Þ
Fig. 8 shows a typical chronopotentiogram of a symmetrical Li/GPE/Li cell. After the current was turned on, the cell voltage increased due to the IR drop and the electrical double layer charge. After that, the potential remained almost constant. Finally, when the concentration of the lithium ions into the electrolyte near the electrode surface fell to zero, the cell voltage suddenly increased. The period of time from the beginning of the test to the steep cell voltage increase is called the transition time s. The equation describing the transition time was first developed by Sand [9]: s ¼ ðFCÞ2 pD=ð2jð1 tÞÞ2
ð5Þ
where C is the initial lithium ion surface concentration, D is the diffusion coefficient, and j is the current
ð1Þ
where Qt is the total charge deposited on the stainless steel electrode, and Qc is the total cycled charge. A GPE with a liquid fraction content of about 76 wt.% obtained by dipping the TSPE in the 0.1 M solution of LTF in PC was used. About 1 Q of lithium was deposited on the stainless steel electrode by flowing a 0.1-mA current for 1 104 s. After the initial lithium deposition, the current (0.1 mA) was reversed every 1000 s. In such a test, it was: Qt ¼ Itd
ð2Þ
Qc ¼ Intp
ð3Þ
where I is the applied current, n is the cycle number, and td and tp are the deposition and pulse times,
Fig. 7. Voltage profiles as a function of time of gelled (PEO)20 – LiCF3SO3 sandwiched between lithium and stainless steel electrodes; 1 Q of lithium was deposited on the stainless steel electrode by flowing a 0.1-mA current for 1 104 s. After the initial lithium deposition, the current (0.1 mA) was reversed every 1000 s.
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Fig. 8. Chronopotentiogram of gelled (PEO)20 – LiCF3SO3 sandwiched between two lithium electrodes. The liquid fraction content (0.1 M solution) was 97% in weight. The current density was 0.2 mA cm2.
density. In Fig. 9, the transition time evaluated for the gelled TSPE with solutions at various concentrations is plotted versus j 2. No great differences are found in the transition times, especially for high current densities. Supposing that the lithium concentration in the GPE is the same as that in the gelling solution, i.e., the concentration of lithium is unchanged, then it is possible to evaluate the GPE lithium diffusion coefficient by using Eq. (5). The diffusion coefficient values are reported in Fig. 10 versus the concentration in a log – log plot. The slope of the curve is very close
Fig. 10. Diffusion coefficient of lithium ions in gelled (PEO)20 – LiCF3SO3 with several electrolyte solutions.
to 2, i.e., the diffusion coefficient decreases exponentially with the concentration as the transition time increases with the concentration. As a consequence, the transition time dose not change by varying the concentration but its value remains almost constant. 3.6. GPE electrochemical stability The electrochemical stability was tested by sandwiching the GPE between a lithium electrode and a blocking electrode and by slowly increasing the cell voltage until the onset of the current flow was recorded. The electrochemical stability was evaluated as the maximum potential value reached before the current begins to flow. In Fig. 11, the current– voltage profile is reported for a GPE containing 100 wt.% of LTF/PC 0.2 M. The current onset was observed at about 3.8 V, but at 4.2 V, the current flow was less then 1 mA cm2. For a further increase of cell voltage, a very high current begins to flow, thus indicating the onset of the electrolyte decomposition process. 3.7. Battery performance
Fig. 9. Transition time versus the inverse square of the current density of gelled (PEO)20 – LiCF3SO3 with several electrolyte solutions.
Although the lithium cycling efficiency of the GPE is too low for practical use in lithium anode batteries, devices were assembled to investigate the behavior of the GPE in contact with a typical battery cathode material. A device was assembled using a CPC that was 30 mg in weight. A solution 0.1 M of PC/LTF
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Fig. 11. Current response during a low-rate voltage sweep of a gelled (PEO)20 – LiCF3SO3 sandwiched between lithium and stainless steel electrodes. The liquid fraction content (0.2 M solution) was 100% in weight. The sweep rate was 0.1 mV s1.
was used to swell the electrode. The final weight of the cathode after dipping was about 61 mg. The electrolyte was a 50-mm-thick GPE with 97 wt.% PC/LTF 0.1 M liquid content. The cell was cycled at various current densities, and the voltage discharge profiles are plotted in Fig. 12 with respect to the specific capacity. Almost the full capacity (based on a practical capacity of 125 mA h g1) was recovered by cycling the cell at 0.2 mA cm1 (C/10 rate), while about 50% of the full capacity was recovered, discharging the cell at the C/4 rate. The severe reduction of the utilization of the active material with increasing current densities can be related to the low values of
Fig. 12. Voltage profiles versus specific capacity for a cell cycled at various current densities.
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Fig. 13. Specific capacity in charge and discharge versus cycle number for a cell cycled at the C/10 rate.
the transition times detected in the constant current tests. The cyclic performance of the battery recorded at the C/10 rate is plotted in Fig. 13. The cell showed a severe capacity fading upon cycling that could be related to a poor cyclability of the active material and of the lithium electrode.
4. Conclusions In this work, we showed that it is possible to obtain GPEs by gelling PEO-based TSPE in liquid electrolyte (PC/LTF) with a substantial conductivity increase. The charge transfer resistance at the Li/GPE interface increases with time due to the formation of a passivation layer. It was seen that the passivation layer can be easily destroyed by applying a constant current between the lithium electrodes. The lithium cycling efficiency was found to be about 85.3%. The transition time was independent of the solution concentration used to gel the TSPE, and its value was found to be not so high to allow high currents to flow, so the current density in the cell has to be kept low. The GPE electrochemical stability was high enough to be used with the most common cathode material. Finally, cells based on a GPE and a gel CPE were assembled. The electrochemical test showed that the cell is able to work at room temperature. The cell was discharged at a C/10 rate, delivering about 125 mA h g 1, while at the C/4 rate, the active material utilization was reduced to about 50%.
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Of further importance, the TSPE and CPC are prepared through a solvent-free procedure that gives great benefit it terms of cost, safety and environment. References [1] K.M. Abraham, in: B. Scrosati (Ed.), Applications of Electroactive Polymers, Chapman & Hall, London, 1993. [2] F. Gray, Polymer Electrolytes, The Royal Society of Chemis-
try, Cambridge, 1997. [3] I.E. Kelly, J.R. Owen, B.C.H. Steel, J. Power Sources 14 (1985) 13. [4] M.Z.A. Munshi, B.B. Owens, Solid State Ionics 26 (1988) 41. [5] R. Xue, H. Huang, X. Huang, L. Chen, Solid State Ionics 74 (1994) 133. [6] J.N. North, European Patent Application, EP0279554 A2. [7] P.P. Prosini, S. Passerini, R. Vellone, W.H. Smyrl, J. Power Sources 75 (1998) 73. [8] G.B. Appetecchi, M. Carewska, F. Alessandrini, P.P. Prosini, S. Passerini, J. Electrochem. Soc. 147 (1999) 451. [9] H.J.S. Sand, Phylog. Mag. 1 (1901) 45.
A lithium battery electrolyte based on gelled polyethylene oxide Pier Paolo Prosini *, Stefano Passerini ENEA, ERG-TEA-ECHI, C.R.E. Casaccia, S.P. 89, Via Anguillarese 301, 00060-Rome, Italy Received 18 May 2001; received in revised form 3 October 2001; accepted 18 October 2001
Abstract Gel polymer electrolytes (GPEs) were prepared by dipping a polyethylene oxide (PEO)-based solid polymer electrolyte in lithium triflate/propylene carbonate (PC) liquid electrolyte solutions. The quantity of the liquid electrolyte gelled in the polymer was monitored as a function of dipping time in several liquid electrolytic solutions characterized by a different salt concentration. The GPE conductivity was measured as a function of the salt concentration and the liquid fraction content. The Li/GPE interface properties were evaluated by monitoring the charge transfer resistance at open circuit voltage and the lithium cycling efficiency under dynamic conditions. Chronopotentiometry measurements were used to study the variations of the lithium ion concentration in the electrolyte near the electrode surface. The transition time and the lithium diffusion coefficient were calculated as a function of the salt concentration of the liquid electrolyte used to swell the polymer electrolyte. The GPE electrochemical stability was measured by slow scan voltammetry sweep. Battery cells were assembled by sandwiching a GPE between a lithium disk and a gelled PEO-based composite cathode. The battery performance was evaluated at various discharge rates, while the rechargeability was tested under galvanostatic conditions at the C/10 rate. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Gel polymer electrolyte; Lithium battery; Polyethylene oxide
1. Introduction In the past years, many efforts have been done to enhance the transport properties of polymer electrolytes (PE) in order to make them feasible for power applications [1]. Although a large number of polymer electrolytes have been described and characterized, it is possible to group all the polymers within of two classes, namely true solid polymer electrolytes (TSPEs) and gel polymer electrolytes (GPEs).
*
Corresponding author. Tel.: +39-6-3848-6768; fax: +39-63048-6357. E-mail address: [email protected] (P.P. Prosini).
The most common TSPE is a complex between polyethylene oxide (PEO) and a lithium salt. The major drawback of PEO-based TSPE is its low ionic conductivity at room temperature. The specific ionic conductivity of about 1.0 104 S cm1 is only attainable at around 100 C. The low ionic conductivity is related in part to the high degree of crystallinity, and in part, to the low solubility of the salt in the amorphous phase [2]. GPEs can be obtained by dissolving low molecular weight organic molecules with a high dielectric constant in a PEO matrix. Propylene carbonate (PC) has a dielectric constant of 64.4 and its addition to PEO leads to a significant increase of the ionic charge carriers along with a gain in ionic mobility. Kelly et al.
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 1 ) 0 1 0 1 2 - 8
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[3] prepared PEO-based electrolytes with PC as the plasticizer, but the conductivities of the resulting materials were not reported. Munshi and Owens [4] plasticized a (PEO)8 –LiCF3SO3 TSPE by adding PC during the cell assembly. The electrolyte conductivity was not measured and there was little information regarding the dimensional stability of the electrolyte. GPEs were also prepared by casting from acetonitrile solutions containing the lithium salt, PC and PEO [5]. North [6] prepared a series of electrolytes of the formula [PEO: (2PC:2EC)]20 – LiClO4 (EC is ethylene carbonate) with a room conductivity of about 1 103 S cm1 but with poor dimensional stability. To summarize, PEO-based GPEs are characterized by higher ionic conductivity with respect to the corresponding host matrix, but their mechanical properties are poor when compared with TSPE. However, we have found that the salt-containing polymer can swell large amounts of liquid electrolyte without loosing their mechanical properties. In the light of these evidences, the aim of this work was (a) to develop a method to obtain a stable freestanding GPE starting from a TSPE, (b) to investigate the effect of the GPE composition on the ionic conductivity, lithium cycling efficiency and Li/GPE interface resistance and (c) to evaluate the ability of GPE to operate in a battery working at room temperature. Later, it will be shown that the mechanical properties of the PEO-based gelled electrolytes can be largely improved by gelling a TSPE, which already contained lithium salt.
2. Experimental The composite polymer cathode (CPC) and the TSPE were prepared by following a dry, solvent-free procedure. The TSPE was prepared following the procedure described earlier [7,8]. LiCF3SO3 (LTF, Fluka 99%) was used as received. PEO (Union Carbide, MW = 4,000,000) and g-LiAlO2 (Cyprus Foote Mineral, HSA10) were dried in dry argon atmosphere (H2O < 5 ppm), respectively, at 55 and 300 C for 24 h. The powders were carefully sieved trough 200 and 400 mesh sieves and gently mixed by ball milling. The mixture was placed in an aluminium dye covered with releasing Mylar films and hot-pressed at about
1000 N cm 2 and 120 C for 30 min. During the hot pressing, the mixture melted, and after cooling, a 1mm-thick ribbon was obtained. The thinner tape was obtained by cold calendering the ribbon. The electrolyte tape was heated at 110 C under pressure to relax the stress induced by the cold calendering. A mechanically stable membrane with an average thickness of 150 mm was produced. The weight fraction of the polymer electrolyte was 70.6% PEO, 12.8% lithium salt and 16.6% g-LiAlO2. The EO/Li (EO = ethylene oxide unit) ratio in the polymer electrolyte was 20. The CPC was prepared by a modification of the procedure described to prepare the TSPE [8]. PEO and LiCF3SO3 were weighted to give an EO/Li ratio of 20 and mixed in a mortar. The active material (LiMn3O6, Seimi) and the carbon (Super-P, MMM Carbon) were dried at 100 C. The active material and carbon were gently mixed in the proportion of 11:1. The blend of powders was added to the polymer – salt mixture and homogenized in the mortar. The electrodes were obtained by a direct cold calendering of the powder. The weight fraction of the composite cathode was 55% active material, 10% carbon, 5% lithium salt and 30% PEO. The TSPE and the CPC were dried in an oven under a reduced pressure at 60 C for 24 h and stored in an atmosphere-controlled room before use. The GPE was obtained by dipping the TSPE in solutions of LTF in PC at various concentrations for a suitable period of time. After dipping, the GPE’s weight, thickness and conductivity were measured. The GPE conductivity and the lithium/GPE charge transfer resistance were evaluated by impedance spectroscopy (IS) using a frequency response analyzer (Solartron 1260) in the range 10 mHz – 10 kHz, sandwiching the GPE between two lithium disks. The cell was pressed between two stainless steel current collectors. To evaluate the lithium cycling efficiency, the GPE was sandwiched between a stainless steel electrode and a lithium electrode. Lithium was first plated on the stainless steel electrode, and 10% of the total charge deposited was then cycled between the two electrodes. The test was stopped when the cell voltage was larger than 2 V. To study the variations of the lithium concentration into the GPE near the electrodes due to the reduction (and oxidation) of the lithium ions on the electrode surfaces, the voltage of the Li/GPE/Li symmetrical
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cells was recorded with respect to the time at various current densities. The electrochemical stability was tested by sandwiching the GPE between a lithium electrode and a stainless steel electrode and by slowly increasing the cell voltage until the onset of the current flow was recorded. The voltage was swept from the OVC towards a more anodic potential with a scan rate of 0.1 mV s1. Battery cells were assembled by sandwiching the GPE between a lithium electrode and a gelled CPC electrode. The cathode diameter was 1.0 cm. The electrochemical system was contained in stainless steel cells. The cells were galvanostatically cycled between fixed voltage values by using a battery cycler (Maccor 2000). GPE preparation and electrochemical test were conducted in a dry room (RH < 0.2% at 20 C).
3. Results and discussion 3.1. GPE preparation To evaluate the quantity of the liquid electrolyte swelled in the TSPE as a function of the dipping time in LTF/PC solutions (at various concentrations), the weight increase was measured and the results are plotted in Fig. 1. It is important to point out that a similar experiment performed on a pure polymer film (prepared in the same way as that of the TSPE) led to
67
Fig. 2. Trend of thickness increase versus the amount of electrolytic solution (0.2 M) gelled in (PEO)20 – LiCF3SO3 films.
the dissolution of the film itself in the liquid electrolyte solution. It is the presence of the lithium salt in the TSPE that forces the liquid electrolyte into the TSPE itself and reduces the tendency of the polymer to dissolve into the liquid electrolyte solution. As a matter of fact, the amount of electrolytic solution incorporated into the GPE is seen to increase upon decreasing the salt concentration. The time required to swell the TSPE with a reasonable amount of liquid electrolyte is in the order of 1 h or more. Such a long time is certainly due to the absence of any porosity in the TSPE film prepared through the solvent-free procedure [8] that requires the liquid electrolyte to diffuse into the TSPE. As a matter of fact, the initial weight increase as a function of time followed a square root law for any electrolyte solution as expected from a diffusion process. However, such a correlation is lost for longer dipping times, most likely as a result of the modification of the material itself. The sample thickness also increased with the liquid fraction content. In Fig. 2, the percentage of the thickness increase with respect to the weight increase for a sample dipped in the 0.2 M solution is plotted. The thickness increased linearly with weight. 3.2. GPE conductivity
Fig. 1. Trend of weight increase versus dipping time in different molarity electrolytic solutions of (PEO)20 – LiCF3SO3 films.
Fig. 3 shows the evolution of the GPE bulk ionic resistance and the percentage of the liquid electrolyte
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liquid fraction content, reduced the number of free ions that are able to transport the charge. Thus, a maximum in the conductivity is found for the polymer electrolyte gelled with the 0.1 M solution. 3.3. Lithium/GPE interface properties
Fig. 3. Specific resistance (circles) and weight increase (squares) of gelled (PEO)20 – LiCF3SO3 films as a function of dipping time in 0.1 M electrolytic solution.
content with respect to the dipping time in the 0.1 M solution. The resistance decreased very quickly for a low liquid fraction content, then reached a constant value for a dipping time longer than 50 min, which is equivalent to a liquid electrolyte content larger than 48 wt.%. The conductivity of the gelled samples for a period of 2 h in several liquid electrolyte solutions at different salt concentrations is reported in Fig. 4. Initially, the conductivity increased with the increasing salt content due to the increase of the charge carriers. However, for a larger salt concentration, the formation of the ion pairs, together with a reduced
Fig. 4. Conductivity of several GPE obtained by dipping (PEO)20 – LiCF3SO3 for a fixed period of time (2 h) in different electrolytic concentration solutions.
To evaluate the interface properties of the GPE in contact with the lithium metal, the charge transfer resistance in the open circuit condition and the lithium cycling efficiency under dynamic conditions were monitored. The passive layer intrinsically present on the lithium foil or formed when contacted with the GPE reduces the electron transfer rate between the GPE and the lithium itself. The charge transfer resistance at the Li/GPE interface increases with the thickness of the passive layer. In Fig. 5, the interface resistance as a function of the contact time for a gelled TSPE in the 0.2 M solution, which liquid content was about 100%, is plotted. From an initial value of about 200 V cm2, the charge transfer resistance increased rapidly during the first 2 days, reaching a value of about 600 V cm2. At a longer time, the resistance was seen to slowly increase, and after 16 days, it reached an almost steady value of 1.5 kV cm2. Fig. 6 shows the impedance spectra of a Li/GPE/Li cell recorded immediately after preparation (curve a), after 4 days at OCV (curve b) and after a current (0.5 mA cm2) was flowed through the cell for a period of 15 s (curve c). The data clearly show that the passivation film grown for 4 days at OCV was easily destroyed by applying a constant current between the
Fig. 5. Specific resistance of gelled (PEO)20 – LiCF3SO3 versus time. The liquid fraction content (solution 0.2 M) was 100% in weight.
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respectively. By substituting Eqs. (2) and (3) in Eq. (1), it reduces to: e ¼ ðntp Þ=ðntp þ td Þ:
ð4Þ
Fig. 7 shows the voltage profiles as a function of time. The cell was able to cycle for 58 times before the stripping voltage passed the 2.0-V limit. By substituting this value in Eq. (4), a coulombic efficiency of 85.3% was calculated. Such a value is certainly too low to allow the use of the GPE in lithium metal batteries. 3.5. GPE chronopotentiometry Fig. 6. Impedance spectra of gelled (PEO)20 – LiCF3SO3 sandwiched between two lithium electrode: as-made (a), after 4 days at OCV (b) and after a current (0.5 mA cm2) was flowed through the cell for 15 s (c). The liquid fraction content (0.2 M solution) was 100 % in weight.
two lithium electrodes. After the current flowed, a new fresh lithium surface was formed, and the interface resistance went back to a value very close to the initial one as shown in Fig. 6. 3.4. Lithium cycling efficiency The lithium cycling efficiency (e) was calculated by using the following equation: e ¼ Qc =ðQc þ Qt Þ
Fig. 8 shows a typical chronopotentiogram of a symmetrical Li/GPE/Li cell. After the current was turned on, the cell voltage increased due to the IR drop and the electrical double layer charge. After that, the potential remained almost constant. Finally, when the concentration of the lithium ions into the electrolyte near the electrode surface fell to zero, the cell voltage suddenly increased. The period of time from the beginning of the test to the steep cell voltage increase is called the transition time s. The equation describing the transition time was first developed by Sand [9]: s ¼ ðFCÞ2 pD=ð2jð1 tÞÞ2
ð5Þ
where C is the initial lithium ion surface concentration, D is the diffusion coefficient, and j is the current
ð1Þ
where Qt is the total charge deposited on the stainless steel electrode, and Qc is the total cycled charge. A GPE with a liquid fraction content of about 76 wt.% obtained by dipping the TSPE in the 0.1 M solution of LTF in PC was used. About 1 Q of lithium was deposited on the stainless steel electrode by flowing a 0.1-mA current for 1 104 s. After the initial lithium deposition, the current (0.1 mA) was reversed every 1000 s. In such a test, it was: Qt ¼ Itd
ð2Þ
Qc ¼ Intp
ð3Þ
where I is the applied current, n is the cycle number, and td and tp are the deposition and pulse times,
Fig. 7. Voltage profiles as a function of time of gelled (PEO)20 – LiCF3SO3 sandwiched between lithium and stainless steel electrodes; 1 Q of lithium was deposited on the stainless steel electrode by flowing a 0.1-mA current for 1 104 s. After the initial lithium deposition, the current (0.1 mA) was reversed every 1000 s.
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Fig. 8. Chronopotentiogram of gelled (PEO)20 – LiCF3SO3 sandwiched between two lithium electrodes. The liquid fraction content (0.1 M solution) was 97% in weight. The current density was 0.2 mA cm2.
density. In Fig. 9, the transition time evaluated for the gelled TSPE with solutions at various concentrations is plotted versus j 2. No great differences are found in the transition times, especially for high current densities. Supposing that the lithium concentration in the GPE is the same as that in the gelling solution, i.e., the concentration of lithium is unchanged, then it is possible to evaluate the GPE lithium diffusion coefficient by using Eq. (5). The diffusion coefficient values are reported in Fig. 10 versus the concentration in a log – log plot. The slope of the curve is very close
Fig. 10. Diffusion coefficient of lithium ions in gelled (PEO)20 – LiCF3SO3 with several electrolyte solutions.
to 2, i.e., the diffusion coefficient decreases exponentially with the concentration as the transition time increases with the concentration. As a consequence, the transition time dose not change by varying the concentration but its value remains almost constant. 3.6. GPE electrochemical stability The electrochemical stability was tested by sandwiching the GPE between a lithium electrode and a blocking electrode and by slowly increasing the cell voltage until the onset of the current flow was recorded. The electrochemical stability was evaluated as the maximum potential value reached before the current begins to flow. In Fig. 11, the current– voltage profile is reported for a GPE containing 100 wt.% of LTF/PC 0.2 M. The current onset was observed at about 3.8 V, but at 4.2 V, the current flow was less then 1 mA cm2. For a further increase of cell voltage, a very high current begins to flow, thus indicating the onset of the electrolyte decomposition process. 3.7. Battery performance
Fig. 9. Transition time versus the inverse square of the current density of gelled (PEO)20 – LiCF3SO3 with several electrolyte solutions.
Although the lithium cycling efficiency of the GPE is too low for practical use in lithium anode batteries, devices were assembled to investigate the behavior of the GPE in contact with a typical battery cathode material. A device was assembled using a CPC that was 30 mg in weight. A solution 0.1 M of PC/LTF
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Fig. 11. Current response during a low-rate voltage sweep of a gelled (PEO)20 – LiCF3SO3 sandwiched between lithium and stainless steel electrodes. The liquid fraction content (0.2 M solution) was 100% in weight. The sweep rate was 0.1 mV s1.
was used to swell the electrode. The final weight of the cathode after dipping was about 61 mg. The electrolyte was a 50-mm-thick GPE with 97 wt.% PC/LTF 0.1 M liquid content. The cell was cycled at various current densities, and the voltage discharge profiles are plotted in Fig. 12 with respect to the specific capacity. Almost the full capacity (based on a practical capacity of 125 mA h g1) was recovered by cycling the cell at 0.2 mA cm1 (C/10 rate), while about 50% of the full capacity was recovered, discharging the cell at the C/4 rate. The severe reduction of the utilization of the active material with increasing current densities can be related to the low values of
Fig. 12. Voltage profiles versus specific capacity for a cell cycled at various current densities.
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Fig. 13. Specific capacity in charge and discharge versus cycle number for a cell cycled at the C/10 rate.
the transition times detected in the constant current tests. The cyclic performance of the battery recorded at the C/10 rate is plotted in Fig. 13. The cell showed a severe capacity fading upon cycling that could be related to a poor cyclability of the active material and of the lithium electrode.
4. Conclusions In this work, we showed that it is possible to obtain GPEs by gelling PEO-based TSPE in liquid electrolyte (PC/LTF) with a substantial conductivity increase. The charge transfer resistance at the Li/GPE interface increases with time due to the formation of a passivation layer. It was seen that the passivation layer can be easily destroyed by applying a constant current between the lithium electrodes. The lithium cycling efficiency was found to be about 85.3%. The transition time was independent of the solution concentration used to gel the TSPE, and its value was found to be not so high to allow high currents to flow, so the current density in the cell has to be kept low. The GPE electrochemical stability was high enough to be used with the most common cathode material. Finally, cells based on a GPE and a gel CPE were assembled. The electrochemical test showed that the cell is able to work at room temperature. The cell was discharged at a C/10 rate, delivering about 125 mA h g 1, while at the C/4 rate, the active material utilization was reduced to about 50%.
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Of further importance, the TSPE and CPC are prepared through a solvent-free procedure that gives great benefit it terms of cost, safety and environment. References [1] K.M. Abraham, in: B. Scrosati (Ed.), Applications of Electroactive Polymers, Chapman & Hall, London, 1993. [2] F. Gray, Polymer Electrolytes, The Royal Society of Chemis-
try, Cambridge, 1997. [3] I.E. Kelly, J.R. Owen, B.C.H. Steel, J. Power Sources 14 (1985) 13. [4] M.Z.A. Munshi, B.B. Owens, Solid State Ionics 26 (1988) 41. [5] R. Xue, H. Huang, X. Huang, L. Chen, Solid State Ionics 74 (1994) 133. [6] J.N. North, European Patent Application, EP0279554 A2. [7] P.P. Prosini, S. Passerini, R. Vellone, W.H. Smyrl, J. Power Sources 75 (1998) 73. [8] G.B. Appetecchi, M. Carewska, F. Alessandrini, P.P. Prosini, S. Passerini, J. Electrochem. Soc. 147 (1999) 451. [9] H.J.S. Sand, Phylog. Mag. 1 (1901) 45.