Highly Conductive Polymer Electrolytes Containing Rigid Polymers

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Highly Conductive Polymer Electrolytes Containing Rigid Polymers
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  Highly ConductivePolymer ElectrolytesContainingRigid Polymers Xiangyun Wei and Duward F. Shriver* Department of Chemistry and Materials Research Center, Northwestern University,Evanston, Illinois 60208-3113 Received March 20, 1998 Revised Manuscript Received July 16, 1998  Introduction . Polymer electrolytes are the newestarea of solid-state ionic conductors to receive wideattention. They are of interest from the fundamentalstandpoint of ion transport mechanism and their ap-plications in electrochemical devices such as batteriesand electrochromic windows. 1 Generally, polymer elec-trolytes are mixtures of salts with soft polar polymerssuch as poly(ethylene oxide) (PEO) and poly(bis-(methoxyethoxyethoxy)phosphazene) (MEEP). Sincetheir discovery in the 1970s, 2,3 the polymer electrolyteshave been intensively studied. 1,4 - 10 It is generallyagreed that ion transport above  T  g  in polymer electro-lytes is strongly coupled to the polymer segmentalmotion. 1,5 - 8 To increase the segmental motion andthereby conductivity of polymer electrolytes, a varietyof polymers with low   T  g  values have been develop-ed. 1,4 - 6,9,10 At present, the highest room-temperatureconductivity for this type of polymer electrolyte isaround 10 - 5 S cm - 1 . Furthermore, many avenues to low  T  g  materials have been explored, so the prospects arenot encouraging for the development of polymers witha  T  g  low enough to obtain a room-temperature conduc-tivity of 10 - 3 S cm - 1 , an often cited goal for practicalapplication in high energy density batteries. 11 More-over, low   T  g  results in poor mechanical properties, whichcomplicates the design of batteries and fuel cells. New materials with unconventional conduction mechanismare clearly needed.Conventional polymer electrolytes, at low tempera-tures, are thought to conduct ions by thermally acti-vated ion hopping. 7,8,12 Recently, a room-temperatureconductivity of 10 - 5.5 S cm - 1 was obtained from a veryrigid polymer/salt system in which the polymer has a T  g  of 315 °C. 13 These developments encouraged us tosearch for new rigid polymers that can provide path-ways for ion hopping. This line of reasoning led us toinvestigate two rigid polymer systems, poly(vinylenecarbonate) (PVIC) and poly((1,3-dioxolan-2-one-4,5-diyloxalate) (PVICOX), which display both favorable con-ductivity and mechanical properties. These systemswere selected because they can be easily synthesized(Scheme 1) and because they contain a high density ofcoordinating sites, which is necessary to dissolve salts.In addition, the high density of the polar groups shouldreduce the activation energy for ion hopping from onepolar site to the next. Of these two polymers, PVICOXis more irregular, and we hypothesize that this propertywill frustrate close packing and thereby increase staticfree volume and conductivity. Experimental Section.  Measurements.  Powderedsamples were pressed into pellets at a pressure of 2.7 × 10 8 Pa. The density of the PVIC pellet is 1.38 g/cm 3 and the PVICOX pellet density is 1.15 g/cm 3 . Imped-ance spectroscopy was performed with a HP 4192Aimpedance analyzer or a Solartron 1250 analyzer. Theconductivity cell was airtight and loaded under drynitrogen. Temperature control was achieved with a SunSystems environmental chamber. DSC data were col-lected on a Perkin-Elmer Pyris 1 DSC. Materials  . Poly(vinylene carbonate) was synthesizedby bulk polymerization under nitrogen in a sealed tubeemploying 0.3%benzoyl peroxide. 14 Dibromoethylenecarbonate was synthesized according to a literaturemethod. 15 PVICOX was synthesized by reaction ofAg 2 C 2 O 4  and dibromoethylene carbonate in DMF/H 2 Oat room temperature. Preparation of Polymer Electrolytes.  Weighed amountsof polymer and lithium triflate were dissolved in sol-vents (DMF for PVIC system, H 2 O for PVICOX system),and the solvent was removed under vacuum. Thepolymer electrolytes were further dried under highvacuum (ca. 10 - 5 Torr) for at least 4 days (at 70 °C forPVIC-based polymer electrolytes and at 60 °C forPVICOX-based polymer electrolytes). ResultsandDiscussion . The new polymer electro-lytes are identified here by the abbreviation of the (1) Gray, F. M.  Solid Polymer Electrolytes: Fundamentals and Technological Applications  ; VCH: New York, 1991.(2) Fenton, D. E.; Parker, J M.; Wright, P. V.  Polymer   1973 ,  14  ,589.(3) Wright, P. V.  Br. Polymer J.  1975 ,  7  , 319.(4) Armand, M. B.  Annu. Rev. Mater. Sci.  1986 ,  16  , 245.(5) Ratner, M. A.; Shriver, D. F.  Chem. Rev.  1988 ,  88  , 109.(6) Bruce, P. G.; Vincent, C. A.  J. Chem. Soc., Faraday Trans. 1993 , 89  , 3187.(7) Killis, A.; LeNest, J . F.; Cheradame, H.; Gandini, A.  Makromol.Chem  .  1982 ,  183  , 2835.(8) Druger, S. D.; Nitzan, A.; Ratner, M. A.  J. Chem. Phys  .  1983 , 79  , 3133.(9) Blonsky, P. M.; Shriver, D. F.; Austin, P.; Allcock, H.  J. Am.Chem. Soc.  1984 ,  106  , 6854.(10) Xia, D. W.; Soltz, D.; Smid, J .  Solid-StateIonics  1984 ,  14  , 221.(11) J ulien, C.; Nazri, G.  Solid State Batteries: Materials Design and Optimization  ; Kluwer Academic: Boston, 1994; p 363.(12) Wieczorek, w.; Chung, S. H.; Stevens, J . R.  J. Polym. Sci. Part B   1996 ,  34  , 2911.(13) Yamamoto, T.; Inami, M.; Kanbara, T.  Chem. Mater  .  1994 ,  6  ,44.(14) Hass, H. C.; Schuler, N. W.  J. Polym. Sci  .  1958 ,  31  , 237.(15) Newman, M. S.; Addor, R. W.  J. Am. Chem. Soc. 1955 ,  77  , 3789. Scheme1 2307 Chem. Mater.  1998, 10,  2307 - 2308 S0897-4756(98)00170-7 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 08/14/1998  polymer followed by a number, which represents themole ratio of the repeating unit of the polymer to lithiumtriflate, LiCF 3 SO 3 . For example, PVIC10 represents apolymer electrolyte which contains poly(vinylene car-bonate) and lithium triflate, with a molar ratio of 10repeating units of poly(vinylene carbonate) to one oflithium triflate. Thermal Properties.  DSC data of these two new systems are presented in Figures 1 and 2. For PVIC,PVIC10, and PVIC5, no  T  g  (glass transition tempera-ture) or  T  m  (melting temperature) was detected up to200 °C, while PVIC2 and PVIC1 exhibit a  T  m  around100 °C. These phenomena also occur with other rigidpolymer systems. For example, the poly(parabanicacid) - LiBF 4  system exhibits no  T  m  for the pure polymerover a range from room temperature to 400 °C, butaddition of a large amount of salt results in a  T  m  of 220°C. 13 These seemingly puzzling observations may resultfrom high-temperature-induced crystallization or mayrepresent the formation of a eutectic mixture. For thePVICOX system, the pure polymer exhibits a  T  m  of 132°C. X-ray diffraction indicates crystallinity for both thepure polymer and the polymer salt complexes. Additionof lithium triflate shifts the  T  m  to lower temperatures.This behavior is similar to that of poly(vinyl alcohol) - lithium salts systems in which  T  m  decreases withincreasing salt concentration. 13 For conventional poly-mer electrolytes, the addition of salt generally stiffensthe polymers with an increase in  T  g  and/or  T  m , 1,16,17 butfor the new polymer electrolytes, addition of salt softensthe polymers and lowers both  T  m  and  T  g . 13 Conductivity  . Conductivity data are presented inFigures 3 and 4, and these are comparable to or higherthan most conventional polymer electrolytes. The high-est conductivity observed at room temperature is 10 - 4 S cm - 1 . The salt concentration which provides optimumconductivity is much higher than that in conventionalpolymer electrolytes. For the PVICOX-based electro-lytes, the maximum conductivity occurs at 1:1 molarratio (repeating unit of polymer to lithium triflate). Inconventional polymer electrolytes, the conductivityreaches a maxim conductivity at a much lower saltconcentration (typically around 4:1 molar ratio). 1,5,18,19 The conductivities of PVIC2 and PVIC1 are muchhigher than those of PVIC10 and PVIC5, suggesting astructural change. Indeed, DSC data show that PVIC2and PVIC1 display a  T  m  of around 100 °C, while PVIC10and PVIC5 exhibit no  T  m  or  T  g  up to 200 °C. Theconductivities of PVICOX system are about 2 - 4 ordersof magnitude higher than those of PVIC system. Webelieve this is due to the larger free volume of PVICOXsystem. In fact, the PVICOX pellet density is about 20%lower than PVIC, indicating that PVICOX has largerfree volume.One interesting feature of these new polymer elec-trolytes is that crystalline phases do not appear tosuppress ion transport. In conventional polymer elec-trolytes, ion transport in crystalline phases is generallylower than that in amorphous phases, 1,20 because re-duced segmental motion in the crystalline phases gen-erally decreases conductivity and ion transport is gener-ally facilitated by the segmental motion. We concludethat ion transport is decoupled from the segmentalmotion in these new rigid polymer electrolytes. Acknowledgment.  We appreciate the support of theArmy Research Office Award No. DAAG55-98-1-0233and the MRSEC program of the National ScienceFoundation DMR-9632472 at the Materials ResearchCenter of Northwestern University. CM980170Z (16) Lascaud, S.; Perrier, M.; Valle´e, A.; Besner, S.; Prud ′ homme,J .; Armand, M.  Macromolecules   1994 ,  27  , 7469.(17) Kim, D.; Park, J .; Bae, J .; Pyun, S.  J. Polym. Sci. Part B   1996 , 34  , 2127.(18) Blonsky, P. M.; Shriver, D. F.; Austin, P.; Allcock, H. R.  J. Am.Chem. Soc  .  1984 ,  106  , 6854.(19) Dupon, R.; Papke, B. L.; Ratner, M. A.; Shriver, D. F. J.Electrochem.  Soc.  1984 ,  131  , 586.(20) Stainer, M.; Hardy, L. C.; Whitmore, D. H.; Shriver, D. F.  J.Electrochem. Soc  .  1984 ,  31  , 784. Figure1.  DSC curves of (a) PVIC, (b) PVIC2, (c) PVIC1. Figure 2.  DSC curves of (a) PVICOX, (b) PVICOX5, (c)PVICOX1/2. Figure 3.  Temperature dependence of the conductivity forthe PVIC - LiCF 3 SO 3  composite: PVIC10 ( 2 ), PVIC5 ( O ),PVIC2 ( 1 ), PVIC1 ( 0 ). Figure 4.  Temperature dependence of the conductivity forthe PVICOX-LiCF 3 SO 3  composite: PVICOX5 ( 0 ), PVICOX2( 2 ), PVICOX1 ( 1 ), PVICOX1/2 ( O ). 2308  Chem. Mater., Vol. 10, No. 9, 1998 Communications 
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