Nanostructured Block Copolymer Dry ElectrolyteAyan Ghosha,*and Peter Kofinasb,zaDepartment of Chemical and Biomolecular Engineering andbFischell Department of Bioengineering,University of Maryland, College Park, Maryland 20742, USAWe report on the synthesis and characterization of a solid-state polymer electrolyte with enhanced lithium transport based on aself-assembled diblock copolymer. The diblock copolymer consists of a poly共ethylene oxide兲共PEO兲 block and a randomcopolymer of methyl methacylate 共MMA兲 and lithium salt of methacrylic acid 共MAALi兲. Lithium bis共oxalato兲borate,LiBC4O8共LiBOB兲 was used as salt in the dry electrolyte. Impedance and temperature studies were carried out to characterize theconductivity performance of the electrolyte. The diblock copolymer 关PEO-b-共PMMA- ran-PMAALi兲兴 with added LiBOB 共in themolar ratio ethylene oxide:LiBOB = 3:1兲 was used to form flexible translucent films, which exhibited an average ionic conduc-tivity value of 1.26 ⫻ 10−5Scm−1at room temperature 共21°C兲. Transmission electron microscopy was performed to characterizethe morphology of the polymer, and differential scanning calorimetry was carried out to study the thermal properties of theelectrolyte.© 2008 The Electrochemical Society. 关DOI: 10.1149/1.2901905兴 All rights reserved.Manuscript submitted November 30, 2007; revised manuscript received February 26, 2008. Available electronically April 9, 2008.In recent years, interest in polymeric batteries has increased dra-matically. Current configurations have a liquid or gel electrolytealong with a separator between the anode and cathode. This leads toproblems with electrolyte loss and decreased performance over time.The highly reactive nature of such electrolytes necessitates the useof protective enclosures, which add to the size and bulk of the bat-tery. The goal of this study is to investigate nanoscale polymerelectrolyte flexible thin films based on the self-assembly of blockcopolymers. Polymer electrolytes are more compliant than conven-tional inorganic glass or ceramic electrolytes. Lightweight, shape-conforming, polymer electrolyte-based battery systems, could findwidespread application as energy sources in miniature medical de-vices, such as pacemakers, wireless endoscopes, implantable pumps,treatment probes, and untethered robotic mobile manipulators.The complex forming capability of poly共ethylene oxide兲共PEO兲with alkali metal salts, introduced by Fenton et al.1has been thestarting point for an abundance of published work on polymer elec-trolytes for use in batteries. A semicrystalline polymer, PEO, hasbeen a focal component in the design of numerous dry solvent-freeelectrolytes involving: blends,2block copolymers,3-6branchednetworks,7ceramic fillers,8-11room-temperature ionic liquids,12,13and specialized salts,14,15to name a few. It is important to carefullytailor the polymer electrolyte matrix to attain appreciable levels ofconductivity in a solid-state medium. In this work, we have investi-gated a nanostructured thin-film battery electrolyte based on adiblock copolymer composed of a PEO block and a randomcopolymer of methyl methacrylate 共MMA兲 and lithium salt ofmethacrylic acid 共MAALi兲. The diblock copolymer关PEO-b-共PMMA-ran-PMAALi兲兴 共Fig. 1兲 with lithium bis共oxala-to兲borate, LiBC4O8共LiBOB兲 as the added lithium salt was used tocreate the dry, solid-state electrolyte films.We selected a PEO-based diblock copolymer because of its abil-ity to solvate alkali metal salts. The second block, which consists ofa random copolymer of methyl methacylate 共MMA兲 and lithium saltof methacrylic acid 共MAALi兲, was chosen for its ability to incorpo-rate lithium ions within the microphase separated spherical domainsof the diblock copolymer 关PEO-b -共PMMA-ran-PMAALi兲兴 共Fig. 2兲,creating a secondary lithium source. The primary focus of this workis the electrolyte performance at room temperature, and the experi-mental results display the role of polymer and salt selection towardthis objective.ExperimentalThe PEO-b-PMMA block copolymer with an average molecularweight 3000:500 of PEO to PMMA and polydispersity index of 1.16was purchased from Polymer Source Inc. 共Canada兲. LiBOB wasobtained from Chemetall GmbH 共Germany兲. All other chemicalsand solvents were purchased from Aldrich and used as is.Hydrolysis was carried out using lithium hydroxide monohydrate共LiOH·H2O兲 as the base. The block copolymer 共PEO-b-PMMA兲and LiOH·H2O were dissolved in a solvent mixture with a molarratio of 2:1 between LiOH·H2O and the MMA units of the diblockcopolymer. The solvent used was a 2:1 mixture of anhydrous 1,4-dioxane and anhydrous methanol. The hydrolysis process was car-ried out at 85°C for 20 h. As a result of the process, the PMMAblock was hydrolyzed into a random copolymer of methyl methacry-late 共MMA兲 and lithium salt of methacylic acid 共MAALi兲. Thisprocedure was adapted from previous work reported by Mikes andPecka.16After the hydrolysis step, the solvent was removed undervacuum using a Schlenk line setup with a liquid nitrogen solventvapor trap. This dried diblock copolymer 关PEO-b-共PMMA-ran-PMAALi兲兴 was then stored in a Mbraun Labmaster 100 argonglove box for further use.Solutions were prepared by adding varying concentrationsof LiBOB salt to the diblock copolymer 关PEO-b-共PMMA-ran-PMAALi兲兴. The solvent used was anhydrous tetrahydrofuran 共THF兲,*Electrochemical Society Student Member.zE-mail: [email protected] 1. Chemical structure of self-assembled diblock copolymer.Figure 2. Diblock copolymer electrolyte morphology.Journal of The Electrochemical Society, 155 共6兲 A428-A431 共2008兲0013-4651/2008/155共6兲/A428/4/$23.00 © The Electrochemical SocietyA428which was degassed using multiple cycles of a freeze-pump-thawmethod. These polymer solutions were then cast into Petri dishescontaining molds of fluorinated ethylene propylene coated alumi-num sheets. The drying process extended over several days resultingin 200–250 ␮m thick films. Circular sections of the polymer elec-trolyte films were cut for conductivity measurements and mountedbetween two 316 stainless steel blocking electrodes. A poly共tetrafluoroethylene兲-based O-ring was placed between the two electrodesto secure the sample thickness and surface area. The test cell assem-bly was sealed, protecting it from oxygen and humidity, before re-moval