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Nanostructured Block Copolymer Dry Electrolyte



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Journal of The Electrochemical Society 155 6 A428 A431 2008 A428 0013 4651 2008 155 6 A428 4 23 00 The Electrochemical Society Nanostructured Block Copolymer Dry Electrolyte Ayan Ghosha and Peter Kofinasb z a Department of Chemical and Biomolecular Engineering and bFischell Department of Bioengineering University of Maryland College Park Maryland 20742 USA We report on the synthesis and characterization of a solid state polymer electrolyte with enhanced lithium transport based on a self assembled diblock copolymer The diblock copolymer consists of a poly ethylene oxide PEO block and a random copolymer of methyl methacylate MMA and lithium salt of methacrylic acid MAALi Lithium bis oxalato borate LiBC4O8 LiBOB was used as salt in the dry electrolyte Impedance and temperature studies were carried out to characterize the conductivity performance of the electrolyte The diblock copolymer PEO b PMMA ran PMAALi with added LiBOB in the molar ratio ethylene oxide LiBOB 3 1 was used to form flexible translucent films which exhibited an average ionic conductivity value of 1 26 10 5 S cm 1 at room temperature 21 C Transmission electron microscopy was performed to characterize the morphology of the polymer and differential scanning calorimetry was carried out to study the thermal properties of the electrolyte 2008 The Electrochemical Society DOI 10 1149 1 2901905 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 dramatically Current configurations have a liquid or gel electrolyte along with a separator between the anode and cathode This leads to problems with electrolyte loss and decreased performance over time The highly reactive nature of such electrolytes necessitates the use of protective enclosures which add to the size and bulk of the battery The goal of this study is to investigate nanoscale polymer electrolyte flexible thin films based on the self assembly of block copolymers Polymer electrolytes are more compliant than conventional inorganic glass or ceramic electrolytes Lightweight shapeconforming polymer electrolyte based battery systems could find widespread application as energy sources in miniature medical devices such as pacemakers wireless endoscopes implantable pumps treatment probes and untethered robotic mobile manipulators The complex forming capability of poly ethylene oxide PEO with alkali metal salts introduced by Fenton et al 1 has been the starting point for an abundance of published work on polymer electrolytes for use in batteries A semicrystalline polymer PEO has been a focal component in the design of numerous dry solvent free electrolytes involving blends 2 block copolymers 3 6 branched networks 7 ceramic fillers 8 11 room temperature ionic liquids 12 13 and specialized salts 14 15 to name a few It is important to carefully tailor the polymer electrolyte matrix to attain appreciable levels of conductivity in a solid state medium In this work we have investigated a nanostructured thin film battery electrolyte based on a diblock copolymer composed of a PEO block and a random copolymer of methyl methacrylate MMA and lithium salt of methacrylic acid MAALi The diblock copolymer PEO b PMMA ran PMAALi Fig 1 with lithium bis oxalato borate LiBC4O8 LiBOB as the added lithium salt was used to create the dry solid state electrolyte films We selected a PEO based diblock copolymer because of its ability to solvate alkali metal salts The second block which consists of a random copolymer of methyl methacylate MMA and lithium salt of methacrylic acid MAALi was chosen for its ability to incorporate lithium ions within the microphase separated spherical domains of the diblock copolymer PEO b PMMA ran PMAALi Fig 2 creating a secondary lithium source The primary focus of this work is the electrolyte performance at room temperature and the experimental results display the role of polymer and salt selection toward this objective was purchased from Polymer Source Inc Canada LiBOB was obtained from Chemetall GmbH Germany All other chemicals and solvents were purchased from Aldrich and used as is Hydrolysis was carried out using lithium hydroxide monohydrate LiOH H2O as the base The block copolymer PEO b PMMA and LiOH H2O were dissolved in a solvent mixture with a molar ratio of 2 1 between LiOH H2O and the MMA units of the diblock copolymer The solvent used was a 2 1 mixture of anhydrous 1 4dioxane and anhydrous methanol The hydrolysis process was carried out at 85 C for 20 h As a result of the process the PMMA block was hydrolyzed into a random copolymer of methyl methacrylate MMA and lithium salt of methacylic acid MAALi This procedure was adapted from previous work reported by Mikes and Pecka 16 After the hydrolysis step the solvent was removed under vacuum using a Schlenk line setup with a liquid nitrogen solvent vapor trap This dried diblock copolymer PEO b PMMAran PMAALi was then stored in a Mbraun Labmaster 100 argon glove box for further use Solutions were prepared by adding varying concentrations of LiBOB salt to the diblock copolymer PEO b PMMA ranPMAALi The solvent used was anhydrous tetrahydrofuran THF Figure 1 Chemical structure of self assembled diblock copolymer Experimental The PEO b PMMA block copolymer with an average molecular weight 3000 500 of PEO to PMMA and polydispersity index of 1 16 Electrochemical Society Student Member z E mail kofinas umd edu Figure 2 Diblock copolymer electrolyte morphology Journal of The Electrochemical Society 155 6 A428 A431 2008 A429 which was degassed using multiple cycles of a freeze pump thaw method These polymer solutions were then cast into Petri dishes containing molds of fluorinated ethylene propylene coated aluminum sheets The drying process extended over several days resulting in 200 250 m thick films Circular sections of the polymer electrolyte films were cut for conductivity measurements and mounted between two 316 stainless steel blocking electrodes A poly tetra fluoroethylene based O ring was placed between the two electrodes to secure the sample thickness and surface area The test cell assembly was sealed protecting it from oxygen and humidity before removal from the glove box for impedance analysis The ionic conductivity of the synthesized block copolymer electrolytes was determined from t RA 1 where t A and R represent the thickness surface area and ionic resistance


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