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Solid Polymer Electrolyte with High Ionic Conductivity via Layer-by-Layer Deposition Mengqi Cui, and Pooi See Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04739 • Publication Date (Web): 10 Apr 2016 Downloaded from http://pubs.acs.org on April 11, 2016
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Chemistry of Materials
Solid Polymer Electrolyte with High Ionic Conductivity via Layer-byLayer Deposition Mengqi Cui, Pooi See Lee* School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798 ABSTRACT: A novel multilayer system via layer-by-layer (LbL) self-assembly by alternatively packing polyethylene glycol (PEG)-α-cyclodextrin (αCD) complex and poly(acrylic acid) (PAA) via hydrogen-bonding is designed and investigated in this work. The PEG-αCD inclusion complex is obtained by supramolecular interaction. Multilayer films with different ratios of αCD are fabricated, and the growth behavior, morphology, thermal properties and electrical properties of the resultant LbL films are systematically characterized. The films with PEG-αCD complex and PAA as building blocks show high ionic conductivity reaching up to 2.5×10-5 S cm-1 under room temperature (52%RH), which is almost two orders magnitude higher than the PEG/PAA films under the same conditions. The PEG-αCD15/PAA films also possess high water retention due to the large amount of hydroxyl groups carried by αCD, which enables the films, after exposed to a high humidity environment, to maintain a high ionic conductivity under low humidity environment. This PEG-αCD/PAA LbL system provides an insight for designing polymer based solid state electrolyte and its application towards electrochemical devices.
INTRODUCTION Solid polymer electrolyte (SPE) films possess high ionic conductivity and low electronic conductivity; high chemical, thermal and electrochemical stability, as well as robust mechanical properties.1, 2 Compared to the liquid and gel electrolyte, SPE could provide wider electrochemical operation window, wider range of working temperature, easier processing and less corrosion issue of the electrodes. These features make SPE promising candidates for all-solid-state electrochemical device applications, such as lithium-ion batteries, fuel cells, sensors, dye-sensitive solar cells, electrochromic displays, smart windows, and flexible devices. First discovered in 1973 and demonstrated in all-solidstate film battery in 1979, poly(ethylene oxide) (PEO)/Li+ system is one of the most classic and widely used systems for SPE because of the high solubility of Lithium salt in PEO matrix.3-5 It is known that the ion transport in the SPE is mostly dominated by the local relaxation and Brownian motion (segment motion) of the polymer chains, which are believed to be more favorable in the amorphous phase of the host polymer.2 However, PEO easily forms the crystalline phase at room temperature (RT), which hinders its applicability where high ionic conductivity at RT is desired. Many approaches have been explored to improve the performance of PEO/Li+ SPEs. Inorganic fillers6 and organic plasticizers7 are widely introduced into PEO matrix in order to promote the formation of localized amorphous regions and further enhance the ionic transportation. Blending low molecular
weight polyethylene glycol (PEG) with high molecular weight PEO is another way to decrease the crystallinity of polymer matrix.8 Recently, Chen et al. demonstrated a novel PEO/Li+ system with nano- channels formed by αcyclodextrin (αCD), in which the PEO/Li+ pair is confined. The nano-channels provide the pathway for the directional transportation of Li+ and, thus the PEO-Li+αCD composite showed high ionic conductivity even though the PEO matrix is highly crystallized.9 Layer-by-Layer (LbL) self-assembly has been proven to be a powerful technique to fabricate thin films with integration of different components as well as functionalities.10-17 Solid polymer electrolyte films made via LbL deposition showed great film uniformity, low degree of crystallinity and high ionic conductivity under high humid environment. Delongchamp and coworker demonstrated the fabrication PEO/poly(acrylic acid) (PAA) LbL multilayer films driven by hydrogen bonding. After exposed to 0.1 M lithium salt solution, the film showed an ionic conductivity of around 10-9 S cm-1.18 Soon after that, Lowman et al. reported electrostatic linked poly(ethyleneimde) (PEI)/PAA LbL films, which showed high ionic conductivity after treated with oligoethylene glycol dicarboxylic acid as plasticizer under low pH.19 However, post-treatment was necessary in these systems to obtain satisfactory ionic conductivity inside the films, which at the same time, will limit the application of these kinds of SPE in integrated devices. Nyugen et al. presented a novel PEI/PAA/PEO/PAA multilayers system, which combined both hydrogen bonding and electrostatic
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Chemistry of Materials
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interaction. Taking advantage of the high mobility of PEI chains, the PEI/PAA/PEO/PAA layers showed ionic conductivity of 10-5 S cm-1 at room temperature.20 Even though no post-treatment was applied, the tetralayer design will extend the processing time of the LbL deposition process. Taking the merits of the LbL technique and aiming at developing an effective way to fabricate SPE with high ionic conductivity, here we demonstrate a novel solid polymer electrolyte film fabricated by alternatively stacking PEG-αCD complex and PAA via hydrogenbonding interaction. PEG, chemically synonymous with PEO, refers to oligomers and polymers with low molecular weight (
