Layer-by-Layer Assembled Solid Polymer Electrolyte for Electrochromic Devices

A new solid polymer electrolyte film fabricated by layer-by-layer (LbL) assembly is presented. The electrolyte film consists of four interbonding layers per deposition cycle, which combines electrostatic and hydrogen bonding in the same structure. Linear poly(ethylene imine) (LPEI) and poly(ethylene oxide) (PEO) are used to enhance the dissolution of lithium salt and the ionic transport through segmental motions of polymer chains. Characterization of film structure and growth shows good incorporation of electrostatic and hydrogen bonding, because of the versatile control of the ionization of poly(acrylic acid) (PAA), which serves as a bridging molecule. Ionic conductivity values, as described by the Vogel−Fulcher−Tammann equation, are found to be above 10<sup>−5</sup> S/cm for the dried electrolyte at room temperature and moderate humidity (52%RH). Thermal analysis reveals two competing processes, namely, cross-linking of LPEI and PAA to form an amide compound and segregation of PEO crystalline phase, which results in a moderate reduction of conductivity in the electrolyte after heating. Demonstration of solid electrochromic devices using the LbL-assembled polymer electrolyte is presented in both transmission and reflection mode with a modulation of 30%−40% in the visible and near-infrared range. The successful fabrication of the LbL-assembled electrolytes enables the realization of completely flexible, polymeric, and solid electrochromic devices.