Non-volatile polymer electrolyte based on poly(propylene carbonate), Ionic liquid, and lithium perchlorate for electrochromic devices

A series of solvent-free ionic liquid (IL)-based polymer electrolytes composed of amorphous and biodegradable poly(propylene carbonate) (PPC) host, LiClO₄, and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM ⁺BF₄^-) were prepared and characterized for the first time. FTIR studies reveal that the interaction between PPC chains and imidazolium cations weakens the complexation between PPC chains and Li ⁺ ions. Thermal analysis (DSC and TGA) results show that the incorporation of BMIM⁺BF₄^- into PPC/LiClO ₄ remarkably decreases the glass transition temperature and improves the thermal stability of the electrolytes. AC impedance results show that the ionic conductivities of the electrolytes are significantly increased with the increase of BMIM⁺BF₄^- amount, the ambient ionic conductivity of the electrolyte at a PPC/LiClO₄/BMIM ⁺BF₄^- weight ratio of 1/0.2/3 is 1.5 mS/cm, and the ionic transport behavior follows the Arrhenius equation. Both PPC/LiClO₄/BMIM⁺BF₄^- and PPC/BMIM⁺BF₄^- electrolytes were applied in electrochromic devices with polyaniline as the electrochromic layer. The PPC/LiClO₄/BMIM⁺BF₄^--based device exhibits much better electrochromic performance in terms of optical contrast and switching time due to the presence of much smaller cations.