The Effects of Processing Conditions and Pressure on Composite Polymer Electrolyte Performance

Solid polymer and composite polymer electrolytes have been investigated as a replacement for liquid electrolytes in lithium batteries and have shown promising conductivities at room temperature. However, the literature reports often do not fully investigate the effects of residual solvent or testing apparatus conditions, leading to discrepancies in reported performance and possible conflation of conductivity values. Using combinations of poly (vinylidene fluoride-co-hexafluoropropylene), succinonitrile, and lithium lanthanum zirconium tantalum oxide, this work aims to develop an understanding of how polymer electrolyte composition, including solvent retention, affects conductivity. A custom-designed controllable pressure test fixture was utilized to determine ionic conductivity of the composite polymer electrolyte films at a wide range of pressures (1.5-18.7 psi) and temperatures 10-90 °C. Applied pressure during testing greatly influences apparent conductivities, with optimal composite film conductivity values ranging from 1.2 × 10^-5 to 4.1 × 10^-5 S cm^-1 at room temperature. At elevated temperatures, the composite films achieved greater than 1 mS cm^-1. The ideal pressure was found to be dependent on the polymer electrolyte additives. Symmetric cell testing showed over 99% Coulombic efficiency for over one hundred cycles at 0.1 mA cm^-2. The results of this work highlight the importance of careful characterization of electrolyte films and controlled test fixture pressure when developing polymer electrolytes.