Multifunctional Properties of Structural Gel Electrolytes

In this materials-based approach, each component of the system serves to bear and/or efficiently transfer load.

October 1, 2009

Army Research Laboratory

Due to the demand for more advanced and lightweight systems, multifunctional composite structures that can function as electrochemical energy converters, while bearing mechanical load, are in development. This research involves a materials-based approach in which each component of the system serves to bear and/or efficiently transfer load. In this iteration, multifunctional structural gel electrolytes were prepared by the integration of conductive pathways using non-aqueous solvents into structural resin networks. Polyethylene glycol (200 MW) and propylene carbonate were used as the non-aqueous solvents, while vinyl ester and epoxy resin were used as structural resins. The monomer and solvents were incorporated together and polymerized to create conductive pathways in cross-linked networks. The impact of chemistry and weight fraction of both liquid and resin were investigated on the electrochemical-mechanical response of the resulting system.

All materials were handled under dry atmosphere conditions during preparation, processing, and testing. To characterize the gel electrolytes, they were cured in the forms of pellets and prismatic bars using silicone molds. Sample dimensions were measured precisely using digital calipers. The pellets were 10 mm in diameter and 3-4 mm thick, while the bars were ca. 60 mm long, 12 mm wide, and 4 mm thick. AC impedance measurements to measure ionic resistivity were performed using a Solartron 1260 Impedance/Gain – Phase Analyzer and Solartron 1287 Electrochemical Interface over the frequency range of 10-10⁶ Hz at room temperature (18-20 °C). Mechanical characterization was accomplished by compressing the pellets in an MTS load frame employing a 5 kN load cell and a cross-head speed of 1mm/min. The reported compressive modulus values are calculated from the initial, relatively elastic portion of material loading curves. Viscoelastic properties were measured using dynamic mechanical analysis on a DMA Q800 at 1 Hz and 7.5 amplitude over the range of -150 to 250 °C. Each specimen was clamped using a dual cantilever configuration.

In the gel samples tested, the conductivity limit was raised by almost two orders of magnitude over solvent-free neat polymer electrolytes, thereby allowing for greater multifunctional performance. The addition of liquid solvent generally improves the performance of electrochemical properties relative to solid-state polymer electrolytes, although at the expense of mechanical robustness. Cross-linking the polymer matrix is employed to improve mechanical performance. The ionic conductivity and compressive modulus were measured by AC impedance and compression, respectively. Dynamic Me chan - ical Analysis has been used to validate the processing conditions and material characterization. There is not a significant difference in multifunctionality when either PEG 200 or PC are used as solvents, nor when either SR209 or SR 494 are used as the structural phase.

Two-component electrolytes were found to be superior to systems in which a third component was added containing a compromise of both conductive and structural properties. Epoxy systems yield better solvent retention and improvement in mechanical performance, but otherwise there was no significant difference resulting from the use of structural matrices or liquid electrolytes with different composition as long as the properties were similar. All systems demonstrated a rapid decline in multifunctional performance at a critical liquid concentration that may indicate a percolation threshold for highly conductive regions. Further studies are underway to examine the microstructure in these systems.

This work was done by P-A. T. Nguyen and J.F. Snyder of the Army Research Laboratory. ARL-0072