Effect of Composition and Processing on the Multifunctional Properties of Single-Phase Structural Electrolytes

As the world becomes increasingly electrically dependent, energy storage is a factor which seriously limits progress. Unfortunately, popular methods of energy storage, such as lithium-ion batteries contribute significantly to unrecyclable waste once their short life cycles are over. Supercapacitors are an attractive alternative method of energy storage with significantly higher cyclability and power density than batteries. Moreover, multifunctional structural supercapacitors have been developed to further reduce the overall weight and size of products. Previous reports utilize bi-continuous phase materials as the electrolytes for structural supercapacitors, which show significant trade-off between the electrical and mechanical properties. Our research group has recently developed a single-phase solid polymer electrolyte that outperforms previous designs. The current work focuses on studying the effect of composition and processing on the multifunctional properties of such electrolytes. We analyze the thermal, electrochemical, and mechanical properties of the polymer electrolytes composed of polyethylene terphthalate (PET) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at different compositions after various processing conditions to determine its suitability for use in the structural supercapacitors.<br/><br/>Different from the conventional solution casting method to prepare polymer electrolyte, our electrolytes were prepared by mixing and melting PET and LiTFSI together. The LiTFSI salt was fully dissolved in PET to form a single-phase solid solution. Samples with designed sizes were fabricated through injection molding. Some samples were exposed to a controlled humidity environment to improve their electrochemical performance. The thermal properties of samples with 0 to 50 wt% of LiTFSI were characterized by differential scanning calorimetry (DSC). The ionic conductivities of various dry and humidified samples were measured by using electrochemical impedance spectroscopy (EIS). The mechanical properties of similar samples were evaluated through 3-point bending tests.<br/><br/>It was determined that increasing the weight ratio of LiTFSI in our polymer electrolyte was related to a decrease in heat flow, decrease in melting point from 247 °C to 227 °C, slight increase in glass transition temperature, and decrease in crystallinity from 26% to approximately 0%. In fact, the polymer-electrolyte became essentially amorphous with more than 35 wt% LiTFSI. EIS tests showed that the ionic conductivity reached a peak value at a composition of 20 wt% LiTFSI for dry samples. The samples showed higher ionic conductivity when hydrated, and a peak ionic conductivity of about 34 µS/cm occurred at a 2 wt% absorption of water from the humid air. Finally, the 3-point bending tests demonstrated that increasing the water content of samples dramatically increased the ability to resist breaking from bending.<br/><br/>In conclusion, the results show that the phase and crystallinity of the single-phase structural electrolyte can be controlled through material composition and thermal processing. The mechanical toughness and ionic conductivity of the multifunctional electrolyte can be simultaneously improved through controlled hydration treatment. The optimization of material composition, thermal processing, and hydration treatment will lead to structural electrolytes with significantly improved overall multifunctionality.