Mutifunctional Epoxy-Based Solid Polymer Electrolytes for Energy Storage Systems

Abstract

For the development of next-generation safe energy storage (batteries or supercapacitors), energy conversion (solar cells), and electromechanical transduction devices (ionic actuators), solid polymer electrolytes (SPEs) having good electrochemical stability are widely considered as promising materials to substitute for conventional organic liquid electrolytes that have flammability and explosion issues. The SPEs could also offer a perfect solution to the enhancement of energy or power density, due to the possibility to use lithium metal anode that has a high theoretical specific capacity, and would even enable their use in multifunctional wearable or structural energy storage systems for portable electronics or transport applications. However, the key challenge facing the development of the SPEs for energy storage applications is to achieve high mechanical performance without sacrificing the requisite ionic conductivity: increasing conductivity typically leads to a reduction of the modulus. We prepare cross-linkable epoxy-based networked SPEs including Li salts (LiTFSI) with either plastic crystals (PCs), tetraglymes (G4s) or ionic liquids (ILs). The epoxy resins are particularly attractive as polymer matrices for solid polymer electrolytes due to their high mechanical performance combined with good adhesive properties and corrosion resistance. The selected electrolyte components (PC, G4 or IL) are allowed to boost ionic conductivity owing to solvating the lithium cation and plasticizing the epoxy matrix. As a result, the thermal curing of a homogeneous mixture of epoxy and electrolyte components can generate a two-phase system in which the epoxy phase is selected to provide mechanical strength and the electrolyte phase is selected to maximize ionic conductivity, via polymerization-induced phase separation, allowing for a bicontinuous microphase separation morphology. To further improve the physical properties of the SPEs, inorganic nanoparticles such TiO2, SiO2, or Al2O3 are incorporated in the nanocomposite SPEs, allowing for the combination of the advantages of both inorganic materials and organic polymers. This can be the strategy to achieve the simultaneous improvement in both mechanical strength and ion conduction of nanocomposite SPEs. In the current investigation, we conduct an investigation of the effect of electrolyte and inorganic types and their concentration on the ion conduction and dielectric and viscoelastic response of epoxy-based networked SPEs to investigate their ion and polymer dynamics. We measure storage and loss moduli, viscosity, ionic conductivity, dielectric constant, Young’s modulus of a series of epoxy-based SPEs, where the concentration of epoxy resin, electrolyte, and inorganic filler is varied, using oscillatory and steady shear, dielectric relaxation spectroscopy, and dynamic mechanical analyzer, respectively. These results are complemented by morphological studies from FESEM, microstructural and physical interaction studies from Fourier transform infrared (FTIR) spectroscopy, intermolecular interaction from Density functional theory (DFT) calculations, and thermal properties from DSC. Our study leads to insight regarding optimal design of multifunctional solid polymer electrolytes for energy storage devices.