Optimizing the dry processing parameters of thermoplastic vulcanizate electrolytes for improved microstructure and its impact on electrochemical stability

Abstract

Solid polymer electrolytes have emerged as a promising option for enhancing safety in lithium metal batteries (LMBs), offering advantages such as non-volatility, reduced flammability, straightforward processing, and electrochemical and chemical stability. Nevertheless, they face significant challenges, including limited ionic conductivity at room temperature and insufficient mechanical strength to inhibit dendrite growth. The deliberate engineering of polymer materials with adequate ionic conductivity and mechanical robustness is crucial for ensuring safe and stable batteries. In this study, thermoplastic vulcanizate (TPV) electrolyte systems based on ethylene propylene diene monomer (EPDM) rubber and polycaprolactone (PCL) were prepared using a dry processing method. Unlike solvent-based methods, extruder blending eliminates the use of toxic solvents, reducing environmental hazards and eliminating the need for solvent recovery or disposal, enabling a greener manufacturing process. The impact of processing conditions, such as order of material feeding into the extruder and shear rate, on the morphology (i.e., rubber domain size), mechanical strength, and ionic conductivity of the electrolytes was investigated. The stability of crosslinked polymer electrolytes with different microstructures was studied by cycling in symmetric Li–TPV–Li cells. These experiments connected increased cell resistance with processing conditions, electrolyte morphology and mechanical strength. Longer cycle life was observed for electrolytes with small domain sizes and high mechanical strength. These morphologies were obtained with lower mixing speeds during processing and salt addition following crosslinking of the elastomeric phase.