Deformation and Tensile Properties of Free-Standing Solvent-Free Electrodes for Li-Ion Batteries

Solvent-free (dry-processed) electrodes offer substantial economic and environmental benefits to Li-ion batteries and are manufactured in a way that requires them to withstand tensile loads during roll-to-roll processing. Electrode sheets comprising graphite particles embedded within a polytetrafluoroethylene (PTFE) polymer fibril network were investigated under tension and exhibited viscoelastic behavior: linear loading, plastic deformation, and sheet failure. The degree of PTFE fibrillation during manufacture impacted final sheet properties, and calendering induced fibril alignment and crystallographic texture and macroscopic mechanical anisotropy. Increasing the PTFE fraction by 3.5 wt % led to remarkable improvements in ultimate tensile strength (+900%) and failure strain (+30%). Increasing electrode temperature (>19 °C) delayed sheet failure as PTFE transformed from a triclinic to hexagonal phase, however, higher temperatures (>80 °C) accelerated failure by fibril elongation, pull-out and widespread fibril fracture.