Understanding solvent-free electrode viscoelastic behaviour and its effect on springback in Li-ion battery manufacture

Solvent-free (dry) processing based on polytetrafluoroethylene (PTFE) fibrillation is a potentially environmentally and cost-effective approach to large-scale, roll-to-roll battery electrode manufacture. We show that the PTFE fibril network controls the mechanical response of graphite-based solvent-free electrodes in compression and that the viscoelastic nature of PTFE causes significant electrode thickness recovery after calendering, known as springback. Springback may be ¿40% and may undermine the strict electrode thickness tolerance required in industrial production if poorly controlled. The impact of electrode composition and calendering temperature/speed on springback was investigated. Warm calendering emerged as an effective method to reduce electrode springback to 10% at 200 °C. Electrode compressive response comprised three mechanical regimes: (i) elastic loading, (ii) a plastic plateau, and (iii) a non-linear compaction region, which were well-described by a phenomenological foam model (PFM). Synchrotron X-ray computed tomography (XCT) at incremental compressive strains revealed dynamic porosity evolution had a three regime behaviour comprising initial elastic compression, particle rearrangement and lastly pore collapse and isolation. The dynamics of subsequent springback were described by a generalised Maxwell model (GMM) with two relaxation time constants, and the implications for production discussed.