Regulating Entanglement Networks of Fibrillatable Binders for Sub-20-µm Thick, Robust, Dry-Processed Solid Electrolyte Membranes in All-Solid-State Batteries

Abstract

Despite significant advancements in all-solid-state batteries (ASSBs), the reliance on thick solid electrolyte (SE) membranes hinders their commercial viability. Although the dry process using polytetrafluoroethylene (PTFE) binder is proposed for thin SE membranes, a comprehensive understanding of the correlation between PTFE fibrillation and SE membrane quality remains lacking. Here an important guidance is provided for producing durable SE membranes that can be reduced to sub-20-µm thickness by regulating the entanglement networks of PTFE. While establishing fabrication parameters; shear time and shear temperature, a dominant effect of the number-average molecular weight (M_n) on PTFE fibrillation is revealed for the first time. Moreover, the degree of PTFE fibrillation is quantified using systematic X-ray diffraction (XRD) analysis. The SE membrane utilizing the high-M_n PTFE binder that achieves a maximum fibrillation degree (≈98%) and thus forms highly entangled fibrous PTFE network interweaving SE particles, exhibited superior toughness. Consequently, the 18 µm thick SE membrane with 0.5 wt% high-M_n PTFE shows a considerably higher areal conductance. The ASSB using this durable, ultrathin SE membrane outperforms its counterparts that used flimsy SE membranes or pure SE pellets. Finally, uniquely thin ASSBs demonstrate significantly improved cell-level energy densities, showcasing the promising potential for practical ASSB fabrication.

Keywords: all‐solid‐state battery; dry‐process; entanglement network; solid electrolyte membrane.