Wet-slurry fabrication using PVdF-HFP binder with sulfide electrolytes via synergetic cosolvent approach for all-solid-state batteries

Polymeric binders are a crucial component of large-format all-solid-state Li batteries (ASLBs) that employ sulfide solid electrolytes. However, the severe dissolution of sulfide solid electrolyte materials into conventional polar solvents restricts the suitability of slurry-processing solvents to less polar molecules, and in turn, the suitability of materials for polymeric binders to rubbers, such as nitrile butadiene rubber (NBR). In this study, a synergistic cosolvent approach is employed, to render poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP), which is sulfide-slurry-incompatible, amenable to slurry fabrication. The synergistic combination of highly volatile ethyl acetate, and less volatile hexyl butyrate allows for the utilization of PVdF-HFP as it is soluble in the former but not in the latter, while avoiding the binder migration problem. A comparative investigation of sheet-type LiNi_0.70Co_0.15Mn_0.15O₂ electrodes comprising PVdF-HFP or NBR, demonstrates the superior performance of the former. Specifically, it is revealed that the effective volume fraction and binder distribution in the electrodes are key factors for determining the electrochemical performance. The advantageous features of PVdF-HFP over NBR are highlighted, particularly at a low temperature of 0 °C or under a low external operating pressure of 2 MPa. Finally, the encouraging performance of pouch-type LiNi_0.70Co_0.15Mn_0.15O₂/graphite ASLB full cells fabricated using PVdF-HFP, is successfully demonstrated.