Development of nano-sized LiFePO4 dry cathodes with enhanced flexibility and mechanical robustness for roll-to-roll dry coating process

Abstract

The polytetrafluoroethylene (PTFE) binder-based roll-to-roll dry coating process has emerged as a promising alternative to conventional slurry-based methods for fabricating thick electrodes in high-energy-density lithium-ion batteries (LIBs). However, applying nano-sized lithium iron phosphate (LiFePO4, LFP) to this process remains challenging, as the high specific surface area of nano-sized LFP leads to the formation of short and thin PTFE fiber network that cannot ensure the mechanical integrity of dry cathode at low PTFE binder content. Consequently, the nano-sized LFP dry cathode suffers from poor flexibility and mechanical brittleness, limiting its applicability in roll-to-roll processing. In this study, we investigated the fibrillization behavior of PTFE binders depending on the particle size of LFP, to elucidate the origin of mechanical degradation in nano-sized LFP dry cathodes. Our results revealed that nano-sized LFP facilitates excessive PTFE fibrillization, generating fragile and weak networks with short and long PTFE fibers, leading to the mechanical degradation of nano-sized LFP dry cathodes. To address this issue, we introduced a two-step extrusion process that promotes the formation of thick and long PTFE fiber networks within nano-sized LFP dry cathodes. This strategy enabled the fabrication of flexible and mechanically robust nano-sized LFP cathode film with only 2 wt% PTFE binder. The developed LFP dry cathodes exhibited excellent compatibility with thick electrode designs and achieved high areal capacities (7 mAh cm-2, 2.7 g/cc), offering a scalable solution for next-generation LFP-based LIBs.