PVDF and PEO Catholytes in Solid-State Cathodes Made by Conventional Slurry Casting

Abstract

All-solid-state Li batteries are desired for better safety and energy density than Li-ion batteries. However, the lack of a penetrating liquid electrolyte requires a much different approach to the design of cathodes. The solid catholyte must enable good Li⁺ conduction, form good interfaces with active material particles, and have the strength to bind the cathode together during repeated volume changes. Catholyte formulation is often simply adapted from Li-ion design principles, adding a Li salt to the PVDF binder. Here we show that such a PVDF binder at 10 wt % loading is a starved catholyte condition that compromises cell performance. By substituting a 70 : 30 blend of PVDF:PEO, performance is improved while maintaining nearly the same areal loading of LFP active material. Increasing the catholyte fraction to 16 % can also improve performance, but in this case the benefit of including PEO is lessened, with PVDF alone being an adequate catholyte. EIS analysis shows that PEO helps to form charge transfer interfaces at 10 % catholyte, but that its inclusion can degrade interfaces when there is ample catholyte at 16 %. It is also shown that catholyte agglomeration can impede bulk Li conduction, indicating that microstructural factors are of critical importance.