The Springback Effect in Pre-Densified Sulfidic Solid Electrolyte/Binder-Sheets and Their Implication as Separators in All-Solid-State Batteries – a First Approach

All-solid-state batteries (ASSBs) are expected to revolutionize the Li-ion battery landscape thanks to multiple benefits, such as improving both intrinsic safety and potentially enabling high energy anode concepts. [1,2,3] Amongst the variety of possible solid electrolyte (SE) materials for ASSBs, sulfidic electrolytes are considered particularly advantageous, as they possess high ionic conductivities and beneficial mechanical plasticity, allowing their pre-densification by uniaxial cold pressing at high fabrication pressures ( p fab ). [4] However, one bottleneck of sulfidic ASSBs compared to conventional liquid-electrolyte batteries is the necessity of a certain operating pressure to provide/maintain an intimate mechanical contact between the various materials during materials characterization measurements (e.g., SE conductivity measurements) and/or battery cycling. For these, a special cell setup is required that is hermetically sealed and facilitates a defined and, ideally, variable application of different compressions (operating pressure, p oper ) on the sample/cell stack. [5] Typically, ASSB characterization cells possess a defined circular geometry, whereby the components are placed in an inert polymer containment (e.g. a PEEK tube) and are electrically contacted via metal dies. [2,5,6] However, a problem which is often neglected in the literature is the poor solid-solid contact of pre-densified SE pellets (done at a high p fab ) and mechanically hard current collector dies to contact the sample for impedance measurements. This leads to impedance artifacts and potentially inaccurately determined solid electrolyte Li + -ion conductivities ( σ Li ). [2] In this contribution, we firstly describe a novel ASSB cell setup enabling the artifact-free measurement of the ionic conductivity of pre-densified SE-binder sheets in rigid cell matrices. By applying this setup, we could decouple effects from fabrication pressure and operating pressure, providing the basis for determining the correlation between the SE separator porosity and its Li + -ion conductivity, comparing it to the correlations predicted by the Bruggeman equation. We use sheet-type separators consisting of Li 6 PS 5 Cl (LPSCl) solid electrolyte and different amounts and types of binder, such as hydrogenated nitrile butadiene rubber (HNBR) and polyisobutylene (PIB), prepared by a slurry-based process. [5] By means of potential-controlled electrochemical impedance spectroscopy (PEIS), the impact