Extracting Interfacial Resistance between Potassium β-Alumina and Polymer Electrolytes for Composite Electrolytes

Potassium metal batteries (PMBs) employing solid-state electrolytes (SSEs) utilize the high abundance and low reduction potential of potassium. The inorganic conductor, potassium β-alumina (BASE), has excellent potassium ion conductivity (σ ion ) but requires a lengthy, high temperature sintering process, which has hindered widespread implementation. Alternatively, potassium salts dissolved into polymers such as poly(ethylene oxide) (PEO) forming dry polymer electrolytes (DPEs) are simple to fabricate but suffer from poor stability against potassium and exhibit low room temperature ionic conductivity. Composite inorganic-polymer electrolytes (CIPEs) combine inorganic and polymer conductors to access advantages from both, while mitigating their individual problems. However, the additional resistance produced at the inorganic–polymer interface can significantly limit conductivity. Through equivalent circuit fitting of electrochemical impedance spectroscopy (EIS) spectra, interfacial resistances for BASE|DPE|BASE trilayers were determined to be 2.0 × 10 3 Ω·cm 2, accounting for up to 48% of the total resistance measured. This work highlights the dominance of interfacial resistance on ion transport within CIPEs, highlighting a barrier to achieving high CIPE ionic conductivity for PMBs while further motivating research toward minimizing interfacial resistance of SSEs.