A new strategy is presented for forming thin solid membranes by creating an in situ derived self-healing polymer matrix penetrating the void space of an inorganic solid. The concept is applied as a separator in an all-solid-state battery with a FeS₂-based cathode and achieves tremendous performance for over 200 cycles. Processing in the dry condition represents a paradigm shift for incorporating high active material mass loadings into mixed-matrix membranes.