This analysis suggests that considering the chemical compatibility of bilayer separators is vital when pairing SEs. However, the integrity of the bilayer separator is only preserved when the cell is operated within the electrochemical stability window (ESW) defined for bilayers by the reductive and oxidative potentials of the anolyte and catholyte, respectively. Outside the ESW, the SE may react and eventually decompose into other thermodynamically stable products. In the case of the Li 3 PO 4 /Li 3 InCl 6 bilayer investigated here, Li 3 PO 4 displays a reductive potential as low as ∼0.7 vs. Li/Li + , whereas Li 3 InCl 6 has a high oxidative potential ∼4.3 vs. Li/Li + (see computed values in Figure ). Thus, the union of the anolyte Li 3 PO 4 reducing potential and the catholyte Li 3 InCl 6 oxidative potentials defines the ESW of the bilayer separator, which occurs in the 0.7-4.3 V vs. Li/Li + range. Outside the 0.7-4.3 V voltage range, at least one SE is expected to decompose into simpler thermodynamically stable compounds at the corresponding potential.