A net reduction of greenhouse gas emissions can be achieved by increasing the use of renewable energy sources, such as solar, wind, hydrothermal, and hydroelectric. However, most of these sources can only supply power intermittently and require energy storage platforms, such as rechargeable batteries, to deliver energy based on demand. Lithiumion batteries (LIBs) are widely used as energy storage platforms in millions of portable devices. Still, LIBs have several shortcomings preventing their increased acceptance in vehicular transportation and large infrastructure installations. For example, LIBs rely on flammable liquid organic electrolytes, which pose a safety risk. The gradual increase in the energy density of commercial LIBs witnessed over the past few decades is expected to level out save for disruptive innovations in materials chemistry and device architectures. One promising path towards safer LIBs is the development of all-solid-state batteries (ASSBs), comprising non-flammable inorganic solid electrolytes (SEs). While maintaining Li-ion conductivities comparable to or better than liquid electrolytes, SEs act as physical separators; they are perceived to improve safety by compartmentalizing the high reactivity of the anode and cathode materials, preventing any leakage and reducing gassing, which is common in liquid batteries. In addition, ASSBs may enable higher energy densities by replacing the graphite anode in LIBs with an energy-dense Li-metal anode, increasing the battery capacity and voltage. Finally, ASSBs enable the stacking of bipolar electrodes, reducing dead weight from casing, and improving the cell-level energy density. Numerous studies have focused on finding inorganic SEs enabling fast Li-ion transport (in the order of mS cm -1 ), with sulfides, oxides, and ternary halides amongst the most widely explored SEs. Sulfide SEs show high mechanical deformability, allowing for their implementation in ASSBs via simple cold-pressing processes. However, sulfides exhibit poor oxidative (anodic) stability, reacting with high-voltage layered oxide positive electrode materials. Oxide-based SEs exhibit wider electrochemical stability windows than sulfide SEs. However, achieving sufficient Li-ion conductivities in oxides require dense pellets obtained through energy-intensive sintering processes, which are difficult to scale. Additionally, the ionic conductivity of oxide SEs is typically lower than that of sulfide SEs. Finally, ternary halide SEs, with formula Li i MX j (X= F, Cl, Br; i and j determined by the oxidation state of metal M) display high electrochemical oxidative stabilities (>4 V vs. Li/Li + ), but are unstable against Li metal. From this analysis, it is evident that none of the SEs mentioned above can simultaneously provide adequate ionic conductivity and good chemical and electrochemical stability against the electrodes, as required to achieve viable ASSBs. The inherent reactivity results in the formation of undesired decomposition products at the electrode/SE interfaces, creating additional interfaces and increasing the interfacial (contact) resistance for Li-ion conduction, with detrimental effects on battery performance. 5, One practical strategy to solve this issue is to combine two SEs with different chemical and electrochemical stabilities to form bilayer composite separators. In these composites, the anolyte is the SE in direct contact with the low voltage negative electrode material (e.g. graphite, or Li-metal, and its alloys), and the catholyte is the SE in contact with the high-voltage (typically, a layered oxide) positive electrode material.