1. Conductivity of different types of solid electrolytes at room temperature^[17]

2. Driving force for interphase formation between electrolyte, and cathode, with varying voltage from 0 to 5 V vs lithium metal [Legend: blue, LCO; red, LMO; green, LFP; thick line, LLZO; thin line, LLTO]. The calculated intrinsic stability windows are marked along the bottom for reference^[50]

3. (a) Typical scanning electron microscope (SEM) image of the interface between composite cathodes and LLZTO electrolyte; (b) SEM image for the surface of LLZTO ceramic; SEM images of the composite cathodes which were measured in (c) the second-electron and (d) the back-scatter-electron mode^[53]

4. Schematic illustration of the synthesis procedure^[57]

5. (a) Schematic illustrations of non-modified and Nb-modified LLZ/LiCoO₂ interfaces. The mutual diffusion between LLZO and LiCoO₂ produces non-Li⁺-conductive phases such as a crystalline La₂CoO₄ phase. Nb-modified LLZ/LiCoO₂ interface suppresses the mutual diffusion and produce Li⁺-conductive amorphous phase; (b) Cross-sectional-HAADF-STEM image of a Nb-modified interface between LLZO and LiCoO₂^[65]. EDX elemental mappings in (b) for Co (red), Nb (purple), and La (green) are overlaid in the dashed-line-enclosed region. The top Pt is a protective layer for FIB processes

6. SEM image of the interface between LFP composite cathode containing 15wt% polymer and the LLZTO composite electrolyte^[72]

7. Solid-state LFP or LFMP/Li cells using PEO/LLZTO@IL membranes^[78]