Abstract: An environmentally sustainable, biodegradable, zwitterion-functionalized sepiolite clay composite membrane suitable for electrochemical energy storage and conversion devices is disclosed. The membrane is synthesized by dispersing naturally abundant sepiolite clay in water, functionalizing the slurry with zwitterions such as betaine, alanine, arginine, proline, or valine, and vacuum drying at ambient temperature to yield a free-standing, flexible composite membrane. The zwitterionic compounds chemically attach to surface silanol sites of sepiolite fibers, partially disaggregating fiber bundles and creating controlled hierarchical porosity. Substantially free of polymeric binders, metals, and carbonaceous materials, these membranes exhibit improved ionic conductivity, thermal stability, and chemical resistance compared to traditional separators.