Flexible, Biocompatible Supercapacitors Weaved From Layered Phyllosilicates and Zwitterions via Ionicity

Abstract

Phyllosilicates, for example, bentonite clay, are among the most abundant natural minerals, yet their brittleness and poor conductivity have limited their use in energy storage applications, typically requiring harmful solvents and synthetic binders to form functional membranes. Here, we report a polymer-free, aqueous-based approach in which betaine, a naturally occurring zwitterion, is intercalated into bentonite clay galleries through ionic interactions between its cationic trimethylammonium group and the negatively charged clay surface. This intercalation bridges the clay galleries, creating ion-conductive pathways within free-standing, mechanically robust membranes, as confirmed by molecular dynamics simulations, rheo-impedance, and electrochemical impedance spectroscopy. Incorporation of transition-metal ions (Fe³⁺, Mn²⁺) introduces redox-active sites that contribute to pseudocapacitive charge storage. The resulting membranes function as both electrodes (113 mAh/g vs. NMC811) and separators in all-clay supercapacitor devices, delivering an energy density of 158 mWh/cm³ and a power density of 5688 mW/cm³, with ∼75% capacitance retention after 30000 cycles. The devices are biocompatible and can power LEDs up to 2.2 V after brief charging. We show, for the first time, that zwitterions and clay provide a simple, scalable, and sustainable route to biocompatible, flexible supercapacitors for low-power applications requiring mechanical flexibility and material sustainability.