High-stiffness Electrodes and Separators for Structural Batteries and Capacitors

Structural energy storage promises to merge the features of structural composites with that of batteries or capacitors, resulting in high-stiffness devices that can bear mechanical loads and resist impact. However, structural energy materials are often faced with tradeoffs in stiffness and energy. This is because structural electrodes and electrolytes are often composites of materials that either store energy or bear loads alone, meaning that simple mixing rules may apply. This talk will examine structural electrodes and electrolytes that provide a synergistic path toward the simultaneous improvement of both mechanical and electrochemical behavior. Structural battery lithium iron phosphate cathodes and silicon anodes based upon a reduced graphene oxide (rGO)/aramid nanofiber (ANF) matrix are examined. The rGO sheets provide conductivity and mechanical stiffness, and the ANFs provide Kevlar-like stiffness and toughness. Further, ANFs assembled into thermally stable, high-stiffness membranes provide robust battery separators. Besides batteries, that same rGO/ANF electrode platform provides a path toward high-stiffness capacitor electrodes, in which chemical modification of the rGO surface yields non-covalent interactions with the ANFs that lead to a dramatic enhancement in stiffness. However, these electrodes may undergo dimensional changes during charging and discharging, leading to the generation of internal stresses. To quantify this, the electrochemo-mechanical coupling of rGO electrodes is analyzed using a custom-built instrument, showing clear trends in the stress evolution during operation. Taken together, this work provides insight molecular-level strategies that address both mechanical properties and electrochemical performance for improved structural energy storage devices.