Structural energy storage devices combine the energy storage properties of batteries and supercapacitors with the mechanical properties of structural composites. Such multifunctional devices may allow for energy to be stored within the body panels of electric vehicles, leading to mass and volume savings. However, energy storage and mechanical properties come at a trade-off, making it challenging to develop electrodes that can both store energy and bear mechanical loads. To balance this trade-off, here, we demonstrate how selected materials, such as reduced graphene oxide (rGO) and Kevlar(R) aramid nanofibers (ANFs), can be processed into structural electrodes with enhanced mechanical properties (i.e. ultimate tensile strength and Young's modulus) by engineering interfacial interactions. Structural supercapacitor electrodes composed of rGO and ANFs were fabricated through vacuum-assisted filtration. rGO and ANFs interact with each other mainly through hydrogen bonding interactions. GO was chemically modified to enhance these interfacial interactions, and the effect of the GO modifications on mechanical and energy storage performance was investigated. Significant improvements on the mechanical performance (up to five-fold increase in Young's modulus and four-fold increase in tensile strength compared to pure rGO (no ANFs)) were observed due to the enhanced interfacial interactions. Small deteriorations in energy storage were observed due to the introduction of defects and ion-diffusion limitations induced by the more compact structures. This work demonstrates that synergistic interfacial interactions can lead to significant improvements in mechanical properties of structural supercapacitor electrodes while maintaining good energy storage. Motivated by our prior work on rGO/ANF structural supercapacitor electrodes, we extended this concept to structural lithium-ion battery electrodes using branched ANFs (BANFs) and battery active materials. We combined BANFs with lithium iron phosphate (LFP, cathode) or silicon (Si, anode) particles, and rGO. Overall, we obtained up to two orders of magnitude improvements in Young's modulus and tensile strength compared to commercial battery electrodes while maintaining comparable energy storage properties. As an alternative to the LFP-containing cathodes, structural cathodes based on polyaniline, BANFs, and carbon nanotubes were also fabricated. This work demonstrates an efficient route for developing structural lithium-ion battery cathodes and anodes with enhanced mechanical properties using Kevlar(R) aramid nanofibers.
