Micromechanics Modeling for Structural Energy Storage Devices

Abstract

Structural energy storage devices, such as structural batteries and structural supercapacitors, reduce the volume and/or mass of portable electronic devices and electrical vehicles by providing structural support while storing electrical energy. Modeling for these multifunctional structural energy storage materials has lagged behind at this early stage due to their complex microstructure and multiphysics nature. In this work, models are developed to understand the behavior of the structural electrodes and predict and explore material behaviors in the experimentally unexplored design space, and to propose optimized composition and microstructure. Specifically, the focus of this work includes examining the Young' modulus, electrical conductivity and electromechanical coupling behavior of supercapacitor electrodes, as well as weight savings and multifunctional efficiency for structural batteries and supercapacitors. The Young’s moduli and electrical conductivities of the electrodes were modeled using analytical micromechanics methods. Key parameters include microstructure, volume fraction, and material properties of different phases in the electrode. Aramid nanofiber (ANF) and reduced graphene oxide (rGO) nanocomposites were used as exemplary structural supercapacitor electrodes. The most influencing factor that determines the Young’s modulus was the microstructure (specifically waviness) of rGO and ANF due to their extreme modulus anisotropy. When determining the electrical conductivity, material property and volume fraction of the interphase were key. The models developed for these structural supercapacitor electrodes can be extended to battery electrodes. Stress development was observed during electrochemical cycling of rGO supercapacitor electrodes. In order to capture this electrochemo-mechanical coupling response in rGO-based supercapacitors, a new micromechanics model was developed. The developed stress due to the electrochemical cycling are attributed to an eigenstrain determined by the charges stored at the interphase between rGO flakes. This model not only predicts the material response accurately, but also captures the microstructure at different scales and connects the eigenstrain developed in the nanoscale to the mesoscale response. With this understanding of the microstructure and coupled phenomenon, a micromechanics-based multifunctional efficiency (MFE) was proposed to quantify the volume and/or mass savings when multifunctional structural energy storage materials replace traditional structural materials and energy storage materials. The complex microstructures, material anisotropy, charging conditions and mechanical loading conditions are reflected in the MFE proposed, while existing metrics based on the rule of mixtures provide a constant upper bound. The models and MFEs can be modified or expanded, and after proper calibration, may predict the mechanical, electrical and electrochemo-mechanical coupling response of any electrochemically active material.