Towards electrochemical-performance evaluation of fiber-based batteries: Fiber-arrangement-based method and FE 2 multiscale framework

Abstract

Conventional battery models (e.g., Pseudo-2D model) were developed especially for particle-based battery electrodes and have limitations in addressing the newly-emerging fiber-based ones. This thesis proposes numerical tools for efficient property evaluation of fiber-based electrodes and for multiscale simulation of battery electrochemical behavior. An efficient computational model is first developed to evaluate percolation threshold, effective electronic conductivity, and capacity of fiber-based electrodes. The electrode is composed of conductive and active fibers mixed in an electrolyte matrix. This model rests with generation of randomly-distributed fibers by Monte Carlo method. The connection between conductive fibers is used to determine percolation threshold and electronic conductivity, while the connection between conductive and active fibers defines the active material utilization and capacity. An optimal active-conductive material ratio is identified to maximize the electrode capacity, and the study of fiber orientation effect reveals that the isotropic distribution leads to the highest utilization of active fibers. For more accurate estimation, a FE2 multiscale framework is further proposed to solve physics-based governing equations. The first part extends the conventional FE2 method suited to a one-equation model to transient diffusion in a two-phase medium described by a two-equation model. The new features include the macroscale equations derived by the volume-averaging method and separate treatment of the two phases in terms of information exchange between macro- and micro-scales and boundary conditions of the microscale problem. The differentiation of the two phases results in additional macroscale source terms upscaled from the microscale interfacial flux. Unlike effective material properties, the tangents of the interfacial flux depend on the microscopic length scale. The second part of the FE2 framework addresses the ionic transport in the pore-filling electrolyte of separators, ignoring the interfacial flux between the electrolyte and the active material. The FE2 method features a macroscale constitutive relation numerically obtained, rather than assumed as in Pseudo-2D model and many of the existing models, from microscale simulation results. This unique feature enables the FE2 method to allow for nonlinear (concentration-dependent) transport properties at the microscale and reflect them at the macroscale without postulation. The well-defined microscale problem setting results in effective transport properties expressed in a tensor format that is indispensable for an anisotropic microstructure.