Coupled diusion and mechanics in battery electrodes

Thesis (Ph.D.)--University of Washington, 2016-08

We are living in a world with continuous production and consumption of energy. The energy production in the past decades has started to move away from petrochemical sources toward sustainable sources such as solar, wind and geothermal. Also, the energy consumption is further adapting to the sustainable sources. For instance, in recent years electric vehicles are growing fast that can consume sustainable electric energy stored in their batteries. In this direction, in order to further move toward sustainable energy, materials are becoming increasingly important for storing electric energy. Although, currently the technologies such as Li-ion batteries and solid-oxide fuel cells are commercially available for energy applications, improvements are crucial for the next generation of many other technologies producing or consuming sustainable energies. A critical aspect of the electrochemical activities involved in energy storage technologies such as Li-ion batteries and solid-oxide fuel cells is the diusion of ions into the electrode materials. This process ultimately governs various functional properties of the batteries such as capacity and charging/discharging rates. The rst goal of this dissertation is to develop mathematical tools to analyze the ionic diusion and investigate its coupling with mechanics in electrodes. For this purpose, a thermodynamics-based modeling framework is developed and numerically solved using two numerical methods to analyze ionic diusion in heterogeneous and structured electrodes. The next goal of this dissertation is to develop and analyze characterization techniques to probe the electrochemical processes at the nano-scale. To this end, the mathematical models are rst employed to model a previously developed Atomic Force Microscopy based technique to probe local electrochemical activities called Electrochemical Strain Microscopy (ESM). This method probes the activities by inducing AC electric eld to perturb ionic activities and measuring the surface vibrations. Dierent aspects of this technique are analyzed and the limitations are discussed. Such limitations moves the dissertation toward development of a new technique for probing the electrochemical activities, to overcome the previous limitations, called Scanning Thermo-ionic Microscopy (STIM). In this method, the local activities are probed by inducing AC temperature oscillations to perturb ionic activities and measuring the surface vibrations. The principle mathematical analysis of the coupled governing equations and the method of probing electrochemical activities are discussed in detail. Also, the method is implemented into the AFM hardware/software and the STIM response is conrmed using experiments on LiFePO4 and Sm-doped Ceria as well-known battery and fuel cell electrodes. The STIM method provides a clean method for analyzing energy storage materials and designing novel nano-structured materials for improved performance. Finally, conclusion of the presented work is discussed in the last chapter and the future works to continue the development of the modeling and experiments are listed.