Towards comprehensive design of electrolytes for electrochemical energy storage

Abstract

Increasing utilization of renewable, but intermittent energy sources like wind and solar necessitates accompanying deployment of energy storage technologies, where one promising approach is to store energy electrochemically using batteries or electrolyzers. At the heart of all electrochemical systems is the presence of an ionically conductive, but electronically insulating electrolyte that enables a chemical reaction to be separated into two electrochemical reactions when electrical current flows through an external circuit. The electrolyte plays a vital role in determining the performance of electrochemical devices, where properties like its ionic conductivity and (electro)chemical stability determine the working voltage window, power and cycle life of electrochemical devices. The electrolyte can also interact with redox reactions occurring at the electrodes, altering their thermodynamics and kinetics. Therefore, understanding how to control the properties of the electrolyte is vital for the development of next generation electrochemical storage devices.

In this thesis, a deeper understanding of the fundamental interactions that govern electrolyte performance is developed. The influence of electrolyte on reaction pathways and kinetics is highlighted through studies on the discharge and charge processes in Li-O2 batteries. A unified picture of the influence of solvent, salt concentration and anion on the oxygen reduction reaction that occurs during discharge is developed based on the solvation energy of Li+ and O2 - ions, which influences the Li+-O2 - coupling strength. During the charging reaction, the thermodynamics and kinetics of LiI-based redox mediators reacting with Li2O2 and LiOH are shown to depend on the solvation strength of Li+ and I- ions. Moreover, I-Br interhalide redox mediators are introduced which allow the oxidizing power of the redox mediator to be tuned independently of the solvent. The role of solvation entropy, as well as the composition- and temperature-dependence of dielectric constant, on the ionic conductivity of liquid electrolytes is investigated. Finally, a unified picture of ion conduction in liquid, polymer and ceramic Li-electrolytes is presented based on microscopic dynamics and the energy landscape. From a stronger understanding of the role of solvation, dynamics and energy barriers on the performance of electrolytes, next generation electrochemical storage devices with enhanced properties can be designed.