Side Reactions in Lithium-Ion Batteries
Lithium-ion batteries store energy through two electrochemical reactions. In addition to these main reactions, many side reactions are possible. The causes and effects of battery side reactions are usually detrimental, sometimes positive, and almost always very complicated. We investigate here both fundamental and applied aspects of several side reactions using experiments and theory.
In Part I, we develop a new method to characterize the solid-electrolyte-interphase (SEI), a passivating film that is the product of side reactions on the graphite electrode, using ferrocene, a redox shuttle. By comparing ferrocene kinetics in the presence and absence of passivating films, the shuttle functions as an electrochemical probe of the mechanism by which the SEI prevents reaction. We develop our method using a model system of a rotating-disk-electrode (RDE) and glassy carbon. Coupling experimental measurements with a physics-based model aids interpretation of the results. We develop both steady-state and transient methods in the model system before applying the methods to highly oriented pyrolytic graphite (HOPG), which more closely resembles the carbon found in an actual battery. We study the effects of the relative fraction of edge and basal planes, the formation voltage, and the electrolyte anion. We also attempt in-situ atomic-force microscopy to validate our electrochemical measurements.
In Part II, we treat several different overcharge reactions theoretically to explain empirical observations, predict system behavior, and suggest considerations for battery design. We develop a two-dimensional model to explain why lithium deposition occurs preferentially at the edges of electrodes and suggest mitigation techniques. In a separate model, we implement a redox shuttle for overcharge protection. We study how the presence of a redox shuttle affects the macroscale current-potential relations of the battery and optimize the redox potential of the shuttle while considering the tradeoffs between current efficiency and overcharge protection.