Dimitrios Fraggedakis (University of California, Berkeley)

Li–ion batteries have significantly impacted the everyday use of portable electronic devices. However, their application in large–scale energy storage and fast charging remains underutilized due to limitations of current materials that comprise their electrodes and electrolytes. To propose strategies for optimizing battery performance, we need to understand the fundamental electrochemical reaction and transport processes that occur during their operation.

In the first part of this talk, I will explore the mechanism of ion intercalation, an essential step in the energy storage process of Li–ion batteries that involves the transfer of ions and electrons. Existing theories for ion intercalation propose a sequential process in which either ion transfer or electron transfer is the rate–limiting step. However, these approaches tend to violate local charge neutrality and are unable to describe experimental electrochemical observations, such as current response upon applied voltage (Tafel plots). To address these issues, I introduce the theory of coupled ion–electron transfer kinetics, where both the ion and the electron are transferred simultaneously during ion intercalation. This theory predicts curved Tafel plots and reaction–limited currents that depend on the concentration of the intercalated ions, electrolyte properties, and the reorganization energy of the intercalation materials. The predictions are tested and verified against experiments for several intercalation materials used in commercial batteries.

In the second part of the talk, I will discuss the onset of glassy dynamics, a phenomenon that is relevant to ion diffusion in batteries. Alternative materials for safer and low–cost batteries often exhibit glassy ion dynamics, resulting in low ionic conductivities. These dynamics occur below an onset temperature in several disordered solids and liquids, where their equilibrium relaxation time grows also in a super-Arrhenius manner. Although the onset temperature marks a transition in microscopic dynamics, its origin is not yet known. To understand the mechanisms behind the onset of glassy dynamics, I focus on two-dimensional systems and introduce the theory of binding-unbinding transition of pure-shear elastic excitations. The theory predicts the value of the onset temperature and the emergence of elasticity in two-dimensional systems, analogous to the Kosterlitz-Thouless-Halperin-Nelson Young theory of dislocation-mediated melting in two-dimensional solids. The predictions of the theory are compared and verified against experiments and simulations of two–dimensional glass formers.