Alkali Metal Plating and Stripping in Liquid Electrolytes

Doctoral thesis, 2025

Batteries have relatively modest energy densities compared to fossil fuels. In the effort to make battery-driven transport solutions and technologies competitive with gasoline-powered alternatives, it is important to develop batteries with higher energy densities. This can be enabled by utilizing electrode materials that charge and discharge through other electrochemical reactions compared to the conventional Li-ion batteries. Metal anodes are one such class of materials. For instance, lithium metal is one of the electrode materials that can enable the highest theoretical energy densities. Further, sodium or potassium metal can help increase the energy density of batteries built using more abundant materials. To charge a metal anode, ions from the electrolyte are electrochemically plated onto the electrode surface, and during discharge, the metal is stripped from the electrode, releasing ions back into the electrolyte. However, the reversibility of alkali metal plating and stripping is currently not sufficient to enable batteries with acceptable cycle lives and high energy densities. The high reactivity and low reduction potentials of alkali metals drive side reactions between the metal and the electrolyte, consuming active material to form an interphase layer. The side reactions are exacerbated by the tendency to form porous structures during plating, increasing the surface area of the electrode, further aggravating the side reactions. Subsequent stripping from the porous structures to discharge the cell risks creating electronically insulated metal regions, preventing full utilization of the electrode. Therefore, the issues of plating/stripping reversibility in alkali metal electrodes depend on the highly coupled issues of electrode interphase formation, and electrode morphology. The interphase layer will affect the homogeneity and morphology of the lithium plating. The morphology of the plated lithium in turn determines how the side reactions inside the cell will progress. In this thesis, in situ and operando characterization methods are used to track how interphases and electrode morphologies interrelate and how they evolve during cycling. Electrochemical characterization methods are combined with neutron reflectometry and X-ray tomography to help build mechanistic models of how different electrode morphologies evolve during cycling.