Nonuniform charging and phase front instability in nickel (oxy)hydroxide thin-film electrodes

Abstract

Some battery electrodes undergo phase transitions during ion insertion and extraction in electrochemical cycling. These transitions often limit electrode capacity, rate performance, and cycle life, making them critical in electrochemical energy storage. Work in this area has largely centered on phase propagation dynamics at the level of individual particles, thereby reinforcing the prevailing paradigm that macroscopic electrode behavior directly arises from the microscopic dynamics of its constituent particles. Yet this paradigm disregards emergent macroscopic effects and self-organization that can shape electrode behavior in complex and nontrivial ways. Here, we uncover an emergent effect that drives lateral propagation – perpendicular to the applied driving force – of a millimeter-scale phase front between charged NiOOH and discharged Ni(OH)₂ during the charging of thin-film (~50 nm) nickel (oxy)hydroxide electrodes. We introduce a novel mechanistic explanation for this counterintuitive behavior, attributing it to long-range Coulombic interactions between charged regions at the phase front. These interactions trigger planar front instability, akin to the forces that drive phase separation and self-organization in bulk heterojunction organic photovoltaic cells. Thus, this study challenges the fundamental understanding of electrochemical phase transitions in ion insertion electrodes, particularly in thin-film systems, uncovering a new perspective on how the delicate interplay between charging rate and charging uniformity governs battery performance.