Electrochemical phase transitions are ubiquitous in energy materials, including rechargeable battery electrodes, electrocatalysts for water electrolysis, and electrochromic windows. Because different phases exhibit vastly different properties, their spatial distribution critically determines electrode performance and device functionality. Consequently, these transitions have long attracted interest in fundamental and applied research.

Most studies on operando imaging of phase transitions during battery cycling focus on individual particles or small particle ensembles, achieving nanometer-scale resolution. While observing phase transitions in single particles in real time is a remarkable achievement, it raises an important question: do single-particle dynamics adequately represent the spatiotemporal evolution across an entire electrode? The prevailing assumption is that macroscopic electrode behavior during cycling simply reflects the collective microscopic dynamics of its constituent particles. As a result, macroscopic emergent phenomena have received little attention. Yet, in other fields—such as catalysis—macroscopic self-organization and complexity play a decisive role, as demonstrated by Gerhard Ertl’s Nobel Prize–winning work. This raises the possibility that similar effects may occur in battery electrodes.

To address this question, we investigated macroscopic spatiotemporal dynamics of electrochemical phase transitions in nickel (oxy)hydroxide electrodes. These materials are electrochromic, changing color from light green nickel hydroxide (Ni(OH)₂) to dark grey nickel oxyhydroxide (NiOOH) upon charging, and reverting upon discharge. This color change enables direct monitoring of the phase transition using simple optical imaging. Although limited to spatial features larger than ~1 μm, optical imaging offers a large field of view encompassing the entire electrode and allows fast video recording, making it well suited for tracking phase propagation in real time.

Using translucent thin-film electrodes, we recorded videos of the phase transition during charging [1]. Strikingly, instead of a uniform color change across the electrode, we observed lateral propagation of a macroscopic phase front across millimeter-length scales, perpendicular to the applied electrochemical driving force. This behavior contradicts the prevailing model of a planar phase front advancing uniformly through the electrode thickness.

The observed lateral phase-front propagation reveals strongly nonuniform charging, with some regions becoming fully charged while others remain largely discharged. This nonuniformity has critical consequences: overcharged regions can initiate premature oxygen evolution, limiting access to the electrode’s full capacity. Oxygen evolution also promotes battery swelling and electrolyte leakage, undermining safety and lifetime. Our findings therefore uncover a critical and previously overlooked aspect of battery electrode behavior—macroscopic nonuniform charging driven by emergent phase-transition dynamics.