Operando spectro-ptychography reveals dynamical charge-storage and degradation pathways in redox-active electrodes

Abstract: Electrochemical reactions at buried electrode-electrolyte interfaces govern how redox-active materials store and release energy. However, these reactions are difficult to visualize because chemical and morphological changes occur simultaneously over distinct length and time scales. Existing operando microscopies often require trade-offs among chemical sensitivity, spatial resolution and temporal resolution. Direct nanoscale tracking of such processes throughout extended timescale has therefore remained out of reach. Here, we develop a fast and robust operando soft X-ray spectro-ptychography platform that delivers chemical-state-resolved spatiotemporal movies of redox-active electrodes over the full battery lifetime. Applied to an alkaline Fe anode, the method reveals that reversible charge storage gives way to degradation through two competing processes: rapid hydroxide insertion that drives early reversible cycling, and slower dissolution-redeposition that redistributes Fe, enlarges FeOOH particles, and ultimately causes capacity loss. By separating fast charge-storage chemistry from slower degradation chemistry in operando and at both single-particle and particle ensemble level, this work establishes spectro-ptychography as a general approach for studying dynamic redox transformations in batteries, electrocatalysts, and other electrochemical materials.

• The paper introduces a fast operando spectro-ptychography platform that tracks reversible hydroxide-based charge storage in Fe anodes with 3-minute acquisition rates.
• Operando measurements reveal key oxidation-state dynamics, distinguishing rapid ion insertion from degradation via dissolution–redeposition and FeOOH particle coarsening.
• Findings offer actionable insights for designing improved battery electrodes by linking nanoscale charge transport, morphological evolution, and degradation pathways.

Operando Spectro-Ptychography for Unveiling Charge-Storage and Degradation in Redox-Active Fe Anodes

Introduction

Electrochemical energy storage in redox-active transition metal electrodes is fundamentally governed by the interplay of ion insertion, dissolution–redeposition, and morphological evolution at buried interfaces. Historically, the dynamic processes occurring during battery cycling have been challenging to resolve due to the limits of spatial, temporal, and chemical resolution in operando imaging modalities. The paper "Operando spectro-ptychography reveals dynamical charge-storage and degradation pathways in redox-active electrodes" (2606.25175) addresses these technical gaps by introducing a fast, robust operando soft X-ray spectro-ptychography platform capable of nanoscale, chemical-state-resolved, spatiotemporal movies of electrode evolution throughout the battery lifetime. The focus is on the alkaline Fe anode as a model system for probing both reversible and degradative processes.

Methodological Advances in Spectro-Ptychography

A critical innovation is the combination of optimized scanning strategy with a custom liquid electrochemical flow cell, facilitating time-lapse soft X-ray spectro-ptychography at acquisition rates (~3 min per stack) sustainable over 20+ hours—covering an entire battery lifetime. Sparse and large-defocus grid scanning radically reduces radiation dose and increases throughput compared to conventional approaches, providing a spatial resolution that surpasses STXM and enabling morphological and chemical tracking of tens of particles under realistic conditions. Spectroscopic maps at three Fe L3 edge photon energies distinguish metallic (Fe^0) and oxyhydroxide (FeOOH, Fe^3+) states, with fitting algorithms that yield pixel-resolved oxidation state, absorption-derived thickness, and Fe areal mass density.

Charge Storage Mechanism: Rapid Ion Insertion

During early cycling (cycles 8–13), the Fe anode exhibits highly reversible charge–discharge behavior, tracked by spatially and temporally resolved oxidation-state maps. Local Fe^0 and FeOOH fractions interconvert over each cycle, with negligible change in total Fe mass density per pixel, demonstrating minimal net Fe transport. These results rule out dissolution–redeposition as a dominant process in the reversible regime and indicate direct hydroxide insertion/intercalation as the primary charge-storage pathway, consistent with transient swelling/shrinking of the film. Spatial heterogeneity is apparent: larger FeOOH-rich particles reduce more slowly, exhibiting local overpotential and persisting oxidation.

Degradation Pathways: Dissolution–Redeposition and Particle Growth

Transitioning from reversibility to degradation (cycles 16–25), spectro-ptychography reveals a shift toward persistent FeOOH particle growth and incomplete reduction. The spatially averaged oxidation state drifts upward over cycles, correlating with capacity fade and progressive failure. In charged states, oxide layers persist, and Fe concentrates in enlarged FeOOH particles, which become increasingly irreducible. Initial capacity is dominated by particle-rich regions; as coarsening proceeds, conductivity and local capacity decline, confirmed by suppressed hydrogen-evolution currents and post-mortem SEM/AFM analyses. Failure arises through slow dissolution–redeposition-mediated Fe redistribution and coarsening, not uniform catastrophic loss.

Particle-Resolved Dynamics and Temporal Heterogeneity

Longitudinal analysis of 70+ particles demonstrates divergent evolutionary trajectories: some particles undergo reversible swelling/shrinking dominated by ion insertion, others accrue Fe via dissolution–redeposition, and a subset exhibit abrupt coarsening after reaching a critical state. Within a single cycle, reversible topographic changes are evident; over multiple cycles, net Fe migration occurs, correlating with particle volume, oxidation state, and capacity. Heterogeneous particle behavior explains spatially non-uniform electrode response as aging progresses.

Implications and Future Prospects

The methodology and findings redefine mechanistic understanding of charge-storage and degradation in redox-active electrodes. By directly disentangling competing fast and slow pathways within the same active region, the study establishes spectro-ptychography as a general approach to probe nanoscale redox transformations in batteries, electrocatalysts, and other electrochemical systems. Key numerical findings include sustained nanoscale chemical imaging over 20+ hours (>350 stacks), detection of oxidation-state drift as a precursor to capacity fade, and spatial correlation between Fe mass concentration and degradation hotspots.

Practically, this framework enables design of electrode architectures and cycling paradigms that minimize dissolution–redeposition and particle coarsening. Theoretically, it facilitates explicit coupling of ion-insertion dynamics, chemical state evolution, and morphology—paving the way for multiscale models of electrode lifetime. Future developments may include integration with in situ spectro-electrochemical platforms, extension to multi-component electrodes, and application in real-time diagnosis and adaptive control of degradation in commercial batteries.

Conclusion

Fast operando soft X-ray spectro-ptychography offers unprecedented capability to resolve chemical state, morphology, and mass transport across length and time scales in electrochemically active materials. For alkaline Fe anodes, reversible charge storage is dominated by rapid hydroxide insertion with minimal Fe migration; degradation is governed by gradual, spatially heterogeneous dissolution–redeposition and FeOOH particle coarsening. This mechanistic separation has broad implications for rational electrode design, prediction of failure modes, and future operando imaging strategies in battery research, electrocatalysis, and corrosion science.