Structure-selective operando x-ray spectroscopy

TL;DR

This study employs structure-selective operando x-ray spectroscopy to directly observe charge-structure mechanisms during phase transformations in sodium-ion battery materials, revealing how local redox processes are linked to long-range structural changes.

Contribution

It introduces a novel operando x-ray spectroscopy method that distinguishes individual crystalline phases during electrochemical reactions, advancing understanding of phase-specific redox mechanisms.

Findings

Redox activity in the P2-phase ceases when Ni4+ ions are no longer stabilized.

Emergence of the O2- structure correlates with the redox process and phase transformation.

Operando spectra reveal the dynamic interplay between local charge states and structural order.

Abstract

The relationship between charge and structure dictates the properties of electrochemical systems. For example, reversible Na-ion intercalation - a low-cost alternative to Li-ion technology - often induces detrimental structural phase transformations coupled with charge compensation reactions. However, little is known about the underpinning charge-structure mechanisms because the reduction-oxidation (redox) reactions within coexisting structural phases have so far eluded direct operando investigation. Here, we distinguish x-ray spectra of individual crystalline phases operando during a redox-induced phase transformation in P2-Na2/3Ni1/3Mn2/3O2 - an archetypal layered oxide for sodium-ion batteries. We measure the resonant elastic scattering on the Bragg reflection corresponding to the P2-phase lattice spacing. These resonant spectra become static midway through the sodium extraction in an operando coin cell, while the overall sodium extraction proceeds as evidenced by the X-ray absorption averaging over all electrochemically active Ni atoms. The stop of redox activity in the P2-structure signifies its inability to host Ni4+ ions. The coincident emergence of the O2- structure reveals the rigid link between the local redox and the long-range order during the phase transformation. The structure-selective x-ray spectroscopy thus opens a powerful avenue for resolving the dynamic chemistry of different structural phases in multi-phase electrochemical systems.