  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.
