  Structural and ion-ordering phase transitions limit the viability of
sodium-ion intercalation materials in grid scale battery storage by reducing
their lifetime. However, the combination of phenomena in nanoparticulate
electrodes creates complex behavior that is difficult to investigate,
especially on the single nanoparticle scale under operating conditions. In this
work, operando single-particle x-ray diffraction (oSP-XRD) is used to observe
single-particle rotation, interlayer spacing, and layer misorientation in a
functional sodium-ion battery. oSP-XRD is applied to
Na$_{2/3}$[Ni$_{1/3}$Mn$_{2/3}$]O$_{2}$, an archetypal P2-type sodium-ion
positive electrode material with the notorious P2-O2 phase transition induced
by sodium (de)intercalation. It is found that during sodium extraction, the
misorientation of crystalline layers inside individual particles increases
before the layers suddenly align just prior to the P2-O2 transition. The
increase in the long-range order coincides with an additional voltage plateau
signifying a phase transition prior to the P2-O2 transition. To explain the
layer alignment, a model for the phase evolution is proposed that includes a
transition from localized to correlated Jahn-Teller distortions. The model is
anticipated to guide further characterization and engineering of sodium-ion
intercalation materials with P2-O2 type transitions. oSP-XRD therefore opens a
powerful avenue for revealing complex phase behavior in heterogeneous
nanoparticulate systems.
