Electrochemical and thermal control of continuous phase transitions in P2-NaxNi1/3Mn2/3O2

TL;DR

This study investigates how electrochemical and thermal stimuli induce continuous phase transitions in P2-NaxNi1/3Mn2/3O2, revealing the coupling between Na+-vacancy ordering and structural symmetry changes, with implications for battery performance.

Contribution

It uncovers the intrinsic coupling between Na+-vacancy ordering and symmetry-changing phase transitions in P2-NaxNi1/3Mn2/3O2 using diffraction techniques and electrochemical analysis.

Findings

Na+-vacancy ordering occurs at specific sodium stoichiometries

Electrochemical desodiation induces symmetry-changing transformations

Phase transitions exhibit second-order behavior near critical compositions

Abstract

Sodium layered oxides often undergo phase transformations involving ordering or disordering of Na+ upon desodiation, i.e., when cycled as a battery electrode. Accurately characterizing these phases is crucial for understanding functional properties, such as chemical diffusivity. In this work, we reveal that Na+-vacancy (dis)ordering in a layered oxide is intrinsically coupled to continuous symmetry-changing transformations of the host structure. We examine the low-symmetry orthorhombic unit cell of P2-NaxNi1/3Mn2/3O2 (NNM) using both neutron and X-ray diffraction. Specifically, special sodium stoichiometries (x = 2/3 and 1/2) exhibit concomitant Na+-vacancy ordering and an orthorhombic distortion from the parent hexagonal unit cell. We then demonstrate that electrochemical desodiation drives symmetry-changing transformations in NNM that are linked to Na+-vacancy (dis)ordering, with evidence of second-order behavior observed near x = 2/3. Variable-temperature synchrotron X-ray diffraction further clarifies the coupling between Na+-vacancy disordering and orthorhombic-to-hexagonal phase transitions in NNM. The temperature-driven phase transitions at both x = 2/3 and 1/2 are also consistent with a second-order mechanism. Our analysis of the phase transitions in NNM has fundamental consequences for sodium chemical diffusivity in the vicinity of the ordered phases. The insights from this work are directly applicable to other layered oxides that exhibit alkali-metal-vacancy ordering.