AUTHOR=Jakobsen Christian Lund , Johansen Morten , Ericsson Tore , Häggström Lennart , Christensen Christian Kolle , Nielsen Ida , Brant William Robert , Ravnsbæk Dorthe Bomholdt 

TITLE=Ion-intercalation mechanism and structural relaxation in layered iron phosphate Na3Fe3(PO4)4 cathodes

JOURNAL=Frontiers in Batteries and Electrochemistry

VOLUME=3

YEAR=2024

URL=https://www.frontiersin.org/journals/batteries-and-electrochemistry/articles/10.3389/fbael.2024.1433241

DOI=10.3389/fbael.2024.1433241

ISSN=2813-4974

ABSTRACT=<p>Layered Na<sub>3</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>4</sub> can function as a positive electrode for both Li- and Na-ion batteries and may hold advantages from both classical layered and phosphate-based electrode materials. Using a combination of <italic>ex-situ</italic> and operando synchrotron radiation powder X-ray diffraction, void space analysis, and Mössbauer spectroscopy, we herein investigate the structural evolution of the Na<sub>3</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>4</sub> framework during Li- and Na-ion intercalation. We show that during discharge, Li- and Na-intercalation into Na<sub>3</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>4</sub> occurs via a solid solution reaction wherein Na-ions appear to be preferentially intercalated into the intralayer sites. The intercalation causes an expansion of the unit cell volume, however at open circuit conditions after ion-intercalation (i.e., after battery discharge), Na<sub>3+x</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>4</sub> and Li<sub>x</sub>Na<sub>3</sub>Fe<sub>3</sub>(PO<sub>4</sub>)<sub>4</sub> undergo a structural relaxation, wherein the unit volume contracts below that of the pristine material. Rietveld refinement suggests that the ions intercalated into the intra-layer sites diffuse to the sites in the inter-layer space during the relaxation. This behavior brings new perspectives to understanding structural relaxation and deviations between structural evolution observed under dynamic and static conditions.</p>