Abstract
This study conveys striking findings regarding the operando structural behavior of the Na/FePO4 system during a charge and discharge cycle. From Rietveld refinements of synchrotron operando X-Ray diffraction data, it appears that the active material presents large, non-stoichiometric domains while undergoing structural phase transformation. The corresponding extended limits of solubility are characterized by continuous variations in the metrics that mirror the entry of Na occupancy values into thermodynamically forbidden regions. A major consequence of this smoothed phase transformation is a significant decrease in the lattice volume mismatch, which could well compensate for the less efficient Na-based systems with respect to SEI and adverse effect of cation size in comparison to Li batteries. Comparison of the lattice volume mismatch on charge and discharge revealed an explanation for the asymmetry of the electrochemical curve.