Mechanistic Origin of Capacity Limitation in Sidorenkite‐Type Na <sub>3‐</sub> <i> <sub>x</sub> </i> Fe(PO <sub>4</sub> )(CO <sub>3</sub>
New classes of materials are required for sodium ion batteries (SIBs) to compete with their Li‐counterpart. Sidorenkite‐type carbonate‐phosphate cathodes, such as Na 3‐ x Fe(PO 4 )(CO 3 ), were predicted to deliver high capacities based on two electron reactions. However, experiments consistently only ac...