Mechanistic Origin of Capacity Limitation in Sidorenkite‐Type Na 3‐ x Fe(PO 4 )(CO 3 ) Carbonophosphate Cathodes for Na‐Ion Batteries

ABSTRACT 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 access one of the predicted two equivalents of Na, leaving the origin of this limitation unresolved. Here, we establish the mechanistic basis for this discrepancy. We show that carbonate stability and Na + mobility are not performance limiting bottlenecks. Instead, the rigid polyanionic framework destabilizes d 4 ‐configured transition metal centers (e.g., Fe 4+ ), generating lattice strain and suppressing electronic conductivity at high states of charge. Validation through isostructural Mn substitution (d 4 Mn 3+ ) confirms this instability as the root cause for the limited capacity. These insights convert theoretical predictions into actionable design rules: maintain crystallinity to protect carbonate integrity and avoid electron configurations prone to Jahn–Teller distortion (d 4 ) via multi‐element substitution. This work provides a clear pathway for advancing high‐energy sodium‐ion cathodes based on carbonophosphate chemistry.