Na‐Site Energetics Reconfiguration Enables Full Capacity Utilization of Sodium Iron Sulfate Cathodes

ABSTRACT Sodium iron sulfates are promising high‐voltage cathodes for sodium‐ion batteries, but their practical performance is hindered by incomplete utilization of structurally available Na + and sluggish electronic transport. Here, we demonstrate a mixed‐polyanion electrostatic modulation strategy that reconfigures the Na‐site energy landscape of the sulfate‐based framework. This chemical regulation drives a fraction of Na + into the kinetically favorable Na2/Na3 sites to increase Na + accessibility, suppressing occupancy of the Fe–Na‐disorder‐prone Na4 site, and facilitating the subsequent extraction of trapped Na1 sites. Coupled with an electronic percolation network, the resulting cathodes deliver 100 mAh g − 1 at 0.1C in the hard carbon||Na 2.7 Fe 1.7 (SO 4 ) 2.9 (PO 4 ) 0.1 full cell, approaching its theoretical capacity of 102 mAh g − 1 , together with capacity retention of 90% at 5C after 1500 cycles. This work reveals hetero‐anion substitution as a site‐energy reconfiguring strategy for unlocking reversible Na + storage in kinetically constrained polyanion cathodes.