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

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_2.7Fe_1.7(SO₄)_2.9(PO₄)_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.