K+ for Na+ guest-ion substitution in nickel hexacyanoferrate enables high-rate aqueous sodium-ion storage via thermodynamic defect suppression

Abstract

Nickel hexacyanoferrate (NiHCF) is a promising cathode material for high-rate aqueous sodium-ion storage, but its rate capability and specific capacity are hindered by water incorporation and ferrocyanide vacancies formed during synthesis. This study demonstrates that these defects can be suppressed by controlling the alkali guest ion (Na+ vs. K+) and chelating agent (citrate) concentration. K+ incorporation under low-citrate conditions reduced water content by 48% compared to Na+, while Fe vacancy remained constant. Under high-citrate conditions, water content and Fe vacancies decreased by 73% and 43%, respectively. The choice of guest ion strongly influences defect suppression when sufficient chelating agent is present. Density functional theory calculations exhibited that water insertion and ferrocyanide vacancy formation are suppressed in K+-substituted NiHCF, consistent with experimental results. NiHCF synthesized with K+ exhibited a rate capability of 82% at current density of 10.0 vs. 0.1 A g(-1), outperforming the 69% of the Na+-intercalated sample. Specific capacity increased from 72 to 80 mAh g(-1) and cycle stability improved from 77% to 83%, reflecting the less water content and reduced ferrocyanide vacancies. These findings underscore that strategic selection of guest ions during synthesis is an effective pathway for improving the high-rate aqueous Na-ion storage behavior of NiHCF cathodes.