Mitigating structural deterioration via partial substitution with Fe in Mn-based Prussian White cathodes for Na-ion batteries

Abstract

Prussian White (PW) is a promising cathode material for sodium-ion batteries (SIBs), which offers high theoretical capacity at low cost while being composed of abundant elements. However, Fe-based PW suffers from relatively low average voltage, and while substitution of one Fe with Mn can increase the voltage, it introduces structural instabilities supposedly due to the Jahn–Teller distortion of Mn³⁺, resulting in poor capacity retention. In this work, we investigate the effect of partial substitution of Mn by Fe in Mn-based PW on electrochemical and structural stability. Two samples, Na_1.82Mn[Fe(CN)₆]_0.96·2.0H₂O and Na_1.82Mn_0.62Fe_0.38[Fe(CN)₆]_0.96·2.1H₂O, were synthesized by coprecipitation and characterized via a multimodal approach combining laboratory and synchrotron-based techniques. We demonstrate that partial Mn substitution by Fe does not eliminate Jahn-Teller distortion but mitigates its negative effects by suppressing the distorted phase formation occurring near end of charge, thereby significantly enhancing capacity retention without sacrificing overall capacity. These results provide fundamental insight into the interplay between redox activity and structural stability in mixed-metal PW and establish partial Mn substitution by Fe as an effective strategy to improve long-term cyclability of PW cathodes for SIBs.