Regulation of low-spin Fe of Mn-iron hexacyanoferrate for boosted potassium ion storage performance

Abstract

K2Fe[Fe(CN)6] (KFHCF) is promising cathode material for potassium-ion batteries (PIBs). However, it suffers from limited capacity, poor cycling, and rate capability during cycling. Here, the low-spin Fe species coordinated with carbon atoms from cyanogen ligands (FeLS-C) are regulated and activated by manganese (Mn) substitution. The regulation is systematically investigated through theoretical simulation and experiments. The original electron configuration (t2g6eg0) of FeLS-C is changed with appropriate Mn-substitution (t2g3eg2), which improves the utilization of FeLS-C, accelerates the redox charge transfer and enhances the electrical conductivity of the material. The resultant K2Mn0.1Fe0.9[Fe(CN)6] (KMFHCF-0.1) delivers an improved reversible capacity of 135 mAh·g−1 at 100 mA g−1 while the capacity of pure KFHCF is only 118 mAh·g−1. It also demonstrates excellent rate performance (76 mAh·g−1 at 800 mA·g−1) and long cycling stability (0.3% capacity degradation per cycle over 200 cycles). Ex-situ measurements verify that the cathode undergoes a highly reversible solid solution process during K-ions insertion/extraction. The full cells with hard carbon anodes demonstrate a high reversible capacity of 110 mAh·g−1 and considerable energy density of 275 Wh·kg−1 as well as excellent cyclability and favorable rate performance. This work expands the design pathways for Prussian blue analogs cathode materials.