The Pillar Effect of Large‐Size Alkaline Ions on the Electrochemical Stability of Sodium Manganese Hexacyanoferrate for Sodium‐Ion Batteries

Abstract Sodium manganese hexacyanoferrate (NaMnHCF) is an attractive candidate as a cathode material for sodium‐ion batteries due to its low cost and high energy density. However, its practical application is hindered by poor electrochemical stability caused by the Jahn‐Teller effect of Mn and the unstable structure of NaMnHCF. Here, this paper aims to address this issue by introducing highly stable AMnHCF (where A = K, Rb, or Cs) through a facile method to composite with NaMnHCF. The findings reveal that all AMnHCFs have a “pillar effect” on the crystal structure of NaMnHCF. It is observed that the degree of pillar effect varies depending on the specific AMnHCF used. The less electrochemically inactive the alkaline ion is and the greater the degree of compositing with NaMnHCF, the more dramatic the pillar effect. KMnHCF shows limited pillar effect due to its rough composition with NaMnHCF and the loss of K⁺ upon (de)intercalation. RbMnHCF has lower electrochemical activity and can be better composited with NaMnHCF. On the other hand, CsMnHCF exhibits the strongest pillar effect due to the inactivation of Cs⁺ and the excellent coherent structure formed by CsMnHCF and NaMnHCF. This research provides a new perspective on stabilizing NaMnHCF with other alkaline elements.