Trace crystal water reinforces Mn−N bonds to achieve structurally stable sodium manganese hexacyanoferrates for sodium-ion batteries

Abstract

Sodium manganese hexacyanoferrates (MnHCF) featuring high specific capacity and low cost are highly promising cathode materials for sodium-ion batteries (SIBs). However, the structural instability of MnHCF severely limits its practical application in SIBs, while the controversial role of trace crystal water in stabilizing its framework further complicates performance optimization. In this work, the effects of varying crystal water content on the structure and electrochemical performance of MnHCF materials were systematically investigated. The results indicated that rhombohedral MnHCF with trace crystal water (R-MnHCF-W) exhibits superior structural stability compared to monoclinic MnHCF (M-MnHCF) with abundant crystal water and anhydrous MnHCF (R-MnHCF-0). The trace crystal water compensates for structural vacancies and strengthens Mn−N bonds, effectively mitigating Mn³⁺ dissolution. Specifically, the R-MnHCF-W cathode exhibits excellent electrochemical performances with a remarkable reversible capacity of 157.0 mAh g⁻¹ and maintains 79.6 % capacity retention after 100 cycles at 0.2 C. Notably, R-MnHCF-W would transform into M-MnHCF when exposed to moist air, and they can be reversibly restored to R-MnHCF through heat treatment, almost retaining the pristine electrochemical performance. This study elucidates the stabilizing role of trace crystal water and provides a strategic pathway for optimizing MnHCF cathodes.