Understanding the stabilizing effect of K+ on the sodium manganese hexacyanoferrate for sodium-ion batteries

The outstanding cycling performance of sodium manganese hexacyanoferrate (NaMnHCF) was obtained by applying NaPF₆/KPF₆ mixed salt solution as an electrolyte. NaMnHCF and KMnHCF are composited at the nanoscale to form a high-quality NaMnHCF/KMnHCF composite, enabling an exceptional cycling performance with more than 90 % capacity retention after 500 cycles. The enhanced cycling performance is attributed to the high-quality NaMnHCF/KMnHCF composition and the pillar effect of KMnHCF. Furthermore, the superlattice of NaKMnHCF was also discovered after treatment in the mixed electrolyte, which may be partly responsible for the acquired stability. The crystal structure of the cathode material remains stable during cycling, and the cathode-electrolyte interface is more resistant to corrosion in the NaPF₆/KPF₆ mixed electrolyte. Our work reports an effective strategy to achieve high-quality NaMnHCF/KMnHCF composites and articulates the mechanism of the stabilizing effect.