Ion-exchange surface modification enhances cycling stability and kinetics of sodium manganese hexacyanoferrate cathode in sodium-ion batteries

Sodium manganese hexacyanoferrate (NaMnHCF) with low cost and high energy density is a promising cathode in Na-ion batteries (NIBs); however, the Jahn-teller effect of Mn3+ and large amounts of Fe(CN)64− defects significantly reduces the cycling life of the battery. Here, we introduce a chemically less soluble and structurally more stable layer to the NaMnHCF surface with a simple ion-exchange method to mitigate the irreversible structural change and side reactions associated with both the low-spin FeII/FeIII and the high-spin MnII/MnIII redox. The NaMnHCF cathode treated in Cu2+ solution shows much better cycling stability (80 vs. 30 mA h g−1 after 1000 cycles at 1 C) and rate performance than the unmodified NaMnHCF. The as-proposed chemical ion-exchange is a well-compatible and low-cost technology to realize surface modification of NaMnHCF and other hexacyanoferrates, which is of great significance for the development of high-performance NIB cathodes.