Enhanced performance of core–shell structured sodium manganese hexacyanoferrate achieved by self-limiting Na+ –Cs+ ion exchange for sodium-ion batteries

Sodium manganese hexacyanoferrate (NaMnHCF) is a promising cathode material for sodium-ion batteries (SIBs) due to its low cost and high energy density. The Jahn–Teller effect of Mn, however, leads to the poor structural stability of NaMnHCF, resulting in undesired electrochemical performance. Herein, we developed a novel coating strategy and obtained a core–shell structured NaMnHCF through facile Na+–Cs+ ion exchange, which naturally produced a robust and insoluble Cs-rich surface layer (CsMnHCF) with several nanometers in thickness on pristine NaMnHCF. It is shown that the Cs-rich surface plays a positive role in the stability of the NaMnHCF structure by prohibiting the leakage of crystal water, stabilizing the solid–liquid interfaces, and solidifying crystal structure. The electrochemical performance of the core–shell NaMnHCF is dramatically improved with a discharge capacity of 76.3 mAh·g−1 after 1000 cycles at 1.0C and a reversible capacity of 87.0 mAh·g−1 at 10.0C, which is much superior to that of the pristine NaMnHCF with only 26.6 mAh·g−1 after 400 cycles and 31 mAh·g−1 at 10.0C. This work reports a new method for the synthesis of core–shell NaMnHCF and provides a novel perspective for the development of advanced NaMnHCF cathode for SIBs.Graphical abstract