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.

Automated summary

In this study, a core-shell structured NaMnHCF cathode material was successfully synthesized using a coating strategy. The core-shell NaMnHCF exhibited improved electrochemical performance compared to the pristine material. The Cs-rich surface layer played a crucial role in enhancing the structural stability of NaMnHCF.