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

Abstract

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

摘要

亚铁氰锰钠 (NaMnHCF) 以其低廉的成本, 较高的能量密度, 成为有潜力的钠离子电池正极材料。 然而, 由于锰的Jahn–Teller效应, 亚铁氰锰钠结构稳定性较差, 这导致较差的电化学性能。 本研究中, 我们采用简易的Na⁺–Cs⁺离子交换方法, 在亚铁氰锰钠表面制备出富Cs壳层, 形成核壳结构的材料。 表征分析显示, 富Cs壳层通过抑制水的脱出, 稳定颗粒-电解液界面和强化晶格结构来稳定亚铁氰锰钠的结构。 电化学测试表明, 核壳结构的亚铁氰锰钠在1.0C的电流密度下, 循环1000周后仍然可以提供76.3 mAh·g^-1的比容量, 相比于原始亚铁氰锰钠在400周后只能提供26.6 mAh·g^-1的比容量, 包覆后的样品循环性能大幅度提升。 除此之外, 在10.0C的电流密度下, 核壳结构的亚铁氰锰钠可以表现出87 mAh·g^-1的可逆容量, 而原始样品只表现出31 mAh·g^-1的可逆容量。