Structural regulation of Mn-based Prussian blue induced by zinc-substitution for enhanced sodium storage performance

Abstract

Mn-based Prussian blue (NMF) is a prospective choice for sodium-ion battery cathode materials because of its high theoretical specific capacity, high charge/discharge voltage plateau, cheap raw materials and eco-friendly. However, it undergoes serious capacity fading during cycling owing to the unstable Jahn-Teller distortion caused by the oxidation of Mn2+ to Mn3+. In this work, the structural regulation of NMF is achieved through zinc substitution, and the effect of structural adjustment caused by zinc substitution on the sodium storage properties of NMF is studied in detail. The experimental results verify that NZMF still maintain cubic structure with high content of sodium, less [Fe(CN)6] vacancies and low level of crystal water. Moreover, the four coordination of Zn-N is lower than six coordination of Mn-N, confining the nucleus growth, thereby reducing the powders size of NZMF. The smaller powder size of NZMF with higher surface area results in shortened sodium ion diffusion path and accelerated the charge transfer. In addition, the electrochemically inactive zinc plays a pillar role in maintaining its structural stability. Consequently, NZMF reveals better rate capacity and cycling stability than original NMF. In particular, its capacity can be maintained at 68.6 mAh g−1 under a high current density of 1600 mA/g, and the capacity retention is 76.5% at 800 mA/g after 1000 cycles. This work provides a new insight to improve the structural stability of Prussian blue.