Core-Shell Hexacyanoferrate for Superior Na-Ion Batteries
Authors: Wan Min 
Published: February 2020

Core-shell Hexacyanoferrate for Superior Na-ion Batteries

Min Wan, Wuxing Zhang*, Yunhui Huang*

Introduction

In recent years, sodium ion batteries (SIBs) have received increasing research attentions as alternative for lithium ion batteries in the application of large-scale electric energy storage (EES) 1-4. SIBs have the advantages of high sodium abundance and low cost, and plenty of materials are investigated for sodium ion storage, such as layered oxides and tunneled oxides 5-10. Except these, Prussian blue and its derivatives (PBAs) with the merits of high specific capacity, facile synthesis and low cost, are also identified as promising electrode materials for SIBs 11-17.
PBAs usually have a face-centered cubic structure: A
x
M1[M2(CN)6]
y
□1-y
•nH2O (0<x<2, 0<y<1), in which A represents alkali cations, M transition metal ions, and □the vacancies of [M2(CN)6] occupied by coordinated H2O18.The 3D open framework of PBAs can provide large interstitial sites for fast Na+ insertion/extraction. Among these PBAs, sodium iron hexacyanoferrate (A=Na, M1=M2=Fe, Fe-HCF) has two electrochemically active sites, which leads to a high theoretical specific capacity of 170 mAh g-1
19. However, Fe-HCF always suffers from low coulombic efficiency and poor cycling stability due to the lattice vacancies in the crystal structure and the side reactions between Fe-HCF and electrolyte 20-22.
In order to solve these drawbacks of Fe-HCF, it is desirable to synthesize high-quality Fe-HCF with few lattice vacancies via a single iron source method. We propose a facile co-precipitation method to synthesize core-shell structured Fe-HCF@Ni-HCF with outstanding electrochemical performance as cathode material for SIBs. The high-quality Fe-HCF@Ni-HCF integrates the advantages of both Fe-HCF and Ni-HCF, which exhibits high specific capacity, excellent cycling stability and promoted rate performance. It retains 78 % capacity at 200 mA g-1 after 800 cycles and delivers an average coulombic efficiency of approximately 100 % during cycling. The result further proves that Fe-HCF is a promising cathode material in the practical application of SIBs.
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