Crystallographic-plane tuned Prussian-blue wrapped with RGO: a high-capacity, long-life cathode for sodium-ion batteries

Abstract

Designing Prussian blue with optimally exposed crystal planes and confining it in a conductive matrix are critical issues for improving its sodium storage performance, and will result in much improved sodium ion adsorption and diffusion, together with improved electron mobility. Here, we firstly illustrate through DFT simulations that the {100} lattice planes and [100] direction of the K_xFeFe(CN)₆ crystal are the preferred occupation sites and diffusion route for sodium ions. In addition, through coupling with RGO, K_xFeFe(CN)₆ electrodes exhibit better electronic conductivity. Accordingly, {100} plane-capped cubic K_0.33FeFe(CN)₆ wrapped in RGO was fabricated using a facile CTAB-assisted method. Due to the highly robust framework, higher specific surface area, greatly reduced number of lattice water defects and conductive RGO coating, K_0.33FeFe(CN)₆/RGO exhibits superior electrochemical performance in sodium-ion batteries. As a cathode, the RGO-coated K_0.33FeFe(CN)₆ yields an initial discharge–charge capacity of 160 mA h g⁻¹ at a rate of 0.5C, and an excellent capacity retention of 92.2% at 0.5C and 90.1% at 10C after 1000 and 500 cycles. Furthermore, XRD, DFT simulation, XANES and EXAFS verified that the structural changes during the Na-ion insertion–extraction processes are highly reversible. All these results suggest that {100} plane-capped K_0.33FeFe(CN)₆/RGO has excellent potential as a cathode for sodium-ion batteries.