Enhancing Structural and Cycle Stability of Prussian Blue Cathode Materials for Calcium-Ion Batteries by Introducing Divalent Fe

Abstract

Calcium-ion batteries (CIBs) have been of interest for rechargeable batteries due to their high energy density and fast ion diffusion in liquid electrolyte originated from the divalence and low charge/radius ratio of Ca ion. However, exploring high performance cathode materials for CIBs is very challenging. In this work, divalent Fe ions are introduced into copper hexacyanoferrate (CuHCF) to construct a new Prussian blue cathode material rich of Fe 2+ by using K 4 Fe(CN) 6 as the precursor instead of K 3 Fe(CN) 6 . The Fe 2+ ions at low-spin states can improve the structural stability of CuHCF during Ca ions extraction and insertion processes effectively. It is found that the lattice parameter change of CuHCF is only 0.23% during charge and discharge, much less than the CuHCF with Fe 3+ . X-ray absorption spectroscopy indicates that the charge compensation of CuHCF(Fe 2+ ) is mainly contributed by Fe 2+ /Fe 3+ redox couple. The octahedral distortion in CuHCF is also suppressed effectively. As a result, the CuHCF(Fe 2+ ) cathode can deliver a reversible capacity of 54.5 mAh g -1 at 20 mA g -1 with a high capacity retention of 90.43% after 1000 cycles.

Keywords: calcium-ion batteries, Cathode materials, Prussian blue, cycle stability, X-ray absorption