Interfacial Anodic Stability of Vanadium Carbide MXene for Enhanced Calcium-Ion Storage Capacity of Nickel Hexacyanoferrate

Prussian blue analogues (PBAs) are a versatile class of open metal-organic framework materials with large intrinsic channels for accommodating ions of different sizes and charges. However, the inherent sluggish reaction kinetics and low conductivity of PBAs limit charge storage capacities and rate performance. In this work, we report on the vanadium carbide-nickel hexacyanoferrate (V₂CT_<i>x</i>-NiHCF) composite electrodes with calciation charge storage capacity up to 140 mAh g^-1 in concentrated (5 M) calcium-ion electrolyte, that is an order of magnitude higher than in dilute (1 M) electrolyte at a current density of 0.5 A g^-1. The suppressed activity of water in the concentrated electrolyte enables anodic stability of V₂CT_<i>x</i> MXene while unlocking the redox storage capacity of both V₂CT_<i>x</i> and NiHCF. Further, the rate performance of V₂CT_<i>x</i>-NiHCF composite electrode is found to be 50% superior to that of the NiHCF electrode in 5 M calcium(II) bis(trifluoromethylsulfonyl)imide [Ca(TFSI)₂] electrolyte. The charge storage dynamics of V₂CT_<i>x</i>-NiHCF composite electrodes are analyzed using 3D Bode analysis. Ex situ Fourier transform infrared (FTIR) studies confirm the electrochemical stability of cyano bridges of NiHCF during continuous insertion/deinsertion of high charge density Ca-ions. This study opens avenues for the design of high capacity MXene based hybrid electrodes in concentrated multivalent metal ion electrolytes.