Mixed copper-zinc hexacyanoferrates as cathode materials for aqueous zinc-ion batteries

Aqueous rechargeable metal-ion batteries have become potentially advantageous for the integration of renewable energy sources into the electric power grid thanks to their high rate capability, low cost, environmental friendliness, and intrinsic safety. In this work, we tried to improve the electrochemical stability of CuHCF and prevent/postpone its aging upon cycling. At first we investigated the phase transformation occurring in CuHCF during intercalation of zinc using XRD, SEM and EDX. We observed that large particles are formed upon cycling, which are depleted from copper and are zinc- or iron-rich. In order to prevent this, we modified the CuHCF structure by partially substituting its transition metals with zinc ions during synthesis. We observed that CuZnHCF mixtures with Cu:Zn ratios of 93:7 exhibited an excellent cycle life up to 1000 cycles, with improved specific charge retention with respect to its CuHCF counterpart. Also in the case of CuZnHCF mixtures the formation of large particles upon cycling is observed, but less extended as in pure CuHCF. It appears that different morphologies of the particles show different compositions in term of zinc, iron and potassium.