Ion-selective copper hexacyanoferrate with an open-framework structure enables high-voltage aqueous mixed-ion batteries

Abstract

Non-flammable and low-cost aqueous batteries operating on “M⁺/N⁺-dual shuttles” (AMIB) offer great opportunities in large-scale utility grid applications. Here, for the first time, we demonstrate a series of high-voltage AMIB (>1.23 V) based on open-framework copper hexacyanoferrates (CuHCFs) as cathode materials, and TiP₂O₇ & NaTi₂(PO₄)₃ as anode materials. Among them, CuHCF/NaTi₂(PO₄)₃ possesses high power density as an ultra-capacitor (3006 W kg⁻¹), but with a higher energy density (56 W h kg⁻¹). Through multiple characterization techniques combined with ab initio calculations, the intercalation chemistry of alkali cations in CuHCF is revealed. With increasing ionic size, the most stable interstitial site changes from the face-centered site (24d) to the body-centered site (8c). The intercalation voltage for the alkali cations follows the order: K⁺ > Na⁺ > Li⁺. Meanwhile, the ion-selectivity among them follows the same order as the intercalation voltage, which accounts for the higher voltage outputs of AMIB in contrast to aqueous batteries operating on one shuttle.