Inner Helmholtz layer control through co-solvent strategies for high-performance copper hexacyanoferrate//zinc battery

Copper hexacyanoferrate (CuHCF) demonstrates high working voltage, convenient synthesis methods, and economic benefits. However, capacity decay of CuHCF//Zn full cells is usually observed in aqueous electrolytes due to the dissolution of Cu and Fe, as indicated by the irreversible insertion of Zn²⁺ ions and the consequent formation of Zn_xCu_1-xHCF. To address these challenges, a cathode-oriented electrolyte engineering design employing a methyl acetate (MA) co-solvent with zinc triflate (Zn(OTf)₂) salt electrolyte is implemented. The MA co-solvent can inhibit the activity of free water, thereby significantly alleviating element dissolution. Moreover, MA, with its low dielectric constant, can reshape the species distribution at the inner Helmholtz layer on the cathode surface, further inhibiting the dissolution of the active material. This novel co-solvent electrolyte system enhances the performance of the CuHCF//Zn full cell, resulting in 100 % capacity retention for 1700 cycles over 2100 h at 0.1 A g^-1. At a low temperature of -50 °C, the cell can retain half of its theoretical capacity for 1200 cycles in the long-cycling test without any capacity fading.