A Dual‐Ion Battery with a Ferric Ferricyanide Anode Enabling Reversible Na+ Intercalation

Dual‐ion batteries have been developed as a promising technology in recent years, but their high self‐discharge rate leading to low coulombic efficiency (CE) limits their practical applications. This work provides a deft strategy to circumvent this issue by using FeFe(CN)6 as the anode material for hosting Na+ cations, in combination with a graphite cathode for accommodating the bis(trifluoromethanesulfonyl)imide anions (TFSI−). The relatively high bonding force between FeFe(CN)6 and Na+ can hinder self‐extraction of ions from the electrodes, thereby decreasing the self‐discharge rate. The FeFe(CN)6 nanospheres, synthesized by a facile solution reaction method, are well crystallized and dispersed. Na+ insertion into the FeFe(CN)6 cube is determined by cyclic voltammetry (CV), galvanostatic charge–discharge, and X‐ray diffraction tests, suggesting a reversible process. Under a current of 0.05 mA cm−2 the batteries present an acceptable discharge plateau within 1.5–0.7 V and a well‐defined capacity of 75.0 mAh g−1, with a high CE above 98.5% (±0.1%); under 0.2 mA cm−2, cells display a high cyclability of 83.0% capacity retention for 100 cycles, with an excellent CE exceeding 99.6% (±0.1%). Moreover, the batteries exhibit a low self‐discharge rate with a resting capacity decay of 0.32% h−1, outperforming many of the reported dual‐ion cells. A FeFe(CN)6 anode for reversible Na+ intercalation in dual‐ion cells is developed. Due to the relatively high bonding force between Na+ and FeFe(CN)6 lattice, the self‐discharge process, caused by the self‐deintercalation of ions from the electrodes, can be suppressed, and therefore the cells achieve a high coulombic efficiency >98.5%, outperforming most of the dual‐ion cells.