K_xCo_1.5-0.5xFe(CN)₆/rGO with Dual-Active Sodium Ion Storage Site as Superior Anode for Sodium Ion Battery

The unique and open large frame structures of prussian blue analogues (PBA) enables it for accommodating a large number of cations (Na⁺, K⁺, Ca²⁺, etc.), thus, PBA are considered as promising electrode materials for the rechargeable battery. However, due to the chemical composition, there are still many alkaline metal ions in the gap within the framework, which puts multivalent metals in PBA in a low valence state and affects the sodium storage performance. To improve the valence of metal ions in PBA materials, precursors prepared by co-precipitation method and hydrothermal method are used to synthesis K_xCo_1.5-0.5xFe(CN)₆ through further chemical oxidation. Through the introducing of reduced graphene oxide (rGO) with excellent conductivity by a simple physical mixing method, the cycle stability and rate performance of the PBA material can be further improved. The K_0.5Co_1.2Fe(CN)₆·2H₂O/rGO anode prepared with 2 h hydrothermal time and further chemical oxidation, named as KCoHCP-H2-EK/rGO, exhibits a super electrochemical performance, delivering initial charge/discharge capacities of 846.7/1445.0 mAh·g^-1, and a capacity retention of 58.2% after 100 cycles at a current density of 100 mA·g^-1. The KCoHCP-H2-EK/rGO outstanding electrochemical behaviors are attributed to the unique dual-active site structure properties and the improved surface conductance of materials by rGO components.