Material Modification of Copper Hexacyanoferrates As Cathode Material for Aqueous Sodium Ion Batteries

Aqueous sodium ion battery (ASIB) is gaining more attention with the growing concern in safety and environmental issues. However, the potential window of ASIB is limited to the decomposition voltage of water. Recently, the potential window of aqueous electrolyte has been reported that it can be extended in high-concentration electrolytes. Since then, researchers are dedicating to finding cathode materials with higher discharging potential which can increase the energy density of aqueous batteries. In this study, we demonstrate a new concept in cathode material design to reach an ultra-high rate capability by modifying the side group of a Prussian blue analogue, copper hexacyanoferrates (CuHCF). The modified cathode material was carefully analyzed by ex-situ XRD measurements, X-ray photoelectron spectroscopy, cyclic voltammetry (CV), and charge-discharge measurement to reveal their electrochemical performances. The diffusivity was also calculated by potentiostatic intermittent titration technique test and Randles-Sevcik equation. The results showed a great rate capacity for more than 80% from 1 C to 20 C accompanied with an outstanding cycle stability (~ 100% after 1000 cycle at 20 C). The high discharge plateau at about 0.7 V (vs Ag/AgCl) indicates that this material is promising in high-energy ASIB applications. This Prussian blue analogue cathode material is further tested in a full cell system with NaTi 2 (PO 4 ) 3 as anode material. This new concept may shed light on the field of increasing cycle stability and rate capability of Prussian blue analogues.