Intercalation Pseudocapacitance Boosting Ultrafast Sodium Storage in Prussian Blue Analogs.

Great expectation is placed on sodium-ion batteries with high rate capability to satisfy multiple requirements in large-scale energy storage systems. However, the large ionic radius and high mass of Na+ hamper its kinetics in the case of diffusion-controlled mechanisms in conventional electrodes. In this study, a unique intercalation pseudocapacitance has been demonstrated in low-vacancy copper hexacyanoferrate, achieving outstanding rate capability. The minimization of the [Fe(CN)6 ] vacancy enables unhindered diffusion pathways for Na+ and little structural change during the Fe2+ /Fe3+ redox reaction, eliminating solid-state diffusion limits. Moreover, the Cu+ /Cu2+ couple is unexpectedly activated, realizing a record capacity for copper hexacyanoferrate. A capacity of 86 mAh g-1 is obtained at 1 C, of which 50 % is maintained under 100 C and 70 % is achieved at 0 °C. Such intercalation pseudocapacitance might shed light on exploiting high-rate electrodes among Prussian blue analogs for advanced sodium-ion batteries.