Escape of lattice water in potassium iron hexacyanoferrate for cyclic optimization in potassium‐ion batteries

Abstract

AbstractPotassium iron hexacyanoferrate (Prussian blue [PB]) is a very competitive cathode for potassium‐ion batteries due to its 3D robust open framework. However, [Fe(CN)6]4− vacancies and lattice water existed in PB lattices aggravate electrochemical performances. Herein, PBs with different content of vacancies and lattice water are obtained under two synthesis temperatures of 0°C and 25°C. Although K1.36Fe[Fe(CN)6]0.74·0.48H2O (PB0) exhibits an outstanding rate capability compared with K1.43Fe[Fe(CN)6]0.94·0.42H2O (PB25), PB25 with less defects shows a lower polarization and superior stability than PB0 during the cycle. Fourier transform infrared (FTIR) spectra results show that lattice water can escape from PB lattices during the cycle, which enhances the diffusion of K+ kinetically in the PB structure. Benefited from this phenomenon, the diffusion coefficient of K+ in vacancy‐less PB25 reaches 10−8 in two reaction platforms. As potassium‐ion battery cathodes, PB25 displays higher capacity retention of 86.5% over 1000 cycles at 5 C than PB0 with 20.1% capacity retention over 600 cycles. This study provides a new understanding of [Fe(CN)6]4− vacancy and lattice water behavior in K‐containing PB structure.