First-principles approach to the structural, electronic and intercalation voltage of Prussian blue (KxFe[Fe(CN)6]) (x=1, 2) as potential cathode material for potassium ion batteries

Prussian blue (PB) is a good candidate as cathode material in potassium ion batteries (KIB) due to its high electrochemical performance. Thus, to verify the performance, the structural and electronic properties of PB were performed using first-principles studies based on the density functional theory (DFT) method. The properties of PB, KPB and K2PB were calculated using the Cambridge Serial Total Energy Package (CASTEP) computer code. From the geometrical optimization of pure PB, the generalized gradient approximation for Perdew-Burke-Ernzerhof (GGA-PBE) functional shows the most comparable structural properties compared to local density approximation by Ceperley and Adler as parameterized by Perdew and Zunger (LDA-CAPZ) and the generalized gradient approximation for Perdew-Burke-Ernzerhof for solids (GGA-PBEsol) functional. In addition, the electronic properties of the pure PB band gap is 0.72 eV which is slightly underestimated from the experimental value. Thus, the Hubbard U was used to broaden the bands crossing the Fermi level. The band gap using GGA-PBE + U is 1.77 eV, whereU for Fe3+ is 6 eV and Fe2+ is 4 eV. The calculations of the total and partial density of states (pDOS) present the Fe, C and N orbitals at the valence band and conduction band. Other electronic properties such as electron density were also calculated. The intercalation voltage with different numbers of K+ in PB is calculated to be 4.33 and 1.40 V for KPB and K2PB, respectively. It was found that the calculated voltage has been improved near the experimental value. Therefore, the first-principles calculation in this work can give more understanding of the behavior of pure PB, KPB and K2PB for its uses as cathode material in KIB.