First principles study on properties and voltages of cathode material nickel doped prussian blue for sodium ion battery / Noor Atiqah Md Nasir

Within this work, first principles techniques have been applied to understand on the fundamental knowledge of cathode material Prussian blue (PB), or iron hexacyanoferrate on structural properties, electronic properties, and voltages for sodium-ion batteries (SIBs). Sodium is an alternative choice in term of large-scale energy storage as it is having similar physical and chemical properties to lithium, other than having abundance resource and cost friendly. All the calculations are performed using density functional theory (DFT) through the computer simulation CASTEP computer code. The pure crystal structures of PB are optimized using different exchange-correlation functional named LDA-CAPZ, GGA-PBE, and GGA PBEsol. GGA-PBE functional is found to be the best functional as it exhibits the most accurate values for lattice parameter, thus is chosen for further calculations. Moreover, the effects of DFT plus Hubbard U (DFT+U) are explored especially on the electronic properties of PB as the transition metals are shown in the material. The band gap for PB is increases from 0.26 eV o 1.76 eV and is proved that Hubbard U correction improved the electronic band gap close to the experimental result. The effects of different number of Na ion intercalation on PB has also been studied. The open cubic framework structure of PB makes the intercalation/deintercalation of Na ion fast and easy although the radius of Na is bigger than Li. Based on the finding, NaFeFe(CN)6 (Na1PB) display higher voltage than Na2FeFe(CN)6 (Na2PB) with 4.21 V compared to 3.01 V which is compliant with the experimental value. Furthermore, the effects of Ni ion substitution on N-coordinated Fe ion into the Prussian blue analogues (PBAs) with stoichiometry Na2NixFe1-xFe(CN)6 at x = 0.01, 0.03, 0.05, 0.10, and 0.15 are investigated. The partial DOS shows the presence of Ni ion among the PBAs atoms. The results of Ni-doped PBA show the increasing in structural stability and producing higher voltage compared to non-doped PBA. The voltages are increase to 3.23 V, 3.27 V, 3.45 V, 4.32 V and 5.65 V with increasing number of Ni substitution. The first principles study in this work can clarify the doping effects in PBA and may improve understanding of PB as cathode material for SIB.