First-principles Study Of Prussian Blue Analogues Of Cathode Material For Sodium Ion Batteries

Due to the abundant of material and low cost,Na-ion batteries have become a potential secondary battery in the field of large-scale energy storage.However,because of the atomic radius of sodium ions is bigger than that of lithium ions,many commercially available electrode materials for Li-ion batteries cannot be directly applied to Na-ion batteries.Therefore,the development of electrode materials for Na-ion batteries with excellent performance is significant for promoting the market application of Na-ion batteries.Recent years,Prussian blue analogues(PBA)NaxMFe(CN)6 have received extensive attention from scientific researchers owing to the admirable properties.However,it has been noted that experimentally prepared PBA will introduce a large number of defects,such as vacancies and lattice water,which not only brings great difficulties to material characterization,but also hinders our understanding of its intrinsic properties.Therefore,based on first-principles method,this work studies the characteristics of PBA NaxMFe(CN)6 compounds as cathode materials for Na-ion batteries,by considering different transition metal elements M to determine their intrinsic properties and performance.The main results are as follows:1.The structural stability,thermal effect and electrical properties of NaxMFe(CN)6 compounds(M=Fe,Cr,Mn,Sc,Zn,V,Ti,Co,Ni,Cu)as a cathode material for Na-ion batteries during charging and discharging process have been studied.The calculation results show that the volume change of NaxMFe(CN)6 compounds during the inserting/extracting sodium ions is closely associated with the Jahn-Teller effect of the framework structure coupled by the chemical interaction of Na-M atom pairs.If this effect is notable,it will lead to severe deformation in cyanide bridge.The voltage plateau depends on the energy of the NaxMFe(CN)6 compounds related to Na ions,that is,the difference in binding energy between two consecutive states or the change in thermal effect.The gradual decrease of the binding energy will retard the decay of the voltage platform during discharging.By considering the temperature effect,we use the thermal expansion coefficient to obtain the change in the lattice volume of the battery electrode material caused by the temperature change in the course of service.2.The diffusion path and barriers of Na ions as well as electronic structures,such as density of states and band structure of the NaxMFe(CN)6 compounds(M=Fe,Cr,Mn,Sc,Zn,V,Ti,Co,Ni,Cu)as the cathode material of Na-ion batteries during the charging and discharging process has been studied.By considering different diffusion mechanisms,we found that the diffusion of Na ions in the NaxMFe(CN)6 compounds are dominated by single-atom diffusion mechanism,and the difficulty of diffusion and the specific path are strongly related to the type of M element.The calculation of the band gap shows that the ability of electron mobility will change with the content of Na ions,and the type of M element plays an essential role.