| In this paper, the LiNixMn2-xO4, LiMxFe1-xO4, and LiFePO4materials weresystematically investigated by means of the density functional theory (DFT) methodusing a periodic model. We used the CASTEP computational code to perform thepopulation, magnetic moment, electron density difference, formation enthalpy andbonding situation electronic structural properties calculations for three system bymethod mentioned above. Meanwhile, further research the effect of electrode material’sgeometry, electronic state, and electrochemical performance for doping transitionmetal(Ni and Zr) and group(-OH), reveal the relationship between the structure andproperties of materials.The computed result shows that Li exists in the form of pure ion and has formedionic bonds with oxygen in the systems of LiNixMn2-xO4, LiMxFe1-xSO4F and LiFePO4.For the system of LiNixMn2-xO4, LiNi0.5Mn1.5O4material has a high embeddedvoltage that is4.65V after Hubbard is revised. It gives priority to resolve electrodematerial into oxidizing material in the working process. On the one side, Ni that isadulterated can reduce the thermodynamic stability of material relative to the element.On the other side, it will be conductive to improving the thermodynamic stability ofelectrode material relative to oxidizing material. The characteristic of d orbital issignificant near Mn and Ni ions, and the characteristic of p orbital is significant near Oion. Mn3dand Ni3dstates are formed into strong covalent bonds with O2p. In the anodematerial of LiNixMn2-xO4(x=0,0.5,1), LiNi0.5Mn1.5O4and Ni0.5Mn1.5O4are twoterminal structures with good cycling stability. As a result, comprehensive performanceof LiNi0.5Mn1.5O4materials is optimal in this system.For the system of LiMxFe1-xSO4F, Zr which is adulterated will reduce embedded voltage and increase thermodynamic stability to the material. Thus, it can be predictedthat transition metals of former3d/4d adulterated in the periodic table of elements willbe adverse to increase the voltage but improve thermostability for the material. However,-OH group that is doped will reduce the voltage and thermodynamic stability to thematerial. The electrical conductivity of LixFeSO4F and LixFeSO4OH (x=0,1) materialsis not good, but S3sand S3pwill have sp3hybridization and form a strong covalent bondwith O2pstate. There can be effective covalent bonds formd between Fe3dand O2p, F2pand OH2p.Jahn-Teller Effect can be produced in the systems of LiFeSO4F andLiFeSO4OH, but not between FeSO4F and FeSO4OH systems. However, since theyhave strong thermodynamic stability, the cycling performance of material is excellent.The electron distribution of covalent bonds will have influence on the calculated valueof atomic charge and magnetism in the system. The magnetism of LixFeSO4F andLixFeSO4OH systems is mainly from Fe ion.For the system of LiFePO4, electron distribution of covalent bonds will haveinfluence on the calculated value of atomic charge and magnetism in the system. In thesystems of FePO4å'ŒLiFePO4, the magnetism of chemical compounds is mainly from Feion. Since d electronic configuration is not only related to the voltage of system, butalso DOS and carrier concentration of material in Fermi energy level. Therefore, it’sfeasible to use theoretical method to predict the voltage and electrical conductivity ofmaterial. P3sand P3pwill have sp3hybridization, and strong covalent bonds whichstronger than Fe-O bonds are formed between O and P ions. Besides, orbital overlap issignificant between Fe3dand O2pstates, and the characteristic of these covalent bonds iscontributive to the thermodynamic stability of two terminal structures of LixFePO4. Theintroduction of counter cation (P) and the change of electron number of transitionmetals have a significant influence on the embedded voltage of material. |
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