First-principles Studies Of Cathode Material LiMPO 4 (M=Mn,Fe) Of Lithium Ion Batteries

The first-principles PAW pseudopotential method within the generalized gradient approximation(GGA) based on the density functional theory(DFT) is employed to investigate mainly the following three questions.1:The geometry and electronic structure of the bulk cathode material LiMnPO₄ have been studied by first principles calculations. The crystal structure, density of states, band structure and valence electron charge density contours has been shown. On one hand, we want to obtain the theory lattice constant and electronic structure; on the other hand, the PAW+(GGA+U) method will be justifed by comparing with results attained by other theoretical or experimental methods. It is found that the results obtained by our calculations are in good agreement with other theoretical results and experimental datas.2:The Jahn-Teller effect of Li_xMnPO_<sub>4(x=0,0.5) and the Jahn-Teller effect to the conduction of Li_xMnPO₄ are studied through first principles calculation for the first time. The result show that Li_0.5MnPO₄ and MnPO₄ have a strong Jahn-Teller effect, the valence of Mn atom change from bivalence to trivalence when the battery is charging, the Mn atom lose an electron which is initial localized at the Mn-3d(x²-y²)orbital, the band lap of Li_0.5MnPO₄ is the smallest compare to LiMnPO₄ and MnPO₄. Although the Jahn-Teller effect have a disadvantage effect to the structural stability of Li_xMnPO₄, but the Jahn-Teller effect in LixMnPO4 is helpful to the electrical conductive performance.3:The first-principle calculations based on the denstiy functional theory also employed to investigate crystal structure and electronic structure of LiFePO4 and its two doped materials LiMnyFe_1-yPO₄ and LiFePFzO_4-z . The result show that although the cation coped method don't improve the electronic conductive of LiFePO₄, but can improve the intercation voltage of material. The intercation voltage of LiMnyFe1-yPO4 is different when the value of y is different, the value of intercation voltage increases when the value of y increases; the property of coped model is better compared to LiFePO₄ when the value of y is 0.75. While in the anion coped model, although the crystal structure was distorted to some extent, but it can obviously improve the electronic conductive of LiFePO₄.