Study On Quantum Chemistry And Electrochemistry Of Cathode Materials

Lithium iron phosphate (LiFePO₄) is a cathode material for Lithium ion battery. This material is non-toxic and environmentally friendly. It has a relatively high theoretical capacity and good circulation performance and so on the merits, so it becomes the research hot spot. The present research of LiFePO₄ concentrates on preparation and character. The research on theory analysis is very few, and there are almost no reports at home. So the study of LiFePO₄ by quantum chemistry theoretical calculation has significance, it can provide the theory basis to know the structure and the performance. On the other hand, electrolytic manganese dioxide has got wide use in Zn/MnO₂ battery as cathode material, and also has certain application prospect in the lithium ion battery. But the electrochemical performance research in neutral solution is rare, therefore the study is still extremely essential.In this dissertation two models (8LiFePO₄ and FePO₄) with 56 and 48 atoms have been calculated with Gaussian03W software. According to calculation speed and the characteristics of the model, the method B3LYP and the basis 4-31G/6-31G* are finally determined respectively. This calculation can fairly explain some experiments results. Total energy, electric charge distribution, atomic overlap population and the density of states can been obtained though calculation. The results have shown that the total energy of lithium iron phosphate material decreases after intercalation lithium, which makes system more stable; lithium has been existing as ionic state in lithium iron phosphate oxides; interaction forces between Li-O atoms is weakest, it helps lithium ion to move freely in crystal lattice. Interaction forces between P-O atoms is strongest, it inserts lithium ion to leave; the average voltages for Li\LiFePO₄ have been calculated by total energy change between non-intercalated and intercalated models. The calculated average voltage is 3.2V, it is basically agreement with 3.4V that experimentally observed; density of state (DOS) reveals that both LiFePO₄ and FePO₄ are typely semiconductors, O atom orbit mainly contributes to the highest valence band near to the Fermi level, Fe atom orbit mainly contributes to the lowest conduction band in LiFePO₄.The contrast research to EMD in the different neutral solution and in the MgSO₄ solution electrochemistry behavior is also studied by microelectrode technology, cyclic voltammetry, A.C. impedance and XRD in this paper. The results prove that there is a special electrochemical performance in MgSO₄ solution of EMD. Cyclic voltammetry behaviors show that magnesium ion impact on the redox process of EMD, new and electrochemical activity substance is found when reduction, it is reversible. AC impedance measurements reveal that the electrode process of EMD is controlled by electrochemical process and diffusion process in MgSO₄ solution. XRD spectra confirm that a few new peaks represent electrochemical activity materials appear in the reduction process, but also a few peaks represent inert material appear, resulting in capacity fading.