Quantum Chemical Study On The Electronic Structure Of Cathode Material For Lithium-ion Battery

The layered LiCoO2, LiNiO2 and the spinel LiMn2O4 are common cathode materials for lithium-ion cells.There are few theoretic articles about the relationship between the electronic structure and performance of electrode material of lithium-ion cells. In this thesis, the rule of electronic structure changes after intercalation and substitution in the layered LiCoO2, LiNiO2 and the spinel LiMn2O4 has been studied by using quantum chemical ab initio and density function theory(DFT). Some models with 17, 41, 56 and 65 atoms have been calculated with Gaussian98W. However, only the results of the two models with 17 and 41 atoms could be gained. In order to determine the calculating method and basis set, the model of Li5Mn4O83+ is selected as an example. According to calculation speed and the characteristics of the model, the method RHF and the basis STO-5G/6-31G* are finally determined respectively. This calculation can fairly explain some experiments results.The main work of this thesis is to study of electronic structure of cathode material. Some structure paramters, such as total energy, atomic net charge, atomic overlap population, of the model Li5Mn4O83+, Li5Mn12O24-, Li7Co6O2015-, Li7Ni6O2015-, Li5Mn2Co2O83+, Li5Mn2Ni2O8 are calculated. The following results have been concluded:(a) Ab initio has its advantages in the calculating of atom cluster containing transition metal , especially in precision and total energy calculations.(b) Lithium has been existing as ionic state in lithium-manganese oxides and the layered materials after losing electron partially.(c) The total energy of manganate-oxides materials decreases after intercalation lithium. The electrovalent bond weakens and the covalent bond strengthen between manganese and oxygen due to ion polarization effect, which makes the spinel structure more stable.(d) The total energy of the layered material decreases after intercalation lithium. The net charge of transition metal atom has decreased as a result of the static effect of lithium ion and effect of energy band of Co and Ni. The lithium ion can stabilize the layered structure by enhancing the interaction between two interlayers of O-M-O.(e) The total energy of LiMn2O4 has decreased, the positive charge of manganese ion with Co and Ni inserted has increased so that the Jahn-Teller effect has beeninhibited. An increase in the strength of the Mn-O bond improves the stability of spinel structure and inhibites the structural transformation during charge-discharge cycle and suppresses the fading of capacity.(f) The average voltages for LiMn2O4, LiCoO2, LiNiO2 have been calculated by total energy change between non-intercalated and intercalated models. There are two steps with 3.205V and 4.002V during the lithium insertion of Mn4O8 Li2Mn4O8 Li4Mn4O8; The average voltages for LiCoO2 and LiNiO2 are 3.495V and 3.046V respectively.