First Principle Study Of Transition Metal Oxides As Electrode Materials For Lithium Ion Batteries

The spinel type AB₂O₄（a=Li,B=Ti,V,Mn,Co）transition metal oxide with the advantages of high stability,low cost and easy preparation is the most attractive electrode material for lithium-ion batteries.However,how to improve the low electronic conductivity and low theoretical specific capacity of spinel electrode materials is an urgent problem.In this thesis,the optimal cell model,lithium insertion voltage,electronic structure and material stability of two kinds of spinel transition metal oxide electrode materials are systematically studied and discussed by the first principle method.It provides a theoretical basis for further improving its electrochemical performance.For the spinel type material Li₄Ti₅O₁₂,the optimal configurations of Li₄Ti₅O₁₂（1,8）and Li₇Ti₅O₁₂（3,6）are determined.The"zero strain"property of the material is proved.When the intercalation amount of lithium is x>4.5,the volume expansion rate is as high as 10.2%,indicating that the cubic spinel structure has been destroyed.It can be seen that Li₇Ti₅O₁₂ can still insert 1.5 lithium atoms.The average lithium insertion voltage of Li₄Ti₅O₁₂is 0.62～1.23 V.According to the band structure and density of states,Li₄Ti₅O₁₂ is a p-type semiconductor and the band gap is 2.532 e V.With the insertion of lithium,the entire system becomes metallic.The shape and energy range of the valence band in the total density of states are very similar.The valence band is mainly contributed by O-2p state,indicating that the main spinel structure remains unchanged during the process of lithium intercalation.It shows that Li₄Ti₅O₁₂ as a negative electrode material for lithium ion batteries should have a larger theoretical specific capacity than 175 m Ah/g and the material has good stability.For the spinel type material Li₄Mn₅O₁₂,the optimal configurations of Li₄Mn₅O₁₂（9,11）and Li₇Mn₅O₁₂（3,6）are determined.Furthermore,the effects of different amounts of lithium intercalation on the structure and properties of Li₄Mn₅O₁₂ were investigated.The results show that Li₄Mn₅O₁₂ can insert up to 3 Li,the average lithium insertion voltage is 2.71 V.according to the electronic structure,the band gap of Li₄Mn₅O₁₂ is 0.007 e V,with the insertion of lithium,Li₇Mn₅O₁₂ exhibits metallicity.The main spinel structure remains unchanged before and after lithium insertion.It shows that Li₄Mn₅O₁₂ has good stability as a positive electrode material for lithium ion batteries.In addition,this thesis first explores the magnesium insertion behavior of Li₄Mn₅O₁₂.The results show that Li₄Mn₅O₁₂ can embed up to three Mg,and its average magnesium insertion voltage is 1.49 V.In theory,Li₄Mn₅O₁₂ should have a non-negligible magnesium insertion capacity in the low-voltage discharge interval,which provides a good theoretical basis for the subsequent application of manganese materials.