First-principles Studies On The Electronic Structure And Properties Of The Energy-Storage Materials

The performance of lithium batteries mainly depends on theproperties of cathode materials. Therefore researchers try toimprove electrochemical properties of cathode materials to gain thehigh performance batteries. This work focuses on the study of theelectronic structures of LiCoO2 and LiMn2O4 via ab initiocalculations, using CASTEP software package. Combining withElectron Energy Loss Spectroscopy (EELS) and M?ssbauerspectroscopy, we studied charge disproportionation and Jahn-Tellereffect in the materials. It was reproted by Tukamoto et al. that the conductivity ofLiCoO2 increases over two orders of magnitude via Mg doping,which can reduce dangerous factors and energy losing duringcharge-discharge due to the high resistance of cathode materials.However there is still no clear explanation to this phenomenon. Thefirst-principles calculations results show that the band structure ofMg doped LiCoO2 becomes more complex, which is due to theincreasing of the number of atoms in the superlattice cell. Mgdoping introduces new levels, which effect the hybridization ofCo-3d and O-2p and cause the band structure complexing.Comparing band structures of both materials, it's found that thereare two main differences after Mg doping. One is the decrease of V吉林大学博士学位论文 摘 要energy gap between valence bands and conduction bands. Theenergy gap and the conductivity can be connected by effectivemass, σ ≈ (1+γ?Eg / Eg0 )σ0 , where 0 < γ < 1. Though narrowerenergy gap corresponds to higher conductivity, a small gapdecrease can't explain the conductivity increase of two orders ofmagnitude. The other is the close of gap at –2 eV after Mg doping.There emerges a continuous valence bands under the Fermi level.Combining with the electron density of states, it was found that thegap close relates to the distributing of Co-3d electron states. Thereis no such change in Al doped LiCoO2. The integration of electron density of states in each energyregion in the valence band corresponds to the occupation states ineach band. The integration changes reflect the changes ofoccupation situation. Density of States (DOS) study on LiCoO2series materials shows that the integration value of LiCoO2 and Aldoped LiCoO2 under Fermi level equals the total valence electronsnumber included in each superlattice, respectively, so that thevalence bands are full filled by electrons. Materials present asemiconductor character. When it comes to the Mg doped LiCoO2,the integration value of valence bands DOS is larger than the totalvalence electrons number, so that there is a hole in valence bands,which makes the excitation much easier and improve conductivity.Thus, Mg doping introduces acceptor levels into the valence bandsof LiCoO2 to increase the conductivity. In order to have a clear view of the dopants effect on theelectronic structure, we calculated the partial density of states(PDOS) of each subshell. The results show that the lithium ions VI吉林大学博士学位论文 摘 要take on a high ionic state, and have no changes with doping. Thechanges of Co-3d density of states in Al doped materials are muchsmaller than that in Mg doped materials. According to the crystalfield theory, in the oxygen octahedron, hybridization of Co-3d andO-2p forms three main energy bands, antibonding band eg , *bonding band eg and nonbonding t2 . The distribution of 3d b gelectron states changes in the Mg doped LiCoO2. The quantitativeanalysis shows that the states in antibonding band eg decrease, the *states in bonding band eg increase, and the states in nonbonding bband t2 decrease. Such changes indicated that elec...