Structural Designs Of Lithium-rich Cathode Materials Based On First-principles Calculations

The Li-rich layered oxides are regarded the promising high energy density cathode material for the next generation lithium-ion batteries due to their high specific capacity and the high working voltage.However,the commercial applications of these cathode materials are hindered with obstacles such as the poor initial coulombic efficiency,the inferior cycling life,poor rate performance and severe discharge voltage fade.The first-principles calculations can be applied to understand the electrochemical behaviors of the material.In this dissertation,the first-principles calculations were performed to select the proper elements for the surface doping in the Li-rich layered oxides,find out the impacts of the surface doping on the material,clarify the effect of the cation disorder on the electrochemical properties of Li₂MoO₃ and figure out the effect of intra-layer cation disorder on the density of states of lithium-rich materials.These predictions were verified with the corresponding experimental characterizations.Surface doping can effectively improve the performance of the typical Li-rich layered oxide Li_1.2Mn_0.54Ni_0.13Co_0.13O₂.Based on the density functional theory（DFT）calculation,appropriate doping elements such as Mg,Ti,Zr,Nb,Ta,Ce,Pm and Er were selected from a large number of elements with the doping feasibility,the variation of the electronic structure and the chemical stability of the O atom in the doped Li-rich material as the selection criterion.Considering the cost of material fabrication and the convenience of the experimental characterization,only Ti and Zr were chosen for further experimental verification.The spherical aberration corrected scanning transmission electron microscopic（STEM）imaging and the soft X-ray absorption spectroscopy（SXAS）verified the reliability of the DFT calculations.The electrochemical evaluations,SXAS and Raman spectra showed that the strong TM-O bond of the surface doping element helps improving the properties and the structural stability of the Li-rich material.The possible cationic disordered structures in Li₂MoO₃ were explored and the roles of the various disorder structures were summarized.The calculations demonstrate that the cation-disordered structures take place on the 3a and 3b sites of the material.The disorders affect the delithiation potential and the delithiation sequence,and therefore,the thermodynamic properties of the material.The 3a disorder hinders the transport of the Li ions in the material,while the 3b disorder promotes the Li-ion transport and the charge transfer in it.The experimental results verify that the material with more disorders displays better electrochemical performances.The influence of the intra-layer disordered structures was further explored on the electronic structures of Li₂TMO₃（TM=Ti,V,Cr,Mn,Fe,Co,Ni）.Upon constructing a series of intra-layer disordered structures,the O and TM atoms were localized in some specific chemical environments in the structure.The DFT calculations show that specific chemical environment enhances the electrochemical activity of the O atoms.The TM atoms in some specific chemical environments effectively reduce the band gap,and thus improve the electronic conductivity of the material.