Computational Research On Orthosilicate And Li-rich Cathode Materials For Li-ion Battery

With the increasing confilict between non-renewable energy sources（such as fossil fuels and biomass）and environmental problems,the development of clean energy economics has become global issues in 21st century.These concerns have led to recommendations on the development of green energy sources,such as solar,wind,and hydroelectric power.This brings in enormous amout of reasarch interests in energy storage.The lithium ion battery regared as a typical way of chemical energy storage has attracted more attentions because of its high density and relatively simple reaction mechanism.The fast developments of electric vehicles,portable electronics and electrified vehicles have increased the demand for high capacities of Li-ion battery.The silicate and Li-rich materials have been considered the most promising candidates for the next generation of cathode materials.In this dissertation,we have performed a systematic investigation on electronic structure,electrochemical behavior,delithiation structures and lithium diffusion of silicate Li₂MnSiO₄ and lithium rich Li₄FeSbO₆ materials.All the computational work is based on the first principles methods.The main contents are as follows:In the first part,the stability of four polymorphs（with space group Pmn21 Pmnb,P21n,Pn）for Li₂MnSiO₄ is studied.At zero pressure,the Pmn21 and Pmnb phase have good stability.The energy difference of the two systems is less than 0.0092 eV.With the increase of pressure,the enthalpy of Pmn21 is the lowest and it is certainly the favored structure.Then we dissuse the electronic structure,delithium properties and the mechanical constant for the Pmn21 structure under the pressure of 0-10 GPa.In the second and third part,we try to use doping strategy to improve the performance of Li₂MnSiO₄.The structures,electronic properties and delithiation process are studied by using the GGA+U scheme when Al,Fe and Mg are doped at Mn site.Al-doping is the best way to improve the conductivity and cyclability of the cathode material Li₂MnSiO₄.The pure Li₂MnSiO₄ has a low conductivity with a large band gap（3.28 eV）.Al-doping leads to metallic characteristics in Li₂MnSiO₄ crystal due to its density of state with spin-up and spin-down crossing the Fermi level.Among the delithiated structures Li_xMnSiO₄（x=1,0）,Al-doping enhances the structural stability and its cyclability is improved by reducing the volume change.Then,we have built three different doping concentrations Li_2-xNa_xMnSiO₄（x= 0.125,0.25,0.5）with Pmn21 symmetry,and the electronic properties and Li+ ion diffusion behavior have been studied.Li and Na ions belong to the same main group elements with similar chemical properties.The conductivity is increased by doping with the band gap reduced to 3.23 eV,3.19 eV and 3.08 eV respectively.For Li₂MnSiO₄,the lithium ion can diffuse in a two-dimensional pathway.When Na is doped,the volume of the crystal structures has a little increase.Along the[100]direction,the longer Li-O bond could result in wider ionic diffusion channels and smaller activation barriers.In the fourth part,the effect of lattice strain on the ionic diffusion and the defect formation in Li₂MnSiO₄ are investigated.Computational results show that Li ion migration barrier is sensitive to the strain,espically strain applied in be plane.The migration energy increases/decreases upon compressive/tensile strain（from-5%to +5%）for both channels.Furthermore,the Li/Mn anti-site defect cannot be produced spontaneously,and the defect formation energy is insensitive to the strain.Thus,a proper strain value can improve the rate performance of Li₂MnSiO₄ effectively in application.In the fifth part,Li₄FeSbO₆ is a new Li-rich layered oxide material with antiferromagent honeycomb structure.In this part,the electrochemical behavior,delithiation structures and oxygen stability in Li_xFeSbO₆（0≤x<4）are studied.During charging process,the structures present excellent crystal structure volume stability.The charge compensation is mainly contributed by oxygen atoms when Li is extracted from Li₄FeSbO₆ and the oxygen release will occur spontaneously at a high voltage.The lithium ion can diffuse in a three-dimensional pathway with the activation barriers from 0.36 eV to 0.67 eV.