Theory Studies On Crystal Structure And Crystal Morphologies Of Li₂FeSiO₄

Polyanionic silicate cathode material Li2FeSiO4 is makes it a strong competitor for the next generation of secondary cathode materials,due to its high specific theoretical capacity,abundant natural resources,low cost and high safety performance.Li2FeSiO4 is susceptible to the influence of the synthesis conditions showing complex polymorphic characteristics,which also features increase the complexity of crystal morphology.Among them,the most stable Pmn21 and P21/n structures are the focus of theoretical and experimental research.In this paper,atomistic simulations are performed to investigate the lattice structure and crystal morphology of Li2FeSiO4,basing on two above-mentioned polymorphs.On the basis of shell model,the interaction potential of atoms is obtained,using the sensitivity method of variance analysis and relaxation fitting technique.We calculated the free energy of the structure with space group Pmn21 and P21/n under different temperatures and pressures,in order to analyse the structural stability under different synthesis conditions,calculated critical pressure of structural phase transition agrees with the result of first-principles calculation in the literature.The diffusion behaviors of lithium ions in crystal are studied by using the Mott-Littleton method,the diffusion pathways,energy barrier and dimension of lithium ion are calculated.Calculated results show that lithium ions have two-dimensional diffusion pathways in these two structures,the diffusion directions of Pmn21 structure are[100]and[001],while[101]and[001]in P21/n.The exposure of the crystal planes,perpendicular to the diffusion direction of lithium ions in nanocrystalline material is closely related to the de/lithiation and the rate capability of the material.Two-dimensional periodic structure model is constructed to simulate the crystal surface of the low miller index,calculate the surface energy and attachment energy of the corresponding crystal surface,investigate thermodynamics and kinetics of crystal morphology.The equilibrium morphology and the growth morphology are similar,the only difference is the surface area of the crystal planes is different.There is no(100)crystal surface on the morphology of Pmn21 structure,and lithium ions can not be de/lithiated in two-dimensional form,which is not conducive to the performance of material rate performance.From the point of the view of thermodynamics,the strong dependence of particle morphology on synthesis method and solvent conditions suggests that such effects may be due to stability of the surfaces.The effect of the crystal surface stability on the morphology was studied by adjusting the surface energy of the Pmn21 and P21/n structures,which provides a theoretical basis for experimental control of Li2FeSiO4 crystal morphology.