Density Functional Theory Studies On Rate Capability Of Li₂FeSiX₄（X=O,S）

Li2FeSiO4 is considered to be one of the most promising cathode materials for lithium-ion batteries due to its theoretical possibilities for reversibly extracting/inserting two lithium-ions from the host structure and thus to increase its capacity to approximately 330 mAh/g.However,The inherent disadvantages of Li2FeSiO4 are the high operating voltage for the extraction of the second lithium ion and the low tolerance for the high current rates.In this paper,density functional theory(DFT)calculations are performed to investigate the rate performance of Li2FeSiO4,from the ionic and electronic conduction to explore the effect way of improving the rate capability of silicate materials.The main research results are as follows:By using the climbing image nudged elastic band(CI-NEB)method,we study lithium ion transport behavior of Li2FeSiO4 with space groups of Pmn21 and Pbn21(cyclced structure).Calculated results show that the lithium ions in Pmn21 structure present two-dimensional mobility,with high activation energies of 0.82 and 0.92 eV;the Li/Fe antisite exchange opens up the structure for effective Li-migration along three crystallographic axes,with the energy barriers are 0.95,0.96 and 0.89 eV respectively.However,the diffusion barriers are higher,hindering the migration of lithium ion and limiting the rate capability of the material.The calculated electronic density of states shows that,Li2FeSiO4 behave as a n-type semiconductor,and LiFeSiO4 and FeSiO4 are concluded as a p-type semiconductor and a half metal respectively.Owing to the difficult extraction of the second lithium Li2FeSiO4 transforms to LiFeSiO4 during the delithiation process,and vice versa.The lithiation-delithiation interface has the characteristic of a reverse biased diode,leading to low electronic conductivity and low current rates,which also limits the performance.The stable structure of Li2FeSiS4 is similar to the cycled structure of Li2FeSiO4,with the space group Pbn21.Calculated activation energies are 0.60,0.45 and 0.63 eV along three crystallographic axes,significantly lower than those of Li2FeSiO4 due to the lower delithiation voltage for the second lithium ion.Density of electronic states shows that Li2FeSiS4 belongs to P-type semiconductor,while FeSiS4 has half metallic band structure,carriers can easily move through the lithiated-delithiated junction.From the point of view of both ionic and electroic conductivities,S substitution for O can be expected to improve the rate capability performance of silicate materials.