First-principles Study On Fe-site Substitution Of Li₂FeSiO₄

First-principles calculations based on density functional theory have been widely used in materials,and it plays an increasingly important role in the design and optimization of electrode materials.Therefore,in order to understand the nature of materials from atom scale and to improve the low conductivity of materials.In this paper,the first principle is used to calculate the cell parameters,electronic conduction and Li-on diffusion energy barrier of P2₁-Li₂FeSiO₄.Then,the modification of Mn-doped and co-doped Mn/Ni at Fe site was studied,and the effects of doping modification on the electrochemical properties of the materials were compared.Using the GGA+U method,the band gap of the P2₁-Li₂FeSiO₄ crystal was calculated to be 3.047 eV,indicating that it was a semiconductor.The diffusion energy barrier of Li⁺along the[101]direction is 0.79 eV.The unit cell volume expansion of the delithiation is large.In addition,when a lithium ion is extracted,the average deintercalation voltages is 3.47 V,corresponding to redox reaction of Fe²⁺/Fe³⁺.while 0.5 lithium ion continues to be removed from LiFeSiO₄,O^2-oxidation leads to high voltage?4.82 V?.The second lithium ion of Li₂FeSiO₄ materials is difficult to deintercalation/intercalation.Based on Li₂FeSiO₄,the material Li₂Fe_0.75Mn_0.25SiO₄ doped with 25%Mn was established,and the cell parameters,electronic structure and lithium ion diffusion energy barrier were compared.The incorporation of Mn causes the Fe-3d spin-down band to move to the low energy,and the Li₂Fe_0.75Mn_0.25SiO₄ band gap decreased to 2.950 V.The[101]plane energy barrier is reduced to 0.77 eV.In the Mn-doped material,the unit cell volume change is still large during delithiation.when a lithium is extracted,the average voltage is increased to3.74 V,correspondingly,Mn²⁺participates in the reaction,When 1.5 lithium is extracted,the average voltage is increased to 4.87 V,Mn-doping still does not solve the problem that its voltage is too high when more than 1 lithium is extracted.When further 25%Ni is co-doped in Fe sites,the Ni-3d orbital in conduction band lower levels result in the band gap of Li₂Fe_0.5Mn_0.25Ni_0.25SiO₄ decrease to 1.130 eV.The Li⁺diffusion energy barrier is reduced to 0.70 eV.The delithiation still have a large expansion.Due to Ni²⁺participates in the electrochemical reaction during the delithiation process,the average extraction voltage of the first stage?2?x?1?reach as high as 4.09 V,and the average extraction voltage of the second stage?1?x?0.5?decreases to 4.79 V,which makes it possible to extracte the second lithium ion.