First-Principles Study On The Electrochemical Properties And Mechanical Stabilities Of Anion-Cation Doped LiFePO₄

The lithium iron phosphate is a type of new cathode that possesses the benefits of combination of high energy density,low cost and high safety.It has a very important role in electric vehicles,energy storage systems and power supply in emergency situations.However,as an electrode material its two major problems of poor electrical conductivity and low diffusion rate of lithium ions significantly restrict its application in Li-ion batteries.In this study,theoretical calculations of lithium iron phosphate and doped systems were carried out by generalized gradient approximation theory following the first-nature principle to research the alteration of the electronic structure of the substance after doping,the specific influence mechanism of the doped atoms,and the effect of different doping positions of doped ions and multi-position composite doping on the conductivity of the material.Conclusions were drawn as follows:（1）The electrochemical properties of the doped systems were obtained by applying density generalized theory to study the doping of transition metal Nb with 4d orbitals at either the Li or Fe sites.It is found that the doping of Nb at either the Li or Fe sites results in a certain change in cell volume,and the rate of volume change for the doping system at the Li site is much higher than that for the Fe site.Analysis of the energy bands of the doping at both positions shows that the doping enhanced the electrical conductivity of the material.Studies of the differential charge density before and after doping shows that Fe doping makes the charge localization near the Fe site reduced,which is favorable to enhance the transportation performance of lithium ions.And a comprehensive comparison revealed that the Nb doping of Fe sites is more beneficial to improve the conductivity of lithium iron phosphate than doping of Li sites.（2）The electronic structure of the doping system of non-metallic element N at the P-or O-site of LiFePO₄ was studied,and calculated to obtain the electronic structure characteristics of its doping system.It is discovered that the volume expansion of the system after doping of N to P or O sites is small,and the dopant does not affect the excellent stability of the material itself.The energy band density of states and differential charge density of the doped system were investigated,and both positions of doping were observed to be favorable for improving the electrical conductivity of LFP.Observation of the density of states reveals that N doping at the O-site,the effect of N is mainly near the Fermi surface;N doping at the P-site,the contribution of N occurs mainly slightly away from the Fermi surface.（3）The electronic structure properties of the composite doped system with Nb at Fe sites and N at O sites were obtained by applying the GGA+U calculation method.All parameters of the cell were found to increase after doping,and the expansion of the system was 2.45%,while the system formation energy calculation also indicated that the stability of the system was good.The doped system shows a significant decrease in the volume change rate and increase in the embedded lithium voltage relative to the intrinsic system during the de-lithium process,indicating that the cyclic stability and specific capacity are improved by doping it.In general,these energy levels are all moving toward lower energies,leading to a reduction of bandwidth from 3.71 e V to 1.38 e V.The mixed doping of metallic and non-metallic elements has a positive effect on their electrical conductivity.（4）The mechanical properties of Nb-N composite doped LiFePO₄ were calculated by applying transition state theory.Both Nb and N doping added to the shear modulus and Young’s modification of the material;and the Poisson’s ratio was slightly reduced,its probability of shear deformation was reduced after doping;Nb and N-LiFePO₄ still had good ductility;compared before and after doping,the anisotropy does not change significantly in the（010）plane,but in the（100）and（001）surfaces both tended to be isotropic,thus inhibiting the generation of microcracks.