First-principles Study On Doping Modification Of Lithium Iron Phosphate Cathode Materials

Lithium iron phosphate（LFP）is popular as a cathode material for lithium-ion batteries in applications such as power batteries.Because of its high capacity,stable working voltage,good stability,good cycle performance,low price,environmental protection and other excellent characteristics,it has become one of the most competitive materials.However,as an excellent cathode material for lithium-ion batteries,LFP has some attracted much attention.In terms of rate,that is,the charge and discharge rate of the battery.For a long time,improving the conductivity and lithium ion diffusion rate of LFP has been a hot topic of researchers from all over the world.Since,the modification of LFP is limited by experimental conditions and various factors,it is a widely used and effective research method to verify experiment and predict performance by simulation.Based on this,in this present contribution,the electrochemical properties of the LFP doping modification system were investigated in order to find modified LFP materials with good rate properties by simulation.The specific work is as follows:First-principles computational studies under DFT framework was used to investigate the structural stability,conductivity and voltage profile of LiFe_1-nN_nP_1-mM_mO₄（N,M=Si or S）electrode materials.It is found that the LiFe P_7/8Si_1/8O₄system has the most stable structure.After doping,the band gap values of the systems decrease gradually,and LiFe_7/8S_1/8PO₄ system has a minimum band gap of 1.553 e V.The LiFe P_7/8S_1/8O₄ system demonstrates the characteristic of n-type semiconductor,and other doping systems have the feature of p-type semiconductor.The covalent property of Si-O bond is enhanced in the LiFe P_7/8Si_1/8O₄system and doping broadens significantly t he channel of Li ion de-intercalation in LiFe_7/8S_1/8PO₄ and LiFe P_7/8S_1/8O₄.Additionally,the results of lithium intercalation potential imply that the voltages of the doping systems fall into the range of2.23V-2.86 V.LiFe_10/12Co_1/12Mn_1/12P_11/12S_1/12O₄（LF（CM）P（S））were designed and the stability,lithium ion diffusion rate,lithium intercalation potential,discharge curve,and electronic performances were systematically explored.The results show that LF（CM）P（S）has the better stability with the formatio n energy of 2.01 e V.The lithium ion diffusion rate in LF（CM）P（S）is 10 orders of magnitude faster than that of intrinsic system.The characteristic transition from p-type to n-type semiconductor is present due to the doping of S,and the doping of Mn and Co leads to the generation of impurity bands and the reduction of band gap from 3.78 to 0.737e V.The effective mass of electrons in the doped system is 1.59m₀,much greater than 1m₀,which is conducive to the improvement of its conductivity.The disappearance of electron enrichment and the decrease of the localization are responsible for the rapid migration of the lithium ion.The high operating voltage,charge-discharge platform of 4.7V and the voltage difference contribute to the promotion of electrochemical performance,which open a window for the commercializ ation application of Li-ion battery.