Theoretical Calculation And Experimental Study On 3d Transition Metal Doped Lithium Iron Phosphate Cathode Material

The applications of lithium-ion batteries are expanding from portable devices to transportation and industrial fields.Lithium iron phosphate（LiFePO₄）cathode material has been expanding in market demand due to its advantages of high theoretical capacity,high operating voltage,safety and stability,long cycle life,environmental friendliness,and low cost.However,the unique crystal structure of LiFePO₄ determines its poor electronic conductivity,low lithium ion diffusion coefficient,and causes the slow charge/discharge kinetic rate and poor rate performance of LiFePO₄ batteries.The electronic conductivity and ionic conductivity of LiFePO₄ can be improved by selective doping of 3d transition metals,but its effect on the structural stability of LiFePO₄ is still uncertain.The intrinsic mechanism of 3d transition metal-doped LiFePO₄ should be fully investigated before any further large-scale application of 3d transition metal-doped LiFePO₄.Therefore,this thesis reveals the effect of 3d transition metal doping on the stability,electronic conductivity,and work voltage of LiFePO₄ cathode material with the help of first-principles calculations and screens out the optimal doping element.Combined with the experimental characterization,it is verified that Ti⁴⁺could be used as the doping element for further large-scale applications of doped LiFePO₄.（1）The structural properties,electronic structure,doping formation energy,and lithium voltage of the 3d transition metal doped Li Fe_xTM_1-xPO₄ model are compared and analyzed by first-principles calculations in the framework of density functional theory.It is found that the Li Fe_xTM_1-xPO₄ models doped with Ti,V,Cr,Mn,Co,Ni,Cu,and Zn have a small change in unit cell volume（<1%）,and the doping system is more stable.The Li Fe_xTM_1-xPO₄ model doped with Sc,Ti,Cr,Co,and Cu has a small band gap（<1.0 e V）,and the conductivity of the doped system is better.The doping formation energies of Sc,Ti,V,Cr,Mn,and Co are smaller（<1.5 e V）,which is easier to obtain by experimental synthesis.The operating voltage of the Li Fe_xTM_1-xPO₄models doped with Sc,Ti,V,Cr,Mn,Co,Ni,Cu,and Zn are all greater than 2.5 V,which meets the requirements of the operating voltage of lithium-ion batteries.After the above screening,the Ti,Cr,and Co doped Li Fe_0.75TM_0.25PO₄ model exhibited better structural stability and electrochemical properties,and Ti was finally preferred as the3d transition metal doping element for further large-scale application of LiFePO₄ in combination with cost considerations.（2）On the basis of theoretical calculations,Ti⁴⁺-doped LiFePO₄ cathode materials are prepared by the carbothermal reduction method using Ti O₂ as the titanium source,and the optimal doping amount（1%）was selected.The results show that the rate performance and cycling performance of the Ti⁴⁺-doped modified materials are improved compared with pure LiFePO₄,in which the sample with the optimal doping amount（LFP-2）has the best electrochemical performance.The capacity is 155.4 m Ah g^-1 at 0.1 C rate.When the rate is increased to 10 C,the capacity is still 93.5 m Ah g^-1,showing excellent rate performance.Moreover,after 300 cycles at 1 C rate,the capacity retention rate is 94.9%,showing stable long-term cycling performance of LFP-2.The excellent electrochemical performance can be attributed to the moderate doping of Ti⁴⁺in the LiFePO₄ lattice.On the one hand,Ti⁴⁺is doped in the Fe site of the material,which reduces the particle size of the material,shortens the diffusion distance of lithium ions in the bulk phase of the material,and enhances the diffusion kinetics of lithium ions.On the other hand,the doped Ti⁴⁺can adjust the band structure of the material,reduce the band gap,and activate the delocalization of electrons on Fe 3d orbitals,thus improving the electronic conductivity of the material.