Synthesis And Modification Of LiFePO₄ As A Cathode Material For Lithium-ion Batteries

Olivine-structured lithium iron phosphate?LiFePO₄?is considered to be one of the most promising cathode materials due to its high theoretical capacity?170 mAh/g?,flat voltage profile,low cost,thermal safety,structure stability and environmental friendliness.However,the poor electronic conductivity and sluggish lithium ion diffusion seriously limit its rate capability,which makes LiFePO₄ difficult to meet the commercial demand for high power batteries.Therefore,in this dissertation,many progressive efforts have been made to overcome these obstacles by doping supervalence metal ions,coating conductive polymer on particle surface,controlling the particle size and morphologies,and thus to speed up the process of its commercialization.Firstly,the crystal models and electronic structure of undoped and Ru-doped LiFePO₄ were simulated and optimized through CASTEP module of Materials studio software.Based on the first-principle of the Density Functional Theory?DFT?and ultra-soft pseudo potential plane-wave method,the electronic structures such as Fermi levels,energy band structure,density of states of undoped and doped samples were calculated.Calculations show that Ru-doped LiFePO₄ maintained single olivine structure.The lattice parameters show a significant decrease in both a and b but a tiny increase in the c direction leading to an overall decrease in the unit cell volume.The redued lattice parameters in b axis would be more beneficial for Li+ mobility along 1D [010] direction.It may cause the enhancement of the Li+ intercalation/de-intercalation,and further resulting to the excellent electrochemical performance of Ru-doped LiFePO₄.Fermi energies increased to the maximum and then decreased with increasing Ru amount.After doping Ru,density of the total states became stronger especially on the peak intensity nears the Fermi level.And the total density shifted to the low energy,indicating the band gap decreases.Meanwhile,the bottom of conduction bands moved to the Fermi level.These would make it easier for electrons to transition from the valence band to the conduction band,which could increase the conductivity of LiFePO₄.According to calculation results,uniform submicro-sized LiFePO₄/C and Ru-doped LiFe_0.99Ru_0.01PO₄/C cathode materials were synthesized by using a surfactant assisted sol-gel method.The surfactant plays a key role in the synthesis,which acts as a blocking agent preventing and minimizing the agglomeration of the lithium iron phosphates particles.The composite particles show approximately globular structure,and uniform submicro-sized particles?¹00 nm?interconnected to form a porous network.The Ru ions have been doped into the LiFePO₄ phase.Ru-doping promotes Li ion diffusion velocity and electrochemical conductivity of LFP,thus indicating excellent electrochemical performance.Compared to the un-doped LiFePO₄/C,LiFe_0.99Ru_0.01PO₄/C exhibits superior specific capacity and high rate capability.Specific capacity could achieve 162 mAh/g at 0.1 C,even at the high rate of 10 C,the cathode delivers a high discharge capacity of 107 mAh/g with a relative stable flat discharge voltage plateau above 3.0 V upon increasing current.The electrochemical impedance spectra?EIS?results show that the lithium-ion diffusion coefficient for the LiFe_0.99Ru_0.01PO₄/C is 2.38 times higher than LiFePO₄/C.Thus,the electrochemical activity of LiFePO₄/C composites can be improved with an appropriate amount of Ru ion doping and with assistance of lauric acid surfactant.To improve the charge/discharge performances of the LiFePO₄ at the high rate,well-shaped and uniformly dispersed LiFePO₄ nanorods cathode material with a length of 400-500 nm and a diameter of about 100 nm,were obtained by using the surfactant assisted solvothermal method without any further heating as a post-treatment.The surfactant acts as a self-assembling supermolecular template,stimulating the crystallization of LiFePO₄ and directing the nanoparticles growing into nanorods between the bilayer of surfactant.LiFePO₄ nanorods with the reducing crystal size along the b axis shorten the diffusion distance of Li+ extraction/insertion,and thus improve the electrochemical properties of LiFePO₄ nanorods.Such prepared LiFePO₄ nanorods exhibites excellent specific capacity and high rate capability with discharge capacity of 151 mAh/g,122 mAh/g and 95 mAh/g at 0.1 C,1 C and 5 C,respectively.LiFePO₄ nanorods have higher lithium-ion diffusion coefficient and smaller Rct than the irregular morphology LiFePO₄.Finally,LiFePO₄ nanorods coated by conductive polymer PPy,was prepared by an in-situ polymerization coating method.The LiFePO₄/PPy with 2.95% PPy exhibites excellent specific capacity and high rate capability with discharge capacity of 153 mAh/g?138 mAh/g and 118 mAh/g at 0.1 C,1 C and 5 C,respectively.The excellent electrochemical performances of the LiFePO₄/PPy composite suggest that conductive polymer could improve the charge/discharge performances of the LiFePO₄ at the high rate.