Preparation And Modification Of LiFePO₄ And Its Electrochemical Performance

With the advocating and supporting new energy in our country greatly,green travel and low carbon environmental protection are important guarantees in sustainable development society and economy.As a kind of bright future electrode material in lithium-ion batteries,lithium iron phosphate?LiFePO₄?has been paid extensively attention in new energy since it had been discovered,resulting from good safety,higher theoretical specific capacity and open circuit voltage,long lifespan,environmentally friendly and low cost.However,the application of this cathode materials such as electric vehicle and hybrid electric vehicle is hampered by the low electronic conductivity and sluggish lithium ion diffusion.In order to take advantage of the superiority of LiFePO₄and overcome the inherent defect,its modification is inevitable.In the research,modifications were carried out on the basis of preparing LiFePO₄ to obtain excellent electrochemical performance.The typical works as follows:Firstly,LiFePO₄/rGO composite was prepared by sodium dodecyl benzene sulfonate?SDBS?assisted hydrothermal approach,in which the SDBS was used as morphology-controlled reagent.Through optimizing the experiment parameter such as the concentration of SDBS,the pH of precursor mixture and addition amount of graphene oxide?GO?,the preparation condition of LiFePO₄/rGO composite with best electrochemical performance was obtained:the optimal concentration of SDBS is 0.225mol/L,the optimal pH of precursor mixture and GO addition amount is 5.8 and 2%,respectively.Simultaneously,rod-like LiFePO₄ nanoparticles are loading on the surface of rGO compactly,which is benefit to increase the conductivity of materials.SDBS was used as morphology-controlled template assisted hydrothermal approach in-situ composite graphene to modify LiFePO₄ is low cost and simple,which could be regarded as a reference for other electrode materials modification.Secondly,LiFePO₄@C/rGO composite was obtained by in-situ composite graphene in ethylene glycol solvothermal approach,and synergistic effect of graphene doping and carbon coating for LiFePO₄ modification has been assessed.The results show that the morphology of LiFePO₄@C/r GO composite is spherical-like,the surface is coated a carbon of layer with the thickness of 2⁵ nm,and the folded graphene sheets“bridged”with carbon-coated LiFePO₄ nanoparticles formed three dimensional conductive network.In addition,the discharge capacity is still 129.5 mAh/g at the rate of 20 C,moreover,no obvious capacity fading is observed after 200 cycles at the rate of10 C and-20?/1 C,manifesting the excellent rate performance and favorable low temperature stability,this originating from the composite own smaller electrode polarization,faster lithium-ion diffusion and smaller charge transfer resistance.Lastly,based on the diethylene glycol solvothermal approach and carbon-coated decoration,sheet-like of nano-scale LiFePO₄ and Mn²⁺occupied Fe-site doping was realized simultaneously,and the sheet-like LiFe_1-xMn_xPO₄/C composite was synthesized eventually.What's more,the influence of Mn²⁺doping on the morphology and electrochemical performance of LiFePO₄ has been investigated.The results show that the improved electrochemical performance resulting from Mn²⁺doping accelerated lithium-ion diffusion,decreased charge transfer resistance and encouraged reaction dynamics process.