The Design And Electrochemical Performance Studies Of Non-stoichiometric Lithium Iron Phosphate

Lithium-ion batteries are the most promsing energy storage devices in today's energy fields.The commercial applications of Li-ion batteries mostly depend on the cost and electrochemical performances of cathode materials.Olivine LiFePO₄exhibits the thermal stability,long cycle life,highly safety,low cost and environmental benignty,which make it become competitive in marketplace.Aiming to deal with the intrinsic low ion diffusivity and electronical conductivity,numerous strategies have been adopted consist of decreasing the particles to nanometer sizes,modifying the particle surface with conductive agents and doping with supervalent ions,etc.Futhermore,the non-stoichiometric lithium iron phosphate have been synthesized to eliminate the negative effect of Li-Fe anti-site defects on the electrochemical performance.Here,we prepared a series of non-stoichiometric carbon-coated lithium iron phosphate?LiFe_xPO₄/C,x=0.96¹.04?by a solid-state reaction.Consequently,LiFe_0.98PO₄/C exhibits the highest rate capability of 163.5 mAh g^-1 at 0.1 C and 93.5mAh g^-1 at 20 C.In combination with XRD,XPS,M?ssbauer,TEM,Raman etc.,it is concluded that the Li-O bond length is elongated with decreasing x values,facilitating the Li-ion migration.At the same time,the existence of Fe₂P and high graphitization degree of carbon layer enhance the electronical conductivity.Furthermore,we continued preparing Mg²⁺doped LiFe1-_yMg_yPO₄/C?y=0⁰.08?cathode materials,whereas the LiFe_0.96Mg_0.04PO₄/C exhibits the highest electrochemical performance.Taking LiFe_0.96Mg_0.04PO₄/C as a standard,Fe-poor LiFe_0.94Mg_0.04PO₄/C and LiFe_0.96Mg_0.02PO₄/C were synthesized by reducing 2%Fe²⁺or 2%Mg²⁺.According to the results of XRD,XPS,TEM and EIS,etc.,it is indicated that the improvement achieved by reducing Fe²⁺is more significant than the other.LiFe_0.94Mg_0.04PO₄/C exhibits 166.7 mAh g^-1 and 90.9 mAh g^-1 at 0.1 C and 20 C,respectively.