Synthesis Of Spherical Lithium Iron Phosphate/Ni Composites

Olivine-structured LiFePO₄ has been regarded as a potential cathode for lithium ion batteries(LIB)because of its advantages such as theoretical specific capacitance (170mAh/g),high voltage,long cycle life,inexpensive and environmental friendly. However,the olivine-structured LiFePO₄ suffers from poor conductivity and poor rate performance,which prevents it from commercial application.Increasing the migration rate of Li⁺ ions and electrons in internal or interface of LiFePO₄ is the key to improve the conductivity of LiFePO₄.The recent studies are focused on the dopping of carbon, metal particles and metal ions.Fine metal particles(eg.Cu,Ag)dopping has been proved to be an effective route to improve the conductivity of LiFePO₄ while the uniformity of metal distribution in LiFePO₄,particle size,etc are affecting on the electrochemical properties.Therefore,the two-step method combined by precipitation method and citrate gel method was firstly put forward in this thesis,to prepare LiFePO₄/Ni composite microspheres materials.Nickel-coated LiFePO₄(LiFePO₄/Ni)and LiFePO₄/Ni/C composite materials were synthesized by different synthesis techniques.The effects of synthesis parameters including sintering temperature,synthesis process and doping content etc,on the electrochemical properties of nickel-coated LiFePO₄ were investigated.The microstructures, morphologies,composition and thermal-decomposition processes of the resultant composites were investigated by SEM,EDS,XRD,FTIR and TG-DSC.Research results show that LiFePO₄/Ni and LiFePO₄/Ni/C composite microspheres with uniform particle size can be preparated by precipitation-citrate-gel process.Nanometer Ni and Ni/C particles were distributed uniformly on the surface of LiFePO₄ and formed a coating structure,which effectively fined the grain size of LiFePO₄.Button cells were assembled from these cathode materials and the charge-discharge results indicate that LiFePO₄/Ni and LiFePO₄/Ni/C composites showed a flatter charge-discharge voltage plateau,more stable cycling performance and higher capacity at the first charge and discharge cycle than that of pure LiFePO₄.In particlullar,the first discharge capacity of LiFePO₄/5at%Ni/C(molar ratio(cirate:Ni²⁺)=2)composite microspheres was upto 141mAh/g,which was 28.2%higher than that of pure LiFePO₄.The discharge capacity after 30 cycles had only 7.1%decay.