Research On Nano-structured LiFePO₄ Cathode Material For Lithium Ion Batteries

Orthorhombic olivine-type LiFePO₄ is a promising cathode material of lithium batteries,due to its high theoretical capacity?170 mAhg-1?,environmental friendliness,thermal stability,long cycling performance?>2000?,resources abundance and low cost.However,the low electron conduction and sluggish ion diffusivity hinder greatly the rate performance of the lithium-ion batteries.In this thesis,nano-structured LiFePO₄ was studied using low-cost industrial materials,and more details are as follows:Nano-structured FePO₄ was synthesized using industrial iron waste water as iron source,?NH4?2HPO₄ as phosphorus source and citric acid as surfactant.In contrast,four different FePO₄ samples were named as FP-A?0 g citric acid?,FP-B?0.2 g citric acid?,FP-C?0.4 g citric acid?and FP-D?0.6 g citric acid?,respectively.The size and surface properties of FePO₄ were characterized by scanning electron microscopy?SEM?and Fourier transform infrared spectroscopy?FTIR?.The size of FP-D sample was the smallest with little agglomeration.LiFePO₄ was prepared by carbothermal reduction method utilizing FePO₄,Li2CO3 and C12H22O11.In contrast,different Li FePO₄ were named as LFP-A,LFP-B,LFP-C and LFP-D deriving from FP-A,FP-B,FP-C and FP-D,respectively.The structure,size and electrochemical performance of LiFePO₄/C were characterized by X-ray diffraction?XRD?,SEM and galvanostatic charge /discharge tests.The size and morphology of LiFePO₄/C can be modified by the size and shape of FePO₄.Nano FePO₄ contribute to prepare size reduced LiFePO₄/C with less agglomeration.It is not helpful to improve the initial discharge capacity of LiFePO₄/C by adding citric acid,however,it can indeed reduce the capacity loss and improve the rate and cycling performances.LiFePO₄ was then doped with different contents of Mn²⁺?0 %,5 %,8 % and 10 %?,respectively.The structure,size and electrochemical properties of different cathode materials were studied by XRD,SEM and galvanostatic charge /discharge tests.It is revealed that doping does not change the structure of the cathode and LiFe0.95Mn0.05PO₄ shows the best initial discharge capacity at 0.1 C.The effect of metal doping on electrode kinetics and transport of lithium ion properties were investigated by cyclic voltammetry?CV?and electrochemical impedance spectroscopy?EIS?.