Preparation And Doping Modification Of LiFePO₄as Cathode Material By Combustion Method

Olivine-type lithium iron phosphate (hereafter referred to LFP) as a cathodematerial for lithium-ion battery has received great attention due to itsenvironment-friendly, high security, widely raw materials and stable cycle performance.Recently many synthesis method of LFP has been explored by domestic and overseasresearchers. However they mostly need to pass into inert gas to prevent the easyoxidation of Fe2+which inceases the production cost. Besides, due to the restriction of itscrystal structure, LFP has poor electronic conductivity and low Li+diffusion coefficientwhich inhibit its electrochemical performance. Studies have shown that surfacemodification, ion doping and fine granulation can effectively improve theelectrochemical performance of LFP.In this thesis, combustion method with lithium nitrate, diammonium phosphate,ferric nitrate as raw materials and citric acid as complexing agent as well as reducingagent was introduced to prepare the undoped lithium iron phosphate (hereafter referred toN-LFP) under no condition of inert gas. The complete reduction of Fe3+was provedthrough the analysis of XPS. The influence of citric acid dosage, buried burningtemperature and buried burning time on the structure and electrochemical performance ofN-LFP was investigated detailedly by XRD, SEM, FT-IR, laser granularity analyzer andconstant current charge and discharge test. The comprehensive analysis indicated theoptimum synthesis condition of combustion method is: citric acid dosage at φ=2, buriedburning temperature at700℃and buried burning time at10h. The synthesized materialexhibited a discharge specific capacity of121.8mAh/g for the first time at currentdensity of0.2C.Combustion method with the above optimum synthesis condition was employed toprepare high metal ions (Mg2+, Zr4+) doped lithium iron phosphate (hereafter referred toY-LFP). The phase structure of Y-LFP was comprehensively investigated by the analysis of XRD, FT-IR, EDS and XPS. Results showed that Mg2+and Zr4+entered into theintracell. And they both existed in the form of solid solution. The results of constantcurrent charge and discharge test showed that the discharge specific capacity of thematerial increased first and then decreased with the increase of doping amount, whichindicated a small amount of doping could improve the electrochemical performance ofLFP. While excessive amount of doping would lead to the decrease of lithium ionconcentration, causing a loss to the specific capacity. The best doping amount of Mg2+Y-LFP and Zr4+Y-LFP were x=0.03and x=0.02respectively. And their best dischargespecific capacities were141mAh/g and141.9mAh/g. The cyclic voltammetry ofLi0.98Zr0.02FePO4and Li0.98Mg0.02FePO4with the same doping amount and doping ionsradius showed the electrochemical performance of Zr4+Y-LFP was superior to Mg2+Y-LFP, indicating the higher the valence state of doping ion was, the better itselectrochemical performance behaved. In addition, the doping conductive mechanismwas preliminaryly discussed. Y-LFP belongs to cationic vacancy type material. Thelithium ion vacancy appeared with the replacement of high metal ions, which improvedthe migration of intimal lithium ion in the intracell. Meanwhile, the material transformedfrom n-type semiconductor into p-type semiconductor, leading to the improvement ofelectronic conductivity.