Synthesis And Modification Of LiFePO₄as Cathode Materials For Lithium-ion Batteries

With the increasing depletion of three major fossil fuels, the development of new energyhas become the research focus of scientists. Currently, the lithium-ion batteries as a cleanenergy were used in communication mobile devices, laptops and electric vehicles etal, Thedevelopment of the cathode material is a key factor in restricing the performance of lithiumion battery, so, it become the main study object. Among of these, LiFePO₄become the firstchoice of lithium ion battery cathode materials, and it is an ideal material for power supply,because of their high energy density, high voltage, good cycle stability and environmentfriendly. The electronic conductivity and ion mobility of pure LiFePO₄were very low, if itwill be applied in life, it is necessary to improve its electronic conductivity, ion mobility andthe electrochemical properties of the material. People achieve to improve the electrochemicalproperties mainly from the following aspects: surface coating （carbon-coated and metalcoated）, metal doping, reducing the particle size. In this paper, we improve theelectrochemical properties by the hydrothermal method to reduce the particle size andcarbon-coated with solid-phase method.1. This article tells the history of lithium-ion batteries, working principles, characteristicsand applications, the research progress of cathode materials and introduces the advances oflithium iron phosphate （LiFePO₄） in details.2. We choose the hydrothermal method to synthesize materials, and explore differenthydrothermal synthesis temperature, different hydrothermal synthesis times affect onsynthetic samples. Through experiment, when the temperature is180℃, XRD diffractionpeak and standard spectrum figure keep in consistent basicly, belongs to olive stone typeLiFePO₄of Pnma space group, and diffraction peak sharp, crystal growth full, Crystal is good;from SEM figure, we know material particles size distribute among60¹00nm, uniformphase. when the temperature is incresased to200℃, the crystal grow too large,and occursaggregates,reduces the material properties. Therefore, we determines the best hydrothermalsynthesis time is180℃. The hydrothermal synthesis time effect on growing of crystal, so, wediscuss the hydrothermal synthesis time effect on sample in180℃, reaction time from4hrise to7h. From XRD, and SEM,we judge the best hydrothermal synthesis time is6h,it canbe proved by electrochemical performance that180℃, and6h is the best hydrothermalsynthesis conditions, discharge capacity is144.1mAhg-1at0.1C.3. We research different carbon sources effect on LiFePO₄with solid method. two carbonsources （starch and polyvinyl pyrrolidone PVP） coat the LiFePO₄. Experiments show that structure of LiFePO₄material will not be influenced by carbon-coated, and it can get a singlephase, completely structure composites LiFePO₄/C. By comparing different samples’s XRD,SEM, Raman and electrochemical performance, we discover LiFePO₄coated with PVPreflects better performance. discharge capacity of160.6mAh/g at0.1C, after100cycles,capacity retention rate keep at96.5%at5C rate. the structure of calcinated carbon impact onelectrochemical properties of LiFePO₄material. PVP is uniform distributed in the earlyresponse, after calcinating, carbon layers uniform distribut on the surface of LiFePO₄, carbonhas experienced cracking, polycondensation in the calcination process and form a network ofconductive structures, so, it improves effectively the electrochemical properties of thematerial.