Study On Synthesis And Modification Of Phosphate Cathode Materials For Lithium Ion Batteries

Polyanionic LiMPO₄（M=Fe, Mn, Co） have been regarded as highlypromising cathode materials for lithium ion batteries due to their outstandingsafety, fairly high capacity and excellent cyclic stability. However, onlyLiFePO₄has been successfully commercialized so far. This thesis focuses onLiCoPO₄and LiMnPO₄, which have higher electrode potential than LiFePO₄,and studies the effect of synthesis method, micromorphology and particle sizeon their electrochemical performances. The low electronic and ionicconductivities of LiCoPO₄and LiMnPO₄are improved by metal ion dopingand multiphase combination. The major research contents are as follows:LiCoPO₄materials were prepared by solvothermal approach in singlesolvent （water and ethylene glycol） and in mixed solvents （water/alcohol,water/ethylene glycol, water/isopropyl alcohol, water/benzyl alcohol andwater/polyethylene glycol400）, respectively, further coated with a carbonlayer via an in situ or ex situ method. The effect of solvent system onmorphology, particle size and electrochemical properties of LiCoPO₄wasanalyzed. Prickly ball-like LiCoPO₄hierarchical microstructure materialconstructed by numerous nanorods in diameter of ca.40nm and length of ca.1μm was prepared in water/benzyl alcohol mixed solvent. The growthprocess of the prickly ball was examined. The prickly ball-like LiCoPO₄ presented an initial discharge capacity of136mAh g^-1at0.1C rate, and85mAh g^-1at5C rate. The excellent electrochemical performance could beascribed to the small particle size and uniform carbon coating.Vanadium doping was used to enhance the electronic conductivity ofLiCoPO₄. The Li1+0.5xCo1-xVx（PO₄）1+0.5x/C （0.03≤x≤0.10） cathode materialswere prepared by high-temperature solid-state reaction. The effects ofvanadium content, calcination temperature and time, and carbon source onmorphology, structure and electrochemical properties ofLi1+0.5xCo1-xVx（PO₄）1+0.5x/C were examined. XRD data revealed that dopingsmall amount of vanadium into LiCoPO₄crystal lattice did not change itsolivine structure. The electronic conductivity and Li-ion diffusion coefficientin LiCoPO₄increased4fold and1.2fold by doping5%vanadium,respectively. Li1.025Co0.95V0.05（PO₄）1.025/C prepared at750oC for16hpresented the highest electrochemical reactivity. Its initial discharge capacityreached to135mAh g^-1at0.1C rate,23mAh g^-1higher than the pristineLiCoPO₄/C. Furthermore, vanadium doping reduced voltage difference ofcharge/discharge and improved lithium deintercalation/intercalationreversibility and cyclic stability. The charge/discharge test confirmed thatlithium extraction–insertion from LiCoPO₄occurs in a two-step mode. Thecell using Li4Ti5O12as anode and Li1.025Co0.95V0.05（PO₄）1.025/C as cathodedelivered an initial discharge capacity of128mAh g^-1with a dischargevoltage plateau around3.26V.LiMnPO₄cathode materials with plate-like, rod-like and dumbbell-likemorphologies were synthesized via a solvothermal process in water/benzylalcohol, water/polyethylene glycol400and water/ethylene glycol mixedsolvent, respectively. The function mechanism of organic solvent and surfactant in the morphology regulation was discussed. The effect of themicromorphology, particle size, and carbon coating method onelectrochemical performance was investigated systematically. After carbonlayer of ca.3⁴nm thick was coated on the surface of LiMnPO₄nanoplate inthickness of ca.35nm by chemical vapor deposition, the material displayed adischarge capacity of142mAh g^-1at0.1C and retained its93%after50cycles. Uniformly coated carbon layer can effectively reduce the dissolutionof Mn in the electrolyte. The mechanism of lithium extraction–insertion fromLiMnPO₄was properly identified with XRD and CV measurements.In order to improve the electrochemical activity of LiMnPO₄, thexLiMnPO₄·yLi₃V₂（PO₄）₃/C composites were synthesized by combiningLiMnPO₄with Li₃V₂（PO₄）₃, which shows higher electronic and ionicconductivity. The relationships of component proportion and crystal structure,micromorphology, capacity, cyclic stability, and high/low temperatureproperties were studied. The electrochemical performances of cathodecomposites prepared in various ways were compared. XRD and EDSmeasurements suggested that LiMnPO₄and Li₃V₂（PO₄）₃in the compositewere doped mutually by V and Mn, respectively, leading to a uniformmixture of LiMnPO₄and Li₃V₂（PO₄）₃nanocrystallines. Charge/dischargetests showed that the composite material had much higher reversible capacityand rate capability than LiMnPO₄/C. The discharge capacity of2LiMnPO₄·Li₃V₂（PO₄）₃/C at0.1C rate was148mAh g^-1, around1.4times ofLiMnPO₄/C.