Study On The Synthesis And Electrochemical Performance Of Micro/nano-structured LiFePO₄ As Cathode Materials

Olivine lithium iron phosphate（Li Fe PO₄） has been widely used as cathode material in lithium ion power batteries due to its environmental benignity, superior safety and low cost. However, the low electrical conductivity andslow lithium ion diffusion restricts its mass applicationsin high power devices such as electric vehicles.The cooperation of both nanosize particlesand carbon coating layer can overcome the drawbacks of Li Fe PO₄. However, nanoparticles showhigher interfacial energy, lower tap density and worse machinability relative to microspheres, which seriously hinders their large-scale applications. In this work, the high-density Li Fe PO₄/Cmicrospherescomposed of primary nanoparticlesare designed and prepared. And the multiple conductive carbon networksare constructed in the Li Fe PO₄/Cmicrospheres, which can simultaneouslyimprove their power performance and specific volumetric capacity.Micro/nano-structured Li Fe PO₄/Cspherical materials with high rate and cycling performanceare synthesized using iron（III） acrylate as precursor for both iron and carbon source. Iron phosphate/polyacrylic acid（Fe PO₄/PAA） nanoparticles are produced by a co-precipitation reaction. The by-products（acrylic acid ions）are in-situ polymerized into a uniform PAA layer coated on the surface of Fe PO₄ nanoparticles. Li-intercalated Fe PO₄/PAA microspherical precursor is prepared by the spray drying technology. The primary nanoparticles with size of 40⁵0 nm are fully coated by a uniform and partially graphitic carbon layer derived from the decomposition of PAA layer. As a result of the unique micro/nano-structure, the as-prepared Li Fe PO₄/C microspheres display high rate performance（162.9 m Ah·g-1 at 0.1 C and 126.1 m Ah·g-1 at 5 C） and excellent cycling stability（97.1% of capacity retention after 500 cycles at 5C/5C）. This work not only obtains the high-density Li Fe PO₄/Csecondarymicrospheres, but also realized the utilityof acrylic acid ions, the by-products, as coated carbon source.In order to further improve the high rate performance, the excellent multiple conductive carbon networks are constructed in Li Fe PO₄/C microspheres. The firstconductive carbonnetwork is constructed by a uniform carbon layer on the surface of Li Fe PO₄ primary nanoparticles, which derives from the carbonation of in-situ polymerized PAA layer. Moreover, the acetylene black added in spay drying process fills the voids among primary nanoparticles toconstruct the second conductive network. Finally, thecarbon from optimized content sucrose used as binder and linker can interconnect primary nanoparticlestogether to form the third network. The synergistic effect among multiple carbon conductive networks andsubtle mesoporous structureformed inside them make Li Fe PO₄/C microspheres achieve simultaneouslythe highly speed electron transfer and lithium ion transportation.And their power performance is significantlyenhanced. In addition,the industrialization preparationtechnologyof micro/nano-structured Li Fe PO₄/Csphericalcompositesis researched. And it is intended to probe the influence of industrialization preparation parameters, including the choice of lithium source, sintering time and CNT content, on the physicochemical properties of the Li Fe PO₄/C compositesprepared.