Synthesis And Nitrogen And Phosphorus Doping Modification Research Of LiFePO₄ As Cathode Material For Lithium Ion Batteries

Olivine structure Lithium iron phosphate as a lithium-ion battery cathode material have advantages of having a source of abundant, inexpensive and environmental friendly. Its theoretical specific capacity is 170 mAh/g and has a stable discharge platform at about 3.4 V and excellent security as well as cycle performance,However, there are some reasons limit its development, which are its low intrinsic electronic conductivity and ion diffusion rate of pure LiFePO₄. So this thesis is committed to synthesize nanoscale LiFePO₄ material and modify it through carbon coated and doped CNTs as well as incorporate heteroatom in the carbon layer to improve its electrochemical properties.1、Prepared nano-LiFePO₄ material by homogeneous precipitation method, at the same time carbon-coating it and incorporated with CNTs, then obtained LiFePO₄/C/CNTs composites by firing furnace, the composite particle size is more smaller than others.The cycle performance and rate performance of 3% CNTs sample is the best, it keep stable charge and discharge platform at 10 C and the capacity is 104.3 mAh/g;2、Nano-LiFePO₄/C composites synthesized through homogeneous precipitation method, then adding appropriate polyphosphoric acid（PPA）, polyethylene imine（PEI）and egg lecithin（EL） to it by mechanical milling method, prepared lithium iron phosphate composites with nitrogen and phosphorus doping in carbon layer. The test results showed that the LiFePO₄/C/EL sample have excellent electrochemical performance at high magnification. There are still relatively stable platform for the charge and discharge at 20 C and the discharge capacity is 70.7 mAh/g, the cycle capacity after 100 times maintain a rate of about 96% at 0.5 C;3、A series of LiFePO₄/C-（N+P） composites prepared by egg lecithin（EL） and LiFePO₄/C through homogeneous precipitation method and mechanical milling method. The electrochemical test results exhibited that with the addition of 15% EL,the sample have an optimal electrochemical performance, the charge and dischargecurve at 20 C while still having a stable platform, and the discharge capacity reached70.7 mAh/g.