Study On Solid-state Method And Modification Of Cathode Material Lifepo₄ Of Lithium Ion Battery

Olivine structure LiFePO4 attracted much attention as a promising cathode material for lithium-ion batteries.The overwhelming advantage of iron-based compounds is that, in addition to being inexpensive and naturally abundant, they are less toxic than Co, Ni, and Mn. Its commercial use has already started and there are several companies that base their business on lithium phosphate technology. Still, there is a need for a manufacturing process that produces electrochemically active LiFePO4 at a low cost. Therefore the interest in developing new approaches to the synthesis of LiFePO4 did not fade.However, it should be noted that there are three intrinsic negative aspects of olivine-type materials for LIBs:(1) the much lower intrinsic electronic conductivity in LiFePO4 which prevents full use of its theoretical capacity, particularly in the Mn-rich phase. It is likely that the low conductivity is related to the FeO6 linking and the large separation between Fe atoms; (2) the lower true volumetric density.(3) low Li-ion diffusion coefficient. These three intrinsic drawbacks of LiFePO4 pose a bottleneck for the commercial applications. In addition, reversible capacity loss at high current density is another fatal shortcoming of LiFePO4. This poor performance is regarded as the direct consequence of polarization arising upon cycling at high current density.Critical to the success of new cathode materials is their preparation,which controls the morphology, particle size and cation order. A major difficulty related to the synthesis of orthophosphate LiFePO4 comes from the existence of two oxidation degrees of iron in nature, namely Fe(Ⅱ) and Fe(Ⅲ), which makes the preparation of thismaterial with reproducible electrochemical properties difficult. Solid-state chemistry has already established as conventional route for obtaining well-crystallized particles of LiFePO4 with ordered structure.(1)The effects of different carbon source on the performance of LiFePO4 were systematically investigated.The results demonstrate that sucrose,added as conductive precursor before the formation of the crystalline phase,can optimize the performance of LiFePO4 effectively.The carbon content was optimized,and the results show that the material obtained by adding 5 wt.%C have much well electrochemistry capability.(2)Using very cheap raw material Fe203 and FePO4 as iron source and using suerose as reduction agent to synthesize LiFePO4/C composites by thermal reduction method.(3)The effects of cation doping on the physicochemical structure and electrochemical performance of produced cathode were investigated.at different doping position.Then we test and compare its XRD,SEM and the performance of electrochemistry.