Synthesis And Modification Study Of Li₂FeSiO₄/C By A Solid-state Method

Compared with the traditional cathode materials, such as LiCoO₂,LiNiO₂and LiMn₂O₄et al, Li₂FeSiO₄has Lower cost, more friendly forenvironment, higher safety, better electrochemical stability. But, thelow intrinsic conductivity and diffusion coefficient greatly limit theirsapplication in lithium ion batteries. Therefore, to improve theelectrochemical performance of the Li₂FeSiO₄, in this thesis, a seriesof modified and comparative studies based on carbon, Mn²⁺, Al³⁺doping have been investigated.Different carbon contents of Li₂FeSiO₄/C material aresynthesized by a vacuum high temperature solid-state method usingglucose as a carbon source. The effects of carbon contents onphase structure, morphology and electrochemical performance ofLi₂FeSiO₄/C are discussed. Compared with the undoping carbonmaterial, carbon doping decreases the gain size of products, andmakes impurities increased. At the same time, thecarbon-containing materials show agglomerations seriously, poorhomogeneities. The results of charge-discharge show thatappropriate carbon content can improve electronic conductivity andelectrochemical performance of the Li₂FeSiO₄materials. When thecarbon content is4.15%, the initial discharge capacity is108.9 mAh/g. After30cycles, the discharge capacity is77.9mAh/g, andthe capacity retention is71.5%. In addition, with the increasingcycles, the differential capacity vs. potential curves of all thesamples show a transition process from high voltage to low voltageand maintain a similar voltage until10cycles. Combined with thefact, besides the improved diffusion coefficient(D_Li+) values thannon-carbon sample，all the samples show a gradual increased D_Li+values during the first9cycles, after this the D_Li+are basically thesame. We guess that the structural rearrangement of Li₂FeSiO₄maybe a process in a gradual change happened in the first9-10cycles.Li₂Fe_1-xMn_xSiO₄/C cathode materials are prepared by a vacuumhigh temperature solid-state method, then theirs phase structure,morphology and electrochemical performance are investigated andcontrasted. The results show that the Mn²⁺doping increases thecomplexity of solid-state reaction and impurities contents in theproducts, the peak of MnO impurity can be observed obviously. Atthe same time, the gain size reduces obviously, the morphology ofproducts presents loose. The electrochemical impedancespectroscopy （EIS） measurements show the Mn²⁺doping hasimproved electrode reaction rate and lithium ion transportcapability of materials. However, due to the reduced contents ofactive materials, the charge-discharge capacity of Li₂Fe_1-xMn_xSiO₄/Cis lower than that of Li₂FeSiO₄/C, except for Li₂Fe_0.8Mn_0.2SiO₄/C （itdisplays an first discharge capacity of190.7mAh/g）. Furthermore,note that the capacity of Li₂Fe_1-xMn_xSiO₄/C materials fades rapidly.Based on the reaserch of Li₂Fe_0.8Mn_0.2SiO₄/C material,Li₂Fe_0.75Mn_0.2Al_0.05SiO₄/C and Li₂Fe_0.7Mn_0.2Al_0.1SiO₄/C materials are synthesized and contrasted. Compared with Li₂Fe_0.8Mn_0.2SiO₄/Cmaterial, after the Al³⁺doping, a small amount of LiAlO₂impurity arefound, while MnO and Li₂SiO₃impurities decrease or even disappear inthe products. However, the Al³⁺doping lowers electrode reactionrate, inhibits the migration of lithium ion to some extent in theactive materials, and makes the discharge capacity reduce slightly.But, the coulomb efficiency and cycle performance are improvedobviously. Especially, the initial discharge capacity ofLi₂Fe_0.75Mn_0.2Al_0.05SiO₄/C sample reached159.3mAh/g, the capacityretention is78.0%after50cycles. Besides, the differential capacityvs. potential curves of Li₂Fe_0.75Mn_0.2Al_0.05SiO₄/C reappear well after2cycles, and the peak patterns are still evident, indicating that theAl³⁺doping promotes the rearrangement process and the crystalstructure stability of Li₂FeSiO₄materials.