Synthesis And Electrochemical Research Of Li₄Ti₅O₁₂ As Anode Materials For Lithium Ion Batteries

Spinel-structured Li₄Ti₅O₁₂ was considered to be one of the most promising anode materials for lithium-ion batteries due to its unique properties like high safety,low price and environmentally friendly compositions,comparing with the traditional anode materials. However,the low electronic and ironic conductivity of about 10^-9 S/cm at room temperature affects the electrochemical performance of Li₄Ti₅O₁₂,especially at a high cycling rate,which hinders its commercialization.In this thesis,Li₄Ti₅O₁₂ was synthesized via a solid-state reaction method and the effects of Li sources,carbon coating,surface modification and Nb/F co-doping were studied.At first,the effect of different Li/Ti ratio on the final product was studied.A series of spinel Li₄Ti₅O₁₂(a=0.8,0.82,0.84,0.86) was prepared by solid-state reaction using TiO₂ and Li₂CO₃ as raw materials.According to the XRD results,peaks of TiO₂ were observed when a≤0.84.As the ratio x increases,the intensity of the TiO₂ peak decreases.And when a=0.86, well-crystallized Li₄Ti₅O₁₂ without any impurities was obtained.The effect of different lithium sources(Li₂CO₃ and LiNO₃) on Li₄Ti₅O₁₂ was further researched.The investigation of XRD and SEM showed that the as-prepared samples have a high purity,good crystallization, and regular distribution with sub-micron size.And the sample prepared by LiNO₃ seems to have relatively smaller size.It was found that Li₄Ti₅O₁₂ synthesized via different lithium sources(Li₂CO₃ and LiNO₃) have high initial specific discharge capacity about 193 mAh/g and 197 mAh/g.According to the cyclic performance test,using LiNO₃ as lithium source is likely to have Li₄Ti₅O₁₂ anode material with better cyclic performance.The decrease of discharge capacity due to the elevated rates is smaller than that of Li₂CO₃ and the capacity retention rate is about 82%at 1 C.Li₄Ti₅O₁₂/C anode materials using different carbon sources(glucose and starch) were prepared via different routes(800℃12h,850℃12h,850℃15h,850℃18h) and electrochemical performance was studied.According to the research,samples prepared at 850℃for 15h had the best electrochemical performance.After 100 cycles at 1 C,the discharge capacity of Li₄Ti₅O₁₂/C prepared by glucose and starch maintains at the value of 122 and 114 mAh/g respectively.It was believed that the crystallization and the extent of graphitization of C together determine the electrochemical performance of electrode material. Then using AgNO₃ as silver additive,sub-micron sized Li₄Ti₅O₁₂/(Ag+C) with regular distribution was obtained by solid-state reaction.The Ag particles distributed on the surface of Li₄Ti₅O₁₂/C which sized between 50 and 150 nm,has positive impact on the electronic conductivity of the anode material.The existence of C is also proved to facilitate the formation of Ag particles.The initial discharge capacity of Li₄Ti₅O₁₂/(Ag+C) reached to a high value of 164 mAh/g at 1C.Undoped,Nb doping,F doping and Nb/F co-doping Li₄Ti₅O₁₂ samples were prepared via solid-state reaction using Nb₂O₅ and LiF as doping sources.And the effect of Nb/F co-doping on Li₄Ti₅O₁₂ was studied.XRD results revealed that the samples with spinel Li₄Ti₅O₁₂ structure were well-crystallized.SEM images showed that Nb/F doping would hardly change the morphology of Li₄Ti₅O₁₂.According to the electrochemical tests,Nb doping,F doping and Nb/F co-doping could effectively decrease the polarization of electrode,improve the reversibility of electrochemical reactions and thus lead to the enhanced cyclic performance. After 100 cycles at 2 C,the discharge capacities of Nb doping,F doping and Nb/F co-doping samples are 82 mAh/g,75 mAh/g and 74 mAh/g,much higher than the undoped sample Li₄Ti₅O₁₂(66 mAh/g).Furthermore,Nb/F co-doping didn't improve the electrochemical performance of Li₄Ti₅O₁₂ to a higher level comparing with Nb doping and F doping.