Synthesis And Modification Of Lithium Titanate As Anode Material For Lithium Ion Batteries

Spinel Li₄Ti₅O₁₂has been known as one of the most promising anode material forlithium-ion batteries, owning to excellent safe, simple synthesis process, cheaperproduction cost and friendly environment. However, its intrinsic electronic conductivityis only10^-9S/cm, deemed as insulating material. Therefore, Li₄Ti₅O₁₂presents a poorrate capability, far from requirement of industrial application. In this thesis, ahigh-temperature solid-state synthesis is used as a basic method to explore the optimalsynthesis parameters. Then, a new wet-chemical method, reverse microemulsion, isapplied for preparing Li₄Ti₅O₁₂nanoparticles. Finally, various Li₄Ti₅O₁₂compounds aresynthesized by coating carbon or Ag so as to improve the electronic conductivity andrate performance.Analytical Li2CO3and anatase TiO₂were weighed, and the mixture was sintered toobtain spinel Li₄Ti₅O₁₂anode. The roles of the reaction temperature and excess amountof lithium had been investigated, which affect purity and electrochemical properties ofthe final products. The experimental results revealed that pristine Li₄Ti₅O₁₂anodematerial obtained at700℃presents other phases, but the other two LTO samples firedat750and800℃do not. Interestingly, we found the best excess amount of lithium is4%and these samples display outstanding chemical performances. Therefore, the bestsynthesis conditions are750-800℃and4%of Li weight excess. It provides a solidfoundation for further prepare and modification Li₄Ti₅O₁₂anode material.Orthogonal experiment method was applied to study the effect of CTAB, water,butyl titanate and acetylacetone four factors on hydration products. The optimalparemeters of the reverse micromulsion system, namely2g CTAB,1ml water,1mlbutyl titanate,6ml acetylacetone,2.4ml n-butanol and11ml cyclohexane, was figuredout to prepare nanosized TiO₂precursor. Then, a small amount of carbon nanotubeswere mixed into the hydration product, and the compounds were sintered usingsolid-state synthesis method to obtain the carbon-coated LTO/X/C/CNTs with minorTiO₂phase. According to SEM images, the net-like structural carbon nanotubes wereadsorbed by a thin layer of carbon-coated LTO nanoparticles with the size of20-50nm.At0.15C, its initial discharge capacity is as high as224mAh/g. When the currentdensity is increased to24C, the capacity of the carbon-coated LTO/X/C/CNTs sample remains about90mAh/g. The life cyclic curve shows that its specific capacity can keepabout105.9mAh/g at12C after800charge/discharge cycles, with the capacity retentionrate of85.6%. These results clearly proved that the carbon-coated LTO/X/C/CNTssample obtained by reverse microemulsion displays excellent rate capability and cyclicperformance.The nitrogen-doped carbon coating layer was utilized to enhance theelectrochemical properties of Li₄Ti₅O₁₂. Sucrose and ammonia were used as the carbonand nitrogen sources, respectively. The nitrogen-doped carbon-coated LTO/C20/N andLTO/N/C20samples were obtained by two novel routes （Solid-state process â… å'ŒSolid-state process Ⅱ） combined with solid-state synthesis. According to TEM andHRTEM results, a tooth structural carbon layer was found and its thickness is less than5nm on the surface of LTO/C20/N. On the LTO/N/C20sample, an uniform compositecarbon layer can be easily observed about4nm. The rate performance profile indicatesthat the modified LTO/C20/N and LTO/N/C20composites reveal110and100mAh/g atthe high current rate of24C, respectively, with excellent rate capability. Thecharge/discharge curves show that the coated LTO materials present relative lowelectrochemical polarization （the difference of the charging platform potential anddischarge plateau potential）. The CV results indicate that the carbon-coated samplesdisplay lower apparent Li-ion diffusion coefficient than that of pure LTO. Neverless, therate performance of LTO anode material is determined by lithium-ion mobility andelectronic conductivity, where a competitive balance point should be chosen for theoptimized performance.The silver mirror reaction was used to obtain Ag-coated LTO composite. Thecharge/discharge curve displays that coated Ag layer can enhance the rate capability ofthe LTO sample. However, in spite of increasing Ag contain to13.7%, the continuousconductive network is difficult to be formed. Whereras, it leads to the specific capacitydown to only about75mAh/g, which is quite less than that of carbon-coated LTOsample.