Synthesis And Characterizations Of TiO₂Anode Materials For Lithium Ion Batteries

Lithium-ion batteries have been widely applied in cell phones, laptop computersand other electronic devices due to their high energy/power densities, low toxicity andlong cycle life. Currently, carbonaceous materials such as graphite, are the mostcommonly used anode materials in lithium-ion batteries. However, they suffer from arather high irreversible capacity during the initial cycles and serious safety problemsduring repeated charge-discharge cycling. Therefore, researchers devote to looking forother materials to replace carbonaceous anode materials. In recent years, titaniumdioxides (TiO2) have attracted great interest as anode materials for lithium-ionbatteries. TiO2have excellent safety performance, superior cycle stability and ratecapacity due to their higher voltage of titanium dioxides (~1.6V vs. Li/Li+) andstable structure. However, currently the electrochemical performance of TiO2does notmeet the applied requirements of the lithium ion battery in others applications such as,hybrid electric vehicles, electric vehicles and smart power grid. Therefore, it isimportant to prepare TiO2with outstanding tap density, superior capacity, excellentcycling and rate performance. In this paper, we systematically study the effects ofmorphologies and structures on the electrochemical performance of TiO2.Firstly, a facile microwave hydrothermal process is developed to prepare anataseTiO2microspheres that maintaining multiple properties including high surface area,high crystallinity, uniform mesoporous, perfect microspheres and uniform particlesize. Using this fine anatase TiO2product, a TiO2/RGO (RGO: reduced grapheneoxide) hybrid material is prepared under UV-light irradiation. The nano sized TiO2crystallites not only provide short pathways for Li+ion diffusion but also enlarge surface area for interfacial Li+ion storage. And their mesoporous microstructure isbeneficial for soaking of the electrolyte, thus facilitating the Li+ion transportationthrough the electrolyte. Incorporation of RGO further improves the electrochemicalkinetics of the TiO2microspheres, which results in superior electrochemicalperformance of the hybrid materials in terms of specific capacity, rate capability andcycle stability. The hybrid material shows a discharge capacity of155.8mAh g-1atthe5C (0.84A g-1) rate. Even at the60C (10.08A g-1) rate, a high discharge capacityof83.6mAh g-1is still obtained, which is two times higher than that of pristinemesoporous TiO2.Secondly, TiO2-B anode for lithium-ion batteries is prepared by the hydrothermalmethod. The samples have a large uniform manoribbons with10-200nm in width andseveral micrometers in length. The styrene butadiene rubber and sodium carboxylmethyl cellulose (SBR/CMC) and polyvinylidene fluoride (PVDF) binders are used toprepare the TiO2-B electrodes. Scanning electron microscope (SEM) andelectrochemical impedance spectroscopy (EIS) show that the electrode prepared withSBR/CMC has better electrode maintainability and electrochemical kinetics whichresult in better electrochemical performance. The optimized SBR/CMC binder contentis proposed to be in the range of12~15wt%. In addition, the1M LiPF6electrolytedissolved in EC: DMC=3:7is more suitable for the TiO2-B electrode. Using thisbinder content and electrolyte, the TiO2-B material exhibits superior capacityretention and rate capability. Even at the10C (3A g-1) rate, the material still shows adischarge capacity of142.5mAh g-1which keeps very well after800cycles. Based onthis work, it is concluded that SBR/CMC is a promising binder for the TiO2-B anodewhich provides not only cheaper and environmental friendly but also much betterelectrochemical performance than PVDF.Then, TiO2-B nanowires are synthesized via microwave assisted hydrothermalmethod at different reaction temperatures. Increasing the hydrothermal temperature,the length of nanowires will decrease, but the width is almost not changed. TiO2-B nanowires have large specific surface area (155.60m2g-1), which not only enhancesthe area between active materials and electrolyte but also facilitates the transportationof Li+ion. The electrochemical performance is affected by the crystallinity and aspectratio of the nanowires. The nanowires synthesized at180°C have best electrochemicalperformance. At1C (0.3A g-1) rate, after100cycles, the discharge capacity of220.5mAh g-1, indicates the sample have superior cycling performance. Even at30C (9Ag-1) rate, a high discharge capacity of107.7mAhg-1is still obtained.Finally, TiO2-B/NG hybrid materials are prepared through the use of in situhydrazine monohydrate vapor reduction and microwave assisted hydrothermalmethods. The nitrogen can enter into the graphene, and the nanowires will anchor ingraphene by Ti-O-C during the hydrazine monohydrate vapor reduction process.Cyclic voltammograms (CV) and EIS results illustrate that the hybrid materials havebetter electrochemical kinetic than TiO2-B nanowires, which results in excellentproperties in terms of cycling performance and rate capability as well as capacity forLIBs. After1000cycles, TiO2-B/NG sample shows the discharge capacity of203.1mAh g-1at10C (0.3A g-1) rate. Even at60C (18A g-1) rate, a high dischargecapacity of140.6mAh g-1is still obtained. This approach would be very helpful inboosting the electrochemical performance of other low electronic conductivitynanomaterials.In this paper, the electrochemical performance of TiO2is adjusted by changevarious kinds of factors which includes morphologies, structures, crystallinity,conductivity of TiO2and the electrodes structure. This works give us a theoretical andtechnical guidance for basic research and practical applications of TiO2anodematerials in lithium-ion batteries.