Synthesis And Electrochemical Properties Of Ti-based Oxides As Anode Materials For Lithium-ion Batteries

With the increasing concerns over increased energy consumption and environmentalissues, the demand for developing renewable and clean energy technologies is becomingmore and more critical. Rechargeable lithium-ion batteries （LIBs）, recognized to be the mostimportant energy storage and conversion equipment with advantages of high energy density,long cycle life and no “memory effect”, have the potential for large-scale applications incommunication facilities, portable electronics, stationary energy storage systems andenormous markets of electric vehicles.The battery performance of LIBs strongly depends on the electrode properties, and thusa strict choice of electrode materials including both the cathodes and anodes plays a greatrole. In general, graphitic carbon is the most common anode material for LIBs, because ofits low cost, high abundance and outstanding kinetics. However, its operating voltage isbelow0.2V versus Li/Li+. This voltage is close to the lithium electroplating potential,especially at high rates, which may cause a safety issue. In addition, a layer of electronicallyinsulating solid-electrolyte interphase （SEI） is inevitably formed on the surface of graphitebelow1.0V versus Li/Li+. A fundamental solution to solve this issue is to find excellentalternatives to graphite with better cycling stability and safety. Ti-based oxides such asLi₄Ti₅O₁₂and TiO₂have been considered as potential alternative materials, due to their goodreversibility and a higher working voltage assuring a better safety of LIBs. Nevertheless,the main disadvantages of Li₄Ti₅O₁₂and TiO₂lie in the low electrical conductivity, poorelectron transport and aggregation tendency of nanoparticles, resulting in the deteriorationof reversible capacity and rate capability, thus inhibiting their applications in LIBs. In thisdissertation, the electrochemical performances of Ti-based oxides were improved by newsynthetic and surface-modification methods. The main contents and results are summarizedas follows:The availability of high-quality nanocrystals underpins a diverse range of applicationsand investigations into size-dependent physical and chemical properties. Effective syntheticmethods that yield uniform nanocrystals are critically important. Here we demonstrate a fastand economical microwave-assisted solid-state method to prepare spinel Li₄Ti₅O₁₂ nanocrystallites in large quantities using cost-effective commercial TiO₂as a raw materialof titanium. This method easily programs the synthetic conditions including temperatureand time, and significantly shortens the synthesis time to minutes. The as-formed Li₄Ti₅O₁₂nanocrystals prepared by a microwave-assisted solid-state method exhibit a distinctivelynarrower particle size distribution without agglomeration, and the particle size ranges from100to350nm with an average size of180nm. When evaluated as an anode material forlithium-ion batteries, they exhibit greatly enhanced electrochemical lithium-storageperformances, including not only high rate capabilities but also a highly reversiblecapability of¹60mA h g–1over500cycles at1C.A fast and economical route based on an efficient microwave-induced solid-stateprocess has been developed to synthesize metastable TiO₂（B） nanobelts on a large scale.This new method reduces the synthesis time for the preparation of TiO₂（B） nanobelts to lessthan half an hour, allowing the screening of a wide range of reaction conditions foroptimizing and scaling up the production and facilitating the formation of metastable-phaseTiO₂（B）. The as-formed TiO₂（B） nanobelts with widths of30–100nm and lengths up to afew micrometers exhibit enhanced lithium-storage performances, compared with theTiO₂（B） product obtained by the conventional heating. This work provides a new way forlarge-scale industrial production of high-quality metastable TiO₂（B） nanostructures.Nanoporous TiO₂spheres coated with N-doped carbon （NC） have been successfullysynthesized via a facile solution-phase process and subsequent heat treatment. Comparedwith nanoporous TiO₂spheres, the as-formed TiO₂@NC nanocomposites shown asignificantly higher specific capacity and rate capability, which can be ascribed to thesynergistic effect of the porous structural configuration and the uniform NC layer with highconductivity. The TiO₂@NC nanohybrids not only delivered a high capacity of¹70mA hg–1at a current density of0.1A g–1, but also maintained an excellent performance as thecurrent density increased to2.0A g–1, demonstrating that the NC coating is a promisingapproach for preparation of high-performance electrode materials for lithium ion batteries.We demonstrate a facile but versatile in-situ photochemical strategy for graftinginorganic conductive molybdenum oxysulfide （MoO_xS_y） clusters in the framework ofnanoporous TiO₂spheres.The as-obtained TiO₂@MoO_xS_ynanohybrids as an anode material for lithium-ion batteries exhibit high lithium storage performances with high specificcapacity and remarkable rate capability. Results show that the conductive MoO_xS_yclustersare uniformly distributed on the surface of nanoporous TiO₂spheres. The synergistic effectof the nanoporous structure and the amorphous conductive layer of MoO_xS_yclusters maycontribute to the enhanced lithium-storage capability of TiO₂@MoO_xS_ynanohybrids. Thiswork presents a new powerful strategy toward preparing high-performance Ti-based anodematerials for next-generation LIBs with high power and energy densities. It is expected thatthe present photochemical grafting method can be extended to design other high-performance and multifunctional TiO₂@shell nanostructures, and more importantly,provide insight into the design of advanced conformal porous nanohybrids.