The Study On The Modification Of Nanosized Titanium-based Anode Material For Rechargable Batteries

Electrode is a very important part, which directly effect the electrochemical performance of the battery. Particularly for the anode, because of the week thermo stability and chemical stability of graphite in the commercial lithium-ion battery, the safety problem is a big concern. In this research, lithium titanium oxide(Li₄Ti₅O₁₂) was introduced as anode material for lithium-ion batteries. Li₄Ti₅O₁₂ is named as “zero strain insertion material” providing with good safety and cycling performance, becoming a promising anode material for high-performance lithium-ion batteries.Due to the rarity and uneven distribution of lithium resource, the price of lithiumion batteries is very high. It is severely limited in the large-scale energy storage system such as smart grids. Sodium-ion batteries have the advantages of sufficient sodium resource and low cost. Therefore, sodium-ion batteries have potential in large-scale energy storage application instead of lithium-ion batteries. The research on sodium-ion batteries is a relatively new field, some issues need to be settled. Because the volume of sodium ion is larger than lithium ion and the reducing voltage is higher than lithium ion, it is more obvious on the problems of volume change and structure instability of electrode after inserting sodium ions. Therefore, the electrochemical activity of sodiumion batteries need be improved as well as the performance of electrode materials. Herein, Li₄Ti₅O₁₂ and TiO₂ were introduced into the research of sodium-ion batteries and the modification method to improve the properties of Li₄Ti₅O₁₂ and TiO₂.Though Li₄Ti₅O₁₂ and TiO₂ are provided with high safety and long life span in lithium-ion batteries and sodium-ion batteries, the low conductivity prevent their potential applications in high-performance sencondary batteries. In order to improve the conductivity, in this thesis, nanostructure design together with carbon-coating and element doping methods were adopted in the titanium-based anode material to study about the working mechanism and modification mechanism. The work was divided into four parts as follows:（1） In order to improve the energy density and electronic conductivity, Li₄Ti₅O₁₂ embedded in carbon nanofiber structure was designed by electrospinning and thermal treatment. The resultant material was used as anode material for lithium-ion batteries.Nanostructure is an effective modification method to improve the electrochemical properties of energy storage materials. Three different temperatures（550oC, 650 oC, 750oC） were used in the thermo treatment process. The morphology and structure of Li₄Ti₅O₁₂ were characterized. Electrochemical tests were also utilized to discuss the influence of carbonization temperature on the physical and electrochemical properties. According to the SEM results, Li₄Ti₅O₁₂ nanoparticles were uniformly distributed in the carbon nanofibers. The electrochemical properties including cycling performance and rate performance, the sample of Li₄Ti₅O₁₂ in carbon nanofiber possessed better results than untreated commercial Li₄Ti₅O₁₂. Nanofiber structure benefit to shortening the distance of ion diffusion and providing pathway of electron transfer, enhancing the electrochemical activity. As a result, Li₄Ti₅O₁₂ in carbon nanofiber prepared at 750oC（LTO@CNF-750oC） achieved high reversible capacity（156.8 m Ah/g）, good cycling stability, high average coulombic efficiency（99.2%）. The capacity maintained 164.7 m Ah/g after 100 cycles. LTO@CNF-750 oC had good rate capability. The capacity was 101.2 m Ah/g when the current density rise to 800 m A/g.（2） Li₄Ti₅O₁₂ is provided with good thermo stability and structure stability, excellent safety for lithium-ion batteries, but there is few report about Li₄Ti₅O₁₂ using in sodiumion batteries. In this part, Li₄Ti₅O₁₂ was applied as anode material in sodium-ion batteries and improved performance by adding with carbon nanofiberand copper element doping method. Based on the Li₄Ti₅O₁₂ in carbon nanofiber, certain amount of copper ions were doping into Li₄Ti₅O₁₂ crystal（Li4-x Cux Ti5O12@CNF, x=0, 0.05, 0.1）. The morphology and structure were characterized. The electrochemical properties were also tested and compared, aim to discover the mechanism of sodium ion inserting into Li4-x Cux Ti5O12@CNF and discuss the influence of element doping on properties of Li4-xCux Ti5O12@CNF anode material. The results showed that copper doping did not change the spinel structure of Li₄Ti₅O₁₂, and the degree of crystallization was reduced with the increase of doping amount. SEM illustrated the size of Li₄Ti₅O₁₂ particles were decreased with the increase of copper doping. When the doping amount was 0.05 mol(Li_3.95Cu_0.05Ti₅O₁₂@CNF), the crystals were uniform and well distributed. In the electrochemical tests, electrochemical impedance spectroscopy（EIS） showed Li_3.95Cu_0.05Ti₅O₁₂@CNF the lowest resistance. Li_3.95Cu_0.05Ti₅O₁₂@CNF possessed highest discharge capacity of 244.2 m Ah/g, highest reversible capacity of 158.1 m Ah/g, good cycling stability, indicating copper doping was effective to improve the electrochemical performance of Li4-x Cux Ti5O12@CNF in sodium-ion batteries and the best doping amount was 0.05 mol.（3） Compared with Li₄Ti₅O₁₂, TiO₂ has advantages of good structure stability, sufficient resource, low cost and higher theoretical capacity, which makes it a promising anode material for sodium-ion batteris. Because of the sluggish of sodium ions, TiO₂ with micro size shows low activity according to the literature, so that nanosized TiO₂ were used in the most research. However, the high surface area of nanosized TiO₂ would prompt the secondary reaction with electrolyte and forming unstable solide electrolyte interface（SEI）, resulting to irriversible capacity and bad cycling performance. Therefore in the experiment, poly（vinyl pyrrolidone）（PVP） was used as carbon resource to coat onto TiO₂. In this way, electronic conductivity would be improved and SEI on the TiO₂ surface would be stablized. The morphology and structure were characterized and the electrochemical properties were also tested to explore the function of carbon coating. Consequently, after carbon coating, the reversible capacity of TiO₂@C was 242.3 m Ah/g. The capacity retention was 87.0% after 100 cycles. At the current density of 800 m A/g, the capacity achieved 127.6 m Ah/g, higher than pure carbon and pure TiO₂.（4） In the previous research, carbon coating was proved an effective way to modified TiO₂. However the cycling performance and rate capability of TiO₂@C still did not meet the requirement of high-performance sodium-ion batteries. It is maybe because TiO₂ particles were not distributed well and did not mix with carbon very well. In this part, the structure of TiO₂ particles distributing in the carbon nanofiber was prepared by electrospinning process for high-performance sodium-ion batteries. Besides, TiO₂ has different crystal structure, which may effect the ablity of sodium ions insertion in TiO₂. In order to study the relationship of TiO₂ crystal structure and electrochemical properties. Different thermo treatment temperatures（450oC, 550 oC, 650 oC, and 750oC） were used to synthesize TiO₂@CNF. The morphology and structure of TiO₂@CNF were characterized to find the relationship of treatment temperature and crystal structure. The electrochemical properties were also tested to discuss the relationship with crystal structure and electrochemical properties. As a result, TiO₂@CNF showed continuous fibrous structure loading with TiO₂ nanoparticles. The crystal structure of TiO₂ in TiO₂@CNF-550 o C was antase, while The crystal structures of TiO₂ in TiO₂@CNF-650 oC and TiO₂@CNF-750 oC were anatase and rutile simultaneously. At the low current density, TiO₂@CNF-550 o C had the best capacity and capacity retention after 100 cycles, while TiO₂@CNF-650 o C possessed the best rate capability.