Fundamental Research Of Ti-O Based Anode Materials For Li/Na Ion Batteries

Titanium is very abundant in the Earth’s crust, which is forth among all metal elements, up to 0.6%, behind aluminium, ion and magnesium. In recent years, due to their special physical and chemical properties, titanium oxides have been successfully applied in capacitor, photocatalysis, solar cell and lithium ion (Li-ion) battery. In the charge and discharge processes, due to its good reversibility and no structural change, the spinel Li4Ti5O12 is also called as a zero-strain lithium insertion material. More and more studies have been focused on this material as an anode material for Li-ion batteries. However, spinel Li4Ti5O12 is a semiconductor, and suffers from a problem of poor rate capability mainly due to its low electronic conductivity. Although at low current density, it shows good electrochemical performance; however, when charging at high current density, its capacity decays quickly. The main methods utilized to improve the electronic conductivity are nanocrystallization and doping.In this work, nano Li4Ti5O12 with different sizes has been synthesized and its electrochemical performance has been tested as an anode material for Li-ion batteries, which revealed the relationship between the battery and capacitor performance. To improve the electronic conductivity, Mg-doping and Ru-doping are also studied. The grain size of Mg-doped Li4Ti5O12 is about 20 nm, and the electronic conductivity is improved. The charge and discharge voltage plateaus of the doped material are more obvious and closer to the theoretical voltage plateau (1.55 V) than the undoped, especially at high C-rates.Li4Ti5O12 doped with Ru is successfully synthesized and forms a solid solution Li4Ti4.95Ru0.05O12. which can also improve the electronic conductivity. Meanwhile, LiaTi4.95Ru0.05O12 is discharged to 0.01 V, and more Li ions are inserted into the crystal structure. The charge and discharge capacities are enhanced and simultaneously long cycle life is acquired. At the current density of 1750 mA g-1, the specific discharge capacity is 242 mAh g-1 over 100 cycles. And after 500 cycles, it can retain 185 mAh g-1, demonstrating an excellent electrochemical performance.However, the mass use of Li will increase the cost of Li-ion batterv. which limits its large-scale application and future development. Sodium element, which belongs to the group one (IA) as lithium, possesses similar chemical property to lithium. Moreover, Na is abundant in nature. Therefore, if Na-ion batteries with excellent performance were commercialized, they will possess more competitive advantages, especially in large scale applications. At present, Na2Ti3O7 is one of the main anode materials for Na-ion batteries for its obvious and low voltage plateau, high capacity and low cost.Its structure consists of zigzag layers of titanium oxygen octahedra with sodium ions in the interlayer space that are easily exchanged. This material shows excellent C-rate and it is also the lowest voltage ever reported oxide insertion electrode for Na-ion batteries. The main problems are as follows:poor performance at high current densities, large irreversible capacity loss, low capacity and capacity retention ratio.First-principles simulations are employed to investigate the insertion of sodium into the Na2Ti3O7 structure by calculating formation energy and volume change. The mechanism of electrochemical reaction and theoretical capacity of Na2TiO7 is obtained. Single crystalline Na2Ti3O7 rods are obtained through sintering a precursor synthesized in a reverse micelle. After tens of cycles, the lattice fringes are still clear, uniform and orderly, which is mainly attributed to the long-range ordered, crystal defect-free and stable single crystalline. In order to optimized the property, a microspheric Na2Ti3O7 material consisting of tiny nanotubes of ca.8 nm in outside diameter and a few hundred nanometers in length has been synthesized by hydrothermal method. This material exhibits excellent C-rates. and the electrical double-layer capacitor mechanism is proposed, which can be applied in fast charge and discharge energy storage systems. NaHTi3O7 nanotubes are synthesized by oil bath, the plane space of which is 0.87 nm, corresponding to the spacing between the neighboring TiO6 octahedron layers of the nanotube walls. NaHTi3O7 nanotubes show good capacity and cycle performance due to the layered crystal structure and nanotube morphology.