Introduction to the basic properties of lithium titanate and preparation methods
Spinel Li4Ti5O12 is a composite oxide of metallic lithium and low-potential transition metal titanium, in the form of white powder, as shown in Figure 1. AB2X4 series, spinel structure, belong to the solid solution Li1+xTi2-xO4 (0≤x≤1/3) series, with cubic structure, space group Fd3m, and three-dimensional diffusion channels for lithium ions. Among them, Li+ occupies the positions of tetrahedron 8a and 1/6 octahedron 16d, the remaining octahedron 16d position is occupied by Ti4+, and O2- is distributed at the position 32e. So spinel Li4Ti5O12 can be written as Li（8a）[Li1/3Ti5/3](16d)O4（32e）, and its structure is shown in Figure 2(a). The positions of the octahedron 16c and the tetrahedrons 8b and 48f are vacant. When discharging, the Li+ meter located at position 8a is transferred to position 16c, and the Li+ meter to be inserted is transferred to position 16c through position 8a, and converted into rock salt phase lithium titanium oxide: Li2（16c）[Li1/3Ti5/3](16d)O4（32e）. In this process, 3/5 of Ti4+ gets electrons and undergoes a reduction reaction to convert to Ti3+; as shown in Figure 2(b) and (c), the opposite is true when charging. The chemical equation of the reaction is as shown in formula (1-1). Under 1.55V (vs.Li/Li+) potential, each molecular unit of Li4Ti5O12 (spinel type) can be embedded with 3 Li+, and its theoretical specific capacity is 175mA·h/g.
Li（8a）[Li1/3Ti5/34+](16d)O4（32e）+Li++e– ↔Li2（16c）[Li1/3Ti2/34+Ti3+](16d)O4（32e） （1-1）
During the conversion of Li4Ti5O12 (spinel phase) to Li7Ti5O12 (rock salt phase), the lattice constant changes from 0.8364m to 0.8353nm, so the volume change is small (the volume change of graphite can reach about 10%), so Li4Ti5O12 is called a “zero strain” material. Due to the small volume change during the charge and discharge process, Li4Ti5O12 has the stability of almost no change in structure. Therefore, the negative electrode material used as the negative electrode of the battery has a long service life. Currently, lithium titanate-based lithium-ion batteries that can be recycled up to 20,000 times have been commercialized. In addition, Li4Ti5O12 has a relatively high lithium insertion potential (1.55V vs. Li/Li, graphite 0.1V vs. Li/Lit), and it is not easy to precipitate lithium dendrites, so the safety during charge and discharge is relatively high. Based on the long life and high safety of Li4Ti5O12, lithium-ion batteries with Li4Ti5O12 as the negative electrode can be used in power batteries, energy storage batteries and other fields.
Spinel Li4Ti5O12 has vacancies at the positions of the octahedron 16c and the tetrahedron 8b and 48f. These vacancies make it a conductor of Li+, but the band gap of empty ti-3d is as high as 2 ~ 3eV, which makes Li4Ti5O12 show low electronic conductivity (~1013s/m) and lit diffusion coefficient (10~1013cm2/s), resulting in poor electrochemical performance of the electrode material at high rate. In addition, the discharge potential of Li4Ti5O12 is relatively high and the specific capacity is relatively low. As shown in Figure 3, the theoretical value is only 175mA·h/g (graphite can reach more than 300mA·h/g). These two shortcomings are the biggest bottleneck of its commercial application. The current main methods for preparing Li4Ti5O12include three methods: solid phase method, sol-gel method and solvothermal method. These three methods have been extensively studied, and their principles and processes have been relatively mature.
1) Solid phase method
The solid-phase method usually uses Li2CO3, LiOH·H2O, TiO2 and Ti(OH)4 as raw materials, uniformly mixed in organic solvents or water, and then dried to form a precursor, and finally calcined at a temperature above 800°C to obtain Li4Ti5O12.
2) Sol-gel method
The sol-gel method for preparing Li4Ti5O12 usually involves adding organic matter as a chelating agent to the reactant solution to make it form a gel, and then heat treating it to obtain Li4Ti5O12. Usually tetrabutyl titanate, titanium isopropoxide, etc. are used as the titanium source, lithium acetate, lithium hydroxide, etc. are used as the lithium source, water, ethanol, etc. are used as the dispersion system, and citric acid, acrylic acid, oxalic acid, etc. are used as the integrating agent.
3) Solvothermal method
The solvothermal method is a method in which the lithium source and the titanium source are reacted in a high-pressure aqueous or organic solution at 100 to 200°C, and then the crystal is further calcined to obtain a finished product.
In addition to the above three methods, there are molten salt method, microwave method, combustion method, template method and electrostatic spinning method for preparing Li4Ti5O12.