Titanium-based materials have been widely studied in the field of rechargeable lithium-ion batteries due to their stable structure, excellent cycle performance, abundant crustal resources, low cost, and non-toxicity . Since the working mechanisms of sodium-ion batteries and lithium-ion batteries are similar, titanium-based materials have also been widely used in the research of sodium-ion battery electrode materials. In addition, titanium-based materials have a lower charge and discharge voltage platform in sodium-ion batteries, which has attracted a lot of attention in their application research in the negative electrode field. At present, the research hotspots in titanium-based materials are mainly concentrated on materials such as titanium dioxide , lithium titanate , sodium titanate and sodium titanium phosphate. In recent years, the relevant work of researchers has mainly been to explore the oxidation/reduction mechanism and optimization of electrochemical performance of titanium-based negative electrode materials. This section mainly introduces the basic properties and crystal structures of several titanium-based negative electrode materials, and explains the modification research, electrochemical performance and sodium storage mechanism of the materials.
Ti02 negative electrode material
Titanium dioxide has attracted widespread attention as a typical embedded negative electrode material for lithium-ion batteries due to its advantages such as stable structure, non-toxicity, low price and abundant materials. Ti02 is composed of Ti06 octahedron and Ti4+. The polymorph of Ti02 is related to the connection mode of Ti06 octahedron. The current research types mainly include anatase, rutile, bronze and brookite types, crystal structure. Rutile Ti02 is a tetragonal crystal system. The Ti atom is located at the center of the lattice, and the six oxygen atoms are located on the corners of the octahedron. Each octahedron is connected to the surrounding ten octahedrons, eight of which share corners and two share edges. Every two Ti02 molecules form a unit cell. The structure of anatase Ti02 is also a tetragonal crystal system, in which each octahedron is connected to the surrounding eight octahedrons, four of which share edges and four share corners. Four Ti02s form a unit cell. Brookite TiO2 belongs to the orthorhombic system. Six TiO2 form a unit cell, and the structure is unstable. Therefore, brookite TiO2 is relatively rare in nature. Among these crystal phases, the rutile phase is the most stable. Anatase and brookite TiO2 can be transformed into the rutile phase through an irreversible exothermic reaction after high-temperature heating. Although the radius of Na+ is larger than that of Li+, since the activation barrier of Na embedded in the anatase lattice is almost the same as that of Li, most of the research results are centered on anatase TiO2. Moreover, among the various crystal forms of TiO2, natural anatase TiO2 is one of the earliest main structures for Li embedding.
Although TiO2 has many advantages such as low price and stable structure, it still has many shortcomings when used in the negative electrode of sodium-ion batteries. For example, since Ti4+ has no d electrons in TiO2, all TiO2 crystal forms are electronic insulators, which greatly limits the electrochemical performance of TiO2 in sodium-ion batteries. At the same time, due to some defects of TiO2 itself, coupled with the large radius of Na+ and slow transmission rate, its sodium storage performance is also not ideal. Therefore, many current studies on TiO2 focus on improving the electronic conductivity and ionic conductivity of TiO2 through material nano-sizing, interface modification, ion doping, different types of C composites and morphology and structure optimization.
Amorphous TiO2 is used as the negative electrode material in sodium-ion batteries. Amorphous TiO2 nanotubes are directly grown on titanium foil, and the obtained material is used as the negative electrode of sodium-ion batteries. The reversible specific capacity can reach 150 mA·h·g-1, and the battery has excellent cycle performance.
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