Synthesis and li-ion insertion properties of highly crystalline mesoporous rutile TiO₂

Mesoporous TiO₂ has attracted great attention as a promising Li insertion electrode material with improved cycling life, rate capability, and high power density. Up to date, mesoporous anatase TiO₂ has been investigated for Li insertion. Recent studies have shown that nanosized rutile could be an excellent candidate for anode materials for higher Li insertion capacity and improved stability. However, synthesis of highly crystalline mesoporous rutile has met with limited success so far. There has been no report on Li insertion of mesoporous rutile TiO₂. In this paper, we report a new low-temperature solution growth of TiO₂ nanocrystals within an anionic surfactant matrix to produce highly crystalline mesoporous rutile and investigate Li insertion properties of the mesoporous crystalline rutile. X-ray diffraction (XRD) patterns and N₂ sorption isotherms reveal that mesoporous structure in the highly crystalline mesoporous TiO₂ directly results from the anionic surfactant templating effects with high surface area (245-300 m²/g) and tunable mesopore diameter ranging from 2.2 to 3.8 nm after calcination. Transmission electron microscopy (TEM) measurements show that framework of the highly crystalline mesoporous TiO ₂ are composed of aligned rutile nanorod building blocks grown along [001] direction. The new mesoporous crystalline rutile can accommodate more than 0.7 Li (Li_0.7TiO₂, 235 mA h g^-1) during the first discharge at a C/S rate between 1 and 3 V versus Li⁺/Li, with a reversible capacity of 0.55 Li (Li_0.55TiO₂, 185 mA h g^-1). The mesoporous crystalline rutile shows excellent capacity retention with less than 10% capacity loss after more than 100 cycles. XRD and TEM characterization on the electrochemically lithiated sample show that the rutile nanorods were transformed into cubic rocksalt LiTiO₂ nanorods, but the mesostructures remained stable after the phase transformation and cycling. Furthermore, the crystalline mesoporous rutile may also have good potential for other applications such as stable catalyst supports.