Synthesis of Nanostructured TiNb2O7 Anode: High Capacity and Improved Rate Performance for Lithium Ion Battery Applications

Titanium-based materials have been regarded as promising alternatives to carbon-based electrode materials due to their high-voltage lithium insertion processes using a Ti⁴⁺/Ti³⁺ redox couple (~1.5 V vs. Li/Li⁺). Within this high voltage region (> 1V), the electrode is safe from the formation of solid-electrolyte interphase (SEI) and a lithium dendrite formation, thus guaranteeing a stable cycle performance and fast charging/discharge performance. However, the most negative point of the anode using the Ti⁴⁺/Ti³⁺ redox couple is their theoretically low charge capacities (Li₄Ti₅O₁₂: 175 mA h g^-1 and anatase TiO₂: 168 mA h g^-1).

 As a new electrode material, TiNb₂O₇ (TNO) was reported by Goodenough’s group in 2011. This material can insert 5 Li ions due to the multiple redox couples (Ti⁴⁺/Ti³⁺, Nb⁵⁺/Nb⁴⁺, and Nb⁴⁺/Nb³⁺), corresponding to a high theoretical capacity of 387.6 mA h g^-1. This value is much higher than that of other Ti- and Nb-based electrodes. However, due to the high formation temperature of monoclinic TNO phase, it was hard to synthesize nano-sized or nano-structured TNO materials. Above 900 °C, primary particles grow to a large size, in the order of 100 nm. Therefore, an effective fabrication method for the nanosized TNO crystals is necessary in order to achieve rapid Li storage, as in the case of other electrode studies.

 In this presentation, we report the synthesis of an ordered mesoporous structured TiNb₂O₇ (OM-TNO) and its high capacity and rapid rate performance as an anode in LIB. The OM-TNO was synthesized using block-copolymer-assisted, one pot synthesis. As a structure-directing agent, Poly (ethylene oxide-b-styrene) interacts with the hydrolyzed Ti- and Nb-sol, resulting in a segregation of PS blocks and PEO/precursors. After heat-treatment at 900 °C, nano-sized TNO crystals are formed inside the pore walls. As an anode, OM-TNO electrode exhibited high reversible capacity of 289 mA h g^-1 at a 0.1 C rate, and high rate capability; 40% capacity retention at a 50 C rate (= 19.35 A g^-1). The enhanced electrochemical performance is attributed to the OM-TNO’s unique features, such as the small crystallites, large mesopores, and high surface area.