The commercial lithium ion batteries based on graphite anode is known for its low rate performance. Titanium oxide appears to be a promising building block for electrodes used in lithium-ion storage owing to its superior charge storage capability, environmental benignity and low cost. Unfortunately, titanium oxide has poor ionic and electronic conductivity as well as substantially low lithium-storage capacity. This poses numerous challenges towards achieving high performance devices.
Several researchers have devoted their efforts explore various polymorphs, including titanium oxide-bronze, rutile and anatase phases. Titanium oxide-bronze exhibit faster lithiation/delithiation kinematics and higher capacity as compared to rutile and anatese. However, it preparation requires harsh and complex conditions making it less attractive for large-scale preparation.
Another promising approach is through structure and morphology control. Including composite architectures based on low-dimensional titanium oxide; Core-shell morphologies such as carbon coated titanium oxide nanoparticles and cable like titanium oxide carbon nanotubes. The low-dimensional titanium oxide offers shortened ion diffusion length and the conductive inclusions enable effective electron transport granting these composites high electrochemical storage performance. However, the titanium oxide nanocrystals are assembled around the conductive moieties that may disassemble easily from the conductive networks and lead to rapid capacity fading. More importantly, previous works with low dimensional crystals often demonstrate very low tapping density (<0.2 g cm3) which would not be feasible in real applications.
Researchers led by professors Yunfeng Lu and Hexing Li at university of California Los Angeles and in collaboration with scientists at Shanghai University of Electric Power and Shanghai Normal University, developed a nanocomposite architecture prepared by in-situ growth mesoporous single-crystal-like titanium oxide particles threaded through by carbon nanotubes. Their work is now published in Nano Energy.
Assembled from sub-10 nm anatase nanocrystals, single-crystal secondary particles in micrometer scale with mesoporous features and threaded by carbon nanotubes (CNTs) was synthesized by one step sovolthermal method. No further high temperature heating required.
The electrochemical performance of the composite was characterized by using coin-type cells with loading of 2-3 mg/cm2. From the galvanostatic charge/discharge curves, the composite achieved 260 mAh g-1 at 0.5C which is among the highest capacities of the reported TiO2 anode. A relatively short voltage plateau (phase charge) with a long slopping “tail” (200 mAh g-1) indicated a high surface charge contribution. Even at an extremely high rate of 30C, it still delivered 120 mAh g-1. The researchers also investigated the composite performance at extended voltage window from 0.005-2.7V. Such lower voltage was often avoided due to formation of solid electrolyte interface (SEI) and further lithium insertion would challenge the robustness of the active materials. With sub-10nm building nanocrystals, the composite achieved 440 mAh g-1 at 50mA g-1. As seen from the cyclic voltammetry diagram, such “extra” capacity below 1V main came from surface contribution. Above 1V to 2.7V, the composite still maintained the reversible anatase phase confirmed by the in-situ X-Ray Diffraction. Overall, high rate charge discharge at 2000 mA g-1 for 1000 cycles was demonstrated with negligible capacity fading. The scanning electron microscopy after cycling confirmed the structure was maintained
By threading single crystal -like titanium oxide mesocrystals with carbon nanotubes, the research team was able to realize high rate anode composite for lithium ion batteries. The composites with iso-orientated primary nanocrystals has successfully enhanced the rate and cycling performance. Importantly, the tapping density (1.12 g cm3) is much closer to the real applications which was often ignored in nano-scale engineering. This method might bring some guidelines for the functional materials and device towards highly efficient energy storage systems.
Yiting Peng, Zaiyuan Le, Meicheng Wen, Dieqing Zhang, Zheng Chen, Hao Bin Wu, Hexing Li, Yunfeng Lu. Mesoporous single-crystal-like TiO2 mesocages threaded with carbon nanotubes for high-performance electrochemical energy storage. Nano Energy, volume 35 (2017), pages 44–51.Go To Nano Energy