Dual phase TiO2 nanotubes for high power electrochemical energy storage devices
Date of Issue2017-05-08
School of Materials Science and Engineering
The development of ultrafast charging electrochemical energy storage (EES) devices with long cycle life is of great importance to the advancement of modern society with sustainable energy generation and consumption. Lithium-ion based EES devices offer high power, high energy and exceptional lifetime and are consistently pursued for such applications. In particular, TiO2 (B) material is promising anode candidate owing to its fast lithium diffusion kinetics and open channel structure. However, the high rate performance of TiO2 (B) is hindered by its poor electrical conductivity originated from large band gap (> 3.0 eV), therefore a large amount of conductive additives (> 15 wt%) are required to unleash the high power capabilities, which decreases the overall energy of the electrode since these additives are electro inactive within the operation window (1-3 V) of TiO2 (B). To address such issues, this thesis aims to improve the high rate performances of TiO2 (B) electrode while maximizing the total contents of active materials through integration of anatase phase. The ultimate scope is to develop fast charging TiO2 electrodes with maximum loading amount of active materials for high power and long life EES devices. To achieve such goal, the properties of TiO2 materials are investigated first in half cells with lithium metal as counter electrode. Applying the additive-free all TiO2 nanotube electrodes as platform, a systematic investigation of the electrodes conduction behavior via in situ electrochemical impedance study (EIS) indicates that much higher lithiation-induced conductivity is achieved in dual phase TiO2 nanotubes than pure phase TiO2 (B) at early stage of reaction, owing to the high conductivity of lithiated anatase. The resultant dual phase TiO2 nanotubes feature superior rate performances: a specific capacity of 112.1 mAh g-1 can be delivered at 24 A g-1 (~72 C), and higher capacity of 131.1 mAh g-1 can be achieved after 10,000 cycles at 30 C. Furthermore, even mechanical mixture of anatase particles to TiO2 (B) nanotubes exhibits much higher rate performances at 24 A g-1 (91.2 mAh g-1) than pure phase TiO2 (B) nanotubes (13.7 mAh g-1). Followed by the discovery of high lithiation-induced conductivity in dual phase TiO2 materials, a pre-lithiation process is carried out to improve the electrical conductivity and first cycle Columbic efficiency of the pristine TiO2 materials for applications in high power lithium-ion hybrid capacitor (LIC). The LIC cells with pre-lithiated TiO2 anode could achieve high energy density of 50.77 Wh kg-1 and high power density of 8.98 kW kg-1 at 20.7 Wh kg-1, compared to only 10.9 Wh kg-1 delivered by pristine TiO2 electrode at power density of 4550 W kg-1. Furthermore, the cycling stability of the pre-lithiated TiO2 materials is also improved with 73% capacity retention after 10,000 cycles. Finally, a water soluble sericin binder is developed for the first time to assist in formation of stable passivation layer in high voltage LiNi0.5Mn1.5O4 cathode to fabricate high energy and high power battery cells with TiO2 anode. The highest energy density of the full cell formulated with sericin binder can reach 687.6 Wh kg-1 (based on TiO2 mass). At higher power of 18.26 kW kg-1, the energy density can still reach 233.4 Wh kg-1, much higher than that of the PVDF binder (89.7 Wh kg-1). Furthermore, the full cell could be cycled at 10 C for 1,000 cycles with 80% capacity retention, further indicating the long-term stability of TiO2 and the sericin binder. In conclusion, this thesis unravels the importance of dual phase TiO2 electrode for ultrafast charging EES devices with ultralong cycle life. The lithiation-induced conductivity plays important roles in the utilization of TiO2-based electrode materials with fast charging capabilities. These findings provide guidance towards commercial adoption of TiO2-based electrode, in fact, large scale prismatic and cylindrical cells have already been fabricated for commercialization evaluation.