Novel Mesoporous Nanotitania/Carbon Composite Electrodes for Electrochemical Energy Storage

Abstract

The development of novel scalable nanoscale materials and fabrication techniques for high performance TiO 2 Li-ion electrodes is investigated in this thesis. The development and study of the novel fabrication methods is approached from the stand point of a binder-free electrode, and its comparison to standard binder-based electrodes, making use of commercial P25 TiO 2 product as active material. The characterization of novel titania synthesized by manipulating the parameters of an aqueous process, is performed for two phases of TiO 2 , namely anatase and brookite, based on comprehensive nanocrystal morphological and electrochemical property assessment. Finally all is brought together by investigating the application of the novel fabrication protocol to the aqueous synthesized anatase to engineer binderfree 3-dimensional electrodes, and studying their Li-ion intercalation performance.The fabrication of binder-free P25 nanotitania/carbon composite electrodes is pursued via formulating pastes and controlled sintering yielding enhanced electrochemical performance in comparison to standard binder-based methods using the same active and conductive components. The new paste formulation and sintering sequence provides a significant increase in dispersion of carbon due to smaller agglomerate size resulting in enhanced inter-particle (active-conduction) mixing and packing density than standard binder-based electrodes. Cyclic voltammetry and galvanostatic charge/discharge proved that the binder-free electrodes possess improved conductivity, specific capacity, and reversibility, as compared to binderbased electrodes. Most significantly, the capacity retention of the binder-free electrode was much higher than that of the binder-based electrode at 94 and 53%, respectively.iii Using an aqueous solution based synthesis process, 2-dimensional brookite nanoplatelets and 6 nm anatase nanoparticles are obtained and examined for their physical and electrochemical properties as Li-ion host materials. Brookite, itself is particularly interesting, as it is the least understood of the nanoscale TiO 2 phases, especially in the form of 2-D crystal structures that are deemed advantageous for Liion diffusion. This study provides insight into the lithiation mechanism of brookite, concluding that a solid-solution is formed upon lithiation corresponding to Li 0.5 TiO 2 that is isostructural with brookite. Further, the brookite nanoplatelets are discovered to undergo unique crystal structure enhancement upon cycling that allows them to quickly relax upon delithiation in an extremely efficient (>99.7% Coulombic efficiency) reversible manner translating to excellent Li-ion intercalation stability.Anatase nanoparticle are described in terms of crystallinity and surface area. Asprepared anatase has low crystallinity (∼70%), but high surface area (∼222 m 2 g −1 ), and when annealed at 300°C, the crystallinity increases to ∼90% but the surface area drops to 120 m 2 g −1 . Electrochemically, this allows the annealed sample with higher crystall...

Binder-Free Fabrication of Nanotitania/Carbon Lithium-Ion Intercalation Electrodes

“…XRD analysis involving Rietveld refinement, as described in detail elsewhere, 57 determined the following P25 phase composition: 86% anatase, 11% rutile and 3% brookite. In addition to XRD, identification of the different phase nanoparticles was made by t-EFSD.…”