Electrospun TiO₂-based nanomaterials for energy storage

Abstract

Energy storage is of great importance in supporting the wide application of two renewable clean energy sources including solar and intermittent wind energy. Batteries, such as Ni-MH, Ni-Cd, lead-acid, and lithium ion batteries (LIBs), become the leading candidates for electrical vehicles (EV) and possible for practical applications to store electrical energy in the form of chemical energy. Among of these batteries, LIBs outperform others due to their high volumetric and gravimetric energy densities, low self-discharge, wide temperature working window, long cycle life and no memory effect. These benefits make them gain remarkable success in portable electronics, such as notebooks, mobile phones, and digital cameras. However, in order to satisfy the demand of new markets in EVs and hybrid electric vehicles (HEV), LIBs are required with higher energy and power densities, higher safety, longer durability and lower cost. Titanium dioxide (TiO₂) has emerged as a promising anode material for LIBs due to its low cost, environmental friendliness, and structural stability during lithium insertion/deinsertion processes. However, poor electron transport limits its practical application, especially at high current densities. This thesis focuses mainly on developing novel TiO2-based anodes with excellent electrochemical performance in terms of high capacity, excellent rate capability and long cycle life by using electrospinning technique. By a simple coaxial electrospinning technique combined with subsequent calcination treatment, one-dimensional porous TiO2-carbon composite nanofibers (ODPTCNs) with plentiful pores as storage regions and high conductivity for the rapid transportation of both electrons and lithium ions was developed. The novel ODPTCNs show a remarkable specific reversible capacity of ~806 mAh g-1 and a high volumetric capacity of ~1.2 Ah cm-3, and maintain the capacity of ~680 mAh g-1 after 250 cycles at a current density of 100 mA g-1 and exhibit an exceptional discharge rate capability of 5 A g-1 while retaining a capacity of ~ 260 mAh g⁻¹ after 1600 cycles. In order to increase the energy density of TiO₂ materials, MnO₂ with a theoretical capacity approximately triple higher than that of the graphite anode was decorated with TiO₂ to form a core/shell TiO₂-MnO₂/MnO₂ heterostructures by combining an electrospinning technique with a hydrothermal reaction. The large surface area of the resulting materials offers sufficient electrode/electrolyte interface to promote the charge-transfer reactions, which yields a better rate capability. The porous structure of TiO₂-MnO₂/MnO₂ nanofibers not only facilitates Li-ion access, but also accommodates large volumetric expansion during the charging/discharging processes, resulting in an excellent cycle performance. This material delivers a high reversible capacity of 891 mAh g-1 at the first cycle and maintains the capacity of 888 mAh g-1 after 50 cycles at the current density of 0.1 A g⁻¹; it also shows a remarkable rate capability of 2 A g⁻¹ while retaining a capacity of 185 mAh g⁻¹ after 500 cycles. Nanotube structure is also a promising structure for LIBs due to its typical advantages such as one-dimensional (1D) and hollow structures, which can improve the electrochemical performance of the electrodes. A novel nanoarchitecture constructed by Co₃O₄ nanoparticles encapsulated in the porous binary Co₃O₄-TiO₂ nanotubes was exploited by simple electrospinning and hydrolysis method, followed by a calcination. The prepared hybrid nanomaterials show enhanced electrochemical performance in terms of remarkable specific reversible capacity of ~1007 mAh g-1 at the current density of 0.1 A g⁻¹ after 160 cycles, and an extraordinarily stable capacity retention of 673 mAh g⁻¹ after 2000 cycles at a current density of 3 A g⁻¹. A novel Sn-nanoparticle in-situ insertion approach by lithiation was exploited to address the problem of large expansion of Sn and thus achieve improved performance. Sn nanoparticles are inserted into the pores of highly stable titanium dioxide-carbon (TiO₂-x-C) nanofiber substrates by lithiation, which can effectively localize the Sn nanoparticles. During lithiation, Sn reacts with Li to form Li4.4Sn alloy accompanying by large volume change, and thus insert into the pores surrounding the initial Sn nanoparticles in the TiO₂-x-C nanofibers. However, the Li4.4Sn alloy cannot recover to the original Sn nanoparticle with diameter about 150 nm after delithiation due to the surface absorption force between Sn nanoparticle and TiO₂-x-C substrate, separating many smaller Sn nanoparticles in the pores of TiO₂-x-C nanofiber. These detached Sn nanoparticles remained in the pores, which were able to accommodate their expansion, thus yielding a very long cycle life. The lithiation-induced size reduction of the Sn nanoparticles leads to their uniform distribution in the fibers, which enhances the electrochemical activation. Batteries containing these porous TiO₂-x-C-Sn nanofibers exhibit a high specific capacity of 957 mAh g⁻¹ after 200 cycles at 0.1 A g⁻¹ and can cycle more than 10,000 times at 3 A g⁻¹ while retaining more than 82.3 % of their capacity (0.177 % decrease per 100 cycles), which represents the longest cycling life of Sn-based anodes for LIBs so far.