Institute of Superconducting and Electronic Materials - Faculty of Engineering
Abstract
The lithium-ion (Li-ion) battery possesses many outstanding advantages over the well known rechargeable battery systems, in particularly higher energy density and longer shelf life, as well as not suffering from the memory effect problems of Ni-MH batteries. Those advantages are making it the greatest energy source of choice for the portable electronic market. Graphite and LiCoO2 are commonly used in commercial Li-ion battery. Despite their widespread utilization, the current electro-active materials have reached to a limit in terms of delivering even higher power, energy density, and longer cycle life for the new, emerging field of large-scale energy storage systems, such as in the automotive industry. Hybrid and fully electric cars need safer, cheaper, and higher performing batteries in order to offer an important alternative to combustion engines. To overcome the shortcomings of the current Li-ion battery, improvements are needed to push the Li-ion technology to the next level. Hence, the motivation for this PhD work is to search for potential electro-active materials, by means of the synthesis of film and powder based electrodes, fine-tuning of the composition of the composite electrodes, and characterization of them for possible application in Li-ion rechargeable batteries. Among the anode candidates studied were free-standing carbon nanotube (CNT) films, tin glycolate, and polypyrrole coated silicon (Si-PPy) nanocomposite materials. Two cathode candidates were also studied: lithium manganese oxide (LiMn2O4) thin film, and polypyrrole coated lithium trivanadate (LiV3O8-PPy) composite. Ionic liquid based polymer electrolyte was also studied to enhance Li-ion battery safety features. Free-standing CNT film electrodes have been synthesized by a simple vacuum filtration method. The free-standing electrodes were produced without any binder or metal current collector, which significantly reduced the total weight. The free-standing CNT film electrodes were also flexible and had good electrical conductivity with the addition of carbon black. Three different types of CNTs were used, i.e. single-wall CNTs (SWCNTs), double-wall CNTs (DWCNTs) and multi-wall CNTs (MWCNTs). The films based on MWCNT are much better than SWCNT and DWCNT films in terms of their electrochemical performance, with stable cycling behavior of 300 mAh g-1 after 40 cycles. A detailed study revealed that MWCNT electrode exhibited a reversible, sharp, and intense peak at approximately 0.15 V vs. Li/Li+ during the Li+ de-intercalation process. A thin solid electrolyte interphase (SEI) layer was observed on the surface of MWCNTs after prolonged cycling. This proved that only multi-wall CNTs have the capability for significant Li+ ion intercalation/de-intercalation. Novel tin glycolate particles were prepared by the polyol-mediated method. The prepared powders consist of fine tin-based particles (80 – 120 nm), encapsulated within tin glycolate shells. When applied as an anode material for Li-ion batteries, the glycolate shells buffered the volume expansion upon Li-Sn alloying, and thus the tin glycolate particles showed a high specific charge of 416 mAh g-1 beyond 50 cycles. Novel Si-PPy nanocomposite was prepared by coating the Si particle surfaces with PPy by the in situ chemical polymerization method. The cycle stability of Si-PPy nanocomposite electrodes was greatly enhanced with 50 wt. % PPy. The loading level of PPy plays a major role in determining the stability of the nanocomposite, and consequently creates a good matrix to improve the electrical conductivity, buffer the volume change during cycling, and prevent cracking and pulverization of the Si. A new approach was developed to rapidly synthesize nanostructured LiMn2O4 thin films by flame spray deposition (FSD) and in situ annealing. The LiMn2O4 films on stainless steel current collector exhibited good cyclability, with two pairs of redox peaks at approximately 4.00 and 4.15 V vs. Li/Li+. The study indicated that spinel LiMn2O4 thin films can be prepared by the fast and efficient FSD method. LiV3O8-PPy composites were synthesized by a low-temperature solution route followed by an in situ polymerization method. For LiV3O8 material, only 24 wt. % of PPy is needed to enhance the electrical conductivity and stability of the composite electrode, which delivered a specific charge of 183 mAh g-1 beyond 100 cycles. A solution casting method was used to prepare IL-PE composite membrane. The composite membrane was then assembled with LiV3O8-PPy (24 wt. % PPy) composite cathode and tested as a lithium polymer battery at room temperature. The cell delivered 200 mAh g-1 with respect to the mass of the cathode material
