Advanced Structural Electrode Materials for Lithium-Ion Batteries and Lithium-Sulfur Batteries.

摘要

Batteries, as one of the most appropriate and promising electrical energy storage systems, are playing a vital role in future use of electrical energy. Significant efforts have been placed on exploring high specific capacity electrode for advanced lithium based batteries, including lithium-ion batteries and lithium-sulfur batteries.;Lithium-ion batteries and lithium-sulfur batteries are two distinguishing energy storage systems that have different energy storage mechanisms. Rechargeable lithium-on batteries have two Li+ intercalation electrodes, in which the electrical and chemical energies are interconverted via a reversible Li+ intercalation/de-intercalation process between the electrodes. Rechargeable lithium-sulfur batteries operate by reducing elemental sulfur in the discharge process to produce a series of soluble lithium polysulfides to ultimately form solid lithium sulfide and converting lithium sulfide back to elemental sulfur in the charge process.;For lithium-ion batteries, the research focused on the use of silicon as an anode material since it has the highest theoretical capacity of 4200 mAh/g, which is four times higher than that of graphite (375 mAh/g). Silicon has the greatest potential to replace graphite for use in next-generation lithium ion batteries. However, challenges from its semi-conductive property and the huge volume expansion (300%) during the lithiation process greatly hinder silicon's application into lithium-ion batteries. The study of how to harness silicon into an electrode with superior chemical performance has been one of the driving factors for this work. Gaining this understanding in regards to silicon may also provide potential processing routes to other active materials, sensitive to volume change in lithium-ion batteries. In this work, silicon and carbon composites, including silicon-carbon nanofibers and silicon-carbon nanotubes, were designed and developed to try to solve the problems caused by the silicon volume expansion, enabling a specific capacity of more than 1000 mAh/g as a promising anode materials for lithium-ion batteries.;In addition to lithium-ion batteries, advanced cathode design of lithium-sulfur batteries was studied and discussed. The motivations for studying lithium-sulfur batteries come from two most important merits of sulfur: (a) abundance and low cost, and (b) high capacity of 1675 mAh/g. Lithium-sulfur batteries have a 3~5 fold higher theoretical energy density than conventional lithium-ion batteries. For sulfur cathodes, the main barriers to commercial production are their short cycle life, low charging efficiency, and high self-discharge rate, which are caused by the non-conductivity of sulfur and the migration of the dissolved sulfur reduction products out of the cathode region. Therefore, the electrochemical properties of sulfur and its cathode structures deserve investigations and discussions in this work. An advanced sulfur electrode was firstly developed and studied in my work, which exhibited a good cycling performance with a high sulfur loading (2.6 mg/cm2) and high sulfur content (65%) as cathodes for lithium-sulfur batteries.