Aramid Nanofiber Composites for Energy Storage Applications

Abstract

Lithium ion batteries and non-aqueous redox flow batteries represent two of the most important energy storage technologies to efficient electric vehicles and power grid, which are essential to decreasing U.S. dependence on fossil fuels and sustainable economic growth. Many of the developmental roadblocks for these batteries are related to the separator, an electrically insulating layer between the cathode and anode. Lithium dendrite growth has limited the performance and threatened the safety of lithium ion batteries by piercing the separator and causing internal shorts. In non-aqueous redox flow batteries, active material crossover through microporous separators and the general lack of a suitable ion conducting membrane has led to low operating efficiencies and rapid capacity fade. Developing new separators for these batteries involve the combination of different and sometimes seemingly contradictory properties, such as high ionic conductivity, mechanical stability, thermal stability, chemical stability, and selective permeability. In this dissertation, I present work on composites made from Kevlar-drived aramid nanofibers (ANF) through rational design and fabrication techniques. For lithium ion batteries, a dendrite suppressing layer-by-layer composite of ANF and polyethylene oxide is present with goals of high ionic conductivity, improved safety and thermal stability. For non-aqueous redox flow batteries, a nanoporous ANF separator with surface polyelectrolyte modification is used to achieve high coulombic efficiencies and cycle life in practical flow cells. Finally, manufacturability of ANF based separators is addressed through a prototype machine for continuous ANF separator production and a novel separator coated on anode assembly. In combination, these studies serve as a foundation for addressing the challenges in separator engineering for lithium ion batteries and redox flow batteries.