Dually Li Ion/Electron-Conductive Materials for Polymer-Based, Printable Lib Electrodes

Tuesday, October 13, 2015: 14:30
213-B (Phoenix Convention Center)
M. B. McDonald (MIT) and P. T. Hammond (Massachusetts Institute of Technology)
Limitations in lithium ion batteries (LIBs) reside foremost in their 1) energy capacity, and 2) size and configuration modulation. Polymer properties are generally attractive for next-generation electronics due to their flexibility, facile tunability with known chemistry, and processability. Specialized polymers, such as conjugated polymers, are electrically conductive, while others, such as poly(ethylene oxide) (PEO), form matrices suitable for Li ion transport. Inert polymers such as poly(ethylene) and poly(propylene) are fabricated to form ordered porous electrode separators (‘Celgard’) that are typically wetted with liquid electrolyte to form lithium polymer batteries. In this work, the desired properties of polymers are bestowed unto LIBs via the modification of these preformed polymer separators, which are used as the skeletons for all-polymer, wet-fabricated LIBs.

First, commercial separators were infiltrated with PEO in place of toxic, flammable solvents to form the solid state electrolyte. For the electrodes, previously, conducting polymers (CPs) have been shown to act as a binding materials to not only adhere active material, but form conductive pathways to bulk charge collectors for Li ion cathodes such as LiFePO4. Herein, the brittle, usually carbon-based scaffolds and binders are removed to form entirely conducting polymer bulk electrodes that are flexible. Active cathode (LiCoO2) and anode (graphite, Si) microparticle materials are incorporated to optimize battery capacity by exploiting excessive polyanion dopant in the commercially available CP suspension PEDOT:PSS as a dispersing agent, yielding “LIB electrode inks” that are printable onto the solid electrolyte support.

Transport of Li ions through the polymer matrix to access active material is systematically investigated both by forming a porous polymer network akin to carbon paper electrodes, and by fabricating composites of PEDOT:PSS with PEO. The mixtures are optimized for electronic conductivity, Li ion conductivity, and capacity through control of the active material content. All-polymer thin LIBs can then be entirely fabricated through wet deposition across the commercial separator scaffold and demonstrate behavior consistent with the current rigid LIB configurations. This novel configuration juxtaposes the all-polymer printed LIB, where dispersions consisting of PEDOT:PSS-PEO and active material as electrodes and a solution-based polymer electrolyte are wet-cast onto a substrate layer-by-layer.