Functional polymers are invisible components of devices used for generating and storage of energy. Examples are proton conducting polymers as membrane electrode assembly in PEM fuel cells and lithium-ion transporting separator membranes in batteries. Separator membrane is a critical component of a battery. It provides a barrier between the anode and the cathode while enabling the exchange of ionic charge carriers from one side to the other. Separators currently used in lithium-ion batteries are made of porous polyolefins. They are rendered porous by a mechanical biaxial extrusion process. As the battery heats up, the protective layer on the anode breaks down followed by breakdown of electrolytes into flammable gases. This, in turn, causes the polyolefin separators to undergo catastrophic shrinkages above 120° C leading to shorting of cells causing sparks that ignite the electrolyte resulting in a fire. The inherent safety risks associated with the separator membranes threaten the continued advances of lithium-ion battery in applications requiring higher energy density.

Polymers are critical to, both, the efficiency of such devices as well as safety in their operation. It is, therefore, not surprising that considerable amount of current research is devoted to the identification of suitable polymer substrates, the ability to transform them into functional materials through chemical and physical methods, as well as better understand the relationship between polymer structure, function and device performance. Functional polymers are, thus, poised to play a significant role in the emerging energy generation and storage devices.

This lecture will provide an overview of this area in terms of the nature of polymers that are of interest and their structure-function relationship [1]. Methods of modifying the polymers will be discussed in relation to their performance in specific applications. The concept of porosity in polymers will be introduced as a key parameter that defines ion mobility across polymer membranes. The nature of lithium cation solvation and ion pairing   in liquid electrolytes as well as its speciation has been probed using   ⁷Li solution state NMR and HOESY NMR experiments as well as ⁷Li NMR self-diffusion measurements [2]. Porous functional polybenzimidazoles (PBIs) with intrinsic porosities as well as macroporous PBIs have been prepared [3,4]. Their applications as separators for lithium ion battery will be discussed. Porous PBIs show useful properties as safe and non-flammable separators for lithium-ion battery applications.  The influence of porosity on electrical conductivity and kinetics of lithium ion transport has been studied.  Identification of an efficient separator polymer material for lithium-sulfur battery is still a challenge. Some early understanding on principles of designing functional porous polymers that can facilitate lithium ion transport and at the same time inhibit the transport of polysulfide ions [5] and all organic polymer anode capable of intercalating lithium-ions will be discussed [6].