This Small Business Technology Transfer (STTR) Phase I project seeks to establish the feasibility of an entirely new class of liquid electrolyte separator for Li ion batteries that has a built-in reversible thermal shutdown mechanism. With increasing use of Li ion batteries, safety is of particular concern as reports of occasional injuries appear in the news due to a short circuit, cell overcharge, or other overheating related catastrophic events. A separator medium having a reversible thermal shutdown mechanism means that Li ion batteries can automatically be shutdown before a thermal runaway incident occurs and can automatically be restarted once a safe thermal environment is restored. A Li ion battery separator with the targeted built-in reversible thermal shutdown capability not only addresses safety, but is also expected to extend the active life of a battery. The technical objectives of this project are to identify a suitable polymer material, optimize its material characteristics, and investigate the factors related to the reversible thermal shutdown, mechanical stability and durability. The polymer nanofiber web will be subjected to thermal cycling and the reversible thermal shutdown characteristics will be evaluated using impedance measurements. In this project, the composition and structure of a durable electrospun polymer nanofiber web with a reversible thermal shutdown characteristic will be identified.
The broader/commercial impact of this project, if successful, will be an improvement in safety for Li ion batteries. This innovation has a direct impact on the safety of users. Moreover, the tendency to overheat also impacts Li Ion battery operations. The associated costs to address both issues have continued to be a barrier for wider adoption of Li Ion batteries, and thus, the successful outcome of this project also has the potential of lowering the cost of the battery technology. Development of a separator with reversible thermal shutdown capability will help lessen the hurdles that are deterring the Li Ion battery technology from achieving its potential in a forecasted $7 billion end user market. This collaboration between the small business and an academic partner involves a post-doctoral scientist at the academic institution, who will gain commercially relevant research experience.
Safety is particularly important issue with Li ion batteries. Separators in an operating Li battery must be capable of shutting down above a certain temperature to avoid thermal runaway in the event of a short circuit, cell overcharge, or some other catastrophic event. Presently, this is accomplished by having the separator impedance increase dramatically and irreversibly, without a loss in mechanical integrity, at a preset elevated temperature that effectively stops ion transport between the electrodes. Such a shutdown mechanism is irreversible, i.e., once the separator melts, it is permanently damaged and the battery is no longer operable. In the present Phase I study it is our intent to identify a polymer nanofiber separator which can reversibly swell/deswell in Li ion battery electrolyte. The advantage of incorporating a reversible thermal shutdown separator in a Li ion battery is that the battery can be reused even after a thermal runaway incident. During the Phase I project we have identified a polyamide based reversible thermal shutdown separator polymer for Li ion batteries. The polymer was cross-linked in order to have controlled swelling and deswelling as a function of temperature and also to improve the mechanical properties during the swelling/deswelling in the electrolyte. The polymer along with the cross-linking agent was electrospun to form a web of nanofibers of with fiber diameter 250 nm and the nanofiber web was cured at optimum temperature. The nanofiber web swell excessively in a Li ion battery electrolyte above 100oC, when cooled to room temperature the extent of deswelling was controlled by the salt present in the electrolyte. With further improvement in the polymer chemistry and scale up and commercialization, the nanofiber based reversible thermal shutdown separators are expected to improve the safety in Li ion batteries and also maintain nation`s industrial competitiveness in global market place in the area of Li ion battery separator technology. The improvement in the safety of Li ion batteries means the broad based usage of them including in electric and hybrid electric vehicles.
