Functional Polymers for Beyond-Li-Ion Batteries: Elucidating Structure-Chemistry-Property Relationships for Engineered Ion Transport in Gel Polymer Electrolytes

To meet the energy needs of tomorrow, next generation rechargeable batteries that have high energy density and consist of sustainable materials are in dire need of development. Of the many so called “beyond-Li-ion” chemistries being explored, the metal-sulfur battery chemistry is especially attractive due to its material sustainability and high theoretical energy density. This thesis presents work to fill critical gaps regarding control of ion transport in next-generation batteries, both for more general concerns such as increasing cationic conductivity, and for concerns specific to sulfur-based batteries like the polysulfide shuttle. Specifically, this work raises the following questions: What are the structure-chemistry-property relationships for crosslinked gel-polymer electrolytes, and can those relationships be used to intelligently engineer ion-regulating functional polymers for better performing batteries? To answer those questions, the use of structural, electrostatic, and dielectric means to control ion transport in crosslinked gel-polymer electrolytes are investigated.

Regarding cationic transport, it is demonstrated here that by changing the crosslinker chemistry in polymer electrolytes designed for Li, Na, K, and Ca systems that cation transport can be enhanced. Specifically, reducing the oxygen density in the polymer network for gel electrolytes was found to enhance cation mobility by discouraging cation coordination with the polymer network. This approach led to an order of magnitude increase in cationic conductivity for select cases.

Now consider the polysulfide shuttle effect, wherein active material intermediates known as polysulfides escape the battery cathode, leading to fast capacity fading. Demonstrated here is that cross-linked polymer gels with the right balance of segmental dynamic inhibiting ionic aggregates, highly dissociable tethered anions, and low polarity crosslinking monomers can repel polysulfides, helping to immobilize them in the cathode of magnesium-sulfur batteries. For Li-S batteries, a similar relationship between capacity improvement and lower polymer dielectric constant is observed. Further, it is shown that unless the cross-linked polymer is porous or sufficiently tough, Li-S batteries using polymer gel electrolytes will suffer quick and catastrophic dendritic failure.

To contextualize the Mg-S results and provide deeper insight, liquid electrolytes from the literature were investigated and a largely undescribed phenomenon of Mg-S self-discharge was observed and explored. Understanding the mechanism and kinetics of this self-discharge reaction helps to interpret both future and past results of polysulfide shuttle inhibiting polymers. With the knowledge of serious self-discharge in Mg-S batteries, a siloxane-based gel polymer electrolyte was designed and found to improve performance owing to the low polysulfide solubility in the polymer gel. The low solubility leads to minor improvements in terms of self-discharge and a boost in cell efficiency.

Taken in totality, the work presented here lays the foundation for directed engineering of gel polymer electrolytes that meet the specific ion transport needs of next-generation rechargeable batteries.