Georgia Institute of Technology Materials Science and Engineering

Committee Members:

Prof. Gleb Yushin, Advisor, MSE

Prof. Faisal Alamgir, MSE

Prof. Meilin Liu, MSE

Prof. Rampi Ramprasad, MSE

Prof. Preet Singh, MSE

Mohan Sanghadasa, Ph.D., US Army Futures Command

Polyphosphazene- and Acrylate-Based Polymer Electrolytes for Lithium Batteries   

Abstract:            

Safety concerns of traditional liquid electrolytes especially when paired with lithium (Li) metal anodes has increased research of solid polymer electrolytes (SPEs) to exploit the superior thermal and mechanical properties of polymers. SPEs are safer than the flammable liquid electrolytes with high vapor pressure used in conventional lithium-ion batteries and many demonstrate good compatibility with Li metal anodes. This creates an opportunity to store more energy per unit volume by replacing traditional graphite anodes with high-capacity Li metal or Li alloy anodes, thus reducing safety concerns while also improving performance.

A promising route to improve performance of SPEs is to anchor the anion to the polymer backbone, thus only allowing movement of Li-ions and eliminating the detrimental effects of polarization that are common in conventional dual-ion conducting SPEs. These types of SPEs, known as single Li-ion conducting solid polymer electrolytes (SLiC-SPEs), exhibit high Li-ion transference numbers (  , which limits Li dendrite growth thus further increasing the safety of SPEs and the cycle life of the battery. However, SLiC-SPEs suffer from inadequate ionic conductivity, small electrochemical stability windows, and limited cycling stability. In this dissertation, the synthesis of novel SLiC-SPEs and their potential application in Li metal batteries (LMBs) has been investigated. The focus of this dissertation is on the influence of chemical structure, Li-ion content, and membrane processing on Li transport and overall electrochemical performance of the synthesized SLiC-SPEs in LMBs.

For this work, two different SLiC-SPE systems were developed. The first being based on the largely unexplored polyphosphazene chemistry. Polyphosphazenes are primarily known for their use as flame retardant materials but have demonstrated high Li-ion conductivity owing to their highly flexible P=N backbone which promotes Li-ion conduction. While polyphosphazenes have not been widely considered as SPEs in the literature, a few existing examples showed promising ionic conductivity and compatibility with Li metal. In this dissertation, three novel polyphosphazene-based SLiC-SPEs comprised of lithiated polyphosphazenes blended with polyethylene oxide are investigated. The SLiC polyphosphazenes were prepared through a facile synthesis route opening the door for enhanced tunability of polymer properties via facile macromolecular nucleophilic substitution and subsequent lithiation.

The second SLiC-SPE system investigated is prepared through infiltration and in situ copolymerization of lithium 1-[3-(methacryloloxy)propylsulfonyl]-1-(trifluoromethane sulfonyl)imide (LiMTFSI) and poly(propylene glycol) acrylate (PPGA) on the LiFePO4 cathode surface. The LiMTFSI co-monomer contributes a highly dissociated Li-ion and the poly(propylene glycol) side chains on the PPGA co-monomer are ideal for solvating and transporting Li-ions. The focus is to improve the notoriously low ionic conductivities of SLiC-SPEs and reduce interfacial resistance between the cathode and SPE for enhanced electrochemical performance. The monomer solution is infiltrated into the cathode and then heated to initiate polymerization and the SLiC-SPE is formed in situ and is well-integrated into the cathode structure providing an excellent interface between the cathode and SLiC-SPE in a simple, yet scalable, manner. The effect of Li+ ion concentration in the copolymer and plasticizing additives on ionic conductivity and cycling performance is probed. Additionally, a synergistic effect between PPGA-co-LiMTFSI and other lithium salts is explored.