Improving the performance of solid polymer electrolytes for lithium batteries via plasticization with aqueous salt or ionic liquid
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The goal of this dissertation is to investigate and enable polyethylene oxide (PEO)-based solid polymer electrolytes (SPEs) for lithium batteries. Specifically, two different strategies to plasticize the PEO matrix for improving ion transport are explored. PEO has a propensity to crystallize below 60C, rendering ion motion too slow to be commercially competitive and constituting one of the main challenges of utilizing PEO SPEs as an alternative to organic liquid electrolytes. ILSPEs incorporating ionic liquids (ILs) were fabricated by blending PEO, IL, and corresponding lithium salt followed by hot-pressing the mixture into a homogenous film. Aqueous SPEs (ASPEs) were fabricated by blending a highly concentrated solution of lithium salt in water (aqueous salt) with PEO followed by hot-pressing in a similar manner. Thermal analysis and electrochemical characterization were carried out for both classes of SPEs to assess their suitability as electrolytes and to optimize their composition for performance. Additionally, engineering the interface between the SPE and electrodes remains challenging and is critical for achieving good cycling performance. Multiple approaches for quality interface creation are proposed and carried out. Optimized ILSPE compositions show resistance to oxidation and were able to achieve room temperature conductivity of 0.96 mS/cm at room temperature, a value suitable for commercial application, as well as good rate performance at room temperature cycling in Li/ ILSPE/ lithium iron phosphate configuration. ASPE compositions exhibit conductivities between 0.68 and 1.75mS/cm at room temperature, with proof-of-concept cycling in a LTO/ ASPE/ LMO configuration.