Committee Members:
Prof. Gleb Yushin, Advisor, MSE
Prof. Preet Singh, MSE
Prof. Faisal Alamgir, MSE
Prof. Matthew McDowell, MSE/ME
Dr. Alexander Alexeev, ME
Host matrices for single and dual ion conducting polymer electrolytes and in-situ polymerization for lithium-ion batteries
ABSTRACT: In the wake of rapid and catastrophic climate change, a global energy economy driven by fossil fuel is seeing a pressing need for transition towards green energy. There was a 584 EJ consumption of primary energy in the year 2019 and only 146 EJ of this was through renewable energy sources of which a mere 3.4% of contributed to the transport sector. The primary energy sector alone causes 34% of all Green House Gas (GHG) emissions, while transportation caused 14% of GHG emissions globally in the year 2018 making them dominant factors increasing global warming and driving climate change. Lithium-ion batteries (LIBs) have proven to be promising candidates for energy storage and have seen extensive advances since its conception by Stanley Whittingham and commercialization by SONY, but major challenges still exist with respect to their safety and hazardous nature. Numerous factors can cause a conventional LIB to fail including, but not limited to internal short circuits, thermal abuse conditions, electrical abuse and mechanical abuse and sensor faults, all of them leading to the highly hazardous thermal runaway reaction risking explosion and fire.
To address the hazardous nature of LIBs, solid state electrolyte systems are being extensively developed as they present direct solutions to the safety problem by completely avoiding the flammable, volatile liquid organic electrolyte systems and replacing them with ionically conducting solids. A major trade-off preventing commercialization of solid-state lithium batteries (SSLBs) is the low lithium-ion conductivity of solid-state electrolytes (SSEs) – both ceramic and polymer electrolytes at ambient temperature. Solid polymer electrolytes (SPEs) have a significant advantage over solid inorganic electrolytes with respect to processibility, versatility with different lithium salts and additives, ease of large-scale manufacturing, light weight, and flexibility making them a viable candidate for a wide range of applications from wearable devices to EVs for the immediate future.
SPE systems have seen extensive research and development for use in LIBs to address major challenges like poor interface (or interphase) contacts, and a low ionic conductivity that have a significant negative impact in the progress of commercializing SPEs and polymer SSLBs (PSSLBs). This proposal outlines the need to address most of these major shortcomings of known SPE systems in 2 different approaches. To address the challenge of ionic conductivity, I focus my research on systematic studies of polyphosphazene (PPZ) based SPEs as they are reported to have a low glass transition temperature (Tg,) flame retardancy, and a facile chemistry for a wide variety of side chains enabling the tuning of their properties with ease. 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 they have not been widely considered as SPEs in the literature, a few existing examples showed promising ionic conductivity and compatibility with Li metal.
To address the interfacial contact problem and to minimize the volume fraction of the SPE needed, I study a one-step manufacturing approach where the liquid monomer(s) are used to achieve intimate interfacial contact with the electrode particles and are then “frozen” into place by in-situ polymerization. With this method, we use a liquid-state SPE precursor to achieve good interfacial contact and convert it into a solid-state electrolyte in the same configuration potentially achieving the best of both worlds – conventional liquid electrolytes (easy processing, good interfacial contact, sufficiently good stability induced by in-situ formed solid electrolyte interphases, high energy and power density) and SPE (superior safety and stability).