Novel Adiponitrile-Based Cocrystalline Solid State Electrolytes for Lithium-Ion Batteries

Abstract

The demand for improved energy storage has and will continue to exponentially increase as technology advances. Electric vehicles, cell phones and laptops predominantly use lithium-ion batteries (LIBs) due to their inherent high energy densities and their ability to be recharged. However, issues such as charging rates, flammability concerns, energy density need to be addressed in the advancement and innovation of LIBs. Current LIBs suffer from acute safety issues due to the use of a volatile liquid electrolyte. These issues occur when lithium metal gets deposited irregularly onto the surface of electrodes during charge/discharge cycling, forming lithium dendrites. The dendrites then form an electronically conductive path between the cathode and the anode, causing thermal runaway. To remedy this dangerous safety issue, solid state batteries have been explored. A solid-state battery (SSB) removes the volatile liquid electrolyte and replaces it with a solid, ionically conductive electrolyte. Historically, many types of solid-state batteries have been researched, all with some critical flaw(s) that prevent them from widespread commercial adoption. Ceramic types such as Lithium lanthanum zirconium oxide (LLZO) have high ionic conductivity (>10-3 S cm-1), but suffer from mechanical brittleness, creating excess interfacial failure modes. However, polymer types such as polyethylene oxide (PEO), tend to have better interfacial contact, though they suffer from poor ionic conductivity. Improving mechanical and chemical interfacial contact between the solid-state electrolyte and electrodes, while maintaining high ionic conductivity, will provide a major improvement to the success of solid-state battery adoption. The work presented in this thesis explores novel soft solid cocrystalline electrolytes as an avenue for improved interfacial electrolyte-electrode contact while maintaining high ionic conductivity. By exploiting hard soft acid base theory, novel dinitrile-based cocrystals containing low-cost starting materials were synthesized and characterized as solid-state electrolytes. The utilization of LiBF4, adiponitrile (ADN) and/or succinonitrile (SCN) in thesecocrystals decreases Li-Li distances compared to previously reported cocrystals, thereby promoting the migration of Li-ions. Select cocrystal also reached ionic conductivity values of > 10-4 S cm-1, nearly matching that of ceramic electrolytes. Novel single-crystal-to-single-crystal (SCSC) phase transition were characterized in both LiBF4 and adiponitrile/LiBF4 cocrystals. The synthesis, thermal, and electrochemical characterization techniques for these compounds include both single crystal and powder x-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, and electrochemical impedance spectroscopy.