Ionic Liquid Assisted Composite Ceramic-Polymer Electrolyte for Lithium Metal Batteries

Abstract

Facing demands for smaller and more powerful batteries to keep pace with technological advances, as the conventional lithium-ion battery (LIB) is reaching its inherent physicochemical limit, new electrode materials must be researched. One attractive anode material is lithium metal which has a specific energy density 11 times higher than conventional graphite anode. However, lithium metal is incompatible with conventional liquid electrolyte and leads to severe impact on cycle life and safety issues. For lithium metal to be feasible as anode material in a secondary LIB, the electrolyte required must be thermodynamically stable against lithium metal, or can decompose and form a thin solid-electrolyte interface layer on the surface of the lithium to prevent further parasitic reactions. The electrolyte chosen must also be able to prevent or suppress the growth of lithium dendrite to avoid penetration, leading to short circuit and severe safety issues. This thesis presents a composite ceramic-polymer electrolyte (CPE) based on solid polymer electrolyte polyethylene (PEO) containing lithium salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Ceramic electrolyte Li1.5Al0.5Ge1.5P3O12 (LAGP) is mixed and dispersed inside the polymer to study and provide additional pathways for lithium ion conduction, which raises both ionic conductivity as well as Li+ transference number (tLi+) due to LAGP being a single-ion conductor. LAGP to PEO ratio was studied and optimized through the “bricklayer” model for ion conduction pathways. Addition of LAGP at a weight ratio of 1:1 relative to PEO (1 LAGP) allows for bulk ionic conductivity at 35°C to increase from 3.61 × 10-6 S cm-1 to 2.49×10-5 S cm-1. To further improve Ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide (EMITFSI) is then added to modify the PEO further through plasticization which decreases both glass transition temperature (Tg) and melting temperature (Tm). This raises the ionic conductivity further to 6.20×10-5 S cm-1 at 35°C and 6.1×10-4 S cm-1 at 50°C. This also results in tLi+ = 0.72 at 50°C, which is an improvement upon PEO-LiTFSI solid polymer electrolyte of tLi+ = 0.46, making it more efficient. The final optimized composite electrolyte was able to initially deliver 139 mAh g-1 in discharge capacity and 115 mAh g-1 after 125 cycles at a charging rate of 0.3 C, with good rate capability of 112 mAh g-1 at 1C while under 50°C environment, which is reduced by 10 to 20°C compared to similar literature, providing a pathway towards a practical polymer based solid state battery with a scalable production method.