Gelation of Cathode Electrolytes for Improved Structural Integrity and Enhanced Electrochemical Performance of Li-O2 Batteries

Abstract

Due to the significantly high theoretical energy density, lithium-oxygen (Li-O2) batteries have been attracting much attention as next generation lithium secondary batteries. Nevertheless, there remain a lot of fundamental challenges to be solved for commercialization of Li-O2 batteries, and the cycling performance of currently reported state-of-the-art Li-O2 batteries is still very poor compared to commercially available Li-ion batteries.[1,2] One of the critical problems for the Li-O2 batteries is the leakage of excess liquid electrolytes from the cathode.[3] During the cell operation, Li2O2 is deposited onto the surface of carbon electrodes, which forces the liquid electrolyte out of the cathode. As a result, the deficiency of liquid electrolytes induces deterioration of the cycling performance. Moreover, the formation of Li2O2 causes serious volume expansion of the carbon cathode, which leads to the structural disintegration of electrodes.[4] To suppress the leakage of electrolytes and disintegration of cathode structures, we are aiming to gelate electrolytes inside the carbon nanotube (CNT) cathodes. Although some gel polymer electrolytes have been reported for Li-O2 batteries so far,[5] gelation of electrolytes inside the cathode has been little investigated. In this study, several types of multi-functional cross-linkers are evaluated to gelate the cathode electrolytes via thermally or UV light-initiated in situ polymerization. By tuning the chemical structure and composition of gelling agents, viscoelastic properties of gels can be broadly controlled. We investigate the effect of gel viscoelasticity and cell restraint pressure on the leakage of electrolyte and mechanical integrity of the CNT cathode, as well as battery performance of the Li-O2 cells. In addition, as another strategy to fabricate CNT-based gel cathodes, solvent-mediated technique is exploited: CNTs, electrolytes, gelling polymers, and volatile solvents are kneaded to form paste films. After evaporation of the solvent, CNT-based gel cathodes were obtained. The relationship between composition and preparation process of the gel cathode and resultant electrochemical performance of Li-O2 cells are studied in detail. We also explore the possibility to reduce the electrolyte amount inside cathodes by gelation toward practically high energy density Li-O2 batteries.