Comparing the Electrochemical Performance of Organic and Ionic Liquid-Based Electrolytes in Lithium-Oxygen Battery

Abstract

Lithium-oxygen batteries (LOB) have been considered promising electrochemical energy storage devices to supersede current rechargeable batteries for next generation electric vehicles. Despite having tremendously high theoretical energy density (5200 Wh/kg), the practically achievable energy density (1700 Wh/kg) is far from commercial application till date. There are several challenges associated with Li anode, porous cathode and electrolyte that need to be resolved to achieve better results. The choice of suitable electrolyte for LOB is one of the biggest obstacles in its development. The various electrolytes that have been developed and studied come with limitations which includes thermal and chemical stability at higher voltages, volatility, viscosity, flammability, capability to dissolve incoming oxygen etc. Therefore, in order to improve current performance of battery, these parameters need to be optimized.This study focuses on the influence of organic and ionic liquid (IL)-based electrolytes on electrochemical performance of Li-O2 battery. [Pyr14][TFSI] and two commercially available organic solvents TEGDME and DMSO were investigated in this study. A series of tests were performed at various operating temperatures with the IL. Vulcan XC 72R coated carbon cloth with 30wt% PTFE is used as cathode. 1 mg/cm2 (+/-0.3mg/ cm2) is used as carbon loading. All experiments were conducted with pure oxygen supply at room temperature (unless stated otherwise). Li|Li Symmetric tests were also performed to measure the stability of electrolyte.The IL was tested at three different operating temperatures including room temperature (22°C), 35°C and 75°C. The results indicate that increase in temperature helps improve discharge capacity of battery. The discharge capacity obtained at three operating temperatures were 0.04 mAh/mg, 0.06 mAh/mg and 0.3 mAh/mg respectively. The improvement is in accordance with the fact that increase in temperature reduces IL viscosity and increases oxygen diffusivity. At discharge cut-off voltage of 2V, the discharge cycles did not show mass transfer limitation therefore, the subsequent tests were conducted with cut-off voltage set at 0V. This significantly improved the discharge capacity from 0.3 mAh/mg to 1.1 mAh/mg with discharge cycles limited by mass transfer effect.Two organic electrolytes TEGDME and DMSO were also tested at room temperature (22°C). The discharge capacities obtained were 4.65 mAh/mg and 5.43 mAh/mg. The higher discharge capacity of DMSO could be associated with its unique properties which includes donor number (DN), conductivity and viscosity. The DN for DMSO and TEGDME are 29.8 kcal/mol and 16.6 kcal/mol respectively. Higher DN improves discharge capacity by promoting growth of Li2O2 as bigger toroidal precipitates on cathode surface as opposed to its counterpart which produces Li2O2 insulating film. Similarly, DMSO has higher conductivity (2.11 mS/cm) and lower viscosity (1.94 cP) than TEGDME (0.3 mS/cm,4.05 cP) which improves species mobility.Finally, Li|Li symmetric tests were conducted at room temperature to measure stability of electrolytes. The results indicate, DMSO could cycle 60 cycles without significant surge in overpotential followed by TEGDME (40 cycles) and IL (20 cycles).