Investigation of properties and performance of three novel highly concentrated ether-functionalised ionic liquid electrolytes for lithium metal batteries

Abstract

Exploring highly concentrated (>50 mol% Li-salt) ionic liquids as electrolytes for lithium metal batteries can lead to the development of safer, high-performance batteries with increased energy density and improved cycling stability. However, at high concentrations of Li-salt, ionic liquid electrolytes can have high viscosity and slow ion transport kinetics. Here, we investigate the use of novel ether-functionalised cations as a promising approach for increasing fluidity and weakening Li+-anion interactions in highly concentrated ionic liquid electrolytes to facilitate faster transport of Li+. Three small ether-functionalised cations, namely 1-methoxymethyl-1,1,1-trimethylammonium ([N111,1O1]+), N-methoxymethyl-N-methylpiperidinium ([C1O1mpip]+) and N-methoxymethyl-N-methylmorpholinium ([C1O1mmor]+) are compared when paired with bis(fluorosulfonyl)imide ([FSI]−) anion at a 1:1 mol ratio with LiFSI. The study provides insights into the relationship between the structure of the ether-functionalised cations and the properties of the resulting electrolytes, including thermal behaviour, ionic conductivity, viscosity, ion diffusivity, Li plating/stripping behaviour and Li+-transference number. The performance of the electrolytes was tested in Li|LiFePO4 (LFP) cells (1.14 mAh cm−2) at 0.57 mA cm−2 (C/2) for 100 cycles at 50 °C. (LiFSI)0.5([C1O1mpip][FSI])0.5 and (LiFSI)0.5([N111,1O1][FSI])0.5, have high Li-transference numbers of 0.47 ± 0.02 and 0.37 ± 0.02, respectively, and both display 0.85 mAh cm−2 areal discharge capacity after 100 cycles, with capacity retention over 98.5% (in comparison to the first cycle after preconditioning cycles), demonstrating the feasibility of these electrolytes in Li-metal cells.