Water-tolerant lithium metal cycling in high lithium concentration phosphonium-based ionic liquid electrolytes. Issue 10 (15th August 2018) - Kerr, Robert
Abstract : Cycling stability at high capacities and water-tolerance are two key properties for the operation of high-capacity lithium (Li) metal–air batteries. Abstract : Cycling stability at high capacities and water-tolerance are two key properties for the operation of high-capacity lithium (Li) metal–air batteries. Here, we have demonstrated the cycling of Li metal at high rates and high capacities in a newly developed family of ionic liquids based on quaternary alkylphosphonium cations and the bis(fluorosulfonyl)imide anion. A high LiFSI salt concentration of 50 mol% gave the most favourable combination of performance and water-tolerance when compared to 33 mol% and 20 mol%. These high salt content electrolytes exhibited stable cycling at 1 mA cm −2, in 1 h steps for up to 250 cycles at room temperature in the presence of up to 5000 ppm water. The two smallest cations, triethylmethylphosphonium (P1222 ) and trimethylisobutylphosphonium (P111i4 ), showed significantly superior cycling capabilities than the larger tributylmethylphosphonium (P1444 ) and trihexyltetradecylphosphonium (P66614 ) cations. Furthermore, the two small phosphonium cations supported high current densities and more stable long-term cycling than the N -methyl- N -propylpyrrolidinium cation (C3 mpyr), which is of similar molecular weight. Fourier transform infra-red spectroscopy was used to characterise the composition of electrode surface layers, while ex situ X-ray photoelectron spectroscopy showed that the presence of water affects the amount of decomposition products at the electrode surface. These benchmark cycling results represent a significant step forward in the development of water-tolerant electrolytes for Li metal-based batteries.