Water-tolerant lithium metal cycling in high lithium concentration phosphonium-based ionic liquid electrolytes

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 (P_111i4), 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 (C₃mpyr), 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.