Enabling a Stable High-Power Lithium-Bromine Flow Battery Using Task-Specific Ionic Liquids

Abstract

Hydrophobic task-specific ionic liquids (TSILs) can be the key to unlocking the potential of energy-dense lithium-bromine batteries for a wide variety of applications such as provision of sustainable power for transportation and the grid. In this paper, we describe a high efficiency catalyst-free lithium-bromine rechargeable fuel cell using highly concentrated bromine catholytes, with higher theoretical energy density than most lithium-ion cathode materials and catalyst-free lithium-air chemistries. Using a novel hydrophobic ionic liquid, we eliminate the organic electrolyte in the original Li-Br flow cell architecture with a flat graphite electrode, proposed by Bai and Bazant, and improve peak power density by ∼4× and the ionic conductivity of the solid electrolyte by almost an order of magnitude. The peak power density of 34.5 mW cm−2 at a potential of 3.45 V is the highest reported value for similar catalyst-free battery chemistries to date. It can also reversibly electrodeposit lithium in a dendrite-free manner at high current densities ∼10 mA cm−2 without any catalyst, with a >95% voltage efficiency for a 1 M Br2 in 9 M LiBr catholyte. Despite having chemical stability to lithium and bromine, suboptimal Li+ chelating affinity of the TSIL results in a depletion of ion concentration in the ionic liquid phase during operation, rendering stable long term cycling challenging. This can be addressed by the design of ionic liquid cations with higher Li+ affinity, while maintaining high chemical stability, hydrophobicity, and reasonable viscosity. This work is a proof-of-concept of the viability of this approach, and has potential for applicability in long-range transportation, beyond current Li-ion and Li-air batteries.