Development of PFAS-Free Locally Concentrated Ionic Liquid Electrolytes for High-Energy Lithium and Aluminum Metal Batteries

Abstract

Lithium-ion batteries (LIBs) based on graphite anodes are a widely used state-of-the-art battery technology, but their energy density is approaching theoretical limits, prompting interest in lithium-metal batteries (LMBs) that can achieve higher energy density. In addition, the limited availability of lithium reserves raises supply concerns; therefore, research on postlithium metal batteries is underway. A major issue with these metal anodes, including lithium, is dendritic formation and insufficient reversibility, which leads to safety risks due to short circuits and the use of flammable electrolytes.

Ionic liquid electrolytes (ILEs), composed of metal salts and ionic liquids, offer a safer alternative due to their nonflammable nature and high thermal stability. Moreover, they can enable high Coulombic efficiency (CE) for lithium metal anodes (LMAs) and allow reversible stripping/plating of various post-lithium metals for battery application, e.g., aluminum metal batteries (AMBs). Despite these advantages, ILEs suffer from high viscosity, which impairs ion transport and wettability. To resolve these challenges, researchers have developed locally concentrated ionic liquid electrolytes (LCILEs) by adding low-viscosity nonsolvating cosolvents, e.g., hydrofluoroether, to ILEs. These cosolvents do not coordinate with cationic charge carriers, thereby reducing viscosity and improving ion transport without compromising the compatibility of electrolytes with metal anodes. However, due to the inherent difference of molecular organic solvents and ionic liquids full of charged species, the most used nonsolvating cosolvents, i.e., hydrofluoroether, are less effective for ILEs with respect to concentrated electrolytes based on conventional organic solvents. Moreover, hydrofluoroether contains environmentally problematic −CF₃ and/or −CF₂- groups, i.e., per- and polyfluoroalkyl substances (PFAS), with their use subject to restrictions.

In this Account, we provide an overview of the endeavors of our research group on the development of PFAS-free LCILEs for high-energy LMBs and AMBs. First, aromatic organic cations and aromatic less/nonfluorinated cosolvents are proposed to weaken the organic cation–anion interaction and strengthen the organic cation-cosolvent interaction, respectively. This is with consideration of the uncovered phase nanosegregation structure of LCILEs that effectively reduces the viscosity and promotes the Li⁺ transport ability with respect to the conventional nonaromatic organic cations and highly fluorinated PFAS cosolvents. Then, the effect of electrolyte components that do not coordinate to Li⁺, including organic cations and nonsolvating cosolvents, on the SEI composition and LMA reversibility is presented, which confirms the feasibility of reaching a high lithium stripping/plating CE up to 99.7% in the developed PFAS-free LCILEs. In the subsequent discussion on cathode compatibility, we present that in addition to LiFePO₄ with high cyclability but inferior energy density, nickel-rich layered oxide and sulfurized polyacrylonitrile (SPAN) can be employed to construct high-energy LMBs for PFAS-free LCILEs with different anodic stability. Additionally, the feasible application of the LCILE strategy to promote the kinetics of AMBs relying on a different anode chemistry is demonstrated. Lastly, future research directions with an emphasis on nonsolvating component optimization, electrolyte dynamics, and electrode/electrolyte interphase formation are provided.