Understanding the Role of Separator and Electrolyte Compatibility on Lithium Metal Anode Performance Using Ionic Liquid-Based Electrolytes

High performance Li|NMC and Li|LFP cells using ionic liquid-based electrolytes have previously been demonstrated, whereby the choice of commercial polyolefin separator was found to play a determinative role in the lithium metal anode's cycling performance and stability. Here, the relationship between the separator properties and the lithium metal cycling behavior has been explored by considering the role and importance of electrolyte chemistry and its resultant interactions with the separator and electrode surface. In this study, an ionic liquid electrolyte (ILE) based on the bis(fluorosulfonyl)imide (FSI) anion was chosen, namely N-methyl-N-propylpyrrolidinium FSI (C3mpyrFSI) with 3.2 mol·kg–1 LiFSI. An organic:IL hybrid electrolyte consisting of 20:80 DME:IL (with the same respective LiFSI molalities) was also prepared using the same IL. Five separators were investigated, namely Solupor 7P03A, Solupor 5P03A, Celgard 3501, Celgard 3401, and Celgard 2500, and the combinations of electrolyte-separator were characterized ex situ, both before and and after cycling, in a Li|Li symmetric coin cell. The interaction between the separator and electrolyte and the subsequent impact on the electrolyte transport properties have been characterized using NMR diffusion and electrochemical impedance spectroscopy measurements (MacMullin number). The evolution of the lithium metal morphology was studied using SEM, revealing the degree to which the deposited lithium grows progressively into the separator during cycling. Identifying the key properties of the electrolyte-separator system (e.g., ion transport) and understanding how they can be modified through choice of separator and electrolyte chemistry to improve the lithium metal anode durability is a critical aspect for the future development of durable, high capacity lithium metal batteries.