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Stable metal anode cycling for high energy density batteries can be realized through modification of electrolyte composition and optimization of formation protocols, i.e., electrode interphase preconditioning conditions. However, the relationship between these and the electrochemical performance is still unclear due to a lack of molecular level understanding of electric double layer (EDL) changes with modification of these two parameters. Herein, we examine the impact of ionic liquid (IL) electrolyte composition (salt concentration and cosolvent) and preconditioning cycling conditions on Li anode performance through EDL changes affecting both the solid–electrolyte interphase (SEI) and deposition morphology. Each electrolyte composition results in a particular interfacial Li-ion solvation environment, which controls the reductive stability, Li deposition potential, and ultimately the composition of properties of the SEI. The latter is dependent on the EDL composition such as the IL cation/Li-anion ratio or the presence of other surface active additives. It is found that in a superconcentrated electrolyte, a high current density (≥10.0 mA cm–2/1.0 mAh cm–2) is beneficial during the metal anode preconditioning step, compared with the case of low Li salt-containing IL. This correlates with a predominance of Li x (anion) y (x > y) at a highly negatively charged interface, which is present when higher current densities are used for preconditioning, as suggested by molecular dynamics simulations. In contrast, for the lower viscosity superconcentrated electrolyte containing 20 wt % of ether cosolvent, a more moderate preconditioning step current density (6.0 mA cm–2/1.0 mAh cm–2) leads to an optimized deposition morphology and improved cycling performance. This is a consequence of the competing processes of ion transport at the interface, which controls the Li+ ion flux and the intrinsic reduction kinetics occurring at the more negative electrode.

Long-term cycling studies of high capacity Li-metal|lithium iron phosphate (LFP, 3.5 mAh/cm2) cells were carried out using two highly concentrated ionic liquid electrolytes (ILEs). Cells incorporating N-propyl-N-methylpyrrolidinium bis­(fluorosulfonyl)­imide (C3mpyrFSI) or triethylmethyl­phosphonium bis­(fluorosulfonyl)­imide (P1222FSI), with 50 mol % lithium bis­(fluorosulfonyl)­imide (LiFSI) electrolytes were shown to operate for over 180 cycles at 50 °C at a rate of C/2 (1.75 mA/cm2). The choice of separator was identified as a critical factor to enable high areal capacity cycling, with the occurrence of cell failure through a short-circuiting mechanism being particularly sensitive to separator characteristics. Several commercial separators were characterized and tested, and their performance was related to membrane properties such as the MacMullin number, pore size, and contact angle. Celgard 3000 series separators were found to support long-term cycling due to their combination of desirable nanoporosity and wettability. The most compatible cell components were assembled into a pouch cell to further demonstrate the feasibility of ILE incorporation into high-capacity lithium metal batteries for commercial purposes.

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Employing high-voltage Ni-rich cathodes in Li metal batteries (LMBs) requires stabilization of the electrode/electrolyte interfaces at both electrodes. A stable solid–electrolyte interphase (SEI) and suppression of active material pulverization remain the greatest challenges to achieving efficient long-term cycling. Herein, studies of NMC622 (1 mAh cm–2) cathodes were performed using highly concentrated N-methyl-N-propylpyrrolidinium bis­(fluorosulfonyl)­imide (C3mpyrFSI) 50 mol % lithium bis­(fluorosulfonyl)­imide (LiFSI) ionic liquid electrolyte (ILE). The resulting SEI formed at the cathode enabled promising cycling performance (98.13% capacity retention after 100 cycles), and a low degree of ion mixing and lattice expansion was observed, even at an elevated temperature of 50 °C. Fitting of acquired impedance spectra indicated that the SEI resistivity (R SEI) had a low and stable contribution to the internal resistivity of the system, whereas active material pulverization and secondary grain isolation significantly increased the charge transfer resistance (R CT) throughout cycling.

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.

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