The film-forming electrolyte additive/co-solvent fluoroethylene carbonate (FEC) can play a crucial role in enabling high-energy-density lithium metal batteries (LMBs). Its beneficial impact on homogeneous and compact lithium (Li) deposition morphology leads to improved Coulombic efficiency (CE) of the resulting cell chemistry during galvanostatic cycling and consequently an extended cell lifetime. Herein, the impact of this promising additive/co-solvent on selected properties of LMBs is systematically investigated by utilizing an in-house developed lithium pretreatment method. The results reveal that as long as FEC is present in the organic carbonate-based electrolyte, a dense mosaic-like lithium morphology of Li deposits with a reduced polarization of only 20 mV combined with a prolonged cycle life is achieved. When the pretreated Li electrodes with an FEC-derived preformed SEI (pSEI) are galvanostatically cycled with the FEC-free electrolyte, the described benefits induced by the additive are not observable. These results underline that the favorable properties of the FEC-derived SEI are beneficial only if there is unreacted FEC in the electrolyte formulation left to constantly reform the interphase layer, which is especially important for anodes with high-volume changes and dynamic surfaces like lithium metal and lithiated silicon.

Each battery cell consists of three main components: the anode, the cathode, and the separator soaked with liquid electrolyte, the medium in the battery that allows charged ions to move between the two electrodes. Besides a wide electrochemical stability window and good compatibility with both electrodes, the electrolyte should also be safe, thermally stable and environmentally benign, showing a high ionic conductivity of the charge-carrying Li ions and finally a low price. This unique combination of properties is impossible to achieve with a simple salt–solvent mixture and usually requires a combination of different electrolyte components, i.e., several liquid solvents and additives and one or more conducting salt(s). For lithium-based batteries, which are the most common electrochemical energy storage devices today, a solution based on lithium hexafluorophosphate (LiPF₆) in a mixture of organic carbonates as the solvent is used. Usually, the conducting salt concentrations used for lithium-based electrolytes are in the range of ≈1 to 1.2 M, but recently, electrolytes with much higher conducting salt concentrations of 5 M and even over 10 M have been investigated as they offer several benefits ranging from increased safety to a broadened electrochemical stability window, thus enabling cheap and safe solvents, even water.

In lithium metal batteries, the cycle life relevantly declines with decreasing electrolyte amount. The capacity decay is kinetically reasoned as shown by rises in cell resistances, in particular for the discharge processes, as indicated by the full capacity recovery during a constant voltage step after discharge at the end of life (EOL). Interestingly, adding fresh electrolyte after EOL only partially recovers the capacity, suggesting a different and more crucial failure origin than the assumed loss of charge carriers due to the electrolyte “dry‐out”. Contrary to the cathode, the anode has higher resistances and a thicker surface layer post mortem, which is also observed in Li‖Li cells. In addition, the resistance portion of the electrolyte itself remains comparatively low during cycling, suggesting that resistance rise is dominated by the Li anode and is confirmed by exchange with fresh Li, where the capacities are recovered toward initial values, again. Based on the observations, a mechanism with a faster dry‐out of Li metal pores is proposed, which decreases the electrolyte‐accessible Li metal surface area, enhances local current densities, and facilitates high surface area and dead lithium. This continuously clogs and blocks the surface, reducing the practical accessible Li and eventually causing the rollover fade.

The rising deployment of portable devices and electric vehicles has accelerated the development of high energy-dense batteries and revived interest in lithium metal as an anode material in recent years. However, by employing the widely-used organic carbonate-based electrolyte formulations in lithium-metal batteries (LMBs), whisker-like Li deposits emerge during galvanostatic cycling, precipitating a rapid active material degradation.¹ Consequently, researchers have shifted their focus towards novel electrolyte formulations aimed at facilitating a dense Li deposition and prolonging the cycle life of LMBs. A promising approach is the application of localized high-concentration electrolytes (LHCEs) due to their unique solvation structure, causing the formation of an effective anion-derived Solid Electrolyte Interphase (SEI) and a dense Li deposition morphology.^2–5 One of the remaining challenges for this novel electrolyte concept is the low ionic conductivity and the resulting declining galvanostatic cycling performance at higher current densities (> 1 mA cm^-2), thus making the increase of the ion mobility crucial to enable LHCEs to become viable electrolyte solutions for LMBs. In our study, we investigated inexpensive heat transfer fluids as effective co-diluents for LHCEs employed in LMBs. Incorporating these low-cost fluids into the electrolyte formulation results in lower viscosity and enhances molecular diversity, thereby improving transport properties and causing superior fast-charging capabilities of the resulting LMB chemistry. While the introduction of these novel co-diluents appears to have no discernible effect on the composition of the interphase layer formation, their presence promotes a more uniform lithium deposition morphology, particularly under higher charging rates. Additionally, the non-flammable nature of these components enhances the safety profile of the LMBs, rendering the novel components as multifunctional co-diluents for LHCEs and concurrently improving several crucial properties of the resulting electrolyte formulation. References: Horstmann, B. et al. Strategies towards enabling lithium metal in batteries: interphases and electrodes. Energy Environ. Sci. 14, 5289–5314 (2021). Cao, X., Jia, H., Xu, W. & Zhang, J.-G. Review—Localized High-Concentration Electrolytes for Lithium Batteries. J. Electrochem. Soc. 168, 010522 (2021). Ren, X. et al. Enabling High-Voltage Lithium-Metal Batteries under Practical Conditions. Joule 3, 1662–1676 (2019). Ren, F. et al. Solvent–Diluent Interaction-Mediated Solvation Structure of Localized High-Concentration Electrolytes. ACS Appl. Mater. Interfaces 14, 4211–4219 (2022). Angarita-Gomez, S. & Balbuena, P. B. Ion mobility and solvation complexes at liquid–solid interfaces in dilute, high concentration, and localized high concentration electrolytes. Mater. Adv. 3, 6352–6363 (2022).