Abstract: Li anodes represent a theoretical 10-fold upgrade over the capacity of graphite at comparable potential, and as such are key anode candidates for next-generation high energy density ( 〉 1000 Wh/L) batteries. However, Li still falls below the 99.95-99.97% CE required for long-life cycling (1) and displays rate capability an order-of-magnitude lower than necessary for fast-charging ( 〉 2 C). (2) Interestingly, intrinsic Li 0 /Li + redox has been reported to be more facile ( j 0 〉 10 mA/cm 2 ) (3) than graphite intercalation ( j 0 = 1-3 mA/cm 2 ) (4) . In spite of this, the chemical exchange of Li + through the native solid electrolyte interphase (SEI) on Li is typically slow (0.5-3 mA/cm 2 ), (5) and as such it can bottleneck Li + /Li 0 redox (3) and increase charge-transfer resistance. (6) The SEI thus manifests itself by substantially decreasing the exchange current j 0 measured on Li down from its intrinsic kinetic value. (7) While multiple recent studies see j 0 as a relevant property in determining Li reversibility, (1) measuring j 0 in the presence of an SEI is not straightforward. Consequently, current literature presents widely varying numerical values of j 0 in the presence of an SEI, making it challenging to discern the relationship between j 0 and CE. To bridge this gap, Li + exchange at the Li anode is here systematically quantified using cyclic voltammetry (CV) at slow scan rates and electrochemical impedance spectroscopy (EIS), both of which allow an SEI to develop natively. To avoid ambiguity with the intrinsic Li 0 /Li + redox exchange current j 0 , exchange rates are here interpreted in the framework of a “pseudo”-exchange current, j 0 p , that represents the total rate of Li + exchange on the electrode. j 0 p was measured across a selection of historically-relevant and modern electrolytes, spanning low (78.0%) to high (99.3%) CE. In both methodologies, a strong dependence of j 0 p on electrolyte chemistry was identified. These differences reflect a strong correlation between CE and j 0 p , with electrolytes that display higher j 0 p typically also displaying higher CE. Upon cycling, a dynamic behavior of Li + exchange on both Cu and Li were observed, with j 0 p typically increasing through cycling, attributed to morphological changes induced by non-uniform plating/stripping inherent to Li electrochemistry. (8) We will discuss the implications of this dynamic behavior on both the formation cycle on Cu, as well as how j 0 p changes report on SEI evolution during cycling. Finally, it was found that cycling Li with current densities j beyond j 0 p leads to substantial capacity loss and low CE, whereas electrolytes that can sustain high j 0 p are insensitive to j . Altogether, our results indicate that Li + exchange plays a dominant role in determining the rate capability and CE of Li anodes, with high- j 0 p electrolytes displaying higher CE and better rate capability than their low- j 0 p counterparts. G. M. Hobold, J. Lopez, R. Guo, N. Minafra, A. Banerjee, Y. Shirley Meng, Y. Shao-Horn and B. M. Gallant, Nature Energy , 6 , 951 (2021). P. Albertus, S. Babinec, S. Litzelman and A. Newman, Nature Energy , 3 , 16 (2018). D. T. Boyle, X. Kong, A. Pei, P. E. Rudnicki, F. Shi, W. Huang, Z. Bao, J. Qin and Y. Cui, ACS Energy Letters , 5 , 701 (2020). Y.-C. Chang, J.-H. Jong and G. T.-K. Fey, Journal of The Electrochemical Society , 147 , 2033 (2000). A. B. Gunnarsdóttir, S. Vema, S. Menkin, L. E. Marbella and C. P. Grey, Journal of Materials Chemistry A , 8 , 14975 (2020). A. Zaban, E. Zinigrad and D. Aurbach, The Journal of Physical Chemistry , 100 , 3089 (1996). J. N. Butler, D. R. Cogley and J. C. Synnott, Journal of Physical Chemistry , 73 , 4026 (1969). J. Z. Lee, T. A. Wynn, M. A. Schroeder, J. Alvarado, X. Wang, K. Xu and Y. S. Meng, ACS Energy Letters , 4 , 489 (2019).

Abstract: Li battery chemistries exploit reactions that occur under very reducing conditions (~ 0 V vs. Li/Li + ) at negative electrodes, which are held below the electrochemical stability window of known carbonate-based electrolyte systems (0.5-1 V vs. Li/Li + ), 1 resulting in solvent decomposition at the surface and leading to the formation of a solid electrolyte interphase (SEI). When formed on Li, however, this interphase is unable to fully protect the electrode, resulting in an SEI composition that changes over time. 2 Due to this dynamic nature, its chemistry is challenging to characterize using conventional ex-situ surface analysis ( e.g. , Fourier transform infrared spectroscopy, FTIR; X-ray photoelectron spectroscopy, XPS). 3, 4 Consequently, identifying the chemical mechanisms through which candidate systems for improving Li SEI stability operate – such as the use of highly fluorinated and/or concentrated electrolytes 5 – remains difficult. Fortunately, interphase reactions are known to release gases that are easily detected by gas chromatography (GC) and mass spectrometry (MS), 6 using which we are able to construct a time-resolved picture of the SEI chemistry. Here we show that these experiments can provide valuable insight into the dynamic chemical reaction pathways on Li, yielding an updated picture of factors that contribute to a good SEI. Our experimental approach consists of a custom-designed electrochemical cell coupled to a GC instrument, which, by sampling the cell headspace periodically, enables operando quantification of CO 2 , CO, H 2 and C 1-2 products evolved at Li electrodes. By comparing gas evolution between rest and polarization regimes, we identified a significant increase in the rate of formation of all observed gases when the electrodes are polarized. Using electrodes that we identified to be gas-inert, such as LiFePO 4 , we identified that the deposition reaction, i.e. , Li + reduction, accounts for nearly all of the gas-releasing interphase reactions, implying that the SEI is only formed electrochemically when Li is plated. We next explored the nature of these reactions by quantifying how the evolution rate of each gas scales with current density, which enabled the distinction between gases that were formed in chemically-limiting reactions and gases that were formed from electrochemically-limiting steps. Because our experiments are able to quantify the abundance of each gas product, we also identified and quantified the branching of specific interphase reactions that occur on Li with conventional carbonate-based electrolyte systems. Then, by rationally tuning the electrolyte ( i.e., changing the salt species and/or introducing fluorinated solvents), we discovered and quantified how each component drives specific interphase reactions, and how the ensuing SEI chemistry affects the Faradaic efficiency of the Li electrode over time. In particular, we observed that pathways that promote decarbonylation and/or decarboxylation ( i.e. , release of CO/CO 2 ) of the solvent upon reduction correlate with higher Faradaic efficiencies. By varying salt concentration, we further explored how interphase formation pathways are affected by the solvation structure of the electrolyte in the contact-ion and solvent-separated regimes. Thus, our experiments provide a mechanistic picture of how organic solvent-derived products affect SEI chemistry and stability, which can be as important as known ionic phases like LiF. 5 1. Gauthier, M.; Carney, T. J.; Grimaud, A.; Giordano, L.; Pour, N.; Chang, H.-H.; Fenning, D. P.; Lux, S. F.; Paschos, O.; Bauer, C.; Maglia, F.; Lupart, S.; Lamp, P.; Shao-Horn, Y., The Journal of Physical Chemistry Letters 2015, 6 (22), 4653-4672. 2. Lin, D.; Liu, Y.; Cui, Y., Nature Nanotechnology 2017, 12 (3), 194. 3. Aurbach, D.; Markovsky, B.; Shechter, A.; Ein‐Eli, Y.; Cohen, H., Journal of The Electrochemical Society 1996, 143 (12), 3809-3820. 4. Dedryvère, R.; Gireaud, L.; Grugeon, S.; Laruelle, S.; Tarascon, J. M.; Gonbeau, D., The Journal of Physical Chemistry B 2005, 109 (33), 15868-15875. 5. Suo, L.; Xue, W.; Gobet, M.; Greenbaum, S. G.; Wang, C.; Chen, Y.; Yang, W.; Li, Y.; Li, J., Proceedings of the National Academy of Sciences 2018, 115 (6), 1156-1161. 6. Xu, K., Chemical Reviews 2014, 114 (23), 11503-11618.

Abstract: Given concerns about the low earth abundance of Li in Li-ion batteries, there is growing interest in developing a beyond-Li materials basis for rechargeable batteries. Divalent batteries based on calcium (Ca) have received attention due to their ~2000-fold higher concentration in the earth’s crust along with attractive theoretical metrics if Ca is used as the metallic anode. At present, only a small number of nonaqueous electrolytes have been identified that allow for Ca electrodeposition, 1-5 limiting the design space for Ca battery development; thus, learning how to tailor Ca 2+ speciation and electroactivity is of central importance to engineer next-generation battery electrolytes. Unlike many Li + analogues in common battery electrolytes, Ca 2+ cations are prone to coordinate with one or both anions in solution even at low concentrations ( 〈 1 M), which can substantially impair their ability to participate in electrochemical processes. Recent studies have shown that in an electrolyte system of growing interest, Ca(BH 4 ) 2 in tetrahydrofuran (THF), unexpected promotion of active CaBH 4 + clusters from neutral Ca(BH 4 ) 2 ion pairs occurs at salt concentrations greater than 1 M, such that higher salt concentrations are necessary to provide charge mobility and high current densities. 6 Learning how to break ion-pairing constraints and more flexibly modulate Ca 2+ speciation over broader ranges of salt concentrations is therefore important to identify handles for next-generation electrolyte design. This study investigates an alternative means to tailor the Ca 2+ -to-BH 4 - ratio, and thus the generation of electroactive Ca ionic species, by using an exemplar dual-salt electrolyte, Ca(BH 4 ) 2 + Ca(TFSI) 2 in THF, at varying anion ratios for a constant total salt concentration of 1 M Ca 2+ . Introduction of a more highly-dissociating source of Ca 2+ in Ca(TFSI) 2 effectively drives re-speciation of the more strongly ion-pairing Ca(BH 4 ) 2 as indicated by a 4x increase in ionic conductivity (Fig. 1a) and Raman spectroscopy measurements (Fig. 1b), 7 generating larger populations of charged species and supporting a ~2x increase in plating current density. We further find that TFSI - enables Ca plating at high current densities when its concentration is less than that of Ca(BH 4 ) 2 , but leads to a dramatic shut-down of plating activity when concentrations exceed that of Ca(BH 4 ) 2 (Fig. 1c), 7 providing direct evidence of the role of coordination-shell chemistry on modulating Ca 2+ plating activity. On the other hand, Ca stripping activity is suppressed by the presence of TFSI - at all salt concentrations (Fig. 1d), 7 which decomposes onto the Ca surface, passivating the deposits with compounds comprising ~30% C, 35% O, and 10% F. Results are compared to that of a second dual-salt electrolyte system, Ca(BH 4 ) 2 + TBABH 4 in THF, which enables enrichment of BH 4 - concentrations to be higher than 2x that of Ca 2+ and similarly experiences a 4x increase in ionic conductivity and ~2x increase in plating current density. This work reveals factors that modulate Ca 2+ coordination and activity and highlights future directions to attain both high plating currents and reversibility for Ca-based electrolyte design. 1 Ponrouch, A., Frontera, C., Bardé, F. & Palacín, M. R. Towards a calcium-based rechargeable battery. Nature Materials 15 , 169 (2015). 2 Wang, D. et al. Plating and stripping calcium in an organic electrolyte. Nature Materials 17 , 16 (2017). 3 Shyamsunder, A., Blanc, L. E., Assoud, A. & Nazar, L. F. Reversible Calcium Plating and Stripping at Room Temperature Using a Borate Salt. ACS Energy Letters 4 , 2271-2276 (2019). 4 Li, Z., Fuhr, O., Fichtner, M. & Zhao-Karger, Z. Towards stable and efficient electrolytes for room-temperature rechargeable calcium batteries. Energy & Environmental Science (2019). 5 Kisu, K. et al. Monocarborane cluster as a stable fluorine-free calcium battery electrolyte. Scientific Reports 11 , 7563 (2021). 6 Hahn, N. T. et al. The critical role of configurational flexibility in facilitating reversible reactive metal deposition from borohydride solutions. Journal of Materials Chemistry A 8 , 7235-7244 (2020). 7 Melemed, A. M., Skiba, D. A., Gallant, B. M. Toggling Calcium Plating Activity and Reversibility through Modulation of Ca 2+ Speciation in Borohydride-based Electrolytes. Manuscript in revision. 8 Rey, I. et al. Spectroscopic and Theoretical Study of (CF 3 SO 2 )2N - (TFSI - ) and (CF 3 SO 2 )2NH (HTFSI). The Journal of Physical Chemistry A (1998). 9 Tchitchekova, D. S. et al. On the Reliability of Half-Cell Tests for Monovalent (Li + , Na + ) and Divalent (Mg 2+ , Ca 2+ ) Cation Based Batteries. Journal of The Electrochemical Society 164 , A1384-A1392 (2017). Figure 1

Abstract: Given concerns about the low earth abundance of Li in Li-ion batteries, there is growing interest in developing a beyond-Li materials basis for rechargeable batteries. Divalent batteries based on calcium (Ca) have received attention due to their ~2000-fold higher concentration in the earth’s crust along with attractive theoretical metrics if Ca is used as the metallic anode. When most common battery electrolytes come into contact with Ca, however, a passivating solid electrolyte interphase (SEI) forms that prevents Ca 2+ transport. 1 At present, only a small number of nonaqueous electrolytes have been identified that form sufficiently Ca 2+ -conductive SEIs for Ca electrodeposition, limiting the design space for Ca battery development. Thus, learning how to tailor Ca SEI formation is of central importance to identify handles for next-generation battery design. Several methods of modifying SEI to improve Ca 2+ transport have been reported, including synthesizing an ex situ artificial layer, 2 or the introduction of boron-containing electrolyte additives that form beneficial SEI compounds. 3 Ca electrolyte modification faces a unique challenge, however, as the large size and divalent nature of Ca 2+ leads to large coordination environments (up to 7 solvating molecules) in solution. 4 Recent studies have begun to identify that co-reduction of these coordinating molecules has a significant impact on Ca electrochemistry. For example, strong coordination between Ca 2+ and the anion TFSI - has been demonstrated to deactivate Ca plating behavior via TFSI - decomposition. 5,6 However, competition between coordinating molecules can be exploited to favor specific Ca 2+ speciation (i.e. BH 4 - displaces bidentate TFSI - --Ca 2+ in favor of BH 4 - --Ca 2+ interactions) and unlock Ca plating behavior. 6 Further mechanistic understanding is needed to identify the specific connections between Ca 2+ coordination, SEI formation, and anode performance (beyond just a binary toggling of plating behavior). To address this, we have systematically introduced chelating glyme-based solvents into a baseline electrolyte, investigating the subsequent changes in SEI composition and electrochemical behavior. This study investigates the preferential coordination of Ca 2+ by glymes in the baseline electrolyte 1 M Ca(BH 4 ) 2 in tetrahydrofuran (THF). THF is systematically replaced by 1,2-dimethoxyethane (G1) or bis(2-methoxyethyl) ether (G2) over a range of Gx/Ca 2+ molar ratios. Microcalorimetry measurements demonstrate that the introduction of both G1 and G2 to the electrolyte is enthalpically favorable, as each glyme displaces THF from the Ca 2+ coordination sphere and modulates Ca 2+ --BH 4 - speciation, as indicated by NMR and Raman spectroscopy. Addition of a small amount of each glyme (1:1 Gx/Ca 2+ ) increases electrolyte ionic conductivity by nearly half; however, glyme addition systematically increases the overpotentials of both Ca plating and stripping while decreasing Coulombic efficiency. The presence of glymes increases the amount of carbon present in both chemically- and electrochemically-formed SEI as indicated through XPS. We further find increased evidence of fragmentation of strongly Ca 2+ -coordinating molecules (G2, BH 4 - ) within the SEI formed during Ca deposition as compared to formed chemically. Calcium-based olefins as well as ionic phases (CaCO 3 , CaC 2 ) are detected in electrochemically-formed SEI through titration gas chromatography. This work reveals factors that modulate both Ca 2+ coordination and subsequent SEI formation, highlighting preferential coordination as a novel approach for Ca-based electrolyte design. 1 Aurbach, D., Skaletsky, R. & Gofer, Y. The Electrochemical Behavior of Calcium Electrodes in a Few Organic Electrolytes. Journal of The Electrochemical Society 138 , 3536-3545 (1991). 2 Hou, Z., Zhou, R., Min, Z., Lu, Z. & Zhang, B. Realizing Wide-Temperature Reversible Ca Metal Anodes through a Ca 2+ -Conducting Artificial Layer. ACS Energy Letters , 274-279 (2022). 3 Bodin, C. et al. Boron-Based Functional Additives Enable Solid Electrolyte Interphase Engineering in Calcium Metal Battery. Batteries & Supercaps n/a , e202200433 (2022). 4 Tchitchekova, D. S. et al. On the Reliability of Half-Cell Tests for Monovalent (Li + , Na + ) and Divalent (Mg 2+ , Ca 2+ ) Cation Based Batteries. Journal of The Electrochemical Society 164 , A1384-A1392 (2017). 5 Hahn, N. T. et al. Influence of Ether Solvent and Anion Coordination on Electrochemical Behavior in Calcium Battery Electrolytes. ACS Applied Energy Materials 3 , 8437-8447 (2020). 6 Melemed, A. M., Skiba, D. A. & Gallant, B. M. Toggling Calcium Plating Activity and Reversibility through Modulation of Ca 2+ Speciation in Borohydride-Based Electrolytes. The Journal of Physical Chemistry C 126 , 892-902 (2022).

Abstract: High-potential, high-capacity redox reactions are of fundamental interest in the development of novel primary and secondary battery chemistries. To date, the most widely explored “advanced” nonaqueous cathode reactions for Li or Na batteries involve two-electron transfers: nonaqueous O 2 reduction in Li-O 2 batteries (2 e - /O 2 ) and reduction of elemental sulfur (16 e - /S 8 or a net 2 e - /S), both of which are based on redox with the chalcogen family [ 1-3 ]. In this talk, we report on redox of molecules from a different family: those containing halogen ligands, with a particular focus on the model perfluorinated gas, sulfur hexafluoride (SF 6 ) as an illustrative example. Perfluorinated gases in general, and SF 6 in particular, are conventionally described as highly stable [ 4-5 ], yet can yield high thermodynamic potentials vs. Li metal with large theoretical numbers of electrons transferred (e.g. SF 6 + 8Li -- 〉 6LiF + Li 2 S, E o r = 3.69 V vs. Li/Li + ) [ 6 ]. In recent work, we have developed a Li-SF 6 primary battery that employs a gas-to-solid reaction at the positive electrode, analogous in some aspects to a Li-O 2 battery that similarly operates on a dissolved-gas cathode reaction. In such a configuration, we have demonstrated that SF 6 , which is virtually chemically inert against Li metal at room temperature, can be activated at an electrified interface consisting of a simple carbon electrode in a nonaqueous electrolyte (0.3 M LiClO 4 in tetraethyleneglycol dimethyl ether (TEGDME)). Upon activation, SF 6 readily undergoes continued reduction and reaction with Li + ions, resulting in the hypothesized 8-electron reduction of the central S 6+ to Li 2 S and the concurrent expulsion of all fluoride ligands to form stoichiometric LiF. The reaction stoichiometry has been validated using a suite of techniques, including pressure-coupled discharge measurements, solid phase analysis (XRD, FTIR and XPS), liquid-phase analysis ( 19 F and 1 H NMR), and electrochemical characterization. The reduction potential, which varies from ~2.0 – 2.4 V vs. Li/Li + depending on the electrolyte solvent, reflects significant overpotentials (~1 V) which are unusual among the currently studied “advanced” multi-electron reactions. In this talk, we will examine the behavior of the Li-SF 6 reduction reaction as a function of parameters including electrolyte solvent, anion composition, and salt concentration, in order to gain mechanistic insight into the dramatic SF 6 gas-to-solid phase transformation. Specifically, we characterize the electrolyte-dependence of the reduction onset potential, electron consumption and Coulombic efficiency, capacity, and nature of the solid phase (chemistry and morphology) and discuss the degree of “tunability” achievable by tailoring the reaction environment. Possible descriptors governing the growth process of the solid phase (LiF), resulting morphology, and corresponding capacity will be examined and compared to the theory developed in a sister field, that of O 2 -to-Li 2 O 2 transformations in Li-O 2 batteries [ 7-8 ]. In addition, we discuss current understanding of the origin of such large overpotentials involving “inert” molecules in electrochemical systems, and outline challenges to be addressed in the development of a practical system. The possibility to broaden this reaction class to include potentially reversible reactions based on halogenated molecules will be discussed. [1] Lu, Y. C., Gallant, B. M., Kwabi, D. G., Harding, J. R., Mitchell, R. R., Whittingham, M. S. & Shao-Horn, Y. Energ Environ Sci 6 , 750-768, (2013). [2] Aurbach, D., McCloskey, B. D., Nazar, L. F. & Bruce, P. G. Nature Energy 1 , 16128-16139 (2016). [3] Bruce, P. G., Freunberger, S. A., Hardwick, L. J. & Tarascon, J. M. Nat Mater 11 , 19-29, (2012). [4] Seppelt, K. Chem Rev 115 , 1296-1306, (2015). [5] Zamostna, L., Braun, T. & Braun, B. Angew Chem Int Edit 53 , 2745-2749, (2014). [6] Groff, E. G. & Faeth, G. M. Journal of Hydronautics 12 , 63-70 (1978). [7] Aetukuri, N. B., McCloskey, B. D., Garcia, J. M., Krupp, L. E., Viswanathan, V. & Luntz, A. C. Nat Chem 7 , 50-56, (2015). [8] Johnson, L., Li, C. M., Liu, Z., Chen, Y. H., Freunberger, S. A., Ashok, P. C., Praveen, B. B., Dholakia, K., Tarascon, J. M. & Bruce, P. G. Nat Chem 6 , 1091-1098, (2014). Figure 1

Abstract: Owing to its high theoretical capacity (3860 mAh/g, over 10 times that of graphite) and its lowest theoretical electrochemical potential (-3.04 V vs SHE), Li metal is an ideal anode material for improving the capacity of rechargeable Li batteries [1]. However, the practical application of the Li anode is severely hindered by its high reactivity with standard electrolyte compositions, which results in limited Coulombic efficiency (CE) in those electrolytes (for instance, 85-90% with 1 M LiPF 6 EC:DEC [2]). Strategies to improve CE have focused on electrolyte engineering, including development of highly concentrated electrolytes (HCE) and more recently, local high-concentration electrolytes (LHCE) [3] . A feature shared by these strategies is the promotion of contact-ion pairing, which enhances anion contributions to the solid electrolyte interphase (SEI). Among the best-performing salts are fluorinated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI), which are believed to promote stable inorganic-rich SEIs and correlate with higher CE [4]. A second strategy of interest is to pu rsue development of additives incorporated at relatively low concentrations ( e.g. , 2-5 wt%) that can favorably alter the SEI, such as carbonates ( e.g. , vinylene carbonate (VC) [5] and fluoroethylene carbonate (FEC) [6] ), sulfates ( e.g. , ethylene sulfate (DTD) and 1,3-propanesulfonate (PS) [7]), and borates ( e.g. , lithium difluoro(oxoalate) borate (LiDFOB) [8] and tris (2,2,2-trifluoroethyl) borate (TTFEB) [9] ). An advantage of this strategy is the possibility to ultimately lower reliance on high fluorinated salt concentrations, which are costly and can be corrosive towards the Al current collector, and potentially expand the versatility of available electrolyte frameworks having competitive CE. However, compared to the principles developed for salt and solvent selection in HCE and LHCE, the mechanism behind successful additive activation needs elucidation. In this work, we examined the potential of "FSI-like," neutral sulfonyl/sulfamoyl fluorides (R-SO 2 F and R-R’-NSO 2 F, Figure 1) as functional additives for Li cycling, motivated by their structural similarity to the high-performance FSI - anion. The examined baseline electrolytes include high-CE systems consisting of LiFSI dissolved in various solvents: fluoroethylene carbonate (FEC, 1 and 4 M salt), 1,2-dimethoxyethane (DME, 4 M), and dimethyl carbonate (DMC, 6 M). The ability for each additive to coordinate with Li + was first examined by NMR, with trends rationalized in part by supporting microcalorimetry data on the degree of solvent coordination strength. We relate these to CE and cycling outcomes in each electrolyte. Interestingly, we find that additives have negligible effect on CE in FEC-based electrolytes, whereas significant impacts were observed in DME and DMC. We relate these diverse outcomes to the SEI chemical compositions and gases evolved during galvanostatic cycling, as characterized by X-ray photoelectron spectroscopy (XPS) and gas chromatography (GC), which help to rationalize competitive reactions among solvent, anion, and additive. Unfortunately, additives had a negative-to-neutral impact on CE in these systems. Thus, we finally examined a LiPF 6 in carbonate electrolyte, 1 M LiPF 6 in EC/DMC (LP40), where we hypothesized that competitive reduction of coordinating additives over problematic carbonate solvent would lead to performance gains. Indeed, significant improvements (up to 94%, compared to baseline 89% over the initial 50 cycles) in CE were observed for two additives, with their structural advantages further discussed. Overall, our findings provide insights into the effects of sulfonyl/sulfamoyl fluoride additive structures on Li metal cyclability and the compositions of baseline electrolytes whose electrochemical cycling stability can be effectively modulated by these additives. Xu, W., et al., Lithium metal anodes for rechargeable batteries. Energy & Environmental Science, 2014. 7 (2):p.513-537. Genovese, M., et al., Combinatorial methods for improving lithium metal cycling efficiency. Journal of The Electrochemical Society, 2018. 165 (13):p.A3000. Hobold, G.M., et al., Moving beyond 99.9% Coulombic efficiency for lithium anodes in liquid electrolytes. Nature Energy, 2021. 6 (10):p.951-960. Suo, L., et al., Fluorine-donating electrolytes enable highly reversible 5-V-class Li metal batteries. Proceedings of the National Academy of Sciences, 2018. 115 (6):p.1156-1161. Aurbach, D., et al., On the use of vinylene carbonate (VC) as an additive to electrolyte solutions for Li-ion batteries. Electrochimica acta, 2002. 47 (9):p.1423-1439. Markevich, E., et al., Fluoroethylene carbonate as an important component in electrolyte solutions for high-voltage lithium batteries: role of surface chemistry on the cathode. Langmuir, 2014. 30 (25):p.7414-7424. Han, B., et al., Poor Stability of Li2CO3 in the Solid Electrolyte Interphase of a Lithium ‐Metal Anode Revealed by Cryo ‐Electron Microscopy. Advanced Materials, 2021. 33 (22):p.2100404. Liu, J., et al., Lithium difluoro (oxalato) borate as a functional additive for lithium-ion batteries. Electrochemistry communications, 2007. 9 (3):p.475-479. Ma, Y., et al., Enabling reliable lithium metal batteries by a bifunctional anionic electrolyte additive. Energy Storage Materials, 2018. 11 :p.197-204. Figure 1. (a) Sulfonyl/sulfamoyl fluoride chemical structures and (b) R-S and S-F bond lengths calculated by density functional theory. Figure 1