Fluorine-free electrolytes for sustainable lithium batteries: a review

Since their commercialization in the 1990s, lithium-ion (Li-ion) batteries have revolutionized the world of energy storage through the electrification of personal electronics and transportation, as well as the integration of renewable resources (e.g., solar and wind) into the grid¹. Despite the massive increase in battery production to meet the ever-growing energy demand, making batteries safer, more sustainable, and easier to recycle remains challenging^2,3. While these goals have been partially fulfilled by some recent progress made on electrode materials, including the development of Co- and Ni-free LiMn₂O₄ (LMO) and LiFePO₄ (LFP) cathodes as well as the recycling and upgrade of LiCoO₂/LiNi_1–x–yMn_xCo_yO₂ (LCO/NMC) electrodes from the end-of-life devices^4,5,6,7, this is generally not the case for electrolytes due to the heavy dependence on fluorine.

Current non-aqueous Li battery electrolytes consist of Li salts, solvents, and additives, where the presence of fluorine seems inevitable⁸. Lithium hexafluorophosphate (LiPF₆), containing more than ¾ of fluorine by weight, has a particular set of attributes such as high ionic conductivity (ca. 10^–2 S cm^–1 at 25 °C), excellent electrochemical stability (>4.5 V vs. Li⁺/Li), and good passivation for graphite anode and Al current collector, which lead to its almost exclusive use in commercial Li-ion batteries^9,10. In addition, other fluorinated salts, such as lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), are widely used not only in the non-aqueous system, but also in aqueous and solid polymer electrolyte systems because of their high solubility, good hydrolytic stability, and improved thermal stability^9,11,12. Moreover, fluorine-containing solvents or additives (e.g., fluoroethylene carbonate, FEC) can stabilize the electrode materials by forming a fluorinated solid-electrolyte interphase (SEI), and/or cathode-electrolyte interphase (CEI), which improves the longevity of Li-ion batteries^13,14.

With such a high fluorine content in the electrolyte, a range of safety and environmental issues are raised. First, fluorinated electrolytes are susceptible to generating corrosive and toxic products (e.g., HF, PF₅, etc.) when in contact with trace moisture or under abuse, damaging the battery performance^15,16. This is in addition to the fact that fluorine-based synthesis is inherently hazardous, environmentally unfriendly, and costly, as toxic HF is commonly used as a fluorinating agent in the industrial production of various inorganic and organic fluorinated salts^17,18,19. Furthermore, the disposal and recycling of fluorine-containing electrolytes, which amount to ca. 5–10% of the total cell weight, also pose serious environmental and health-related concerns^7,20. Notably, in 2023, the European Chemicals Agency (ECHA) proposed to ban around 10,000 per- and polyfluoroalkyl substances (PFAS), including popular electrolyte components like LiTFSI, 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), and electrode binders such as poly(vinylidene fluoride) (PVDF), and poly(tetrafluoroethylene) (PTFE)²¹. Of course, to attain entirely fluorine-free batteries, the choice of binder should also be optimized. There have been promising efforts in achieving fluorine-free binders^22,23,24,25, but this lies beyond the scope of the present review.

In response to the aforementioned challenges, the battery research community has placed growing emphasis on developing sustainable fluorine-free electrolytes. However, the principles and efficient design of fluorine-free systems remain to be further elucidated. In this review, we provide fundamental insights and discuss recent advances of fluorine-free electrolytes for lithium-based batteries that are complementary to, and potentially competitive with their fluorinated counterparts. Here we focus on the non-aqueous system as it is the most commercially relevant, where the examples provided are arranged into fluorine-free salts, solvents, additives, and the resulting interphases (Fig. 1). Meanwhile, the anode materials discussed are not limited to conventional graphite anode. Promising fluorine-free electrolyte designs for advanced silicon and lithium metal anodes are also presented to meet the high energy density requirement. To provide a realistic outlook, we conclude with both the challenges and opportunities of current fluorine-free electrolytes, with the ambition to motivate the battery community to move beyond the costly, toxic fluorination schemes towards more sustainable paradigms for future battery designs.

Design of sustainable non-aqueous lithium battery electrolytes with fluorine-free salts, solvents, additives, and the resulting interphases.

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Over the past decades, the available choice of lithium salts for electrolyte applications has been rather limited, with fluorine remaining integral to the anionic components^8,9,19. The unique position of fluorine comes from: (1) the highest electronegativity (4.0 on the Pauling scale)²⁶, which helps delocalize the negative charge on anions, thereby facilitating the dissociation between cations and anions; (2) low polarizability that reduces intermolecular forces, particularly ion–dipole interactions with solvent molecules, which in turn improves the transport properties of electrolytes with low viscosities¹⁹; and (3) strong electron-withdrawing capability that reduces the electron density around the anion and thus promote the oxidative stability^19,21. At present, LiPF₆ remains the dominant Li salt in commercial electrolytes due to its overall superior performance, with global annual production exceeding 100,000 tons¹⁹. Notably, fluorinated organic anions like FSI^– and TFSI^– are receiving growing attention in recent years, where the negative charges in sulfonimide are effectively delocalized by the conjugation of four oxygens and one nitrogen in the anionic core [i.e., –SO₂–N^(–)–SO₂–], rendering superior solubility in aprotic solvents (>5 M)²⁷. In an attempt to decouple from the fluorine chemistry, the design of fluorine-free anions has been realized by alternative electron-withdrawing groups and/or conjugated structures^19,28,29. Several candidates to be discussed in this section include boron-based anions, lithium perchlorate (LiClO₄), lithium nitrate (LiNO₃), and cyano- (or nitrile-) based anions.

Among all fluorine-free salt alternatives, lithium bis(oxalate)borate (LiBOB) stands out for several reasons. The conjugation of four electron-withdrawing carbonyl groups on the BOB anion helps delocalize the charge and facilitate salt dissociation, leading to moderate solubility and ionic conductivity³⁰. LiBOB exhibits higher thermal stability (up to 302 °C) and generates more benign thermal decomposition products, such as CO₂, CO, and solid Li₂C₂O₄ and LiB₃O₅, compared to the toxic PF₅ released from LiPF₆³¹. Additionally, it shows good water tolerance due to the formation of LiBOB·xH₂O compounds, which consume trace moisture in the electrolyte³². Earlier efforts towards developing LiBOB-based electrolytes focused on their interaction with graphite anodes. Xu et al. discovered that LiBOB can effectively stabilize graphite anodes even in strongly exfoliating solvents such as propylene carbonate (PC)³³. The reduction of BOB anion generates semicarbonate-like compounds and orthoborates, which collectively form an oxygen-rich SEI that renders graphite stronger protection from solvent cointercalation³⁴. A possible multistep decomposition mechanism for BOB anion is shown in Fig. 2a. Recently, Teoh et al. dissolved LiBOB in ƴ-valerolactone (GVL), a bio-based green solvent, and assessed the electrochemical performance of this electrolyte in graphite half cells³⁵. The use of this electrolyte resulted in excellent rate capacities of graphite anode as opposed to the commercial LiPF₆ electrolyte even with the addition of vinylene carbonate (VC) additive, which could be attributed to the rich chemistry of LiBOB-derived SEI layer. In addition to conventional graphite-based Li-ion batteries, cells combining silicon-graphite composite anodes with NMC111 cathodes provided higher discharge capacity and longer cycle life at C/10 when using a LiBOB electrolyte with VC additives compared to the highly fluorinated electrolyte containing LiPF₆, FEC, and VC³⁶. The authors attributed such performance improvement to the formation of a stable SEI layer, a lower overpotential, and a slow increase in cell resistance. However, they also reported that LiBOB-based electrolytes exhibited higher interfacial resistance at high current rates, which unfortunately limited the cell power capability. Moreover, Samarasingha et al. reported the passivation of a thin Li foil under a constant flow of pure O₂³⁷. Combining lithium oxide coating and LiBOB electrolyte resulted in extended Li metal cycling and inhibited dendrite formation as opposed to the LiPF₆ electrolyte, even under extensive galvanostatic cycling at varying current densities.

a Proposed decomposition mechanism for BOB anion. Reprinted with permission from ref. ³⁴, IOP Publishing. b Schematic showing the formation of the CEI by the LiBOB decomposition. Reprinted with permission from ref. ³⁸, Elsevier. c SEM images of Al current collectors in respective electrolytes after the anodic dissolution test. Reprinted with permission from ref. ⁴⁰, John Wiley and Sons. d Schematic illustration of a bi-functional fluorine-free LiBH₄ electrolyte for the micro-Si anodes (left); The comparison of ICE and capacity achieved through different strategies (right). Adapted with permission from ref. ⁴⁵, John Wiley and Sons. e Schematic illustration of the working mechanism of the APA-LC system on the cathode (left); Corresponding self-discharge curves in Gr||NMC811 full cells, where the cells were fully charged to 4.2 V and held for a long-term period at 25 °C (right). Adapted with permission from ref. ²², Elsevier. f Long-term cycling of symmetric Li||Li cells in different electrolytes at 0.5 mA cm⁻² and 1 mAh cm⁻² at −20 °C. Reprinted with the authors’ permission from ref. ⁴⁹.

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On the other hand, LiBOB was also reported to stabilize the cathode materials with an oxygen-rich CEI, where the ion–dipole interaction between electron-deficient boron atoms and electron-rich species such as PF₆^– may inhibit excessive electrolyte decomposition (Fig. 2b)³⁸. However, it was noticed that Co might play a role in catalyzing the decomposition of BOB anion, which accounted for the poor electrochemical performances of LiBOB electrolyte in LCO and NMC cathodes compared to Co-free LMO and LFP cathodes³⁹. In another work, Kreth et al. prepared a LiBOB/ethyl isopropyl sulfone (EiPS) electrolyte and evaluated its interaction with Al current collectors⁴⁰. As shown in Fig. 2c, no anodic dissolution was observed from Al discs when using 1 M LiBOB in EiPS at 60 °C, confirming the exceptional capability of the LiBOB to passivate the Al surface. The benchmark LP 30 (1 M LiPF₆ in ethylene carbonate [EC]/dimethyl carbonate [DMC]) electrolyte can also protect Al from corrosion but with a slightly higher anodic dissolution current. In clear contrast, 1 M LiTFSI in EiPS and/or EC/DMC failed to passivate the Al current collector, resulting in severe pitting corrosion and high anodic dissolution currents due to the formation of soluble Al(TFSI)₃. The superior stability of 1 M LiBOB/EiPS electrolyte enabled exceptional high-temperature performances in Li||LFP and Li||graphite half cells as opposed to the LP 30 electrolyte.

The derivatives of LiBOB, including lithium malonato-oxalatoborate (LiMOB) and lithium bis(malonato)borate (LiBMB), have also been studied using quantum mechanical calculations⁴¹. These orthoborate anions were proposed by introducing a –CH₂– group between the two carbonyl groups on one side of the BOB anion (i.e., LiMOB) or symmetrically on both sides (i.e., LiBMB). However, both of them have stronger coordination to Li⁺ due to the reduced conjugation effect by having CH₂ in the molecules, resulting in poor solubility and conductivity in common aprotic solvents³⁰. Similarly, lithium tris(oxalato) phosphate (LiTOP) was studied as an analogue of LiBOB, where the six carbonyl groups readily delocalize negative charges on the anion, leading to high solubility and conductivity in carbonate solvents⁴². For the electrochemical performance, LiTOP was reported to form an SEI on graphite at a high potential of ~2.15 V (vs. Li⁺/Li), and Li||graphite half cells made with this salt accomplished high capacity retention and Coulombic efficiency (CE) for 9 cycles⁴². Lithium tris[1,2-benzenediolato(2-)-O,O’]phosphate (LiTBP) is another example of phosphate compound⁴³. When dissolved in the mixture of EC and tetrahydrofuran (THF), it showed inferior conductivity than LiPF₆ due to the high viscosity of the solutions. The oxidative stability of LiTBP was reported to be 3.7 V vs. Li⁺/Li, which restrains the salt from high-voltage applications⁴³. It’s worth noting that the long-term cycling performance of phosphate-based fluorine-free salts remains to be validated due to the lack of relevant experiments.

To continue the efforts of developing new alternative salts, Giri et al. investigated superhalogens as potential building blocks of halogen-free electrolytes via first-principle calculations⁴⁴. The authors proposed three criteria for the design of new electrolytes. First, they should be halogen-free to improve safety. Second, the binding energy between Li⁺ and anions should be small so that Li ions can move easily from one electrode to another. Third, the affinity of the electrolyte to water should be low so that the battery cycle life can be extended. Among all halogen-free superhalogens investigated in their work, LiCB₁₁H₁₂ had the most desirable characteristics, while other metal borohydrides such as LiBH₄ and LiB₃H₈ were also potential candidates. As an example of experimental validation of lithium borohydride (LiBH₄) based electrolyte for battery applications, Li et al. designed a bi-functional LiBH₄/THF-MeTHF (2-methyltetrahydrofuran) electrolyte to stabilize the interphase and improve the CE of micro-sized silicon (mSi) anodes⁴⁵. Benefiting from the strong reducibility of LiBH₄, this electrolyte can chemically pre-lithiate the native oxide layer of mSi and relieve the pressure during SEI formation/accumulation to preserve the internal conductive network (Fig. 2d). For the first time, a fluorine-free electrolyte enabled the mSi electrode (80 wt% Si) to have a high specific capacity of 2900 mAh g^–1, a record-high initial CE of 94.7%, and a stable cycling performance with 84.7% capacity retention after 400 cycles at 0.2 C.

Though plagued by the safety concerns due to the high oxidation state of chlorine (VII), LiClO₄ has nevertheless been extensively studied as a fluorine-free alternative for its low cost, high solubility, and high ionic conductivity^{9,22,46,47,48,49}. In a recent study, Nam et al. demonstrated an entirely fluorine-free Li-ion battery design by replacing the conventional LiPF₆-based electrolyte (LP) and PVDF binder with LiClO₄-based electrolyte (LC) and a newly synthesized aromatic polyamide (APA) binder²². The LC electrolyte had superior oxidation stability up to 5 V, and the polar group-rich APA binder offered higher ionic conductivities and strong hydrogen binding interactions with active material particles and current collectors (Fig. 2e). As a result, the graphite||NMC811 cell in the APA-LC system exhibited a slow self-discharge rate over three weeks, while that of the PVDF-LP system experienced a sharp potential drop only after three days. Consequently, graphite||NMC811 full cells based on the APA-LC system displayed remarkable cycling stability with 75.2% capacity retention after 200 cycles between 2.8 to 4.3 V. Moreover, LiClO₄-based electrolytes have been assessed in high-energy-density Li metal batteries. Zhang et al. evaluated the low-temperature (LT) Li stripping/plating stability in LiTFSI/DOL (1,3-dioxolane)-DME (1,2-dimethoxyethane), LiTFSI/DOL, and LiClO₄/DOL electrolytes, where the latter stood out with the lowest and most stable overpotential over 500 hours cycling at –20 °C (Fig. 2f), possibly resulting from the reduced binding energy between Li⁺ and ClO₄^–49. One critical problem of LT batteries is the rapid increase of internal resistance that leads to performance deterioration. In this regard, the LiClO₄/DOL electrolyte can potentially be employed under extreme temperature environments with minimal voltage fluctuation. Indeed, after incorporating a Lewis base additive dimethyl sulfoxide (DMSO) to regulate electrolyte solvation and Li deposition, the authors were able to achieve impressive LT Li||LFP cell performance down to –80 °C.

Besides LiClO₄, LiNO₃ is also considered a viable candidate to potentially replace LiPF₆ for its excellent SEI-forming ability, superior thermal and moisture stability, low cost, and environmental friendliness^50,51,52. However, due to the poor solubility of LiNO₃ in common aprotic organic solvents, it has mostly been used as an additive in prior works instead of the main salt^52,53,54. Fortunately, recent studies have demonstrated several high donor number solvents that are capable of dissolving LiNO₃ in sufficient amount^{55,56,57,58,59,60}. Details of successful examples will be discussed in the following section to highlight the contribution of solvents for LiNO₃-based fluorine-free electrolytes.

Another category of fluorine-free Li salts is with cyano- (or nitrile-) based anions. They were proposed due to the extensive electron-withdrawing nitrile groups, which allow the dissociation of the salt in typical aprotic solvents and provide moderate solubility and ionic conductivity²⁹. Scheers et al. dissolved lithium tetracyanoborate [LiB(CN)₄] and lithium 4,5-dicyano-1,2,3-triazolate (LiDCTA) in poly(ethylene glycol) dimethyl ether (PEGDME) and used polyacrylonitrile (PAN) membranes to absorb the electrolytes⁶¹. Both electrolytes were able to cycle Li||LFP cells over 15 cycles, though the authors mentioned that for them to be truly competitive, the relatively low oxidation stability and elevated temperature performance must be improved. Likewise, Kerner et al. evaluated LiDCTA-based electrolytes in high flashpoint adiponitrile (ADN) with sulfolane (SL) and EC as co-solvents, aiming at extending the liquid temperature range of the electrolytes without lowering the flashpoint⁶². Due to high electrolyte viscosities and slow mass transport, the electrochemical cycling performances of Li||LFP cells were moderate. However, the authors claimed that the combination of ADN and SL would be promising for cell operations at elevated temperatures (>60 °C) to compensate for the moderate transport properties and rate capability. Besides their role as the counterion in Li salts, cyano-based anions are also important to the development of fluorine-free ionic liquid (IL) electrolytes, and more relevant examples will be presented in the next section.

Overall, there has been significant progress in developing fluorine-free anions, and some key properties (e.g., transport properties, anodic stability, etc.) of the fluorine-free electrolytes could compete with or sometimes surpass their fluorinated counterparts. While concerns over the safety, environmental impact, and economic challenges of fluorine-containing salts are well justified, it is equally important to acknowledge the inherent limitations of certain fluorine-free alternatives, such as limited chemical stability (e.g., ClO₄^–, carboranes), potential toxicity during synthesis and recycling (e.g., use of cyanating agents), and the high cost associated with scaling up and purifying complex anions. To provide a complete picture of the basic properties of Li salts discussed, we compared various factors, including physicochemical properties, electrochemical stability, cost, toxicity, and environmental impact in Table 1, with respective salt structures shown in Fig. S1, Supporting Information. Furthermore, a high-level comparative overview is provided in Table 2 to highlight the key distinctions between salt anions with and without fluorine.

While the fluorinated Li salts are widely used in current non-aqueous battery electrolytes, the solvents, however, do not necessarily need fluorination⁹. Generally, the dissolution of Li⁺ conducting salts in non-aqueous solvents could be considered as a multi-step ligand-exchange reaction^19,28. Upon the attack of lone pair electrons of aprotic solvents (e.g., the polar carbonyl and ether groups), the cation–anion pairs within the crystal lattice gradually dissociate, leading to the formation of various Li⁺ solvation clusters depending on the content and property of solvents, including aggregates (AGGs, i.e., multiple Li-ions associated with one anion), contact ion pairs (CIPs, i.e., a single Li⁺ directly interacting with an anion), solvent separated ion pairs (SSIPs, i.e., a Li⁺ surrounded by solvent molecules with anions excluded), and free anions^19,63,64,65. In the context of fluorine-free salts, the dissociation of Li⁺ cations and anions generally requires more solvating power from the solvent due to the lack of fluorine that can delocalize the charge on the anion^28,29. Therefore, in this section, we first look into several strong-solvating (fluorine-free) solvents that can dissolve a sufficient amount of fluorine-free salts. Additionally, we will discuss current efforts in developing non-solvating fluorine-free cosolvents/diluents to replace fluorinated ethers in emerging localized high-concentration electrolytes (LHCEs), as well as the development of fluorine-free IL electrolytes for Li batteries.

Considering non-aqueous battery electrolytes, the Gutmann donor number (DN) is a commonly used parameter to characterize solvents, as it measures their electron-donating properties and, consequently, their ability to interact with electron acceptors such as Li⁺ cation⁶⁶. The higher the DN, the greater the basicity of the solvent, which suggests stronger coordination with hard Lewis acids like Li⁺. While the DN of Li salts is rarely discussed as most anions exhibit similar and relatively low DN values, LiNO₃, however, is a notable exception with a rather high DN (Table 1). As mentioned earlier, LiNO₃ exhibits strong Li⁺–anion interactions, which hinders its dissolution in typical carbonate and ether-based solvents^66,67,68. On the other hand, there exist some high DN solvents, namely DMSO, 1,3-dimethyl-2-imidazolidinone (DMI), trimethyl phosphate (TMP), triethyl phosphate (TEP), etc., which can dissolve LiNO₃ at practical concentrations (Fig. 3a)^{55,56,57,58,59,60}. Nevertheless, all these high DN solvents with strong electron-donation ability have poor reduction stability, leading to continuous decomposition at the anode^69,70. Chen et al. employed an electron-donation modulation (EDM) strategy to prepare a LiNO₃-based electrolyte, in which the DMSO with a high DN was to dissolve a high concentration of LiNO₃ while PC with a low DN served to regulate the solvation structure and stabilize the electrolytes (Fig. 3a)⁵⁹. As a result, the LiNO₃-DMSO@PC electrolyte exhibited excellent electrochemical compatibility with graphite anodes, as well as the LFP and LCO cathodes, leading to stable battery cycling over 200 cycles. Likewise, Zhou et al. prepared a fluorine-free electrolyte composed of 3 M LiNO₃ in DMI, where the Li⁺ formed CIPs and AGGs with NO₃^– and DMI, suppressing the side reaction between free solvent molecules and Li metal⁶⁰. The concentrated LiNO₃ was reduced before Li deposition and contributed to the initial SEI formation due to the unique solvation structure (Fig. 3b). The authors claimed that the SEI layer consisted of abundant inorganic N-containing components, which possess high Li⁺ conductivity and mechanical strength. As shown in Fig. 3b, dendrite-free columnar Li deposition can be achieved even at a high current density (2 mA cm^–2) and areal capacity (4 mAh cm^–2) conditions.

a DN values of NO₃⁻ and different solvents (top left); Schematics of corresponding electrolyte solvation structures (top right); The dissolution behavior of LiNO₃ in various solvents (bottom). Reprinted with permission from ref. ⁵⁹, John Wiley and Sons. b Schematics of the columnar Li deposition in a 3 M LiNO₃/DMI electrolyte (top); Corresponding SEM images of deposited Li in top-down and cross-sectional views (bottom). Reprinted (adapted) with permission from ref. ⁶⁰. Copyright (2021) American Chemical Society. c ESP and binding energy of Li⁺ with DEE and PhH molecules (top); Optical photographs of Li chips immersed in FB and PhH for 14 days at room temperature (bottom). Reprinted with permission from ref. ⁷³, John Wiley and Sons. d Resonance structures of selected organic compounds with and without resonance effect, and the corresponding average CE and β values. Adapted from ref. ⁷². Copyright (2022), The Authors.

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Despite most of the solvents in battery electrolytes being fluorine-free, fluorination of carbonates and ethers can be beneficial. For example, FEC is widely used as a film-forming additive to enhance battery performance. In addition, fluorinated ethers, such as TTE, bis(2,2,2-trifluoroethyl) ether (BTFE), tris(2,2,2-trifluoroethyl)orthoformate (TFEO), etc., are becoming common co-solvent/diluent in LHCEs for Li metal battery research^71,72. The highly electronegative fluorine withdraws electrons from adjacent oxygen on the ether chain, resulting in low solubility to Li salts, thereby preserving the locally concentrated ion-aggregate structures, and hence, the anion-derived SEIs⁷². Despite this, the high cost and environmental concerns of fluorinated ethers have motivated the research into non-fluorinated co-solvent/diluent for LHCEs^72,73,74. Hai et al. adopted the simplest aromatic hydrocarbon, benzene (PhH), as the diluent to modulate electrolyte solvation⁷³. PhH does not contain any electron-donating groups, and therefore, can only weakly coordinate with Li⁺. This is evidenced by its lower binding energy compared to 1,2-diethoxyethane (DEE) (Fig. 3c). In addition, due to the lack of electron-withdrawing groups, PhH has superior reduction stability when in contact with Li metal. As shown in Fig. 3c, the Li foil immersed in PhH maintained a silver metallic surface over 14 calendar days with no change in pH. In contrast, a dark finish was observed on the Li foil when left in fluorobenzene (FB), where a pH value of 5 indicated the formation of HF, which is known to deteriorate the battery performance. When using PhH as the diluent for LHCE, Li ||single crystal NMC811 cells with a cathode loading of 9 mg cm^–2 achieved 87.3% capacity retention after 450 cycles. In another work, Moon et al. proposed a design principle for the ideal non-fluorinated non-solvating cosolvents (NFNSCs) based on the resonance effect of solvent molecules (Fig. 3d)⁷². Specifically, anisole, ethoxybenzene, and furan were identified with resonance structures where the lone pair electrons on the oxygen atom endowed the solvents with optimal non-solvating and miscible characteristics, exhibiting lower Kamlet-Taft Lewis basicity (β) and higher average CEs as opposed to the non-resonant counterparts. Consequently, Li metal anodes with NFNSC-containing electrolytes demonstrated superior cycling performance over 350 cycles (CE = 99.0%, ethoxybenzene), 500 cycles (CE = 98.5%, anisole), and 1400 cycles (CE = 99.0%, furan), respectively. Following on the previous work, Liu et al. developed a locally concentrated ionic liquid electrolyte (LCILE) consisting of LiFSI and EmimFSI (Emim: 1-ethyl-3-methylimidazolium) diluted by nonfluorinated anisole cosolvent⁷⁴. They found that anisole not only promoted the ion transport in the electrolytes via a nanophase-segregation structure but also modulated the SEI by affecting the deposition of organic cations and anions on the Li metal anode. As a result, LCILE with optimized anisole content enabled Li metal anodes with high CEs and stable cycling under practical conditions.

ILs have been used as alternative solvents in battery electrolytes because of their intrinsic non-flammability, low volatility, and high ionic conductivity⁷⁵. Typical ILs consist of organic cations such as pyrrolidinium (Pyr) and imidazolium (IM) derivatives, accompanied by inorganic or organic anions, such as PF₆^–, TFSI^–, and FSI^–75. The use of fluorinated anions ensures low basicity and a high degree of charge delocalization, which in turn facilitates the easy dissociation of ions in the solvent⁷⁶. However, because of the safety and environmental concerns of the fluorine-containing species, there is a growing trend towards the development of fluorine-free IL electrolytes^{77,78,79,80,81,82,83,84,85,86}. Yoon et al. reported the use of IL electrolytes containing dicyanamide (DCA) and two related anions based on the cyano-moiety⁷⁷. When LiDCA was dissolved in N-methyl-N-butylpyrrolidinium dicyanamide ([Pyr14][DCA]), the electrolyte showed good compatibility with Li metal, Li₄Ti₅O₁₂, and LFP electrodes. In particular, the Li||LFP cell delivered over 130 mAh g^–1 of discharge capacity without a significant capacity reduction after 20 cycles at 50 °C. Likewise, Karimi et al. prepared Pyr14-based ILs with DCA and tricyanomethanide (TCM) anions, as well as their mixtures with respective Li salts as electrolytes for Li metal batteries⁷⁸. The electrolytes exhibited high ionic conductivity (5 mS cm^–1) at ambient temperature and an electrochemical stability window up to 4 V. In a later study, the same group applied LiDCA/[Pyr14][TCM] IL electrolyte with 5 wt% VC additives for the electrochemical alloying of silicon nanowire (Si NW) anodes, where a highly reversible Si-Li alloying process can be achieved with capacities of 1,760 mAh g^–1 after 200 cycles (88% retention) and 1,500 mAh g^–1 after 500 cycles (75% retention), respectively⁷⁹. Overall, the study of fluorine-free IL electrolytes is still in its early phases, where most of the research on the design of new anions is focused on their physicochemical properties with little or no information on the systematic battery performance^{80,81,82,83,84,85,86}. We summarize the chemical structures of the fluorine-free IL anions developed so far in Fig. S2, while future works are needed to validate their electrochemical performance in prototype battery cells.

In short, compared to the limited choice of Li salts, there are a wide spectrum of aprotic organic solvents that could make possible fluorine-free electrolytes. Given that most fluorine-free salts (except for LiClO₄) exhibit relatively poor solubility in conventional carbonate and ether solvents, high DN solvents with strong basicity are often required, though their highly reactive nature poses significant challenges to negative electrodes (e.g., Li metal) and thus demands careful electrolyte engineering. Additionally, several solvent candidates, such as benzene and cyano- (or nitrile-) containing compounds, exhibit potential toxicity and carcinogenicity, which may limit their practical applicability. For reference purposes, Table S2 lists the basic properties of all solvents discussed, along with their toxicity and hazard statement, while the respective solvent structures can be found in Fig. S3.

Another potential source of fluorine in battery electrolytes is the additive. For example, FEC has predominantly been used as an additive to improve the SEI/CEI qualities^14,87,88. The unique position of FEC comes from its preferential decomposition over other organic solvents during redox reactions, resulting in facile defluorination (and decarboxylation) that passivate electrodes⁸⁸. However, a wide variety of non-fluorinated additives that can potentially fulfill the same role have also been investigated. Selected examples in this section include unsaturated carbon-containing compounds (i.e., VC), as well as representative additives classified by their central elements such as P, Si, N, S, etc.

VC is a popular electrolyte additive that positively affects the cathode and particularly the anode^89,90. Due to the presence of a C═C bond, VC is preferentially reduced to form poly(VC) and CO₂, whereas the latter could be further converted to Li₂CO₃, Li₂C₂O₄, and HCO₂Li^91,92. Several studies have identified the polymerization of VC (i.e., poly[VC]) as the key to improving the SEI elasticity (and hence the anode performance)^91,92,93,94. A possible reaction mechanism for the formation of poly(VC) is presented in Fig. 4a. Ivanov et al. studied the volume change of composite graphite anode during the initial formation cycles by electrochemical dilatometry in electrolytes with and without VC⁹⁵. The authors observed that the additive-free electrolyte cannot effectively terminate the electron charge transfer on the active graphite surface. This led to the continuous incorporation of decomposition products into the composite layer during subsequent cycles and caused irreversible volume expansion. After incorporating VC, the modified electrolyte formed a stable SEI layer with poly(VC) that narrowed the interparticle composite pores. In this case, the transport of electrolyte decomposition products inside the graphite layer was limited, and the irreversible expansion of the composite coating was largely reduced. Besides graphite, Si anodes suffer from uncontrolled SEI growth and cracking due to the large volume expansion of Si particles during lithiation^93,96. Jaumann et al. discovered that the VC additive for nano-silicon anodes outperformed FEC in terms of lifetime and efficiency because of the formation of a dense and flexible SEI that survived the large volume changes of Si without surface cracking propagation (Fig. 4b)⁹⁶. However, they also claimed that the VC-derived SEI exhibited higher resistance to Li⁺ migration due to the absence of any defects, which to some extent made it unfavorable for high-power applications. In another work, Jin et al. investigated the SEI formation on binder-free Si nanowire (SiNW) electrodes in pure VC electrolyte⁹³. Using ²⁹Si dynamic nuclear polarization (DNP) enhanced solid-state nuclear magnetic resonance (ssNMR), the authors were able to probe the interfacial region for the first time, where the polymeric SEIs consisting mainly of cross-linked poly(ethylene oxide) (PEO) and aliphatic functional groups along with additional carbonate and carboxylate species. They confirmed that some of the organic SEI was covalently bonded to the Si surface upon cycling, and both the polymeric SEI structure and the nature of its adhesion to the redox-active materials were vital to the capacity retention and CE of Si anodes.

a Proposed reaction scheme for the formation of poly(VC). Reprinted (adapted) with permission from ref. ⁹². Copyright (2016) American Chemical Society. b Schematic illustration of VC and FEC-derived SEIs on the nano-silicon anodes. Reprinted with permission from ref. ⁹⁶, Elsevier. c Proposed mechanisms for phosphite-based additives for HF removal. Reprinted with permission from ref. ⁹⁹, Elsevier. d The volume of gas evolved, reversible capacity loss, and irreversible capacity loss of graphite||NMC442 pouch cells with different electrolyte additives, the cells were charged to 4.5 V and stored at 60 °C. Reprinted from ref. ¹⁰¹, CC BY 4.0. e Comparison of S-content in various sulfur-containing compounds as the electrolyte additive in LIBs (left); TEM of cycled NMC532 cathodes in control electrolyte (middle) and electrolyte with MMDS additive (right). Adapted with permission from ref. ¹⁰⁵, John Wiley and Sons.

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Phosphite (III) derivatives possess lone pair electrons with nucleophilicity and basicity, thus acidic compounds such as HF can be removed via effective protonation of phosphites^97,98,99. Song et al. investigated a family of phosphite-based additives, including triphenyl phosphite (TPP), trimethyl phosphite (TMP), tris(2,2,2-trifluoroethyl) phosphite (TFEP), and tris(trimethylsilyl) phosphite (TMSP), for the stabilization of high voltage LiNi_0.5Mn_1.5O₄ (LNMO) batteries (Fig. 4c)⁹⁹. The authors claimed that the molecular structures of phosphites could influence their ability to scavenge HF and to form adequate CEI on the cathode. For instance, TPP with bulky triphenyl groups showed poor HF scavenging capability, which could be attributed to the increased steric hindrance to forming TPP–HF coordination. On the other hand, TMSP with silyl ether motifs can form pentavalent silane intermediate by coordinating with fluorine anion^99,100. As these silyl ether groups in TMSP served as additional sites to scavenge HF, TMSP therefore achieved superior HF removal performance than TFEP, TMP, and TPP. Moreover, Li||LNMO cells with TMSP-added electrolytes outperformed other additives in a series of high C-rates and high-temperature storage tests, suggesting that TMSP-derived CEI is electrochemically and thermally robust to alleviate the interfacial cracking as well as the oxidative decomposition of the electrolyte at 60 °C.

Cyano- or nitrile-based (–C ≡ N) electrolyte additives have been studied for their high oxidative stability and strong electronegativity to bond with transition metals on the cathode, as well as trace moisture/acid in the electrolyte^{97,98,101,102}. Kim et al. examined a series of nitrile-based electrolyte additives, including succinonitrile (SN), adiponitrile (ADN), and pimelonitrile (PN), on the performance of Li-ion batteries¹⁰¹. As shown in Fig. 4d, the addition of 2 wt% SN reduced the gas generation and reversible capacity loss when stored at 60 °C, and the gas volume became smaller as the chain length of the nitrile additives increased. The authors also reported that nitrile electrolyte additives can slightly increase cell impedance and irreversible capacity loss, which could be suppressed by further electrolyte modifications. Overall, SN can improve the high-voltage cell storage (>4.4 V) and the long-term cycle life of graphite||NMC cells by suppressing electrolyte oxidation at the cathode. In another work by Lee et al., ADN was employed as a bi-functional additive to produce a conductive and robust Li metal/electrolyte interface, as well as to reduce the parasitic reactions between the electrolyte and Ni-rich cathode via strong Ni⁴⁺–ADN coordination¹⁰². Such synergistic effect enabled excellent Li||NMC battery performance with an unprecedented capacity retention of 75% over 830 cycles.

Sulfur (S)-containing compounds are attractive electrolyte additives for Li-ion batteries^98,103. Compared to carbonates, S-containing additives have lower lowest unoccupied molecular orbital (LUMO) for their preferential decomposition¹⁰⁴. In addition, depending on sulfur’s valence state, S-containing compounds can be tailored for various applications with rich organic chemistry. Common S-containing additives include ethyl sulfite (ES), propane sultone (PS), prop-1-ene-1,3-sultone (PES), 1,3,2-dioxathiolane-2,2-dioxide (DTD), and methylene methyl disulfonate (MMDS)^104,105. Among them, MMDS shows an ultrahigh S-content of 34.04% (Fig. 4e)¹⁰⁵. Fan et al. proposed that the electrochemical performance of batteries was highly dependent on the S content of additives, and MMDS was therefore selected to modify the electrolyte chemistry¹⁰⁵. To verify their hypothesis, DTD with a relatively low S content of 25.81% was chosen for comparison. Through coupled experimental and theoretical studies, they discovered that MMDS took part in the Li⁺ solvation structure via strong PF₆^––MMDS interactions and preferentially decomposed as an ultrathin but robust S-rich interphase. As shown in Fig. 4e, the NMC532 cathode cycled in the control electrolyte was covered with an uneven and thick CEI in ca. 115 nm, indicating severe electrolyte decomposition and accumulation of side products. In contrast, a uniform and compact CEI layer with a thickness of ca. 8 nm was observed in the MMDS system, suggesting the effective passivation of the cathode without excessive electrolyte oxidation. The CEI layer formed in the DTD system was relatively uniform but a little bit thicker (ca. 13 nm). The authors claimed that the MMDS-derived interphase was critical in accelerating the Li⁺ transfer and inhibiting gas evolution, transition metal dissolution, and microcracks on the cathode. As a result, the high-voltage graphite||NMC532 pouch cell with MMDS additive realized 87.99% capacity retention after 800 cycles, whereas that of DTD and control electrolyte were 79.88% and 54.68%, respectively.

It should be noted that some fluorine-free salts, such as LiBOB, LiNO₃, etc., can also be considered as additives when used in low concentrations. Their attributes and associated working mechanisms would largely follow the cases discussed in sections 2–3. In addition, although the examples in this section arguably use LiPF₆ as the primary Li salt, non-fluorinated additives are indeed compatible with fluorine-free electrolytes and have been verified in several cases described in previous sections^36,49,79,106.

The creation of SEIs on the anode materials, including graphite, silicon, and Li metal, pre-eminently determines the performance and functionalities of Li batteries. It is broadly accepted that the SEI consists of inorganic and organic phases, with the former including lithium fluoride (LiF), lithium carbonate (Li₂CO₃), lithium oxide (Li₂O), and lithium nitride (Li₃N) depending on salt; and the latter including semi-carbonates, alkoxides (ROLi), and poly/oligomers depending on the solvent¹⁰⁷. Current electrolyte design emphasizes fluorination for high stability and CE, which is largely based on the perceived benefits of LiF in SEI, such as chemical inertness, mechanical strength, low electronic conductivity, and high interfacial energy¹⁰⁸. Yet, LiF is the most resistive phase for Li⁺ transport among common inorganic SEI species (Fig. 5a), and mounting evidence has questioned whether LiF has a clear chemical benefit in the SEI, which therefore motivates a deeper look into other possible phases that provide the major SEI functionality^{108,109,110,111,112}.

a Comparison of band gaps and ionic conductivities of inorganic SEI species (LiF, Li₂CO₃, Li₂O, and Li₃N). Reprinted from ref. ¹¹², CC BY-NC 3.0. b HOMO–LUMO energy gap of typical electrolyte decomposition products and their corresponding binding energy with Li⁺. Reprinted with permission from ref. ¹⁰⁵, John Wiley and Sons. c Cryo-TEM images of mSi anodes after first delithiation in the LiBH₄ electrolyte with the inset showing the enlarged crystal lattice fringe (left); Corresponding FFT pattern (top right); Schematics of Cryo-TEM images with labeled interphase thickness (bottom right). Reprinted with permission from ref. ⁴⁵, John Wiley and Sons. d 3D views of SEI composition detected by TOF-SIMS sputtering on the surface of deposited Li using optimized LiBOB/LiNO₃/DME electrolyte at 0.5 mA cm⁻² and 3 mAh cm⁻² at 60 °C (left); Schematic diagram of the B/O/N hybrid SEI structure on the LMA (right). Reprinted (adapted) with permission from ref. ¹⁰⁶. Copyright (2024) American Chemical Society. e Illustration of the experimental workflow used for Li₂O quantification based on an alcohol-based titration followed by Karl Fischer analysis (top); Rank CE and rank phase (%) for LiF and Li₂O across diverse electrolyte systems (bottom left); Summary of CEs measured for LHCEs containing LiBF₄, LiPF₆, LiClO₄, and LiNO₃ as salts (bottom right). Adapted with permission from ref. ⁴⁸, Springer Nature. f Cryo-TEM images of SEI formed in LiClO₄/DOL electrolyte (left); The color-coded graphs (middle); Corresponding selective area electron diffraction pattern (right). Reprinted with the authors’ permission from ref. ⁴⁹.

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Fan et al. calculated the highest occupied molecular orbital (HOMO)–LUMO energy gap and corresponding binding energy for typical decomposition products in ester-based electrolytes, including LiF, Li₂CO₃, (C₂H₂O₃Li⁺)₂ (LEDC), and two S-containing species, i.e., (C₂H₄SO₃Li⁺)₂ (LCSO) from MMDS and (C₂H₄OSO₃Li⁺)₂ (LCOSO) from DTD (Fig. 5b)¹⁰⁵. DFT calculation showed that the HOMO–LUMO gap of LCSO and LCOSO (7.12 and 6.80 eV, respectively) was slightly higher than that of LEDC (6.51 eV), suggesting the high electronic insulation and chemical stability of decomposition products from MMDS and DTD. Moreover, higher binding energy values of LCSO–Li⁺ and LCOSO–Li⁺ indicated fast Li⁺ diffusion, which is beneficial to the high-rate performance. An in-depth analysis of the SEI composition on graphite was conducted using X-ray photoelectron spectroscopy (XPS). The authors found that the introduction of MMDS reduced the intensities of C–O, C═O, and Li₂CO₃, suggesting the inhibition of solvent decomposition. In addition, lower contents of fluorides (including Li_xPF_y, Li_xPO_yF_z, and LiF) were observed with MMDS additive, which indicates the suppression of excessive decomposition of PF₆^–. They also detected high sulfide intensities, such as ROSO₂Li and ROSO₃Li, confirming the formation of an MMDS-derived S-rich SEI, which successfully inhibited continuous electrolyte decomposition. In another work, Li et al. investigated the pre-lithiation and cycling of mSi anode with a fluorine-free LiBH₄ electrolyte, and the corresponding interfacial structure was revealed by Cryogenic Transmission Electron Microscopy (Cryo-TEM)⁴⁵. As shown in Fig. 5c, the as-formed interphase was homogeneous, thin (ca. 7 nm), and compact, where nanocrystalline (111) Li₂O domains were evenly distributed in a thin organic matrix, implying a suppressed solvent decomposition due to the intrinsic stability of the LiBH₄ electrolyte. Such uniform and inorganic Li₂O-rich interphase facilitated the isotropic alloying/de-alloying reactions and maintained the structural integrity of the mSi electrode after cycling.

Besides graphite and Si anodes, fluorine-free SEIs have also been validated in Li metal batteries. Jiang et al. designed a fluorine-free electrolyte composed of 1 M LiBOB + 0.5 M LiNO₃ in DME, capable of generating a stable B/O/N hybrid SEI with fast Li⁺ transport¹⁰⁶. As shown in Fig. 5d, the SEI structure on the Li metal surface was characterized using time-of-flight secondary ion mass spectrometry (TOF-SIMS). The strong Li⁻ signal indicated predominant inorganic Li salt decompositions on the Li surface. Subsequent analysis of the O₂⁻, B⁻, LiO₂⁻, BO₂⁻, and NO₂⁻ signals confirmed that these inorganic Li species primarily consisted of Li₂O, LiB_xO_y, and LiN_xO_y. In addition, the C⁻ and CH⁻ signals reflected the existence of a thin organic layer on top of this inorganic layer, possibly originating from solvent decomposition. The authors proposed that Li₂O and LiB_xO_y were the main building blocks within the SEI, contributing to mechanical stability, while the presence of dispersed LiN_xO_y created efficient pathways for Li⁺ transport. Furthermore, the inclusion of a small amount of organic compounds imparted the necessary SEI elasticity. Consequently, when tested at 60 °C, the Li ||LFP battery with LiF-free SEI demonstrated a rapid charging/discharging rate of 100 C with a capacity exceeding 80 mAh g^–1 as well as a stable cycling performance over 500 cycles at 50 C. On the other hand, inorganic N-rich components exhibit superior Li⁺ conductivity and remarkable mechanical strength, which can contribute to uniform and rapid Li⁺ diffusion in the SEI^112,113,114. Encouraged by these attributes, Zhou et al. developed a fluorine-free electrolyte consisting of 3 M LiNO₃ in DMI, which enabled micron-size dendrite-free Li deposition in a compact columnar structure⁶⁰. XPS results revealed that N-rich inorganic components (Li₃N and LiN_xO_y) were formed on the surface of the Li anode, inhibiting side reactions with the electrolyte and regulating the Li deposition behavior. The columnar Li deposition showed remarkable electrochemical reversibility with a high average CE of 96% over 240 cycles. Further, a Li||LFP cell with this electrolyte exhibited excellent cycling durability over 700 cycles.

To investigate the relevance of Li₂O vs. LiF for high CE Li batteries, Hobold et al. developed an alcohol-based titration followed by Karl Fischer analysis to reveal Li₂O content in cycled Li anodes, enabling this previously titration-silent phase to be compared statistically with a wide range of other leading SEI constituents including LiF⁴⁸. Contrary to conventional understanding, the authors discovered that Li₂O was the most consistently abundant phase at high CE and the strongest CE descriptor (with a statistical correlation coefficient ρ = 0.903), surpassing LiF (ρ = 0.758), even in highly fluorinated electrolytes (Fig. 5e). To demonstrate the possibility of achieving higher CE using oxygenated electrolytes than the fluorinated ones, the authors first prepared TTE/DME-based LHCEs with fluorinated, oxygen-free LiBF₄ and LiPF₆, which yielded poor CEs of 65% and 91%, respectively. Then, the salt was changed to LiClO₄, which reduced the level of fluorine in LHCE and achieved a higher CE of 98.7%. Furthermore, when TTE was replaced with anisole, an entirely fluorine-free electrolyte was formed with an even higher CE of 98.9%. This ‘defluorination’ strategy was then applied to the 1 M LiTFSI DOL/DME + 3 wt% LiNO₃ system with LiTFSI replaced by LiClO₄, yielding yet another fluorine-free electrolyte with high CE (99.1%). Zhang et al. characterized the microstructure of the SEI formed in a fluorine-free LiClO₄/DOL electrolyte by Cryo-TEM (Fig. 5f)⁴⁹. The authors found that a compact and thin SEI layer (~10 nm) composed of large amounts of (111) Li₂O nanocrystallites evenly distributed in an amorphous matrix on the surface of Li metal, where additional (220) and (311) Li₂O diffraction rings were identified in the selective area electron diffraction (SAED) pattern. Such unique Li₂O-dominated SEI rendered an ultralow overpotential (<0.02 V) for Li deposition even at –20 °C, while that of fluorinated electrolytes exhibited a much larger overpotential of more than 0.1 V. In another work, Guo et al. observed that nanostructured all-Li₂O SEI grown on Li foil possessed an approximately two times higher Li⁺ conductivity than all-LiF SEI, thus supporting comparatively more homogeneous Li⁺ flux, and rendering Li₂O more beneficial to SEI transport¹¹¹. The results above indicate that electrolytes yielding Li₂O-rich, LiF-free interphases are promising alternatives to attain high-performance Li metal batteries.

Bringing together organic, inorganic, and structural analyses, we provide the fundamental considerations and current advances in fluorine-free non-aqueous Li battery electrolytes with respect to salts, solvents, additives, and interphases. To highlight the properties of “successful” fluorine-free electrolytes, such as ionic conductivity and cycling performance, Table S3 is provided for a more thorough comparison.

During the past decades, most of the electrolyte design, development, and optimization for Li-based batteries has been conducted with fluorinated electrolytes containing LiPF₆ (and many of which contain FEC as well), as this category of electrolytes still gives the overall best performance. However, the incorporation of fluorine-containing components raises serious safety and environmental concerns. Also, the responsible disposal and recycling of current fluorinated electrolytes are far from satisfactory. In this context, there is a growing trend towards the development of sustainable and environmentally friendly alternatives with fluorine-free salts, solvents, additives, and thus interphases. This paper provides fundamental insights and summarizes nonaqueous fluorine-free electrolytes developed so far, highlighting multiple possibilities available for them to be complementary to and potentially competitive with their fluorinated counterparts. It shows that fluorine is not strictly necessary to either achieve high ionic conductivity or form effective SEI/CEI layers, and therefore, it is reasonable to deviate from the conventional belief of fluorinated components being indispensable in Li batteries and explore alternative fluorine-free options.

Still, we acknowledge that the research on fluorine-free electrolytes is in an early stage. Though some fluorine-free electrolytes have shown promising performance, such as formulations based on LiBOB, LiNO₃, etc., they are, to a large extent, not fully optimized. Various challenges, including safety concerns associated with the strongly oxidizing LiClO₄, potential toxicity of benzene and some cyano-containing compounds, limited high-rate cell performance due to low salt conductivity and/or high solvent viscosity, as well as the poor anodic and cathodic stabilities of certain salts/solvents, require further research and improvements. As we have shown throughout the paper, what proves effective for conventional fluorinated electrolytes may not necessarily translate to fluorine-free systems. Therefore, fluorine-free electrolytes may not be simply treated as derivatives of the conventional systems; the choice of salts, solvents, additives, and thus the resulting complex interphases needs to be carefully studied in their own formulations and testing protocols.

The key to replacing fluorine in new electrolyte systems is the capability to form passivating SEI (and CEI) layers; therefore, accurately determining the SEI composition and functionality becomes essential to the design and evaluation of fluorine-free electrolyte formulations. Much of the current understanding of SEI properties stems from XPS, which has low depth sensitivity, resulting in the widespread use of depth profiling that can create compositional artifacts such as fictitious LiF enrichment from beam-induced decomposition of electrolyte salts and fragments^115,116,117. In the future, it is crucial to pursue advanced non-destructive material characterizations to probe the interfacial structure with innate, well-preserved compositions.

Indeed, it may not be practical to entirely replace fluorinated components in current non-aqueous electrolyte systems, at least in the short term. However, it is feasible to reduce and/or partially replace the fluorinated components when designing new electrolyte formulations. Tailoring how fluorine is used and distributed in the bulk electrolyte and interphases would allow an effective reduction of the total amount necessary¹³. Established examples include the use of low-concentration (3–6 wt%) VC to replace high-concentration (10–25 wt%) FEC in LiPF₆-containing electrolytes, as well as employing non-fluorinated non-solvating diluents for LiFSI-based LHCEs^72,73,74,118. Further efforts are required to better integrate the fluorinated and nonfluorinated components to achieve both anodic and cathodic improvements while balancing parallel demands regarding corrosion, viscosity, cost, and sustainability.

Last but not least, sustainable fluorine-free electrolytes are important and applicable not only for current Li-ion chemistries, but also for a range of emerging clean technologies, including aqueous/solid-state batteries, sulfur/air-based batteries, as well as sodium/potassium-based batteries^{11,12,19,119,120,121,122}. Many examples presented in this work would serve as a good starting point for further investigations in other battery systems. As both the government and industry are seeking to move beyond costly, toxic fluorination schemes that governed the past decades of battery development, it is a great opportunity for the community to innovate and explore alternative electrolyte systems, which would eventually lead to a safe and sustainable energy future^19,21,29,48.