Enhancements of Interfacial Properties via Organic Sulfonyl Fluoride Electrolyte Additives for High‐Nickel Lithium Batteries

ABSTRACT

High‐nickel lithium nickel cobalt manganese oxide (NCM) cathodes suffer interfacial instability that impairs initial efficiency and long‐term cycling. To overcome those problems, we evaluate a series of five sulfonyl fluoride (–SO₂F) electrolyte additives—Ethenesulfonyl fluoride (A), 2‐(Benzylthio)‐2‐phenylethane‐1‐sulfonyl fluoride (B), 2‐Chloroethane‐1‐sulfonyl fluoride (C), Methyl 2‐(fluorosulfonyl)acetate (D), Methanesulfonyl fluoride (E) for high‐nickel NCM cells. Additives C and E yielded the highest initial Coulombic efficiencies (ICE) (∼90%), markedly higher than other additives. Additives B and E showed higher specific discharge capacity (∼230 mAh/g). X‐ray photoelectron spectroscopy (XPS) revealed that the effective additives E form F‐rich and S‐containing interfacial layers (e.g., LiF and organosulfur species), in contrast to a negligible S signal without additives. These findings demonstrate that –SO₂F functional additives can significantly enhance the initial efficiency and interphase stability of Ni‐rich cathodes. The results correlate the molecular structure of additives to their film‐forming behavior: proper small alkyl–SO₂F (E) effectively form protective interphases. This study qualitatively evaluated the influence of SO₂F moieties on high‐energy lithium battery performance depending on the surrounding materials and provides material‐function‐performance insights for the design of functional electrolyte additives to enhance battery performance.

The proposed interfacial architecture enables rapid Li⁺ transport through continuous ion‐conduction pathways while maintaining intimate contact between the Li metal and current collector. The engineered surface structure improves interfacial compatibility, facilitates uniform ion flux distribution, and promotes stable electrochemical reactions, leading to enhanced charge‐transfer kinetics and improved cycling performance.

1. Introduction

Nickel‐rich layered oxides such as LiNi_0.9Co_0.05Mn_0.05O₂ (NCM955) offer high capacity for next‐generation lithium‐ion batteries, but they face persistent challenges in cyclability, rate capability, and thermal stability [1]. A major issue is the unstable cathode–electrolyte interface (CEI) that forms on Ni‐rich cathodes upon charge [2]. During high‐voltage operation, Ni^{4 +} at the surface is highly reactive and catalyzes electrolyte decomposition, leading to buildup of cathode surface films, gas evolution, transition‐metal dissolution, and rapid impedance growth [3, 4]. Conventional carbonate electrolytes (e.g., LiPF₆ in ethylene carbonate (EC) blends) cannot completely prevent continuous side reactions on Ni‐rich cathodes [5]. This results in significant irreversible capacity loss during initial cycling and accelerates performance degradation.

One effective approach to stabilize high‐Ni cathodes is the use of functional electrolyte additives that form protective interphases. For example, early studies found that adding 1,3‐propane sultone (PS) reduces the CEI impedance and improves NCM cathode storage and cycle performance [6, 7]. The PS additive decomposes on charged cathode surfaces to create a passivating film, thereby mitigating electrolyte decomposition. However, a single additive can have trade‐offs; in the case of PS, while it lowers cathode interfacial resistance, its reduction on the anode side can produce a thick SEI, increasing anode impedance [8]. This highlights the need to develop additives that beneficially modify both electrodes’ interfaces. Recent reports have shown that carefully designed additives can simultaneously protect the cathode and anode.

For instance, p‐toluenesulfonyl fluoride (pTSF), a sulfonyl fluoride additive, was demonstrated to be oxidized on an NCM622 cathode and reduced on a graphite anode to form robust S‐containing interphase films on both [9]. These sulfur‐rich interfacial films suppressed electrolyte decomposition, lowered the interfacial impedance, and inhibited Ni/Mn dissolution from the Ni‐rich cathode [10]. Such results indicate that organosulfur additives with –SO₂F functional groups can significantly improve the stability of high‐energy cells by reinforcing the CEI and SEI.

The –SO₂F (sulfonyl fluoride) functional group is highly reactive toward nucleophiles and can undergo reductive decomposition to yield LiF and sulfonate/sulfite species on electrode surfaces [11, 12]. This makes sulfonyl fluoride compounds promising as film‐forming additives, since LiF is a known beneficial component of interphases (chemically inert and mechanically robust), and organosulfur moieties can polymerize or cross‐link into the interface [13]. Indeed, various sulfonyl‐containing additives have been explored: Che et al. showed that pTSF‐derived films effectively lowered cell impedance and improved cycle life [9], and Dahn's group reported a cyclic sulfate additive (ODTO) that decomposed into sulfur‐containing species at both electrodes, increasing coulombic efficiency and capacity retention [13].

Aromatic sulfonyl fluorides have also proven effective; for example, 2‐fluorobenzenesulfonyl fluoride was found to preferentially form a uniform, low‐resistance CEI on LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811), stabilizing the cathode interface under high voltage [14]. Another well‐known additive is fluoroethylene carbonate (FEC), which is primarily used to stabilize anode SEI. FEC is included in most high‐Ni cell electrolytes to protect graphite or Li‐metal anodes. Notably, FEC can also participate in cathode interphase formation: it is oxidized at the positive electrode to produce LiF‐rich compounds that help stabilize the CEI [15].

However, FEC alone is often insufficient to passivate Ni‐rich cathodes, especially at the elevated upper cutoff voltages needed to utilize full capacity [16]. Therefore, there is strong motivation to identify co‐additives that specifically reinforce the CEI on Ni‐rich oxide cathodes without causing detrimental side effects on the anode.

In this work, we investigate a set of six sulfonyl fluoride‐containing additives (A–E) as 5 wt.% co‐additives in a standard carbonate electrolyte for NCM955‖Li half‐cells in Table 1. The additives vary in structure to probe structure–function relationships in interphase formation:

TABLE 1.

Various sulfonyl fluoride additives for NCM955‖Li.

Label	Additive Name
A	Ethenesulfonyl fluoride—a vinyl (unsaturated) sulfonyl fluoride
B	2‐(Benzythio)‐2‐phenylethane‐1‐sulfonyl fluoride [17]
C	2‐Chloroethane‐1‐sulfonyl fluoride—containing a chloro (–Cl) substituent
D	Methyl 2‐(fluorosulfonyl)acetate—an ester‐functionalized sulfonyl fluoride
E	Methanesulfonyl fluoride—a simple alkyl (methyl) sulfonyl fluoride

2. Results and Discussion

2.1. Initial Coulombic Efficiency (Ice) Comparison

All these additives possess the –SO₂F group, but with different electronic and steric characteristics. The selection of these sulfonyl fluoride‐based additives was not arbitrary, but rather intentionally designed to optimize electrochemical interfacial stabilization for high‐Ni NCM955 cathodes under high‐voltage operating conditions. Sulfonyl fluoride functional groups (–SO₂F) are known to possess strong electron‐withdrawing characteristics, which increase oxidative stability and promote preferential interfacial decomposition before severe bulk electrolyte degradation occurs. This enables the formation of inorganic sulfur‐containing CEI species, such as lithium sulfite and sulfate compounds, that can suppress continuous electrolyte oxidation and transition‐metal dissolution on the cathode surface.

In addition, the molecular structures were systematically modified using different substituents to tune polarity, electron density, steric effects, and interfacial adsorption behavior. Therefore, the molecular design strategy was aimed at controlling both decomposition reactivity and CEI chemistry through rational functional‐group engineering.

More specifically, the selected molecules contain distinct structural motifs intended to induce different electrochemical interphase characteristics. Aromatic substituents were expected to improve oxidative stability and interfacial adsorption on the cathode surface through delocalized electronic structures, whereas aliphatic and chlorinated groups were introduced to modify reduction/oxidation kinetics and increase interfacial reactivity.

The presence of fluorinated sulfonyl groups was also expected to facilitate LiF‐rich interphase formation through fluorine‐derived decomposition pathways, especially when combined with FEC. Consequently, these additives were designed to operate cooperatively with FEC by reinforcing inorganic CEI formation while maintaining stable charge‐transfer kinetics. All sulfonyl fluoride additives were synthesized in our laboratory and conducted to characterize chemical structure of synthesized products by NMR spectra (Figure S1). In addition, we conducted to survey HOMO‐LUMO energy levels of electrolyte components and additives (Figure S2).

From the summary, all sulfonyl fluorides exhibit deep HOMOs (∼–8 to –12 eV) and very low LUMOs (∼–1 to –3 eV), with more electron‐withdrawing substitutions further stabilizing the LUMO, indicating that a low LUMO makes the –SO₂F group a strong electrophile on the cathode surface during first charging to generate stable interfacial layer, while a deep HOMO makes the compounds generally kinetically stable until activated or catalyzed on the anode.

Based on the chemical property, by comparing their electrochemical impacts on initial Coulombic efficiency (ICE), interfacial impedance, and surface film chemistry (via XPS), we aim to elucidate how molecular structure influences an additive's ability to form a beneficial CEI in high‐Ni cathodes.

The first‐cycle Coulombic efficiency of NCM955‖Li cells was strongly influenced by the choice of electrolyte additive. For an usage of Li metal anode, we implemented constant FEC amount (5 wt.%) to avoid sacrificing additional EC electrolyte. Figure 1 (schematic representation) and the data summarized in Table 2 show the trend in ICE for the different additives: E > C > D >B >> A. The strength in specific discharge capacity demonstrated the trend: E ≈ B > C > D >> A. In the baseline electrolyte (5% FEC, no additional additive), the NCM955 half‐cell exhibited a moderate ICE (approximately 85%, typical for Ni‐rich cathodes).

FIGURE 1.

Chemical structures of various sulfonyl fluoride additives.

TABLE 2.

Comparisons of specific charge and discharge capacity, and initial Coulombic efficiency in NCM955||Li with various sulfonyl fluoride additives.

Additive	Specific charge capacity (mAh/g)	Specific discharge capacity (mAh/g)	Initial Coulombic efficiency (%)
A	104.2	69.1	66.3
B	265.4	229.5	86.5
C	244.1	218.5	89.5
D	230.9	203.6	88.2
E	252.5	230.3	91.2

Both cases of adding additive B (benzythio‐phenylethanesulfonyl fluoride) or E (methanesulfonyl fluoride) slightly increased the specific discharge capacity to the ∼230 mAh/g, indicating a reduction in initial irreversible losses (ICE over upper to 85%) despite quite different HOMO level (−8 vs. −11.5 eV). This result imply that the more discharge capacity corresponds to a lower LUMO property than the EC electrolyte to generate stable CEI interphase.

Additives C (chloro) yielded intermediate specific discharge capacity values in the ∼215 mAh/g with high ICE values in the ∼90% range, roughly on par with or slightly above the baseline. Additive D (fluorosulfonyl acetate) provided only a marginal improvement, with the specific discharge capacity around ∼200 mAh/g with high 80s ICE.

In stark contrast, additive A (ethenesulfonyl fluoride) drastically degraded the first‐cycle efficiency—cells with A had ICE well below 70%, meaning over 30% of the Li injected during the first charge was consumed by side reactions and was not recovered on discharge. Especially, over 80 mAh/g, a steep overvoltage was presented due to the decomposition of additive on the Ni‐rich cathodes.

This pronounced variability in ICE reflects how each additive participates in (or interferes with) the solid‐electrolyte interphase formation on the anode and the cathode‐electrolyte interphase on the cathode during the initial cycle. A higher ICE implies fewer irreversible processes (e.g., surface film formation, electrolyte decomposition) consuming lithium during formation. Additives B and E clearly enabled the most efficient first‐cycle behavior, suggesting they form protective interphases rapidly and prevent excessive electrolyte decomposition from the outset. In contrast, the very low ICE with additive A indicates that A likely triggers additional side reactions that consume lithium (and electrolyte) without providing an effective passivation.

These observations are consistent with known behaviors of similar additives. Sulfonyl fluorides that are appropriately reactive can sacrifice a small amount of lithium to form a stable interphase, thereby saving more lithium from continued parasitic reactions. For instance, Dahn's group reported that adding 3% ODTO (a cyclic sulfate additive) increased the initial coulombic efficiency and long‐term capacity retention of NMC‐based cells by forming protective sulfur‐containing interphases [13].

The additives B and E presumably act in a similar beneficial manner, consuming a bit of lithium to generate a resilient LiF/S‐rich CEI and SEI, which in turn minimizes further electrolyte decomposition. The net result is a higher recoverable capacity on the first cycle (improved ICE). On the other hand, certain additives can over‐consume lithium if they react uncontrollably. Additive A, containing an activated vinyl group, appears to fall in this category. Vinyl sulfonyl fluoride is a potent Michael acceptor and may polymerize or react with nucleophiles (e.g. the lithium metal or SEI species) too aggressively during the formation phase. This likely led to a thick, lithium‐consuming polymeric film, analogous to the behavior of PS additive on anodes causing large SEI impedance [18, 19]. The excessive consumption of lithium and electrolyte by A's decomposition accounts for its deleterious effect on ICE.

Moreover, the differences between additives highlight the balance needed: an additive must be reactive enough to form a passivating film, but not so reactive as to induce uncontrolled side reactions. Additive E (CH₃SO₂F) appears to strike this balance well. It has a relatively low reduction/oxidation threshold—thus it is consumed preferentially over the bulk electrolyte—but being a small molecule, its decomposition is limited primarily to forming a simple LiF and organosulfite layer, without generating voluminous polymeric byproducts. Literature on p‐toluenesulfonyl fluoride (which is structurally similar, an aryl‐SO₂F) showed that even a small dose could construct a protective film on both electrodes, dramatically improving cell cycling [9]. Our results for E corroborate that a small sulfonyl fluoride can indeed improve initial efficiency, likely by quickly producing a LiF‐rich interphase that halts further electrolyte attack.

In summary, the ranking of specific discharge capacity (E ≈ B > C > D >> A) demonstrates that additive efficacy is highly structure‐dependent. The best‐performing additives (B, E) minimized initial irreversibility, whereas the worst (A) exacerbated it. We next correlate these efficiency outcomes with the interfacial resistances and surface film compositions for each additive, to more deeply understand the underlying mechanisms (Figure 2).

FIGURE 2.

Initial charge and discharge profiles in NCM955||Li with various sulfonyl fluoride additives.

2.2. Cell Performance and Interfacial Properties

Based on the previous initial properties, cell performance, such as C‐rate and retention was evaluated in Figure 3. Table 3 was summarized in 0.3C, 5C specific discharge capacity, and 1C 100cycle retention properties. Additive A shows low discharge property in 0.1C. After repeated cycling to 0.2C, the discharge capacity was increased with unstable capacity tendency.

FIGURE 3.

Retention property in NCM955||Li with various sulfonyl fluoride additives.

TABLE 3.

Comparisons of several discharge capacity (0.3 and 5C), retention property in NCM955||Li with various sulfonyl fluoride additives.

Additive	Specific discharge capacity @ 0.3C (mAh/g)	Specific discharge capacity @ 5C (mAh/g)	Retention @1C, 100cycle (%)
A	218.3	0.4	—
B	207.5	0.4	31.4
C	—	—	—
D	222.9	46.6	75.9
E	212.5	144.0	95.1

According to increasing current density, more severe discharge capacity fluctuation was observed. As shown in Figure 1, additive A occurred side effect of hindering lithium intercalation and de‐intercalation. Additive E exhibited the highest capacity retention, followed by D > B > >> A > C, consistent with their ability to form stable interphases. Notably, Additive E maintained over 95% of its initial capacity after 100 cycles at 1C, whereas Additive A suffered catastrophic fading within 30 cycles. This performance hierarchy reinforces the correlation between interfacial chemistry and rate/retention behavior across the additive set.

Electrochemical impedance spectroscopy was used to evaluate the resistance of the interphases formed with each additive. Figure 4 (Nyquist plots after first cycle) illustrates the typical impedance response after formation (0.1C and 0.2C × 3 cycles) cycle and after total (0.3C, 0.5C, 1C, 2C, 5C × 3 cycles, 1C × 100 cycles) cycle, indicating cells with higher retention property (>75%) such as B, D, and E additives.

FIGURE 4.

Cell resistance (a) after formation and (b) after total cycles in NCM955||Li with various sulfonyl fluoride additives.

All cells showed a high‐frequency intercept (∼5–25 Ω) representing the electrolyte/bulk resistance (similar across cells since electrolyte ionic conductivity is nearly the same). The diameter of the first semicircle (high‐to‐mid frequency) corresponds to the combined interface resistance (R_f) and charge‐transfer resistance (R_ct) for lithium intercalation at the cathode. Significant differences are evident in this impedance: cells with additives B and E exhibit much smaller semicircles than those with D.

Quantitatively, the interfacial/charge‐transfer resistance (summed as R_int for discussion) for D and E additives was around 50–150 Ω, substantially lower than the B additive cell (∼250 Ω after formation). These trends mirror the ICE results, reinforcing that the additives yielding high ICE also produced low‐impedance interphases.

A low interfacial impedance after formation is typically a sign of a thin, uniform, and ion‐conductive passivation film on the electrodes [20, 21]. In the case of D and E, their decomposition likely created robust yet Li⁺‐permeable films, which facilitate charge transfer and reduce polarization. The presence of LiF in the SEI/CEI (as will be confirmed by XPS) may contribute to this: although LiF itself is electronically insulating, a finely distributed LiF‐rich layer can be beneficial. It provides a dense coverage that stops parasitic reactions while still allowing Li⁺ transport through grain boundaries or a mosaic structure.

Prior studies have noted that S‐ and F‐containing interphases can improve interfacial conductivity. For example, the sulfonyl‐derived films from pTSF were shown to significantly reduce the interfacial impedance on both NCM cathodes and graphite anodes [10]. Similarly, propane sultone additive was credited with lowering CEI impedance on Ni‐rich cathodes [2, 22, 23]. Our EIS results align with these reports. Additives D and E, which form sulfonyl‐derived interphases, achieved the lowest resistance, consistent with effective passivation that prevents build‐up of thick resistive layers.

In contrast, the extremely high impedance observed with additive B (2‐(Benzythio)‐2‐phenylethane‐1‐sulfonyl fluoride) suggests the formation of a very resistive interphase. As hypothesized earlier, A likely underwent polymerization or uncontrolled side reactions. The resulting film could be thick, electrically insulating, or poorly conducting for Li⁺, which greatly hinders charge transfer. This explains both the low ICE (lots of side reaction) and high impedance (difficult for Li⁺ to pass through the film). Essentially, additive B created a “too much of a good thing” scenario—instead of a thin protective film, it produced an excessive layer that impedes cell kinetics. Therefore, additive B is excellent property in the initial cycle, while a lower retention property is observed.

Overall, the EIS analysis supports the conclusion that additives D and E produce the most favorable interphases on NCM955 and Li, reflected in both high initial efficiency and low interfacial resistance. Effective CEI/SEI formation by these additives likely stabilizes the electrode surfaces and facilitates charge transfer. In the next section, we examine the surface chemistry of the cathode interphase via XPS to directly identify the species contributing to these differences.

2.3. Surface Anlaysis (Depth‐Profile NCM of Additive D and E)

For a deeper understanding of high‐performance sulfonyl fluoride additives (D and E) effect on the surfaces of NCM955 cells, we conducted a depth‐profile XPS analysis on NCM955 and surface XPS analysis on Li metal surface after cycling. Figure 5 shows XPS results of S 2p, F 1s peak on the cycled NCM955 electrode with (a, c) additive D electrolyte and (b, d) additive E electrolyte.

FIGURE 5.

XPS results of S 2p, F 1s peak on the cycled NCM955 electrode with (a, c) additive D electrolyte and (b, d) additive E electrolyte.

In S 2p region, for Additive D, the S 2p spectrum shows a well‐defined peak centered around ∼169 eV, consistent with sulfonyl (–SO_2–) species originating from intact additive or its reduction products. The relatively sharp and symmetric profile suggests stable sulfur bonding with limited side‐reactions. Furthermore, according to increasement of etching time, in Additive E, the S 2p signal is weaker and broader, indicating less defined sulfur species. The diminished intensity implies either reduced incorporation of sulfur into the solid–electrolyte interphase (SEI) or partial decomposition into non‐XPS‐active fragments.

In F 1s region, Additive D and E produce a strong F 1s feature at ∼685.5 eV, assignable to metal–fluoride (LiF) formation, along with a higher‐binding‐energy shoulder from residual S–F or C–F species. This indicates that additive E effectively contributes to robust LiF‐rich SEI formation, which is beneficial for interfacial stability. With the same intensity scale, the intensity of LiF at E is higher than that at D on the surface (green line), sustaining a similar intensity of LiF at both additives (red line). A higher intensity of LiF strongly correlates with higher battery retention property [24, 25]. Additive D exhibits comparatively weaker variation of F 1s intensity, dominated by LiF but with less contribution from intact –SO₂F functionality. The lower intensity suggests less efficient fluorine incorporation, correlating with a thinner or less protective fluorine‐rich SEI.

Therefore, XPS results reveal that Additive D generates pronounced S 2p and F 1s signals, corresponding to sulfonyl‐derived sulfur and LiF‐rich fluorine species, indicating effective interphase participation. In contrast, Additive E displays weaker and broader S 2p and higher intensity of F 1s peaks, reflecting less defined sulfur chemistry and increased fluorine incorporation. These results highlight the superior SEI‐forming ability of Additive E relative to Additive D.

Despite both sulfonyl additives (additive D and E) in the electrolyte, sulfur peak appeared only at additive D in the survey of XPS on cycled Li surface (Figure S3), showing different electrochemical reaction (initial Coulomb efficiency and retention property) depending on the electrolyte additive, as shown in Figures 2 and 3 and Table 3. Furthermore, ‐SO₂‐derived decomposition, metal‐sulfur (M‐S), and Li₂S were observed on cycled Li surface (Figure S4a,b). The peak of LiF and Li_xPO_yF_z (F 1s) in additive E also was stronger than the one in additive B, indicating higher retention property in additive E system (Figure S4c,d).

XPS O 1s (Figure S5) has an interesting aspect about the cell results of the surface results. XPS O 1s analysis revealed that Additive D (Methyl 2‐(fluorosulfonyl)acetate) preserved a relatively clean cathode surface, with a dominant lattice oxygen peak/oxygen vacancy (∼528.5 eV) and only a minor high‐binding energy tail above 532 eV. In contrast, Additive E (methanesulfonyl fluoride) produced broader O 1s features with stronger contributions from defect oxygen and carbonate species (∼531–533 eV), indicating a thicker cathode–electrolyte interphase (CEI). Despite this less “pristine” O 1s signature, long‐term full‐cell cycling demonstrated superior capacity retention for E relative to D in Figure 3.

This apparent discrepancy can be explained by the system‐level balance between cathode and anode stabilization. Methanesulfonyl fluoride readily generates LiF‐ and sulfur‐containing inorganic species that form a robust, low‐resistance SEI on the Li metal anode, significantly suppressing Li inventory loss, dendrite growth, and HF‐induced corrosion.

Additionally, the sacrificial oxygenated CEI observed with E likely serves as an early passivation layer, slowing continuous oxygen release and transition‐metal dissolution during extended cycling. In contrast, the thinner, cleaner CEI generated with D does not sufficiently protect the Li anode, leading to faster Li depletion and impedance growth. Thus, while Additive D is effective in mitigating cathode surface reconstruction, Additive E provides a more holistic passivation across both electrodes, ultimately delivering higher cell‐level retention.

The P 2p (Figure S6) spectra corroborate the system‐level passivation promoted by methanesulfonyl fluoride (E). With E, the PF₆ ⁻/POF₃‐like signal near ∼137 eV progressively gives way to pronounced bands at ∼135 eV (Li‐fluorophosphates) and ∼133–134 eV (Li₃PO₄‐like species), evidencing acid/salt‐by‐product conversion into stable inorganic phosphates/fluorophosphates. These species, together with LiF, form dense interphases that suppress HF‐driven transition‐metal dissolution at the cathode and limit Li inventory loss at the anode, consistent with superior retention. In contrast, additive D shows weaker growth of these inorganic P phases, aligning with its thinner CEI and inferior long‐term capacity despite a cleaner O 1s signature.

Therefore, O 1s (thicker oxygenated CEI with E) and P 2p (more Li_xPO_yF_z/Li₃PO₄ with E) tell a coherent electrochemical story: Additive E builds a more inorganic, sacrificial‐but‐protective interphase on both electrodes, which wins at the NCM955 cell level.

Li 1s spectra (Figure S7) further substantiate the distinct interphase chemistries induced by the two additives. With methanesulfonyl fluoride (E), Li 1s evolves from ∼55.6 eV toward a stronger 56.0–56.3 eV component, characteristic of LiF and Li‐fluorophosphates, in line with the growth of Li_xPO_yF_z signatures in P 2p and the oxygenated CEI seen in O 1s.

In contrast, Methyl 2‐(fluorosulfonyl)acetate (D) maintains a narrower Li 1s envelope near 55.5–55.7 eV, indicating carbonate/oxide‐rich interphases with limited LiF content. The inorganic‐rich (LiF/Li_xPO_yF_z) layers formed by E provide more durable passivation of both Li and NCM surfaces, explaining the superior capacity retention despite a less pristine O 1s profile.

Ni 2p spectra (Figure S8) reveal additive‐dependent surface reconstruction on NCM955. With Methyl 2‐(fluorosulfonyl)acetate (D), the Ni 2p₃/₂ line remains centered near ∼855.5 eV with only a minor ∼854.5 eV shoulder and weak satellite growth, indicating limited conversion to Ni^{2 +}. In contrast, methanesulfonyl fluoride (E) produces a pronounced low‐binding‐energy shoulder (∼854–855 eV) and stronger ∼861–862 eV shake‐up satellite, consistent with increased Ni^{2 +} and rock‐salt/oxyhydroxide–like surface formation. These trends agree with the O 1s evidence for a thicker oxygenated CEI under E.

Crucially, P 2p and Li 1s show that additive E concurrently converts salt/acid by‐products into LiF and Li‐fluorophosphates, yielding inorganic‐rich SEI/CEI that suppresses transition‐metal dissolution and loss of Li inventory. Thus, although Ni 2p indicates more initial surface reconstruction with additive E, the resulting inorganic passivation across both electrodes dominates the long‐term behavior and explains the higher capacity retention.

C 1s spectra (Figure S9) corroborate additive‐dependent interphase chemistries. With methanesulfonyl fluoride (E), both the O–C═O band at ∼289.9 eV and the C–O/C═O envelope (286–287.5 eV) increase with cycling, accompanied by a weak ≥291 eV shoulder, indicating progressive formation of alkyl‐carbonate/alkoxide species together with F‐rich environments.

These trends match the O 1s high‐BE growth (531–533 eV, Figure S3) and coincide with the emergence of LiF/Li‐fluorophosphates seen in P 2p and Li 1s, consistent with a hybrid organic–inorganic CEI/SEI. In contrast, Methyl 2‐(fluorosulfonyl)acetate (D) shows a sharper but less evolving O–C = O component and weaker ≥291 eV features, indicative of a thinner, more organic‐lean layer. Although E yields a thicker oxygenated CEI, the concomitant inorganic scaffold effectively passivates both electrodes and limits Li‐inventory loss, explaining the superior capacity retention.

Those compositions can be described as a mixed inorganic‐organic layer: inorganic LiF and Li₂SO₃/Li₂SO₄ providing a scaffolding that resists further electrolyte attack, and organic fragments (from the additive's alkyl or aryl groups) potentially polymerized into a matrix that reinforces the film. Such a hybrid interphase can be both electronically insulating (preventing oxidative electrolyte decomposition) and ionically passable, aligning with the low impedance observed [26, 27].

These findings are in line with prior work showing sulfur‐containing additives convert to protective species at the interface [13, 28, 29]. For instance, XPS by Ma et al. on cells with ODTO additive showed that ODTO “mainly converted to sulfur‐containing species at both electrode surfaces after formation” [13], which parallels our observation for additive E. The ability of additive E to generate a LiF‐ and sulfite‐enriched CEI is a key reason for their excellent performance in the NCM955‖Li cells.

2.4. Surface Analysis (Cycling Effect of Additive E to NCM Cathode and Li Anode)

We demonstrated to cell tests with additional two cases: (1) Ref: no sulfonyl additive case (1 M LiPF₆ EC: EMC (3:7, v/v) + FEC 5 wt.%) and (2) Rep: already reported sulfonyl additive case (p‐toluenesulfonyl fluoride, pTSF, in Figure S10) reported in [3], indicating cell performance (Figure S11) and EIS results (Figure S12). Despite a generation of excellent interphase in both NCM cathode (Ni ∼ 80%) and Li anode in [3], our NCM955 cell results lower retention (37.5%) and almost zero discharge capacity at 5C‐rate test, showing higher interfacial resistance in more nickel contents NCM cathode by EIS. Therefore, we conducted additive E and reference electrolyte analysis by XPS. The XPS was performed on NCM955 cathodes from cells after 100 and 200 cycle (Figure S13).

Ref electrolyte cell shows no discernible S 2p features, whereas E produces pronounced doublets at ∼170 eV and ∼168–169 eV, assigned to sulfate (‐SO₃ ⁻)/sulfonate/fluorosulfonate and sulfite (SO₃ ²⁻)/sulfinate species, respectively. In spite of no sulfur elements in NCM955 cells, a little sulfur chemical between 168 and 170 eV may come due to NCM surface residual sulfates or sulfonates of process/atmospheric origin [30].

The dominance of high‐BE sulfur under E evidences methanesulfonyl fluoride conversion to inorganic SO_xF_y/Li₂SO₄‐like passivation that, together with LiF/Li_xPO_yF_z (P 2p at Figure S14, Li 1s at Figure S15), builds a robust CEI/SEI and correlates with markedly improved capacity retention (∼ 90% at 1 C, 200 cycles) compared with the sulfur‐free reference (∼ 55%). The obtained sulfate at both NCM955 and Li after 100 cycles continually observed after 200 cycles, while there was no observation at both electrodes in reference electrolyte, indicating that stable and well‐defined sulfate chemicals on the both electrodes correspond to higher retention property.

F 1s spectra reinforce the interphase shift induced by additive E. In Ref cells the 686–687 eV Li_xPF_y signal dominates and LiF at ∼684.7 eV is modest, whereas Ref + additive E exhibits a much stronger LiF component that increases with cycling while Li_xPF_y exhibits a little increase at both electrodes. Together with S 2p (SO_xF_y/sulfate in Figure 6), P 2p (Li_xPO_yF_z in Figure S14) and Li 1s (LiF/Li‐fluorophosphates), these results evidence an inorganic‐rich, HF‐scavenging SEI/CEI that passivates both electrodes. This chemistry explains the markedly higher capacity retention of the additive E formulation despite its more oxygenated O 1s (Figure S16) and reconstructed Ni 2p (Figure S17) signatures.

FIGURE 6.

XPS results of S 2p at every 100 cycles on both NCM955 electrode and Li with reference electrolyte and additive E electrolyte.

In addition, C 1s spectra (Figure S18) show that methanesulfonyl fluoride increases both O–C = O (∼288.2 eV) and C–O/C = O (286–287.5 eV) with cycling, indicating a thicker oxygenated interphase [31]. Crucially, concurrent LiF and Li‐fluorophosphate formation (F 1s in Figure 7/Li 1s in Figure S15), phosphate conversion (P 2p in Figure S14), and oxidized sulfur species (S 2p in Figure 6) reveal a hybrid organic–inorganic CEI/SEI that passivates both electrodes, reconciling the more ‘oxygenated’ O 1s/C 1s signatures with the markedly superior capacity retention.

FIGURE 7.

XPS results of F 1s at every 100 cycles on both NCM955 electrode and Li with reference electrolyte and additive E electrolyte.

Transition‐metal core levels support the methanesulfonyl fluoride (MSF, additive E)‐enabled passivation mechanism. In the reference electrolyte, Co 2p exhibits a pronounced 786–789 eV satellite and broadening of the 2p₃/₂ line (Figure S19), while Mn 2p develops a low‐binding‐energy shoulder at ∼640–641 eV and a stronger ∼646–647 eV shake‐up (Figure S20), evidencing Co^{2 +}/spinel‐like reconstruction and Mn^{2 +} formation caused by oxygen loss and HF attack. In contrast, Ref+MSF maintains a Co^{3 +}‐dominated 2p₃/₂ near ∼780 eV with a weaker satellite (Figure S19) and preserves a Mn^{4 +}‐dominated 2p₃/₂ (∼642 eV) with only minor Mn^{2 +} (Figure S20). Together with S 2p/P 2p signatures of acid scavenging and F 1s/Li 1s evidence of LiF/Li‐fluorophosphate buildup, these results show that MSF forms an inorganic‐reinforced CEI/SEI that suppresses TM reduction/dissolution, consistent with the markedly superior long‐term retention.

In addition to a deeper understanding of the additives decomposition mechanism, the FT‐IR spectra in Figure S21 obtained after formation indicates that FEC and additive E (‐SO₂F) undergo distinct decomposition pathways on the Li(Ni_0.9Co_0.05Mn_0.05)O₂ cathode surface, leading to the formation of a hybrid CEI structure. In the FEC‐containing electrolyte, the broad absorption bands observed in the carbonate‐related region are attributed to the formation of lithium alkyl carbonate and polycarbonate species generated through the ring‐opening decomposition of FEC.

These decomposition reactions are also known to promote the formation of LiF‐rich interphases, although LiF itself is weakly IR‐active. The generated LiF/polycarbonate hybrid CEI can effectively suppress electrolyte oxidation and mitigate transition‐metal dissolution on the Ni‐rich cathode surface. Therefore, the FT‐IR results suggest that FEC decomposition primarily contributes to the formation of an organic–inorganic fluorinated interphase during the formation process.

In contrast, the electrolyte containing additive E (‐SO₂F) exhibited characteristic SO_x‐related vibrational features in the 1200–1000 cm⁻¹ region, indicating the formation of sulfur‐containing interphase species. These peaks are associated with lithium sulfite/sulfate compounds generated through the oxidative decomposition of Me‐SO₂F at the cathode surface. The formation of sulfate‐rich inorganic CEI layers is considered beneficial for stabilizing the highly reactive Ni‐rich cathode/electrolyte interface under high‐voltage conditions.

Furthermore, the simultaneous observation of carbonate‐related and SO_x‐related peaks in the FEC + Me‐SO2F electrolyte suggests the formation of a synergistic hybrid CEI composed of LiF/polycarbonate and sulfate‐based inorganic species. Such a chemically integrated CEI layer is expected to improve interfacial stability by suppressing electrolyte decomposition, reducing impedance growth, and enhancing the electrochemical stability of the NCM955 cathode during cycling.

Furthermore, to confirm the synergetic effects between FEC and additive E, we conducted experiments without FEC additive. Based on the additional only E additive test in Figure S22, the electrochemical results clearly demonstrate that additive E (‐SO₂F) alone cannot effectively stabilize the highly reactive NCM955 cathode interface during long‐term cycling, whereas the simultaneous use of FEC and additive E provides a remarkable synergistic effect on interfacial stabilization. The electrolyte containing only additive E exhibited severe capacity degradation, retaining only 17.7% after 100 cycles (Figure S22a), accompanied by a dramatically increased interfacial resistance of approximately 720 Ω (Figure S22).

In contrast, the FEC + additive E electrolyte maintained 95.1% (Figure 3) capacity retention with a significantly lower interfacial resistance of approximately 140 Ω after cycling (Figure 4b). These results indicate that the sulfur‐containing interphase generated solely from additive E (‐SO₂F) decomposition is insufficient to suppress continuous electrolyte decomposition and interfacial deterioration on the Ni‐rich cathode surface. Therefore, the improved electrochemical stability cannot be explained by an independent contribution of additive E alone.

Instead, the results strongly suggest a cooperative and synergistic interaction between FEC and additive E during CEI formation. FEC likely acts as the primary interphase‐forming additive by generating a LiF‐rich and polycarbonate‐based protective CEI, which effectively suppresses parasitic interfacial reactions and stabilizes charge‐transfer kinetics. Under this stabilized interfacial environment, additive E can further contribute by forming sulfur‐containing inorganic species such as lithium sulfite/sulfate, thereby reinforcing the mechanical and chemical stability of the CEI. Consequently, the observed low interfacial resistance and excellent capacity retention are considered to originate predominantly from the FEC‐derived fluorinated interphase, while additive E serves as a secondary synergistic stabilizer rather than a competing additive. Therefore, the relationship between FEC and additive E should be interpreted as a synergistic interfacial stabilization mechanism rather than a competitive decomposition behavior. The combined additive system forms a chemically integrated hybrid CEI that is significantly more stable than the interphases generated from either additive alone.

To investigate the concentration‐dependent effect of the electrolyte additive system, additional electrochemical evaluations were conducted using 1 wt.%, 3 wt.% additive concentrations in Figure S23. The cycling results showed a strongly non‐linear behavior, where the capacity retention after cycling was 46.3% for 1 wt.%, 38.2% for 3 wt.%, and dramatically improved to 95.1% for 5 wt.%. A similar trend was observed in the post‐cycling EIS analysis, where the interfacial resistance values were approximately 330, 420, and 140 Ω for the 1, 3, and 5 wt.% conditions, respectively. Interestingly, despite these substantial electrochemical differences, the bulk structural analysis obtained from XRD showed no significant changes depending on additive concentration. These results indicate that the observed performance differences are not primarily associated with bulk crystal structural changes, but rather originate from concentration‐dependent interfacial stabilization behavior.

The results suggest that the synergistic interphase stabilization between FEC and additive E becomes sufficiently effective only above a critical additive concentration. At lower concentrations (1 and 3 wt.%), the additive‐derived CEI appears insufficient to fully suppress continuous electrolyte decomposition and interfacial degradation, resulting in relatively high impedance growth and rapid capacity fading. In contrast, the 5 wt.% condition enabled the formation of a significantly more stable and conductive hybrid CEI, which effectively reduced charge‐transfer resistance and stabilized long‐term cycling behavior. The absence of major XRD peak changes further supports that the dominant degradation mechanism is governed by surface/interface instability rather than bulk lattice collapse. Therefore, the combined electrochemical and structural analyses confirm that the concentration of FEC and additive E critically influences CEI formation chemistry and interfacial stability on the high‐Ni cathode surface.

The TOF‐SIMS analysis clearly demonstrates that the introduction of additive E (Me‐SO₂F) significantly modifies the cathode electrolyte interphase (CEI) chemistry on the NCM955 surface compared with the reference electrolyte (1 M LiPF₆ in EC:EMC = 3:7) in Figure S24. In the reference electrolyte, the detected surface species were primarily associated with conventional carbonate decomposition products and unstable LiPF₆‐derived interfacial compounds.

In contrast, the Ref + additive E condition exhibited substantially enhanced sulfur‐containing fragment signals, indicating the preferential decomposition of Me‐SO₂F and the subsequent formation of sulfur‐rich interfacial species. The enhanced distribution of SO_x‐related fragments strongly suggests the generation of lithium sulfite/sulfate‐type compounds within the CEI layer. These results indicate that additive E actively participates in CEI formation and alters the surface reaction pathway of the Ni‐rich cathode interface.

Furthermore, the TOF‐SIMS depth‐profile behavior suggests that the additive‐derived CEI possesses a chemically distinct and more stable interfacial structure compared with the reference condition. The additive‐containing electrolyte exhibited stronger inorganic fragment signals together with suppressed unstable carbonate‐related species, implying that the decomposition of Me‐SO₂F promotes the formation of a denser inorganic‐rich interphase.

Such sulfur‐containing interfacial species are considered beneficial for suppressing continuous electrolyte oxidation and mitigating transition‐metal dissolution from the highly reactive NCM955 surface. In particular, the coexistence of fluorine‐containing fragments and sulfur‐derived species suggests the formation of a hybrid inorganic CEI composed of LiF‐rich and sulfate‐rich components. Therefore, the TOF‐SIMS results support that additive E does not merely function as a sacrificial additive, but instead plays a critical role in regulating interfacial chemistry and stabilizing the cathode surface during electrochemical cycling.

Finally, we conducted a full‐cell test using a graphite electrode. The electrochemical performance of the NCM955/graphite full cell further confirms the practical effectiveness of the proposed additive system under realistic battery configurations. The high capacity retention rate of 96.3% at 1C and the effect of reducing interfacial resistance and total resistance even after cycling were sufficiently verified in the full cell, confirming commercial viability in Figure S25.

2.5. Structure–Function Relationships of Organic Sulfonyl Fluoride Additives

Combining the electrochemical and surface chemistry results, we can discern clear relationships between the molecular structure of the additives and their effectiveness in the NCM955‖Li system [32, 33]:

2.5.1. Additive A (Ethenesulfonyl fluoride)

The presence of a vinyl (C = C) group made A highly reactive. It likely underwent polymerization via its activated double bond or reacted in an uncontrolled manner with nucleophiles on the Li metal anode. This led to an overly thick and insulating SEI, as evidenced by A's low ICE and high impedance. A's –SO₂F moiety may not have efficiently produced LiF or durable sulfite species on the cathode; instead, much of A's reaction could have formed long‐chain organics (poly‐sulfonates, PS) that did not adhere well or consumed excessive Li. Thus, the unsaturated structure of A, while intended to promote polymeric film formation, resulted in excessive irreversible reactions and poor performance. This mirrors literature observations that additives which form too much SEI (e.g., excess PS reduction) can increase cell impedance [18].

2.5.2. Additive B (2‐(Benzylthio)‐2‐phenylethane‐1‐sulfonyl fluoride)

B is the most complex and bulky additive in the series, featuring both an aromatic ring and a benzylic thioether attached to the sulfonyl fluoride‐bearing carbon. Despite (or because of) its complexity, B was as effective as E. B's dual functional groups likely confer a synergistic effect. The electron‐rich benzylthio (PhCH_2–S–) substituent can stabilize intermediate radicals or anions formed when B oxidizes, possibly facilitating the formation of a cross‐linked, polymer‐like film.

In addition, we speculate that when B decomposes (probably primarily on the cathode at high voltage given its aromatic stability), the –SO₂F cleaves to form LiF and a sulfonyl radical or cation. The presence of the benzylthio and phenyl groups could lead to further reactions: for instance, the thioether might oxidize to a sulfoxide or sulfone, and the benzyl group could couple with another aromatic ring. The outcome would be a highly aromatic, cross‐linked CEI containing aryl‐sulfone and aryl‐sulfide linkages along with inorganic LiF. Such a film would be mechanically robust and resist dissolution at high temperature (aromatic structures have low solubility and high thermal stability). However, we confirmed that the cell resistance increases after the formation cycle, and the formation of a high resistance layer at the electrode‐electrolyte interface does not produce significant results in improving the performance of high‐energy density electrodes (NCM955|Li).

2.5.3. Additive C (2‐Chloroethane‐1‐sulfonyl fluoride)

Additive C also improved ICE to a moderate extent. The chloro substituent is electron‐withdrawing and may undergo a side reaction (e.g., elimination of HCl or formation of LiCl). If C reacts, it could produce LiCl and a sulfonyl radical that forms the CEI. LiCl (if formed) is another inorganic salt that could contribute to passivation. In C's chloro group, any LiCl produced might be partially soluble or not as beneficial as LiF. The CEI from C did contain LiF and sulfur species, but perhaps not in the optimal proportions as D/E. In short, additive C provided a moderate benefit; its structure was sufficiently reactive to form some protective film, but the nature of its byproducts (Cl‐containing) might be less ideal (e.g., LiCl is less stable than LiF at high voltage).

2.5.4. Additive D (Methyl 2‐(fluorosulfonyl)acetate)

This additive has a bifunctional character: a –SO₂F group attached to an acetate (–CH₂COOCH₃). The electron‐withdrawing ester likely increased the stability of D, possibly requiring a higher potential to oxidize or reduce. D's relatively poor ICE improvement suggests it did not react as readily during formation. It might decompose at the cathode (since anode reduction could be hindered by the ester, which raises the reduction potential). If oxidized on the cathode, D could form LiF plus methyl sulfate or other sulfate species. However, the presence of an ester also means any organics D produces may remain soluble (e.g., methyl sulfate could dissolve) rather than deposit on the surface. This could explain why D showed only minor improvement—it may not leave enough solid film behind, or it might decompose into products that do not strongly passivate the cathode. Additionally, the ester could undergo hydrolysis or transesterification side reactions, consuming electrolyte but not contributing to a stable interphase. Thus, D's structure appears not well‐suited for effective film formation in our system.

2.5.5. Additive E (Methanesulfonyl fluoride)

E is the simplest structure in this series, with a small methyl attached to –SO₂F. This simplicity seems to be a virtue: E performed best in boosting ICE and lowering impedance. E's mechanism can be envisioned as follows: upon first charging, E either reduces at the anode (producing LiF and CH₃SO₂ ⁻ anions) or oxidizes at the cathode around the high‐voltage region. In either case, the S–F bond in E will break to yield LiF, and the CH₃SO₂ fragment will likely form lithium methanesulfinate (Li CH₃SO₂) or related species. Lithium methanesulfinate can further oxidize to lithium methanesulfonate (LiCH₃SO₃) on the cathode. The end result is a CEI/SEI containing LiF and Li–sulfinate/sulfonate. These products are insoluble and adhere to electrode surfaces. Our XPS confirmed abundant LiF and S–O species from E.

The small size of E means the film components are compact and able to distribute evenly. In essence, E acts much like pTSF (toluenesulfonyl fluoride) did in previous studies, creating a protective S–F rich interphase on both cathode and anode [9, 34, 35]. But unlike pTSF (which has a bulky aromatic ring), E has no large moiety that could increase film impedance—it yields a very thin inorganic/organosulfur layer. This likely explains why E's CEI was highly effective at passivation without penalty to ion transport. The excellent performance of E underscores that a small, single‐function sulfonyl fluoride can be an optimal additive for forming LiF‐rich, low‐impedance interphases.

In conclusion, the structure–function analysis reveals that small, highly labile sulfonyl fluorides (like E) and bulky, multifunctional sulfonyl fluorides (like B) are both promising routes to effective interphase formation, albeit likely via different mechanisms. E leans on straightforward, complete decomposition to LiF + sulfinate, providing an inorganic‐rich passivation. B leverages a complex decomposition that forms a robust polymeric/aromatic film. Intermediate structures (C, D) either lack sufficient reactivity or produce suboptimal byproducts, and unsaturated structures (A) risk over‐reactivity. These insights suggest that the ideal additive should either (1) decompose cleanly at a low potential to form primarily LiF and a compact inorganic/organosulfur layer, or (2) decompose at high potential in a controlled polymerization that yields a cohesive, Li⁺‐permeable film. Achieving one of these outcomes without adverse side reactions is key.

Our findings here align with the broader understanding that effective electrolyte additives often function as sacrificial agents that react in preference to the electrolyte, thereby protecting the electrode surfaces with a tailored interphase [36, 37, 38]. The differences among A–E highlight how the additive's functional groups direct its sacrificial reaction pathway. By tuning these functional groups, one can optimize whether an additive primarily fortifies the SEI, the CEI, or both, and how much irreversible capacity loss is incurred during this process. In the present NCM955 cells, additives E provided the best compromise, significantly improving the cathode interface stability with only minimal first‐cycle sacrifice, as shown in Figure 8.

FIGURE 8.

The role of methanesulfonyl fluoride additive on both high‐nickel NCM and Li metal after cycling.

3. Conclusion

Incorporating a sulfonyl fluoride additive is a promising strategy to stabilize high‐Ni NCM cathodes. This work has demonstrated that sulfonyl fluoride (–SO₂F) electrolyte additives can markedly influence the initial efficiency and interfacial stability of NCM955‖Li cells. Through a comparative evaluation of five additives with the common –SO₂F functional group but different substituents, we found that a methanesulfonyl fluoride (E) are highly effective co‐additives in a high‐voltage carbonate electrolyte.

At 5 wt.% each (with 5% FEC also present), E raised the initial Coulombic efficiency of NCM955 half cells to ∼90% (versus ∼85% baseline) and significantly reduced the electrode interfacial impedance after formation. XPS analysis revealed that these additives decompose to form favorable CEI compositions rich in LiF and sulfur‐oxy species (sulfites/sulfonates) that passivate the Ni‐rich cathode.

Our findings provide a comparative framework for structure‐guided selection of electrolyte additives for Ni‐rich cathode systems and emphasize that the design of electrolyte additives must balance reactivity and stability: the additive should decompose preferentially to form a needed interphase, but not so violently as to consume excessive lithium or create insulating layers. The lessons from additives A–E suggest that focusing on functional groups that produce LiF and polymeric sulfones (while avoiding those that lead to runaway polymerization or soluble byproducts) can guide the next generation of electrolyte additive design for lithium‐ion batteries.

Author Contributions

J.O. and M.J.L. conceived and designed experiments with electrochemical analysis. S.B.L. and H.Y.B. synthesized sulfonyl fluoride additives. J.O., M.J.L., Y.K. and J.K. (CNU) discussed results of chemical analysis. J.K. (YU), C.L. and E.N. fabricated coin cells and analyzed surface properties. J.O. and H.Y.B. supervised the projects. All the authors participated in the discussion of the data analysis and manuscript writing. J.O., M.J.L., and H.Y.B. wrote the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: smll74510‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.