Elastic Interfacial Layer Enabled the High-Temperature Performance of Lithium-Ion Batteries via Utilization of Synthetic Fluorosulfate Additive

The key to producing high-energy Li-ion cells is ensuring the interfacial stability of Si-containing anodes and Ni-rich cathodes. Herein, 4-(allyloxy) phenyl fluorosulfate (APFS), a multi-functional electrolyte additive that forms a mechanical strain-adaptive solid electrolyte interphase (SEI) comprising LiF and polymeric species, and a thermally stable cathode–electrolyte interface containing S  O and S  F species. The radical copolymerization of vinylene carbonate (VC) with APFS via electrochemical initiation creates a spatially deformable polymeric SEI on the SiG-C (30 wt.% graphite + 70 wt.% SiC composite) anode, with large volume changes during cycling. Moreover, the APFS-promoted interfacial layers reduce Ni dissolution and deposition. Furthermore, APFS deactivates the Lewis acid PF 5 , thereby inhibiting hydrolyses that produce unwanted HF. These results indicate that the combined use of VC with APFS allows capacity retentions of 72.5% with a high capacity of 143.5 mAh g − 1 in SiG-C/LiNi 0.8 Co 0.1 Mn 0.1 O 2 full cells after 300 cycles at 45 ° C.


Introduction
High-energy Li-ion batteries (LIBs) are essential for powering electric vehicles (EVs). [1][2][3][4][5] Typically, building EV-adaptable batteries that cover 500 km with a single charge requires high-capacity Si-containing anode materials and Ni-rich layered oxide cathodes with a high reversible capacity of over 200 mAh g −1 . However, the The electrochemical performance deterioration of Ni-rich NCM cathodes is mostly attributed to the severe degradation of interfacial structures. Cathode-electrolyte interface (CEI)forming additives that contain heteroatoms have been the subject of several studies concerning Ni-rich NCM cathodes. In this regard, previous studies have revealed that the addition of 1,3-propane sultone (PS) and VC significantly improves the high-temperature performance of LiNi 1/3 Co 1/3 Mn 1/3 O 2 (NCM111)/graphite full cells. [23,24] In addition, sulfur-containing additives, such as 1,3,2-dioxathiolane-2,2-dioxide and 4-propyl- [1,3,2]dioxathiolane-2,2-dioxide, have been introduced to construct a thermally robust Li alkyl sulfonate-based CEI and stabilize Ni-rich layered oxides. [25,26] Furthermore, boroncentered Li bis(oxalate)borate (LiBOB) has been presented to enhance the electrochemical behavior of NCM111/mesocarbon microbead full cells at elevated temperatures and increase the thermal stability of 4.3 V-charged NCM111 cathodes. [27] Therefore, the electron-deficient boron atom in the LiBOB-derived CEI is capable of stabilizing PF 5 , which is a reactive substance that would otherwise produce unwanted HF in the LiPF 6 -based electrolyte. [28] Although PS is effective in forming a thermally stable SEI with inorganic species, such as Li 2 SO 3 on graphite anodes, its suitability for Si-containing anodes and Ni-rich NCM cathodes is yet to be verified. Moreover, LiBOB is less likely to form an LiF-rich SEI, which can endure the mechanical stress induced by Si lithiation. Therefore, developing electrolyte additives that create appropriate interfacial layers for Si-containing anodes and Ni-rich NCM cathodes is crucial in improving the performance of batteries powering EVs.
Herein, we describe the incorporation of a functional fluorosulfate-based additive, 4-(allyloxy)phenyl fluorosulfate (APFS), to create elastic and thermally stable interfacial layers on electrodes and to enhance the electrochemical properties of SiG-C (30% graphite + 70 wt.% SiC composite)/LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) full cells. Cycling studies reveal that using APFS in VC-added electrolytes facilitates stable operation of a 20.5 mg cm −2 high-mass-loaded SiG-C/NCM811 full cell. Meanwhile, spectral studies reveal that the combined formulation of VC and APFS allows tuned deformability of the SEI for SiG-C anodes and enhanced inhibition toward the structural degradation and Ni ion loss of NCM811 cathodes.

Results and Discussion
The multi-property-inspired concept for the APFS structure is depicted in Figure 1a. Here, APFS bearing the vinyl and fluorosulfate groups, as a functional electrolyte additive, was synthesized through successive allylation, sulfurylation, and fluorination. The detailed synthetic procedures and nuclear magnetic resonance (NMR) data are presented in Figure 1b and Figure S1 (Supporting Information). [29]

Cycle Performance of SiG-C/NCM811 Full Cells
Further, the orbital energy levels (highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)) of solvents and additives were theoretically compared.
The APFS additive presented a LUMO energy level of −1.009 eV and a HOMO energy level of −6.780 eV, which were the lowest and highest, implying better tendencies for reductive and oxidative decompositions than others at the SiG-C anode and NCM811 cathode, respectively (Figure 1c).
In addition, structure and electronic properties of Li + -solvent/ additive were investigated in the complex forms ( Figure S2, Supporting Information). As a result, Li + -APFS complex exhibited the most reactive and stable tendencies for complex products in both oxidation and reduction states. [30][31][32] Furthermore, the deformation energy of APFS during its reduction was relatively lower than that of other components (Figure 1d), indicating the favorable decomposition of reduced APFS. In addition, the F dissociation energies, which denote the amount of energy required to detach a fluorine atom from an additive, of all components were compared. Notably, APFS is expected to be a more effective source of LiF [33] than FEC because its F dissociation energy is lower than that of FEC, a fact that is also supported theoretically ( Figure 1e). Typically, LiF in the SEI is an effective component that can ensure the interfacial stability of Si-containing anodes, which suffer from volume changes during cycling. LiF can favorably bind with the Li ions of the organic-rich SEI components and lithiated anodes. [34] Figure 2a-e depicts the cyclic performance of SiG-C/ NCM811 full cells with 1 wt.% VC-, 1 wt.% VC + 2 wt.% FEC-, and 1 wt.% VC + 0.5 wt.% APFS-incorporating electrolytes at 45 °C and 1 C rate under an electrolyte mass to cell capacity ratio (E/C ratio) of 5.6 mg mAh −1 . The VC-containing electrolyte exhibited severe capacity decay after 50 cycles and an inferior capacity retention of 33.2% while delivering a discharge capacity of 65.2 mAh g −1 after 300 cycles at 45 °C (Figure 2a). Conversely, the VC + APFS-containing electrolyte exhibited a significantly improved capacity retention of 72.5% and presented a discharge capacity of 143.5 mAh g −1 , which was slightly better than that of the VC + FEC-containing cells. Furthermore, VC + FEC reached a 99.5% Coulombic efficiency after 50 cycles, whereas VC + APFS rapidly approached 99.5% within 10 cycles (Figure 2b-e). Because the electrochemical performance of SiG-C/NCM811 full cells was evaluated under extremely challenging conditions (high mass loading of the cathode active material corresponding to an areal capacity of 4.21 mAh cm −2 ) in comparison with previous studies, our cycling test could support the beneficial effect of VC + APFS in LIBs (Table S1, Supporting Information). As shown in Figure S3a-c (Supporting Information), an optimal content of 0.5 wt.% APFS is required to ensure the optimal interfacial stability of the electrodes and mitigate the harmful effects of reactive species (e.g., PF 5 generated from LiPF 6 -based electrolytes).
Importantly, enhanced cycle performance of the SiG-C/ NCM811 full cells was achieved in VC + APFS, even after APFS depletion, indicating conservation of the SEI and CEI structures ( Figure 2a; Figures S4, S5, Supporting Information). Notably, APFS decomposed reductively at 1.1 V versus Li/Li + during the initial cathodic scan (lithiation) of the anode. Moreover, the VC reduction peak disappeared for the VC + APFS electrolyte, indicating the co-decomposition of APFS and VC (Figure 2f). Importantly, the anodic current produced by the sacrificial oxidative decomposition of APFS started to increase at 2.9 V versus Li/Li + , and the resulting APFS-derived CEI contributed www.afm-journal.de www.advancedsciencenews.com

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© 2023 Wiley-VCH GmbH to the improvement in the oxidation stability of the electrolyte ( Figure S3d, Supporting Information). The VC + APFS electrolyte effectively mitigated the undesired decomposition of electrolytes at electrodes, resulting in a capacity loss, and drastically enhanced the cycle stability of SiG-C/NCM811 full cells at 25 °C and 1 C (Figure 2g-j). The cell with VC + APFS enhanced the fast-charging performance of the SiG-C/NCM811 full cells at charge rates of 2 and 3 C compared with the VC-containing cell at 25 °C (Figure 2k). Moreover, SiG-C/NCM811 full cells containing VC + APFS exhibited outstanding discharge capacities at a high charge rate of 2 C compared to cells containing VC alone ( Figure S6a,b, Supporting Information). The interfacial impedance (surface film resistance + charge-transfer resistance) was decreased from 33.6 to 25.4 Ω using VC + APFS ( Figure 2l). The SiG-C/NCM811 full cells with the VC + APFS electrolyte exhibited enhanced cyclic performance at 0 °C compared to cells with VC, indicating the formation of less resistive interfacial layers on the electrodes, favoring the migration of Li + ions ( Figure S6c,d, Supporting Information).

APFS-Derived SEI on the SiG-C Anode
The beneficial role of the VC + APFS electrolyte in the SEI structure of the SiG-C anode was examined using ex situ X-ray photoelectron spectroscopy (XPS). As APFS has a relatively low LUMO energy level, it is expected to preferentially undergo electrochemical reduction at the SiG-C anode. The decomposition of APFS began at 2.7 V while producing pronounced peaks attributed to the sulfur-based species (Figure 3a). For the VC + APFS electrolyte, a relatively strong LiF signal was detected for the anode compared to the VC-containing electrolyte (Figure 3b,c). Based on this result, we surmised that the reduction of APFS causes LiF formation. The fluorosulfonyl functional group may release fluorine anions to form LiF, and the detachment of the SO 2 F group from the APFS structure may lead to sulfur-rich SEI formation. The peaks corresponding to the SO 4 2− /SO 2 F − and SO 3 − moieties induced by APFS reduction at the SiG-C anode appeared at 169.8 and 168.0 eV, respectively ( Figure 3c). [35] Since VC does not deactivate PF 5 in the LiPF 6 ↔ PF 5 + LiF reaction, the PF 5catalyzed ring opening of EC occurs to form poly(ethylene oxide)-like species and CO 2 . [36] The formed CO 2 may be converted to Li 2 CO 3 by reacting with a Li-ion and electron at the anode during the charging process ( Figure S7, Supporting Information). In contrast to the VC-derived SEI, which is composed of a poly(VC)-containing inner SEI and P-F/LiF-rich outer SEI, VC + APFS comprises a sulfur-rich inner layer, an organic-rich middle layer containing polymer species, and mechanically robust LiF-based outer SEI. Moreover, the C 1s XPS profiles of the SiG-C anode confirm the disparity in the reductive decomposition of VC and VC + APFS ( Figure S8, Supporting Information). Both electrolytes exhibit poly(VC) species formation at 3.0 V. Owing to the copolymerization of VC and APFS at the SiG-C anode, the VC-derived SEI is found to be primarily composed of poly(VC) species, whereas the VC + APFS-derived SEI has a lower proportion of poly(VC). The relatively strong CC signal corresponding to graphite indicates that VC + APFS creates an SEI that is thinner compared to VC-derived SEI, and facile Li-ion transport is expected. The expected reaction mechanisms for the formation of each inorganic and organic SEI, represented as paths I and II, respectively, are shown in Figure 4. The reaction mechanisms were investigated via density functional theory (DFT) calculations to elucidate the contribution of each functional moiety of APFS on the build-up of the SEI. Based on the low F dissociation energy (Figure 1e), the direct formation of LiF is expected for path I. Furthermore, the one-electron reduction on APFS initiates another pathway of LiF formation. This results in the SO bond breakage at the core of APFS and decomposition into SO 2 F and APE-O groups ( Figure S9, Supporting Information). The distributed charge of each group was −0.988 and −0.012 e in the SO 2 F and APE-O groups, respectively. Therefore, it was assumed that an electron is mainly assigned to the SO 2 F group, resulting in the formation of a SO 2 F − anion and an APE-O radical. Subsequently, the SO 2 F − anion is expected to contribute to LiF formation by interacting with the Li + ion. The energy diagram of the SO 2 F − anion decomposition (Figure S10, Supporting Information) was traced using the most stable structure of the Li + -SO 2 F − ionic complex obtained by comparing the relative energies according to the various adsorption sites of the Li + ion on the SO 2 F − anion ( Figure S11, Supporting Information). Notably, the energy barrier and heat of reaction are thermodynamically favored in the presence of the Li + ion. As SO 2 has a much lower LUMO level (−3.67 eV) than APFS (−1.01 eV), the SO 2 F − anion (0.53 eV), and LiF (0.61 eV) ( Figure S12, Supporting Information), it is more favored to accept an electron to form the SO 2 − anion. [22] Along path II, the APE-O radical is expected to contribute to the copolymerization of VC and APFS. First, the polymerization may proceed unfavorably through the direct reaction of the APE-O radical with the sp 2 carbon in the vinyl group of VC and APFS, as indicated by the generated endothermicity (0.29-0.47 eV) ( Figure S13, Supporting Information). By contrast, the APE-O radical (O-centered) may lead to the formation of an APFS radical (C-centered) through a hydrogen atom transfer (HAT) reaction. [37] Thus, the HAT reaction mechanisms of the APE-O radical with the carbonate solvent molecules, VC, and APFS were investigated. Further, hydrogen sites for the most favorable pathway were identified among the possible HAT reaction pathways ( Figure S14, Supporting Information). The reaction with APFS, where relative energies for each hydrogen site were checked ( Figure S15, Supporting Information), exhibited the lowest energy barrier (0.01 eV) and heat of reaction (0.07 eV). Hence, the APE-O radical was more likely to react with APFS, which formed the APFS radical. Remarkably, the APFS radical could lead to the propagation step of the radical copolymerization of VC and APFS ( Figure S16, Supporting Information). The reactions of the APFS radical with VC and APFS exhibited energy barriers of 0.60 and 0.59 eV and heat of reaction values of −0.02 and −0.20 eV, respectively, indicating the plausible occurrence of the reaction owing to exothermicity and the low energy barrier. [38] Notably, successive reactions with VC and APFS were shown to be possible with low activation energies (0.18-0.30 eV) and exothermic heat of reaction values (−0.98 -−0.67 eV) (Figure S17, Supporting Information). Thus, it is theoretically shown that the reductive decomposition of APFS contributes to the build-up of the inorganic outer SEI based on LiF and the organic inner SEI based on the radical copolymerization of VC and APFS on the SiG-C anode. The SEI structure is considered to be based on an outer inorganic SEI and an inner organic SEI, effectively enhancing the cyclic stability of the SiG-C/NCM811 full cells. To explore the copolymerization of APFS with VC, poly(VC + APFS) was synthesized through a model radical reaction ( Figure S18, Supporting Information) and was characterized using gel permeation chromatography (GPC) and NMR analysis. The samples in entries 1 and 6 were successfully synthesized through precipitation. Based on the GPC analysis, the M n values of poly(VC) and copolymerized VC + APFS were found to be 1.820 × 10 6 and 7.668 × 10 5 g mol −1 , respectively (Figure S19, Supporting Information). Furthermore, the peak broadness in the GPC chromatogram of the copolymer indicated the characteristic low molecular weight. In the 1 H NMR spectrum of poly(VC + APFS), the aromatic CH peaks of APFS were observed at 6.8-7.8 ppm, whereas the peaks corresponding to olefinic moieties ≈6 ppm disappeared, indicating copolymerization between APFS and VC. In the 19 F NMR spectrum, the fluorosulfate group with a peak at 37.4 ppm remained intact after the radical polymerization process (Figures S20 and S21, Supporting Information). Based on these combined computational and experimental studies, the formation of the mechanically stable and deformable SEI on the SiG-C anode can be associated with the copolymerization process between the VC and APFS additives. To understand the contribution of copolymerized poly(VC + APFS) to ion conduction through the SEI, a poly(VC + APFS) containing liquid electrolyte (LE, 1 M LiPF 6 in EC/DEC/EMC (25/30/45 vol.%)) was prepared, and its resistance was measured using a frequency response analyzer, as depicted in Figure S22 (Supporting Information). The compositions of the polymer and LE were designed considering the polymer species in the partially swollen SEI (caused by the LE). The ion conductivities of poly(VC)/LE were lower than those of the LE because poly(VC) tends to make the ion conduction channels tortuous, leading to a decrease in the ionic conductivity. For poly(VC + APFS)/LE, the ionic conductivities were slightly higher than poly(VC)/LE (Table S2, Supporting Information). Notably, according to the Vogel-Fulcher-Tammann equation, the ionic conductivities of polymer electrolytes are affected by the free volume and segmental dynamics associated with the glass transition temperature, Tg. [39][40][41] It is thought that polymer electrolytes with a low Tg exhibit large free volumes that can provide sufficient ion conduction channels. Based on our analysis, poly(VC + APFS) with the LE exhibited a relatively low Tg of −127 °C compared to poly(VC) with the LE (see differential scanning calorimetry (DSC) curves in Figure S23, Supporting Information). We, therefore, confirmed that the electrolyte-rich phase in the APFS-driven polymer matrix facilitates the migration of Li ions. Therefore, the less dense SEI layer formed by VC + APFS allows facile ion conduction to the anode, leading to better cyclic performance of the SiG-C/NCM811 full cells.
Previous studies have shown that the F atom in LiF forms a strong bond with Li of the organic species in the SEI and the lithiated Si anode. [34,42,43] Additionally, we conducted DFT calculations to elucidate the role of LiF ( Figure S24, Supporting Information). We found that strong binding energy of LiF with both SEI and Si anode systems, which could be indicative of gluing effect. The amount of APFS in the electrolyte solution and SEI structure was investigated through 1 H NMR and FT-IR analyses to probe the contribution of the APFS additive to the SEI creation on the anode. In this case, the 1 H NMR spectra (400 MHz, tetrahydrofuran (THF)-d 8 ) showed a decrease in the characteristic resonances of the vinyl (CC) group at ≈6.1 ppm and the phenyl ring at ≈7.1 ppm during charging, indicating that both the APFS moieties are consumed to form the SEI (Figure 5a). The decreased CC peak intensity is in line with the polymeric SEI formation on the SiG-C anode, indicating the consumption of vinyl moieties within the APFS structure through the radical copolymerization between VC and APFS ( Figure 4). The reductive decomposition of the APFS additive is evident from the phenyl structure present in the SEI, as illustrated in Figure 5b. Furthermore, the FT-IR spectra of the SiG-C anode with the VC + APFS electrolyte show an intense CC stretching signal of the aromatic ring and more pronounced CH and CH 2 signals of the vinyl group, indicating that the APFS additive underwent reductive decomposition at the anode and contributed to the formation of the SEI. Figure 5c shows the proposed polymeric structures produced by the electrochemical reduction of the VC and VC + APFS electrolytes. The phenyl moieties bonded to the poly(VC) backbone may impede the tight packing of poly(VC) chains, leading to a dense and rigid SEI structure. The COC groups existing on the side chain of the polymer backbone formed by the copolymerization of VC and APFS can increase the free volume of the SEI through their conformational rotation. [44] Owing to the reduced rigidity of the SEI, the VC + APFS-promoted SEI may have a high adsorption ability toward the volumetric expansion of SiC particles of the SiG-C anode. The mechanical properties of the VC-and VC + APFSderived SEI layers were investigated using contact mode atomic force microscopy (AFM) ( Figure S25, Supporting Information). Figure 5d shows that the Young's modulus of the VC-containing anode substantially increased. The increased modulus may be attributable to the high infiltration level of pulverized SiC particles into the SEI during cycling. This is probably because the VC-derived rigid and dense SEI does not endure the large volumetric stress of SiC particles and incurs physical damage, further increasing the thickness and stiffness of the SEI through continuous electrolyte decomposition and electrical isolation of pulverized SiC particles (Figure 5e). The undesired SEI thickening causes a squeezing-out action of the anode electrolyte, whereas a localized compression force is applied to the cathode side, leading to nonuniform reactions and microcracking of the cathodes ( Figure S26, Supporting Information). By contrast, the anode with the VC + APFS electrolyte presented low values compared to that with the VC electrolyte, along with no significant increase in the Young's modulus. A low Young's modulus indicates high SEI elasticity. [45] In this regard, a flexible spacer in the SEI with a lower Young's modulus is expected to accommodate the volumetric stress induced by the lithiation-delithiation of the Si-containing anodes. Clearly, the more elastic VC + APFS-derived SEI absorbs the mechanical stress resulting from the lithiation of the SiG-C anode and mitigates the pulverization and electrical isolation of SiC materials. Notably, the VC + APFS electrolyte leads to the creation of a well-balanced SEI structure. This structure either contains mechanically stiff inorganic species such as LiF or sulfur-containing species and flexible polymeric species based on poly(VC + APFS) to achieve high electrochemical reversibility of the SiG-C anodes ( Figure S27, Supporting Information).
Comparative scanning electron microscopy (SEM) studies on the SiG-C anodes with the VC, VC + FEC, and VC + APFS electrolytes after 100 cycles at 45 °C revealed that the cycled SiG-C anode with the VC + APFS electrolyte had a lower volume expansion of 109.5% compared to the anodes with the VC (113.5%) and VC + FEC electrolytes (112.0%). This result can be attributed to the combined use of the VC and APFS additives manipulating the interface structure of the SiG-C anode (Figure 6a,b; and Figure S28b, Supporting Information). The VC electrolyte induced the build-up of a thick SEI on the outermost anode surface (Figure 6d,f), and the anode particles lost their electronic connection.
This finding suggests that VC does not entirely preserve the SiG-C anode structure in the full cell, and the Li-storage ability of the anode is considerably diminished (Figure 6e,g). The beneficial effects of the VC + APFS electrolyte on the interfacial stabilization of the SiG-C anode and the suppression of dissolved Ni from the cathode and Ni plating on the anode are illustrated in Figure 6c. The uncontrolled CEI formation on the cathode with VC induces microcracking of the NCM811 secondary particles, resulting in impregnation of the electrolyte inside the cathode particles. By contrast, the VC + APFS electrolyte constructs a stable CEI on the NCM811 cathode and a robust SEI on the SiG-C anode, collectively mitigating Ni deposition-induced SEI damage (Figure 6c,h). The positive impact of the VC + APFS electrolyte on the SEI of the SiG-C anode was examined via transmission electron microscopy (TEM) observations after 100 cycles at 45 °C ( Figure S29, Supporting Information). The SiG-C anode composed of graphite and SiC particles contained a relatively thin SEI for the VC + APFS electrolyte, whereas a thick SEI was observed for the VC electrolyte. Graphite and SiC particles with VC revealed that during the cycles, the SiG-C anode exhibited severe accumulation of SEI layers, whereas particles with VC + APFS exhibited smaller expansions: 10 nm for graphite particles and 4 nm for SiC particles. The line energy dispersive spectroscopy spectra revealed that the permeation of the electrolyte decomposition byproducts into the Si nanolayer was hindered, and the original layered structure was maintained with the VC + APFS electrolyte. Furthermore, the SEI with VC + APFS was thinner compared to the VC-derived SEI. TEM observations of the SEI layer showed that the combination of VC and APFS mitigated the reductive decomposition of used electrolyte components, such as EC and LiPF 6 .

Improvement in the CEI Stability with the VC + APFS Electrolyte
In addition to improving the SEI quality of the SiG-C anodes in full cells, the XPS analysis revealed a significant role of VC + APFS in modifying the CEI of the NCM811 cathodes (Figure 7). The S 2p spectra revealed a significant difference between the XPS results of the NCM811 cathodes with VC and VC + APFS. Here, residual APFS predominately contributed to the CEI formation through oxidative decomposition at a charge voltage of 4.2 V after contributing to the sulfur-rich SEI formation below 4.0 V. The relative fraction of sulfur-containing species increased in the CEI (Figure 7b). A possible CEI structure is shown in Figure 1.
The prominently increased fraction of the MO species on the cathode with the VC electrolyte at a charge voltage of 3 V may be attributed to the loss of cathode surface coverage owing to CEI instability. No discernible change in the MO peak of the cathode with the VC + APFS electrolyte was observed. This finding implies that APFS promotes CEI formation, which is electrochemically robust enough to protect the NCM811 cathode. Detailed CEI formation mechanisms on the NCM811 cathode with the APFS additive are illustrated in Figure S30 (Table S3, Supporting  Information); the less dense polymer-like species may preserve the CEI structure during the rapid movement of Li ions at high charge rates ( Figure S31, Supporting Information). In the FT-IR spectra, the increased CC stretch at 1500 cm −1 in the aromatic ring, SO signals at 991-1012 cm −1 in the RSO 2 Li moieties, and CO bonds at 1790-1800 cm −1 in the poly(VC + APFS) structure indicate that the oxidative decomposition of APFS at the cathode contributes to the CEI formation ( Figure S32, Supporting Information). Considering the relatively higher HOMO energy level (i.e., −6.780 eV) of APFS compared to EC (i.e., −8.68 eV), FEC (i.e., −9.06 eV) and VC (i.e., −7.29 eV) (see also Figure 1c), the oxidative path of APFS was investigated through the DFT calculation. Here, we considered two cases of oxidative reaction of APFS with EC and LiOH. At the former reaction, unstable APFS-derived species (i.e., APE-OH + ) were yielded, however, the latter reaction, in which oxidized APFS decomposed into the APE-O and Li-HOSO 2 F groups, exhibited more exothermic heat of reaction compared to EC (i.e., −1.19 and −0.91 eV, respectively) ( Figure S33, Supporting Information). Furthermore, the overall energy barrier of APFS with LiOH was lower than that with EC (i.e., 0.27 and 0.56 eV, respectively) except for the first step, which shows a marginal difference. Therefore, the SO bond breakage is expected to occur more plausibly in reaction with LiOH than EC. The distributed charge of the APE-O and Li-HOSO 2 F groups was + 0.02 and + 0.98 e, respectively, resulting in the formation of APE-O radical and Li + -HOSO 2 F cation. Thus, the O-centered APE-O radical may involve in HAT reaction and radical polymerization processes with VC (analogous to Path II in Figure 4), leading to the formation of polymeric species with free volume in the CEI layer composed of sulfur-containing species. The beneficial effect of APFS on the interfacial stabilization of the NCM811 cathode was confirmed through TEM observations after 100 cycles at 45 °C ( Figure S34, Supporting Information). An 8 nm thick CEI layer was observed on the NCM811 cathode in the VC electrolyte, whereas a relatively thin CEI layer was formed for the VC + APFS electrolyte. A computational study indicated that the APFS additive deactivated PF 5 , producing corrosive HF by an undesirable reaction with a trace amount of moisture in the electrolyte (Figure 8a). In the presence of APFS, the heat of reaction and energy barrier increased. The mitigation of HF generation in the electrolyte may aid in stably maintaining the interfacial layers on the SiG-C anode and NCM811 cathode during cycling. 19 F NMR data confirmed the role of APFS in deactivating the PF 5 molecules (Figure 8b,c). By introducing APFS into the electrolyte, the relative intensity of the HF peak at −191 ppm was reduced from 24.9% to 13.1%. Additionally, the relative intensity of the corresponding peaks of PO 2 F 2 − at −80.8 and −83.4 ppm also decreased. This can be explained by the deactivation ability of APFS resulting from its complex formation with PF 5 . The undesirable formation of acid compounds (HF and HPO 2 F 2 ) aggravates the leaching of transition metal (TM) ions from the NCM811 cathode at elevated temperatures.
The dissolved TM ions migrate toward the lithiated anode when a completely charged full cell is stored at 60 °C. The VC + APFS-containing cell exhibited a discharge capacity retention of 86.7% (171.8 mAh g −1 ), which was comparable to that of the VC + FEC-containing cell (171.7 mAh g −1 (86.1%)) ( Figure S35a,b, Supporting Information). In contrast to conventional electrolytes, the VC + APFS electrolyte demonstrated enhanced capacity retention and recovery with reduced cell resistance after storage at 60 °C for 20 d (Figure S35c,d, Supporting Information). After high-temperature storage, the capacity loss was associated with Li-ion extraction from the lithiated SiG-C anode; it is, therefore, logical that the VC + APFS-promoted SEI is robust enough to maintain the charged state of the SiG-C anode without severe thickening of the SEI, causing cell resistance.
Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis was conducted to examine the quantity of dissolved TM from the delithiated NCM811 cathode with the VC-and VC + APFS-induced CEI in contact with 1 M LiPF 6 EC/ EMC/DEC (25/45/30 vol.%) for 3 d at 60 °C. The Ni ions dissolved in the delithiated NCM811 were significantly reduced in the APFS electrolyte ( Figure S36, Supporting Information). The onset temperature of the exothermic peak of APFS-applied delithiated cathodes was considerably delayed compared to that for the VC and VC + FEC electrolytes. Furthermore, the total heat released reduced to 0.88 kJ g −1 , confirming that the CEI formed using APFS effectively mitigated the exothermic surface reaction of the NCM particles ( Figure S37a, Supporting Information). In addition, the combination of APFS with VC improved the thermal stability of the SEI and lithiated SiG-C anodes ( Figure S37b, Supporting Information). The structural stability of the NCM811 cathodes with VC, VC + FEC, and VC + APFS was examined (Figure 9a-d; Figure S38c,d, Supporting Information). Notably, the intergranular cracking of the NCM811 cathode particles was effectively suppressed in the VC + APFS electrolyte compared to that in the VC and VC + FEC electrolytes. This finding reveals that the CEI formed by VC + APFS helps enable the homogeneous delithiation and lithiation of the NCM811 primary particles without severe intergranular cracking. Conversely, the CEI formed on the cathode with the VC electrolyte does not lead to uniform delithiation and lithiation of the NCM811 primary particles, leading to severe morphological degradation of the NCM particles (Figure 9a,b). Although the VC + FEC electrolyte exhibits a better cycle life at 25 and 45 °C than VC, the thickening of the cathode with the accumulation of decomposed electrolyte byproducts still occurs.
Scanning transmission electron microscopy (STEM) with fast Fourier transform (FFT) and electron energy loss spectroscopy (EELS) analyses confirmed the beneficial effect of APFS in mitigating the irreversible phase transformation of the NCM811 cathodes at the atomic level (Figure 9e-h). Notably, the thickening of the phase transformation layer to the rock-salt phase in the R3_m space group impedes Li-ion transport into the layered structure of the cathode and decreases the Li-storage capability of the cathode. APFS drastically reduced the phase transformation thickness to 3.1 nm while maintaining the layered structure at site B (Figure 9f). Notably, FEC, which demonstrated similar discharge capacity retention with APFS after 100 cycles ( Figure S39a, Supporting Information), did not mitigate the severe phase transformation of the cathode. The EELS analysis confirmed the interfacial stabilization effect of APFS on the distinguishing oxidation state of TM in the NCM811 cathode (Figure 9g,h). The variation in the oxidation state of the TM ions was characterized by two indices: the energy loss difference (ΔE) of the O K-edge and the intensity ratio (L 3 /L 2 ) of the TM ions. [46] The ΔE of the O K-edge is defined as the energy loss change from the main-edge peak (≈540 eV) to pre-edge peaks (≈530 eV). Because the O K-edge peaks demonstrate the local vacant 3d state of TMs, their movement to lower energy losses represents the oxidation of the TM ion. The VC + APFS electrolyte led to a high ΔE value, indicating mitigation of the reduction of the Ni 4+ ions to lower valance states (Ni 2+ and Ni 3+ ) on the surface [47][48][49] (Figure S39d, Supporting Information). In addition, a distinct O K-edge peak appeared at a relatively low depth for the NCM811 cathode cycled with the VC + APFS electrolyte, indicating a low level of oxygen loss (Figure 9g,h; Figure S39c, Supporting Information). Notably, the Ni 2+ ions formed in the delithiated states are prone to migration into vacant Li slabs, causing undesired Li/Ni cation mixing at the NCM811 cathode during cycling. After 100 cycles, the Ni L 3 /L 2 ratio decreased more rapidly from the surface to bulk in the re-lithiated state of the NCM811 cathode with the VC + APFS electrolyte ( Figure S39e, Supporting Information). This result reveals that the thickening of the inactive rock-salt structure induced by the migration of the Ni 2+ ions into Li slabs at the surface region was effectively mitigated by the electrochemically robust APFS-derived CEI on the NCM811 cathode. The X-ray diffraction patterns implied that the inhomogeneous VCderived CEI induced unwanted Li consumption at the cathode and caused irreversible structural changes in the cathode lattice ( Figure S40a-c, Supporting Information). Conversely, the APFS-modified CEI effectively suppressed the undesired electrolyte oxidation at the NCM811 cathode and improved the mechanical integrity of the cathode.

Conclusion
In this study, we demonstrated the mechanism of action of APFS to form SEIs and CEIs with remarkable characteristics in SiG-C/NCM811 full cells with practical mass loading. In particular, the introduction of APFS into the VC electrolyte formed an SEI with LiF-rich and deformable polymeric species that can withstand severe volume changes of the SiG-C anode during electrochemical cycling. In addition, the synthetic fluorosulfate additive contributed to the formation of SEI and CEI comprising thermally robust sulfur species and less dense polymer-like constituents, enabling facile Li-ion transport. The sulfur-containing CEI restrained further electrolyte oxidation at the cathode, effectively reducing the release of exothermic heat from the delithiated cathode at high temperatures, and suppressing the dissolution of TM from the NCM811 cathode. Moreover, the ability of the APFS to deactivate PF 5 mitigated the interfacially induced instability of the NCM811 cathode and SiG-C anode. This study may provide direction for designing novel electrolyte additives and opportunities to replace conventional electrolyte additives, such as FEC, which exhibit thermal and chemical instability in LiPF 6 -based electrolytes.
Electrochemical Measurements: Here, 2032 coin-type full cells, which were manufactured in a glove box filled with Ar gas (O 2 and H 2 O < 1.0 ppm), were cycled at 45 °C and 1 C after precycling from 2.5 to 4.2 V at 0.1 C and 25 °C using a battery performance evaluation device (TOSCAT-3100, TOYO System Co., LTD.). For a comparison of the electrochemical reduction behaviors of the electrolyte additives, SiG-C/Li half-cells were precycled from 1.5 to 0.05 V versus Li/Li + at C/20. An Al 2 O 3 -coated polyethylene membrane with 49.2% porosity and 15.1 µm thickness was employed as a separator. The cells were stored at 60 °C for 20 d to explore their self-discharge properties. Subsequently, the cell impedance was measured by the direct current internal resistance method at a state of charge = 50. The impedance of the SiG-C/NCM811 www.afm-journal.de www.advancedsciencenews.com

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© 2023 Wiley-VCH GmbH full cells after the precycle was measured by AC impedance analysis using an electrochemical potentiostat (VSP-2e, Biologics) over a frequency range of 0.01-1 MHz.
Characterization: For analysis, all procedures were conducted in a glove box filled with inert Ar gas. The cells were disassembled, and the remaining electrolyte from the electrodes was eliminated using dimethyl carbonate. The elemental compositions of the SEI and CEI layers were determined by XPS (ESCALAB 250Xi System, Thermo Fisher Scientific) analysis, which was conducted using Al-ka (hv = 1486 eV) X-rays under ultrahigh vacuum conditions. The samples of the cycled electrodes for XPS measurements were packed in a tight aluminum pouch in a glovebox and transferred to the XPS chamber before measurement to prevent contamination by air and moisture. The decomposition of the electrolyte additives at different charged states during precycling and the amount of PO 2 F 2 − with HF generation after storing the electrolyte were examined by 400 MHz 1 H NMR spectroscopy (AVANCE III HD, Bruker) using THF-d 8 (99.5%, NMR grade, BK Instruments Inc.). The relative amounts of PO 2 F 2 − and HF generated during the storage of the electrolyte solutions (at 25 °C for 10 d) were calculated based on the exact amount of 1 wt.% hexafluorobenzene (internal standard) for the 19 F-NMR analysis.
The mechanical properties of the VC-and VC + APFS-promoted SEI on the SiG-C anode were examined by comparing Young's modulus through the contact mode of AFM (MultiMode V, Veeco). FT-IR spectroscopy (670/620-IR Series, Varian Inc.) was performed to analyze the polymeric species formed by the co-decomposition of VC + APFS. The morphological changes in the SiG-C anodes and NCM811 cathodes with VC, VC + FEC, and VC + APFS were explored using SEM (SU8230, Hitachi). The TEM samples were prepared using a focused ion beam system (Helios Nanolab 450, FEI). Randomly selected particles of the NCM811 cathode were coated with a Pt layer, whereas the particles of the SiG-C anode were coated with carbon or Pt. High-angle annular dark-field STEM (HAADF-STEM, Titan G2 60-300, FEI) analysis was conducted to determine the phase transition behavior of the NCM811 cathode with different electrolytes. EELS analysis was conducted to evaluate the oxidation states of the TM ions, and the EELS profiles were recorded from the surface to the bulk electrode at a 1 nm interval.
Computational Details: To investigate the orbital energy levels, deformation energies, F dissociation energies, and reaction mechanisms, DFT calculations were performed with the DMol 3 program. [51,52] All calculations were performed in an implicit environment using the conductor-like screening model while using an estimated dielectric constant (=10.757) of the solvent mixture (EC/DEC/EMC (25/30/45 vol.%)). [53] The detailed DFT calculations are described in the Supporting Information.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.