Covalent and polyfluorinated lithium salt for stable LiCoO_2 batteries at high temperature and high voltage

Domain: physics
Published: 2025-11-24
Editorial outcome: major_revision

Abstract
Elevating the charge cut-off voltage of the LiCoO2 positive electrode beyond 4.5 V has already been the focus to unlock its energy for portable electronics, whereas the severe phase transitions during the delithiation of LiCoO2 from 4.5 V to 4.7 V can ruin the electrode structure. Besides, the poor thermal and hydrolytic stabilities of traditional lithium salt (LiPF6) prevent batteries from working at high temperatures and increase production costs and environmental pollution. Here, we show that using covalent lithium nonafluoro-n-butanesulfonate as a fluorine-rich lithium salt to create a robust LiF-rich cathode electrolyte interphase, which effectively impedes the surface destruction, we successfully realize a stable LiCoO2 battery at a high voltage of 4.7 V and demonstrate a 2.14 Ah Li||LiCoO2 pouch cell with a stack-level specific energy of 518 Wh kg−1 (without packaging). Furthermore, its satisfactory thermal stability empowers LiCoO2 to work at a harsh condition of 60 °C and 4.6 V. Even more, its anti-hydrolytic stability enables the LiCoO2 battery to work with the electrolyte with 200 or even 1000 ppm water contamination. Lithium nonafluoro-n-butanesulfonate presents itself as a potentially viable lithium salt for advanced lithium batteries, offering the prospect of high voltage, improved thermal stability and eco-friendliness.

Introduction
Lithium-ion batteries (LIBs), as the most favorable energy storage portable electronics, such as cell phones, laptops, earphones, and devices, have achieved significant progress and are widely used. smartwatches1,2.

Among the various positive electrode materials in LIBs, lithium cobalt Though the LCO has a high theoretical specific capacity of oxide (LiCoO2, LCO), which has the highest volumetric energy density, 274 mAh g−1, the commercial LCO electrode can only deliver a capacity is employed as the prevailing positive electrode in batteries for of about 185 mAh g−1 at a charge voltage of 4.5 V1,2. In order to extend the working lifetime of batteries, improving the stability at high charge harmful O1 phase, thus enabling the Li||LCO battery to achieve satis- voltage of the positive electrode will release a much higher specific factory cycling stability of 360 cycles at a high voltage of 4.7 V. And the capacity and energy. Nowadays, much effort has been made to obtain a anion NFBS− which is only composed of tightly covalent bonds, makes stable LCO electrode at 4.6 V, but realizing a decent 4.7 V LCO still LiNFBS stable until about 400 °C before thermal decomposition and remains a challenge due to the irreversible phase change at a highly also empowers LiNFBS to coexist with water peacefully. As a result, the delithiated state. During the charging process beyond 4.5 V, the LCO Li||LCO battery with LiNFBS-based electrolyte can stably operate at a electrode will undergo a transition from the O3 hexagonal phase to the high temperature of 60 °C along with high voltage, and the battery also hybridized O1–O3 hexagonal phase (abbreviated as H1-3 phase), which shows negligible influence on the cycling performance when con- has been regarded as a highly harmful phase with significant shrinkage taminating the electrolyte with 200 or even 1000 ppm water. Our of the c-axis and collapse of O–Co–O slabs3–5. When further elevating electrochemical performances and experimental evidence highlight the charging voltage to extract the residual Li+, a more detrimental O1 the potential of LiNFBS for the next-generation batteries to realize phase will appear. Such H1-3 and O1 phases will lead to the collapse of versatile requirements.

Results
Physical properties
The molecular structures of LiPF6 and LiNFBS are shown in Fig. 1a. PF6− is a coordination complex with a coordinate bond (F→P) in which one of the F-atoms donates a lone pair electron to P, forming the equilibrium of LiPF6 ⇌ PF5 + LiF. Since PF5 is a strong oxidant, the reactions between PF5 and organic solvents will become increasingly violent at elevated temperatures, leading to the equilibrium shifts to the right with the vanishing of PF6−. Besides, trace contaminated or residual water in the electrolyte will catalyze the decomposition of LiPF6 with the formation of aggressive PF5, POF3 and highly corrosive HF (LiPF6 → PF5 + LiF, PF5 + H2O → POF3 + 2HF)18–22, thus leading to the premature failure of the battery. The moisture-sensitive feature of LiPF6 also brings great difficulties to the production process and recycling, exacerbating environmental pollution and going against green chemical engineering for the modern battery industry (Fig. 1a). In contrast, NFBS− only consists of sturdy covalent bonds, so LiNFBS is anticipated to exhibit satisfactory thermal and hydrolytic stabilities, and we also place great hope on the fluorine-rich NFBS− for producing LiF-rich CEI to stabilize the LCO electrode (Fig. 1a). Therefore, LiNFBS maybe a competitive candidate of future lithium salt for high-voltage (>4.6 V) and high-temperature lithium battery with environment- friendliness (Fig. 1b). Though LiNFBS has been used in previous reports26,27, its detailed physical and chemical properties have not been fully explored, and its advantage for stabilizing high-voltage battery (>4.6 V) at a diluent concentration (~1 M) has not been exploited.

Then, thermogravimetry-differential scanning calorimetry (TG- DSC) tests were first carried out to evaluate the thermal stabilities of the two lithium salts, LiNFBS and LiPF6. Unlike LiPF6 which suffers a severe mass loss above about 70 °C accompanied by a large amount of heat absorption, LiNFBS presents good stability until about 400 °C (Fig. 1c and Supplementary Fig. 1). Then the thermal and hydrolytic stabilities of the practical electrolytes with 1 M lithium salt (LiNFBS and LiPF6) in fluoroethylene carbonate (FEC) and ethyl methyl carbonate (EMC) were determined, and the electrolytes were sealed in vials under an Ar atmosphere. After resting the electrolytes in an incubator at 60 °C or storing the electrolytes with 1000 ppm H2O for 4 days, the LiPF6-based electrolytes turned dark brown while the LiNFBS-based electrolytes remained unchanged (Fig. 1d, e). Subsequently, 19F nuclear magnetic resonance (19F-NMR) spectra were employed to track the evolution of the lithium salts in electrolytes. From Fig. 1f, we can clearly see that the signals of PF6− were weakened and nearly disappeared after thermolysis and hydrolysis, respectively. Besides, the peaks of O=PFxOy z- were intensified, wherein the strong peaks at −73.6 ppm belonging to O=PF(OH)2 emerged, and some small peaks at −76~(−82) ppm can be assigned to the signals of decomposed solvents, such as O=PF(OC2H5)2 and OPF2(OC2H5)18,22. However, no difference can be observed in 19F-NMR spectra for the LiNFBS-based electrolyte, whether after thermolysis or hydrolysis (Fig. 1g), unambiguously proving the thermolytic and hydrolytic superiority of the LiNFBS.

Considering that the anodic stability of the electrolytes on the Al current collector is the premise to guarantee the battery operates stably at high voltage, Li||Al cells were assembled. Encouragingly, the LiNFBS and LiPF6-based electrolytes exhibit little anodic current without observable Al corrosion, in contrast to the phenomenon that LiFSI and LiCF3SO3 based electrolytes tend to severely corrode the Al foil (Supplementary Figs. 2 and 3). Moreover, the LiNFBS-based electrolyte manifests a better wettability on the Celgard membrane (Supplementary Fig. 4), and shows a pretty high Li+ transfer number (0.92) with a moderate ionic conductivity (2.4 mS cm−1) due to the relatively slow movement of the large-size anion (Supplementary Figs. 5 and 6).

LCO electrochemical performance
Based on the high oxidation stabilities of LiNFBS and LiPF6-based electrolytes beyond 5.0 V (Supplementary Fig. 2a), we checked the compatibility of the electrolytes for high-voltage (>4.6 V) LCO battery with a quasi-practical areal capacity of about 2.2 mAh cm−2 (i.e., 235 mAh g−1) at 0.1 C (1 C = 200 mA g−1), and the cut-off voltage of the Li||LCO battery is challengingly fixed on 4.7 V to fully unlock the capacity of the LCO electrode. Pushing the charging voltage to 4.8 V can further release the capacity of LCO, but the tremendous O1 phase above 4.7 V can irreversibly ruin the electrode structure, which is still a great challenge even now. From Fig. 2a, a pair of plateaus at the final charge and initial discharge of the battery with LiPF6-based electrolyte can be observed, which can be assigned to the phase transition of LCO between H1-3 phase and O1 phase. It’s widely known that the O1 phase is regarded as the most destructive phase transition, with a huge shrinkage of c-axis and collapse of O–Co–O slabs, releasing O2 from the lattice and forming the irreversible spinel phase3–5. Then the formed oxidized oxygen and oxidant Co4+ are prone to attack the organic solvent, forming a thick CEI layer to impede the charge transfer (Supplementary Figs. 7 and 8). As a consequence, the collapse of the crystal structure and increased charge transfer resistance jointly contribute to the fast decay of the battery. As can be seen from Fig. 2a–c, the LiPF6-based battery with the emergence of the O1 phase attenuates quickly at a high voltage of 4.7 V, while the LiNFBS-based battery with a mild phase transition can cycle for 360 cycles with a capacity retention of 70%. Besides, the LiNFBS-based battery also shows a higher average Coulombic efficiency (CE), i.e., 99.88% for LiNFBS and 98.35% for LiPF6 (Fig. 2c), and low CE in the LiPF6-based electrolyte can be imputed to the continuous parasitic reaction between the LCO electrode and electrolyte (Supplementary Fig. 9). Encouragingly, our cycling performance at a high voltage of 4.7 V is competitive compared with the reported Li||LCO batteries (Supplementary Fig. 10 and Table S1)7,10,13,28–32.

Then an ampere-hour-level Li||LCO pouch cell with thin Li foil (50 μm Li, N/P = 1.9), high LCO mass loading (~23.5 mg cm−2) and lean electrolyte (2.0 g Ah−1) was assembled to verify the practicality of LiNFBS-based electrolyte for realizing high specific energy (Fig. 2d, e and Supplementary Fig. 11). As can be seen from Fig. 2d, the 4.7 V Li|| LCO pouch cell delivers a capacity of 2.14 Ah during the activation process at 0.1 C, which the specific capacity of LCO electrode and the specifc energy of the pouch cell can be calculated to be 232 mAh g−1 and 518 Wh kg−1 (Supplementary Table S2), respectively, and the pouch cell can cycle about 50 cycles at 0.2 C (Fig. 2d, e). Though LiNFBS can stabilize the Li metal to some extent compared with LiPF6 (see detailed discussion in Supplementary Figs. 12–20), insufficiently high Li plating/ stripping CE restrict the cycling lifetime of the practical pouch cell in such harsh conditions, and we believe that using advanced solvents or extra additives to stabilize the Li negative electrode will prolong the cycling lifetime of the pouch cell.

LCO structure evolution
To unravel the phase evolution during the charging process, in-situ X- ray diffraction (XRD) measurements were conducted on the LCO positive electrodes (Fig. 3a–d and Supplementary Fig. 21). Since the (003) peak related to the layer distance of the c-axis is prone to change violently during the delithiation, the (003) peak is usually chosen to track the structure evolution of LCO2,3,32, and the intensities of pristine (003) peak were normalized to the same value. For the LCO electrode charged in LiPF6-based electrolyte, the (003) peak shifts to a much larger angle with a high proportion of harmful H1-3/O1 phase (67.2%) (Fig. 3a, b), intergrowth of H1-3 and O1 phases4,33. However, the (003) peak of the LiNFBS-based LCO electrode shows a small shift with a weak H1-3/O1 phase (41.5%) (Fig. 3c, d). Besides, a stronger peak of H2 intermediate belonging to the H2 phase at the initial charging process in LiNFBS can be noted (Fig. 3c, d)34, which may contribute to the delay of phase transition.

To explore the detrimental effects of H1-3 and O1 phase transitions, we use a scanning electron microscope (SEM) to visualize the morphology changes. From Fig. 3e, f, and Supplementary Fig. 22, we can observe obvious cracks on the LiPF6-based LCO electrode due to the internal stress-induced lattice oxygen release, while the LiNFBS- based LCO electrode remains intact. Then, in-situ differential electrochemical mass spectrometry (DEMS) tests were carried out to check the gas evolution during the charging process (Fig. 3g, h). For the LiNFBS-based LCO electrode, no obvious gas evolution can be detected (Fig. 3h). In contrast for the LiPF6-based LCO electrode, O2 from the oxidized lattice oxygen evolves accompanied by the generation of CO2 from the oxidative decomposition of electrolyte (Fig. 3g). The release of lattice oxygen will produce oxygen vacancies, and the electron trapped in oxygen vacancy or the unpaired electron in oxidized oxygen (Oα-2, 0 < α < 2) can be detected by electron paramagnetic resonance (EPR) spectra, which shows sharp absorption with a g-factor of about 2.0035,36. Due to the partial overlapping of O 2p and Co 3d orbitals, a lattice oxygen will be partially oxidized at high voltage37. When charging the LCO electrode in LiPF6-based electrolyte at 4.7 V, the LCO electrode exhibits a distinct EPR signal of oxygen vacancy or oxidized oxygen, and the signal still remains at a high intensity after discharge, which means the irreversible oxygen oxidation or the release of lattice oxygen forms permanent oxygen vacancies/radicals. In contrast, though the LCO electrode charged in LiNFBS-based electrolyte shows some EPR signal, it nearly disappears after discharge (Fig. 3i and Supplementary Fig. 23), indicating the reversible oxygen redox enabled by LiNFBS.

Then, soft X-ray absorption spectroscopy (sXAS) tests were carried out to investigate the transition of electronic structure at the surficial layer of LCO. As shown in Fig. 3j, the peaks associated with the excitation of photoelectrons from the O 1s orbital to Co 3d-O 2p hybridized orbitals can be observed. More specifically, the highest peaks ~530.6 eV can be assigned to the O 2p hybridized with the eg of Co3+, while the peaks before 530 eV can be assigned to the O 2p hybridized with the eg and t2g of Co4+9,38. The presence of massive Co4+ in the LiPF6-based LCO electrode can be attributed to the formation of spinel phase from the collapse of LixCoO2, which can block the diffusion channel of Li+39, thus impeding the reversible redox of Co4+/Co3+ due to the lack of charge compensation. The structure deterioration of the LiPF6-based LCO electrode has been confirmed by the high- resolution transmission electron microscope (HRTEM). As can be seen in Supplementary Fig. 24, the original layer structure of the LCO electrode was transformed into a spinel phase in LiPF6-based electrolyte, while that is well preserved in LiNFBS-based electrolyte, further demonstrating the superiority of our LiNFBS-based electrolyte for stabilizing the high voltage LCO electrode.

CEI characterization and analysis
To disclose the lithium salt-induced difference in LCO cycling performance, X-ray photoelectron spectroscopy (XPS) was used to analyze the components of CEI on the LCO surface. From Fig. 4a, Supplementary Fig. 25 and Supplementary Tables S3–S6, we can see the distinct decomposed products of lithium salts. The F 1s spectra display the LiPF6-derived CEI shows the dominant proportion of LixPOyFz with little LiF (Fig. 4a and Supplementary Table S3)40,41, but the LiNFBS- derived CEI layer is composed of a large amount of LiF along with the partially decomposed -CFx (Fig. 4a and Supplementary Fig. 25a)41,42. The decomposition of lithium salts can also be validated by the Li 1s, P 2p and S 2p spectra (Supplementary Fig. 25b–d) because the corresponding lithium salt anion is the unique source of P or S element in the positive electrode and electrolyte. The CEI of LCO in contact with LiPF6 electrolyte with an ignorable amount of LiF cannot inhibit the decomposition of organic solvent, thus leading to the much stronger C–O signal in C 1s spectra (Supplementary Fig. 25a and Supplementary Table S4). The roughnesses and mechanical properties of the CEI on both LCO electrodes have been assessed by atomic force microscope (AFM), which indicates that the CEI on the LCO in LiPF6 electrolyte is uneven with significant variations in height (Supplementary Fig. 26a), and the organic-rich CEI shows a much lower Young’s modulus (Supplementary Fig. 26b). In contrast, LiNFBS-derived LiF-rich CEI exhibits a much higher Young’s modulus, enabling it to stabilize the LCO surface with a smoother morphology (Supplementary Fig. 26c, d). Therefore, the LiF-rich CEI enabled by the LiNFBS naturally endow the LCO electrode with prominent cycling stability at high voltage. Though no report claims the CEI can directly inhibit the positive electrode phase transition, lots of scientists report robust CEI or coating layer on the positive electrode surface cannot only impede the continuous side reactions between the positive electrode and electrolyte but also prevent the lattice oxygen release and cobalt dissolution43–46, thereby avoiding the irreversible formation of spinel phase to retain the integrity of positive electrode and restrain the phase transition to some extent. Additionally, we provide a different viewpoint on the CEI- induced inhibition of phase transition as follows. Assuming the conversion of the O3 phase to H1-3 and O1 phases (O3 ⇌ H1-3 ⇌ O1) is reversible, once the lattice oxygen in metastable H1-3 or O1 phase releases and subsequently reacts with electrolyte (H1-3/O1 + electrolyte → spinel + byproducts), the H1-3 or O1 phase will irreversible form spinel phase and promote the conversion of O3 phase to H1-3 or even O1 phase. Based on the above analysis, we can believe that our LiF-rich CEI can indirectly restrain the phase transition to a certain extent by stabilizing the LCO electrode surface, thus endowing the LCO electrode with a sterling cycling lifetime at a high voltage of 4.7 V.

Then, time-of-flight secondary-ion mass spectrometry (TOF-SIMS) was utilized as a complementary technique to XPS to further characterize the chemical composition of CEI and visualize its distribution in a 3D viewer. Then, we can deduce the CEI components from the mass spectra and visualize the relative content of CEI from their intensity. As presented in Fig. 4b, c, we can clearly observe the signal of F- fragments dominating from the surface to the bulk of CEI, which mainly originates from LiF. However, the signals of organic components such as CHO2- and C2HO- are sparsely distributed in the CEI layer. Furthermore, we can observe that the signal of S- from NFBS− is much stronger than that of Li2CO3- and LiCO3- from organic solvent, demonstrating that the LiF-rich CEI endowed by the LiNFBS inhibits the decomposition of organic carbonate solvent. Though the -SO3- derived CEI may also contribute to protection of the positive electrode43–46, its low content compared to LiF diminishes the actual effectiveness in CEI.

Though the Highest Occupied Molecular Orbital energy level obtained from density functional theory (DFT) calculation is highly associated with the oxidation potential of the molecule, their difference cannot be ignored47,48. Accurate redox potential can be obtained from the Nernst equation (ΔG = –nFEabs), where Eabs is the potential versus absolute vacuum scale and can be converted into Li+/Li scale by a conversion factor of 1.4 V47–49. The optimized molecular structures are presented in Supplementary Data 1–4. As the cycling voltage range of the Li||LCO batteries is restricted to 3.0–4.7 V, the Fermi level of the LCO electrode can be 3.0–4.7 V vs. Li+/Li. As shown in Fig. 4d and Supplementary Tables S7, and 8, NFBS− delivers the lowest oxidation potential (5.08 V vs. Li+/Li), which conforms to the Linear sweep voltammetry (LSV) results in Supplementary Fig. 2a, indicating the preferential decomposition of NFBS−. Based on the Gibbs free energy changes from DFT calculations, the energetically favorable decomposition pathways of NFBS− to CEI can be illustrated in Fig. 4e. With the 4.7 V voltage applied on the Li||LCO battery, the energy for oxidizing NFBS− (C4F9SO3- - e- = C4F9 + SO3) on the LCO electrode can decrease from 6.48 eV to 0.38 eV, which makes the energy gap easy to be surpassed. Then the Ox- from lattice oxygen or decomposed electrolyte can react with SO3 and C4F9 to form Li2SO4, C3F7, LiF, Li2CO3, CO2, etc. Additionally, C3F7 can be further degraded into a lower homologue (CxFy) or completely decomposed in a manner similar to C4F9. The downtrend of Gibbs free energy during the decomposition means the decomposition reactions are thermodynamically feasible, and the decomposition products were evidenced by XPS spectra and TOF- SIMS results (Fig. 4a–c and Supplementary Fig. 25). Therefore, we can firmly believe that LiNFBS-derived LiF-rich CEI endow the high LCO electrode with satisfactory stability.

LCO stability at high temperatures and with water
To meet the demand for high-temperature batteries in various applications, such as electronics used in summer or confined spaces with poor heat dissipation, improving the thermal stability of the electrolyte is a prerequisite for ensuring the battery performs well at high temperatures. However, the parasitic reactions between the positive electrode and electrolyte can be accelerated at elevated temperatures, exhausting the electrolyte and forming a thick passive layer on the positive electrode surface. Besides, LiPF6 is prone to decomposing into PF5 at elevated temperatures, which will further promote the decomposition of electrolytes and facilitate the side reactions at the positive electrode surface18–21. Consequently, realizing a stable cycling performance of the battery at high temperatures is very tricky, to say nothing of enabling a battery to stably work under both high temperature and high voltages simultaneously.

After evaluating the thermal stability of the electrolytes (Fig. 1c–g), the two electrolytes were used to check their feasibility in practical batteries under high voltage and high temperature (Supplementary Fig. 27). When we increase the temperature to 60 °C and elevate the cut-off voltage to 4.6 V (Fig. 5a–c), the LCO battery with LiPF6-based electrolyte quickly attenuates within 39 cycles in this harsh environment. Besides, when we rest the charged LCO batteries at 60 °C, the open circuit voltage of the LiPF6-based battery decreases quickly (Supplementary Fig. 28). Thanks to the satisfactory thermal stability of LiNFBS and its ability to form LiF-rich components to stabilize the positive electrode and negative electrode surfaces (Supplementary Fig. 29 and Supplementary Table S9), our LiNFBS-based electrolyte enables the battery to cycle 200 cycles with a capacity retention of 70% and a high Coulombic efficiency of 99.75% (Fig. 5b, c).

In the commercial production and storage of the commonly used lithium salt LiPF6, the humidity of the environment should be strictly controlled, and the water content of the prepared electrolytes should be less than 10 ppm, which notably raises the commercialization costs. Besides, LiPF6 can also react with organic solvents to produce water when the battery is exposed to hot weather or undergoes thermal runaway. The water will catalyze the decomposition of LiPF6 with the formation of PF5 and HF (LiPF6 → PF5 + LiF, PF5 + H2O → POF3 + 2HF), and the aggressive PF5, POF3 and HF are apt to attack the electrolyte and corrosive the positive electrode with the formation of additional water18–22, leading to vicious circle and the premature death of the battery. After checking the hydrolytic stability of the electrolytes in Fig. 1e–g, we further examined the cycling stabilities of the LCO batteries under a high cut-off voltage of 4.7 V with the two electrolytes in the presence of water. As shown in Fig. 5d–f, after adding 200 ppm water to the LiPF6-based electrolyte, the charging curves of the LCO battery fluctuate along with the emergence of the overcharging behavior (Fig. 5d), making the battery quickly expire with a low average CE of 61.23% (Fig. 5f). In sharp contrast, owing to the satisfactory anti-hydrolytic stability of LiNFBS and its originated LiF-rich CEI, despite adding 200 ppm water to the LiNFBS-based electrolyte, the LCO battery shows comparable cycling performance with the battery using the pristine LiNFBS-based electrolyte (Fig. 2c, and Supplementary Fig. 30). Even if exposing the LCO battery to the LiNFBS-based electrolyte with 1000 ppm water, the cycling lifetime of the LCO battery still shows negligible inferiority comparing with the battery using dry electrolyte (Figs. 2c, and 5e, f). It should be noted that we just intend to show the superiority of our lithium salt under harsh environments in this paragraph, and adding extra water to the real battery is not advisable because water can react with either Li metal electrode or lithiated graphite electrode50,51. Pretreating the electrolyte with molecular sieve can also lower the water content50,51, whereas our lithium salt (LiNFBS), with significantly higher thermal and hydrolysis stabilities, can lower production and preservation costs, as well as ensure the electrolyte withstands the hydrolysis of water originating from high temperature and parasitic reactions.

Methods
Materials
Lithium perfluorobutanesulfonate (LiNFBS, 99.6%) was purchased from Tokyo Chemical Industry, battery-grade LiPF6 (99.99%), fluoroethylene carbonate (FEC, >99%), ethyl methyl carbonate (EMC, 99.9%) and N-methylpyrrolidone (NMP, >99.5%, anhydrous) were purchased from Aldrich. Lithium bis(fluorosulfonyl)imide (LiFSI, 99.9%) and lithium trifluoromethylsulfonate (LiCF3SO3, 99.9%) were purchased from DoDoChem. Polyvinylidene difluoride (PVDF) and SuperP carbon black were purchased from Kejing Materials Technology Co., Ltd. The solvents were dried with molecular sieves for at least one week, and the electrolytes were prepared by dissolving the lithium salts into FEC/EMC (v/v=3:7) to obtain 1 M solution. LCO was purchased from Ronbay Technology. The LCO slurries were prepared with NMP and blended by an automatic mixer, and the weight ratio of the LCO powder, PVDF and SuperP in the LCO electrodes is 96.4:1.8:1.8. The mass loadings of the LCO electrodes in the coin cell (single-side coated) and pouch cell (double-side coated) are about 9.6 and 23.5 mg cm−2, respectively, and the tap densities of the LCO electrodes are 3.5 g cm−3. Fresh Li chips (99.9%; diameter: 15.6 mm, thickness: 450 μm) and Li-Cu composite foils (99.9% Li and 99.99% Cu; 50 μm Li on both sides of 6 μm Cu) were purchased from China Energy Lithium Co., Ltd. All the negative/positive electrodes and electrolyte solutions were sealed and used in an Argon-filled glove box (H2O or O2 purity: <0.1 ppm). Al2O3-coated polyethylene membrane (12 μm PE and 2 μm Al2O3; 60 mm width) and polypropylene membrane (Celgard 2500, 25 μm) were purchased from Guangdong Canrd New Energy Technology Co., Ltd, and the Al2O3-coated side was in contact with the negative electrode. In the Li||LCO pouch cell, 9 pieces of Li@Cu electrode (45×56 mm2) were stacked with 8 pieces of LCO electrodes (45×56 mm2), separated by the Al2O3-coated membrane. The storage of cell components and the assembly of cells were conducted at 25 ± 1 °C.

Characterizations
LSV, chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS) tests were performed on CHI760 electrochemical workstation. TG-DSC tests were carried out on HITACHI STA200 under N2 atmosphere, and the temperatures were increased from room temperature to 450 °C at a heating rate of 5 °C min−1. 19F-NMR spectra were recorded on Bruker 600 MHz, and fluorobenzene was used as the internal standard. In-situ XRD measurements were performed on D2 PHASER XE-T X-ray Diffractometer System with a scan speed of 2° (2θ) min−1, then the evolutions of (003) peak of the LCO electrodes can be tracked during the charging process of Li||LCO batteries at 0.2 C rate. In-situ DEMS tests were performed on QMG220 (Linglu Instrument Co. Ltd) to record the gas evolution during the charging process, and the charge current was 0.25 C, and the flow rate was 0.8 mL/min. Only XRD and DEMS tests were performed with in-situ methods, whereas the other characterizations were carried out with ex-situ methods. For ex-situ characterizations, the cycled coin-type Li||LCO and Li||Li cells were disassembled in an Argon-filled glove box to avoid the parasitic reactions with moisture and oxygen in air, then the cycled LCO electrodes and Li negative electrodes were washed with anhydrous dimethyl carbonate to remove the residual electrolytes. After drying the electrodes under vacuum, they were transferred to the specimen chamber of the characterization equipment with vacuum transfer chambers to prevent electrode contamination. The morphologies of the cycled LCO positive electrode and Li negative electrode were conducted on Quattro S SEM and an atomic force microscope (AFM, Bruker Dimension Icon). X-ray photoelectron spectra (XPS) tests were performed on Thermo Scientific Nexas with Al K-Alpha X-ray source, and C 1s spectrum at 284.8 eV was adopted to calibrate the spectra. The X-ray spot size was set to 400 µm, and the working voltage and current were 12 kV and 6 mA, respectively. High-resolution XPS was collected 5 times to improve the signal-to-noise ratio. HRTEM tests were conducted on FEI Helios 5. TOF-SIMS analysis of the detached positive electrode was carried out on ToF.SIMS 5–100 instrument (IONTOF GmbH) with high mass resolution mode, 30 keV Bi+ ion beam was used for depth profiling and 1 keV Cs+ ion beam was used for sputtering the cycled positive electrodes. The total sputtering time and area were 1200 s and 200 μm×200 μm, respectively, and the acquisition area was 50 μm×50 μm. Bruker EMX plus-6/1 was utilized for EPR spectra tests to detect the unpaired electron and defect in the LCO electrode. Soft X-ray absorption spectra tests were performed at Beamlines MCD-A and MCD-B (Soochow Beamline for Energy Materials) of the National Synchrotron Radiation Laboratory (NSRL) in the University of Science and Technology of China. The spectra data were acquired in total electron yield mode under vacuum and calibrated with NiO.

Electrochemical measurements
All the coin cells were CR2032-type, made with a 304 stainless steel case and spring, and assembled in an argon-filled glove box (H2O or O2 purity: <0.1 ppm). The storage, assembly and electrochemical measurements of cells were conducted at 25 ± 1 °C. The Coulombic efficiencies (CEs) for the Li||Cu cell are determined by the ratio of discharge capacity to charge capacity. In contrast, for the Li||LCO cell, the calculation is reversed. Potentiostatic polarization at 5.0 V was conducted by Li||Al cells to evaluate the electrochemical stability of the electrolyte on the Al foil at high voltage, and the potentiostatic polarization at 4.7 V was conducted by Li||LCO cells to determine the electrochemical stability of the electrolyte on the highly delithiated LCO electrode. Here, Li metals were used as negative electrodes and reference electrodes for LSV and CA tests, thus, the potential can be referenced to Li+/Li. The ionic conductivities of the electrolytes were measured using SS||SS coin cells (where SS denotes stainless steel disc with a diameter of 15.6 mm) by potentiostatic EIS with a voltage amplitude of 5 mV, calibrated with 0.1 M KCl in water52. Galvanostatic intermittent titration technique tests were conducted on LAND CT2001A with a current pulse at 0.2 C for 20 min and a relaxation time of 2 h, one data point was acquired every second during relaxation. Coulombic efficiencies (CEs) were evaluated by Li||Cu cells with Aurbach’s methods at 0.5 mA cm−253. External pressure of 1 MPa was applied on the Li||LCO pouch cell during test, and the current densities during activation and cycling process were 0.1 C and 0.2 C, respectively.

Density functional theory calculations and molecular dynamics simulations
All DFT calculations were carried out by Gaussian 16A software54. Structure optimizations were performed at the level of B3LYP-D3(BJ)/ 6-311g(d,p) under a solvation environment, and the solvation effects were quantified by SMD (solvation model based on density) solvation model with methyl ethanoate as the solvent and 22.7 as the dielectric constant55,56. High-accuracy electronic energies were calculated at the level of B2PLYP/ma-def2-TZVP, then we can get the redox potential from the Nernst equation (ΔG = –nFEabs), where Eabs is the potential versus absolute vacuum scale and can be converted into Li+/Li scale by a conversion factor of 1.4 V47–49. Classic molecular dynamics simulations were conducted on Gromacs package, version 2018.857. For the LiNFBS-based electrolyte, it contains 100 NFBS−, 100 Li+, 359 FEC, and 592 EMC, and the LiPF6-based electrolyte includes 100 PF6−, 100 Li+, 383 FEC and 629 EMC (See Supplementary Table S10 for details). The RESP2 charges of the optimized molecules were calculated by Gaussian at the level of B3LYP-D3(BJ)/def2-TZVP with the help of Multiwfn program58. FEC, EMC, NFBS− and PF6− were modeled by the general Amber force field, and Li+ was modeled by the Universal force field. After initial energy minimization at 298.15 K and 1 atm, 1000 ps with a timestep of 1 fs was used to equilibrate the simulation system, and 500 ps with a timestep of 1 fs was used for production runs. The particle-mesh Ewald method with a Coulomb cutoff of 1.0 nm was employed to account for the long-range electrostatic interactions, and Verlet cut-off scheme was used as the default buffer size. The radial distribution functions were calculated by Visual Molecular Dynamics software with a step size of 0.02 nm. The initial and final configuration of simulation results are shown in Supplementary Data 6–8.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "LiCoO2 (LCO), as the prevailing cathode material in portable electronic devices, has attracted lots of interest among scientists, and elevating the charging voltage of LCO is becoming one of the research hot topics to fulfill the demand for long-endurance electronics. To improve the high-voltage stability of the highly delithiated LCO cathode, the authors have developed a fluorine-rich lithium salt, lithium nonafluoro-n-butanesulfonate (LiNFBS), to form a sturdy surface with LiF-rich components. Therefore, the LCO battery shows excellent cycling stability at 4.7 V, and the authors also fabricated a 2 Ah Li||LCO pouch cell with LiNFBS to convince the reviewers and readers. Besides, the authors also demonstrate the compatibility of the LCO battery with high temperature (60 ˚C) and contamination of water, the content of this work is well-organized and logical. Based on my viewpoint, I believe that LiNFBS shows great promise as a lithium salt replacement for LiPF6, enabling high-temperature battery applications and benefiting the eco-friendly battery industry. Therefore, I would like to recommend this work to be accepted after addressing the following minor questions.\n1. The EPR signal of the LCO cathode after the 50th discharge in LiNFBS-based electrolyte seems to disappear, and the y-axis of Fig. 3i should be enlarged to show more details.\n2. The authors claim that NFBS--derived LiF-rich CEI can enable the LCO battery to work at 4.7 V or 60 ˚C, so can the LCO battery operate at 4.7 V and 60 ˚C simultaneously?\n3. Why do the LiNFBS-based LCO batteries with 0, 200 and 1000 ppm water show different charge profiles in the first cycles? And why do the LiNFBS-based LCO batteries with water deliver much higher discharge capacity than those with blank electrolytes?\n4. Why do the molecular numbers of two electrolytes differ in molecular dynamic simulations?\n5. DFT calculations should be carried out to confirm the molecular dynamic simulations which indicate that NFBS- has a higher coordination tendency with Li+.\n6. As shown in Fig.3a,c, why is the initial charge voltage of the LCO battery using LiNFBS much higher than that of the battery with LiPF6? Is the charge capacity of the two batteries consistent in in-situ XRD tests?\n7. More information about the CEI should be discussed to show the superiority of LiNFBS.", "reviewer_2": "Review for Manuscript NCOMMS-25-14418\nThe manuscript entitled “Covalent and fluorine-rich lithium salt for high-temperature, water-immune and ultrahigh-voltage LiCoO2“ by Qi Xiong et al. is focused on the study of the performance of LiCoO2 (LCO) cathode material by using two kinds of electrolytes consisting of LiPF6 salt and LiNFBS. Various electrochemical techniques, in situ-XRD, SEM, DEMS, XPS, EPR, NMR, XAS, TOF-SIMS, TG-DSC, as well as DFT calculations were applied to understand the reason of an increased stability of Li/LiNFBS/LCO battery at 4.7 V.\nThe reviewer’s comments:\nIn spite of high cost of cobalt, its scarcity, price volatility and ethical concerns, the topic undoubtedly has a high potential due to demand to develop high energy density and long-term stable Li ion batteries. The stable performance of the LCO battery at 4.7 V is a very good achievement, because commercial LCO-based batteries are limited by cut off of 4.2 V. Therefore, Reviewer could recommend the manuscript for publication in Nature Communications, but after major revision.\nFirstly, the experimental conditions should be clear specified. It is clear pointed that XRD was in situ performed, whereas the conditions (in situ or ex situ) for other techniques are not specified.\n- 30 kV of Bi+ ion beam (p.12, line 321) can lead to the change of composition and oxidation states at the surface that might impact the battery performance. The authors should comment the issue.\n- The potential should be specified with respect to a reference electrode across the manuscript and in relevant Figures.\nSecondly, the authors should consider the role of SOx, which is formed on the surface of LCO (Fig. 4c), for protection of the cathode against TM dissolution or/and oxygen release. See, for example, Adv. Energy Mater. https://doi.org/10.1002/aenm.202101864.\nThirdly, the authors added 200 ppm and 1000 ppm of water into LiNFBS electrolyte (p.11, lines 291-292). However, water impurities in the electrolyte should have a strong parasitic effect also on metallic lithium which is used as the anode, which might lead to its oxidation and, therefore, impact the battery performance. The authors should discuss the issue. Was the same amount of water added to LiPF6 based electrolyte?\n“During the production and preservation process of LiPF6, the requirement of an ultra-dry environment (less than 10 ppm) substantially exacerbates the cost of chemical engineering” (p.4, lines 75-77). Reviewer agrees only partially with the statement. However, there is a cheap way to remove water residuals from liquid electrolytes at laboratory conditions, namely to use molecular sieves as reported in the article: Angew. Chem. Int. Ed. 2021, 60, 15572. https://doi.org/10.1002/anie.202104124. A short relevant discussion and citation of the article is needed.\nRegarding the discussion and conclusion about extraction of residual lithium salts by water, and recycling of batteries (p. 4, lines 77-80, 104, 105, 279-280 and 294-295). This should be supported by additional experiments. Reviewer supposes that the relevant discussion and conclusion are above the main scope of the manuscript. Therefore, they can be deleted that will make the main message more clear for the readers.\np.8, lines 198-202. Reviewer is not convinced about the assignment of the C peak at 529.3 eV to Co4+ in the O K edge (Fig. 3j). This peak can be assigned to oxygen loss. This might correlate with CO2 evolution which is higher for the LCP/LiPF6 system (Fig. 3g). The authors also assign the C peak with Co2O3. However, the spinel structure is formed from Co2+/Co3+ mixed oxidation states, but not from Co4+. Further, the peak A is shifted after cycling. What is the reason? The authors should plot the O K edge in an extended range to see the contribution of other oxygen related compounds to the spectrum.\nDid XRD confirm the spinel structure after cycling? To make these contradiction statements clear, Co L-edge XAS of LCO before cycling and after cycling (in accordance with O K-edge XAS) need to be shown.\nAll photoelectron peaks should be identified in the fitted spectra (see for example, Fig.4a,4b,4c). Fitting of the photoelectron spectra should be done in a proper way: the FHWM of the C-O component (Fig. 4d) is more broad compared to other peaks in the C1s spectra. Also, some fitted components of the photoelectron spectra show very close binding energies. To make clear for the readers, the binding energies of the photoelectron peaks (Fig.4a-d, Figs.S17, S20 should be specified/plotted whether across the manuscript, or in SI.\np.9, lines 220- 224. Recent works reported on inhibition of the phase transition, oxygen release and transition metal dissolution via the surface treatment or coating of high-voltage cathode materials should be cited:\nJoule https://doi.org/10.1016/j.joule.2021.06.014;\nAdv. Sci. https://doi.org/10.1002/advs.202413054;\nElectrochim. Acta. https://doi.org/10.1016/j.electacta.2024.143862;\nACS Appl. Energy Mater. https://doi.org/10.1021/acsaem.4c02930\np.9, lines 214-216. It is not clear from the manuscript and Fig.S15: Why NFBS should be decomposed? The authors probably need to consider HOMO and LUMO of NFBS together with the energy levels of LCO to make the conclusion about the electrolyte decomposition at the LCO surface. How does it follow from Figure S15 that LiF is specifically formed due to the NFBS decomposition? What about other elements which constitute the molecule?\np.3, line 42. The sense should be improved: a) the authors probably mean “working lifetime of batteries”, because “working lifetime of electronics” might depend on various factors; b) “improving the charge voltage” -> the authors probably mean “improving the stability (or performance) at high charge voltage”.\np. 4, lines 81-82. “fluorine-rich salt anion is only composed of covalent bonds” -> something wrong with the determination. Reviewer suggests the authors probably mean “atoms form covalent bonds in the salt”, or something like that.\np.4, line 83. “ultrahigh voltage”\np.4, lines 83-84. “Owing to the fluorine-rich nature of LiNFBS, it can endow the CEI with abundant LiF components”. This statement is not obvious. The authors should cite the relevant articles.\np.4, lines 88-89. “decomposition”. Do the authors mean “thermal decomposition”?\np.4, line 89. “empower LiNFBS to coexist with water peacefully”. How was this confirmed?\np.5, line 112. “two lithium salts”. -> the salts should be specified.\np.5, line 115. “1M lithium salt” -> the kind of salt should be specified.\np.6, line 126. “proving the superiority of the LiNFBS” -> the authors probably mean superior both thermal stability of LiNFBS and its stability with respect to water residuals.\np.6, line 137. “ultrahigh-voltage“ -> the value of voltage should be specified.\np.6, line 139. The reason of challenging to fix on 4.7 V should be explained.\np.7, line 162. “about 50 cycles (Fig. 2e). -> The maximal amount of cycles plotted in Fig.2, is 30 cycles. 50 cycles is shown in Fig. 2f.\np.9, lines 216-218. “the CEI in LiPF6 electrolyte”. The CEI is formed between the LCO surface and electrolyte, but not in LiPF6. Therefore, the sentence should be rewritten in a proper way.\nMinor remarks:\nAbstract (line 28): LCO should be decoded."}, "author_response": "Thanks for your constructive suggestion and kind recommendation, your suggestions and comments are very helpful for us. We have carefully revised the manuscript according to your advices, and sincerely hope for its acceptance for publication.\n\nComment 1. Firstly, the experimental conditions should be clear specified. It is clear pointed that XRD was in situ performed, whereas the conditions (in situ or ex situ) for other techniques are not specified.\nResponse: Thanks for your suggestions. In this work, only XRD and DEMS tests were performed with in-situ methods, whereas the other characterizations were carried out with ex-situ methods.\nSupplementary notes have been added to the experimental section of the revised manuscript.\nReviewer comment R1: Still it is not clear what the authors mean “ex-situ”? Were the samples exposed to air before the XPS measurements? XPS is the surface sensitive technique (~10 nm analyzing depth). Exposure of samples to air might strongly change the surface composition of CEI. Therefore, the conclusion about the CEI properties can be erroneous. The description, conditions, setup of the XPS measurements are not provided in the experimental part. In addition, it is not shown neither in the main text or in captions to Figures, which the XPS spectra were measured before the battery cycling and which the spectra after battery cycling, or all the spectra were measured on cycled cathodes.\n\nComment 2. 30 kV of Bi+ ion beam (p.12, line 321) can lead to the change of composition and oxidation states at the surface that might impact the battery performance. The authors should comment the issue.\nResponse: We thank the reviewer for the question. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis in my work was conducted in an ex-situ manner. Specifically, the coin cells after the cycling tests were disassembled to detach the cathode, and the obtained cathode was then used to check the surficial chemical composition with TOF-SIMS. A 30 kV Bi+ ion beam was employed to etch the diamantine chemical components, such as LiF, Li CO , and Li O, allowing us to obtain the spatial distribution of the CEI components along the cathode surface. Similar methods can also be used in other reports (Ref: Nat. Energy, 2022, 7, 946-954; Nat. Energy, 2021, 6, 495-505; Nat. Nanotechnol., 2018, 13, 715-722). Although the Bi+ ion beam may damage the structure or alter the oxidation state of the LCO cathode, the cathode was not reused for any electrochemical tests after TOF-SIMS characterization. Therefore, the post-mortem analyses of the cathode will not impact the accuracy of our battery performance.\nWe apologize for the unclear description and have supplemented more details to the experimental sections.\nReviewer comment R2: Reviewer is not convinced with the approach which relies on spatial distribution of elemental composition measured after the Bi ion bombardment of the cathode. Besides preferential removal of oxygen by the bombardment that would lead to reduction of Co ions, the surface formed by particles of the cathodes is commonly a very rough. Therefore, etching of the upper surface layers might will not change the surface areas where particles touch each other deeper from the surface plain, and this area can be also visible by XPS. Thus, XPS sees the contribution from both the sputtered and unchanged areas. This Reviewer´s comment is not so much to battery performance, but rather to the interpretation of the CEI properties based on which the authors explain the improved battery performance.\n\nComment 3. The potential should be specified with respect to a reference electrode across the manuscript and in relevant Figures.\nResponse: We appreciate the reviewer for this valuable suggestion. All the reference electrodes in LSV and CA tests were lithium metal, so the electrode potentials were referenced to Li+/Li, and additional remarks have been included in the corresponding figure captions (pages 3 and 19 in the Supplementary Information) and experimental sections (page 14 in the main text).\nReviewer comment R3: the response is accepted.\n\nComment 4. Secondly, the authors should consider the role of SOx, which is formed on the surface of LCO (Fig. 4c), for protection of the cathode against TM dissolution or/and oxygen release. See, for example, Adv. Energy Mater. https://doi.org/10.1002/aenm.202101864.\nResponse: We thank the reviewer for this insightful question. We nearly neglected the role of LixSOy, which may also contribute to the protection of the cathode against TM dissolution and/or oxygen release, so the reference (https://doi.org/10.1002/aenm.202101864) has been included in the reference list. For our lithium salt, LiNFBS (C4F9SO3Li), it contains nine F atoms but one -SO3- group, which could yield 9 LiF and 1 LixSOy if it can be fully decomposed. From the TOF-SIMS results (Fig. R8 or Fig. 4b,c), abundant F-, LiF- and LiF2- signals derived from decomposed -C4F9 can be observed, which can be indexed to the CEI components of LiF. In contrast, the signal from -SO3- derived CEI components is rather weak compared to that of LiF. Based on previous reports, indicating that LiF is a good component for suppressing the TM dissolution or/and oxygen release (Ref: Nat. Nanotechnol., 2018, 13, 715-722; Nat. Commun., 2018, 9, 4918; Angew. Chem. Int. Ed., 2022, 61, e202202731; Nat. Commun. 2023, 14, 2211). Although the formed SOx may be conducive to the inhibition of TM dissolution or/and oxygen release, its relatively low content will diminish the actual efficacy.\nThe reference has also been mentioned and discussed in the main text; please see the highlighted section on page 10.\nReviewer comment R4: the response is accepted.\n\nComment 5. Thirdly, the authors added 200 ppm and 1000 ppm of water into LiNFBS electrolyte (p.11, lines 291-292). However, water impurities in the electrolyte should have a strong parasitic effect also on metallic lithium which is used as the anode, which might lead to its oxidation and, therefore, impact the battery performance. The authors should discuss the issue. Was the same amount of water added to LiPF based electrolyte?\nResponse: We appreciate the reviewer for this instructive comment. Adding water to the electrolyte is not our primary purpose; we have merely tested the stability of the water-contaminated electrolyte in a real coin cell. It is important to note that water is highly reactive with Li metal, which can automatically produce LiOH and H (2Li + 2H O = 2LiOH + H ). Even if graphite is used as the anode, the water can still react with lithiated graphite (2Li C + 2xH O = 2C + 2xLiOH + xH ; Ref: J. Power Sources, 2015, 287, 253-260). Therefore, we concur with the reviewer that adding water to the battery electrolyte is not advisable, as it does not provide any benefits to the battery. However, in certain urgent situations, such as thermal runaway or overheating, LiPF6 tends to decompose into PF5 and POF3, which can easily attack the organic solvent to form water and HF (Ref: Nat. Mater., 2022, 21, 455-462; J. Power Sources, 2013, 242, 832-837; ACS Energy Lett., 2021, 6, 2096-2102). These by-products are highly corrosive and will further accelerate the decomposition of the electrolyte, releasing significant amounts of heat into the battery and leading to rapid battery decay, which poses a substantial risk to the electronics. Therefore, utilizing a lithium salt with excellent thermal and hydrolytic stability can significantly enhance the safety of the battery.\nIn my work, we do not test the LCO battery using LiPF6-based electrolyte with 1000 ppm water because significant overcharge of the LCO battery with only 200 ppm of water was observed due to severe electrolyte decomposition (Fig. 5d,f). Interestingly, the cycling lifetime is well preserved for the LCO battery using LiNFBS-based electrolyte with 200 ppm water (Fig. 5f and Fig. S28). Even when 1000 ppm water was added, the cycling times only decreased from 360 to 336 at a capacity cut-off of 70% (Fig. 5e,f). Due to the high stability of LiNFBS toward water, the contaminated water will react with Li metal or be oxidized at the cathode surface, as the theoretical oxidation potential of water is 4.27 V vs. Li+/Li (1.23 V vs. SHE). The charge profiles of the LCO battery with and without 200 ppm water were plotted below (Fig. R9). A large charge overpotential can be seen in the first charge profile of the LCO battery with 200 ppm water, which can be supposed to the chemical and electrochemical decomposition of water. After exhaustion of water and passivation of the electrodes, the subsequent cycling profiles return to normal (Fig. R9 or Fig. S28).\nWe apologize for the imprecise description, and to avoid misleading readers, our intention has also been clarified and discussed in the revised manuscript. Please see the highlighted part on page 12 in the main text.\nReviewer comment R5: the response is accepted.\n\nComment 6. \"During the production and preservation process of LiPF , the requirement of an ultra-dry environment (less than 10 ppm) substantially exacerbates the cost of chemical engineering\" (p.4, lines 75-77). Reviewer agrees only partially with the statement. However, there is a cheap way to remove water residuals from liquid electrolytes at laboratory conditions, namely to use molecular sieves as reported in the article: Angew. Chem. Int. Ed. 2021, 60, 15572. https://doi.org/10.1002/anie.202104124. A short relevant discussion and citation of the article is needed.\nResponse: We thank the reviewer for this good suggestion. Removing trace amounts of water from the electrolyte using molecular sieves is a cost-effective method to ensure an ultra-dry environment for the battery, which we have already implemented and detailed in the experimental sections of our manuscript. In the mentioned literature (Ref: Angew. Chem. Int. Ed. 2021, 60, 15572), the authors used the molecular sieve as a coating layer on the cathode to sieve the solvation sheath of lithium ions, thereby enhancing the anti-oxidation stability of the electrolyte under high voltage. Despite the coating layer can slightly reduce the energy density of the battery, it can undoubtedly be used as an alternative solution to simultaneously lower down the water content and improve the cathode stability. For these consideration, the mentioned report has been cited into the main text to inspire more readers. In my work, molecular sieves were solely utilized to eliminate the residual water content in the electrolyte. In comparison to LiPF6, LiNFBS, which possesses significantly higher thermal and hydrolysis stabilities, can lower production and preservation costs and provide another pathway to ensure the electrolyte withstands the hydrolysis of water originating from high temperature and parasitic reactions.\nThe reference has also been mentioned and discussed in the main text; please see the highlighted section on page 12.\nReviewer comment R6: the response is accepted.\n\nComment 7. Regarding the discussion and conclusion about extraction of residual lithium salts by water, and recycling of batteries (p. 4, lines 77-80, 104, 105, 279-280 and 294-295). This should be supported by additional experiments. Reviewer supposes that the relevant discussion and conclusion are above the main scope of the manuscript. Therefore, they can be deleted that will make the main message more clear for the readers.\nResponse: We appreciate the reviewer's kind suggestion. Although LiNFBS with high hydrolysis stability may hold great promise for recycling, we currently lack direct experimental evidence in the context of battery recycling. Given that substantial work is needed to support our claim, we would like to heed your advice and remove the relevant content from the manuscript.\nReviewer comment R7: the response is accepted.\n\nComment 8. p.8, lines 198-202. Reviewer is not convinced about the assignment of the C peak at 529.3 eV to Co4+ in the O K edge (Fig. 3j). This peak can be assigned to oxygen loss. This might correlate with CO evolution which is higher for the LCO/LiPF system (Fig. 3g). The authors also assign the C peak with Co O . However, the spinel structure is formed from Co2+/Co3+ mixed oxidation states, but not from Co4+. Further, the peak A is shifted after cycling. What is the reason? The authors should plot the O K edge in an extended range to see the contribution of other oxygen related compounds to the spectrum.\nResponse: We appreciate the valuable comment by the reviewer. The O K-edge XAS, arising from the emission of an electron from the O 1s to O 2p hole, can be easily influenced by the hybridized state of O 2p and Co 3d orbitals. For the pristine LCO (LiCoO ) cathode, the valence of Co is +3 and there are 6 electrons in the split Co 3d orbitals (t and e ), so most of the Co3+ exist in a low-spin state (t6 e0 ) with the lowest energy (Ref: Phys. Rev. Lett. 2013, 111, 056404; Chem. Phys. Lett. 1999, 309, 14). Then, the main peak (peak B, Fig. R10) can be assigned to the final state of [O 1s1c + Co 3d7(t62ge1g)], where c means the hole in O 1s. As for the peak C (Fig. R10), it can be assigned to the hybridized state of O 2p with the t2g and eg states of Co4+ (Ref: Phys. Rev. Lett. 2013, 111, 056404; Chin. Phys. B 2018, 27, 107802; ACS Energy Lett. 2023, 8, 4806-4817; Adv. Energy Mater. 2020, 10, 2001413). The accumulated Co4+ could originate from the restriction of electron transport by the structural distortion (Ref: Adv. Energy Mater. 2020, 10, 2001413). The sXAS of CoO has also been tested and incorporated into Fig. 3j, resembling that of LiCoO2. This similarity can be attributed to the same electron excitation, from O 1s to the hybridized state of O 2p with the eg of Co 3d orbital. Therefore, we cannot differentiate CoO and LiCoO2 based on the O K-edge spectra. Nevertheless, the significant alteration of peak C can illustrate the superiority of LiNFBS in preserving the structural integrity of the LCO cathode.\nThe small shoulder peak (peak A, Fig. R10) in the sXAS of the pristine LCO could be attributed to the trace amounts of metal oxide coated on the LCO surface, which can also be found in previous reports but has not been clearly interpreted or reached a consensus (Ref: Chin. Phys. B 2018, 27, 107802; Adv. Energy Mater. 2020, 10, 2001413; ACS Energy Lett. 2023, 8, 4806). However, the increase of the shoulder peak (peak A) was assigned to the oxidized oxygen with higher energy (Ref: J. Power Sources 2007, 174, 938-943), or the interfacial reactions (Ref: Chin. Phys. B 2018, 27, 107802), therefore it is really difficult to determine the actual change in electronic configuration and we could not explain the reason for the shifting of peak A. The XAS with extended range can be seen in Fig. R10, the exciting range of 536-546 eV can be assigned to the hybridization of O 2p and Co 4sp state, which has been rarely discussed in previous reports, so that we cannot give any compelling explanations.\nWe apologize for some inappropriate interpretations of the sXAS data, and the revised analysis of the sXAS data has been summarized and updated in the main text (page 8). To avoid any misreading by readers, the excitation energy in the x-axis was limited to 532 eV (Fig. 3j). Besides, the irreversibly formation of spinel phase in LiPF6 electrolytes has been evidenced by HRTEM images (Fig. S22), please see Response to Comment 9 for details.\nReviewer comment R8: Reviewer does not concur with the interpretation of the O K XAS and the statement provided in the revised version (p.8; lines 206-207). Firstly, the O K XAS of LCO presented in the cited article (Ref. 36) were measured also at Li-content below x=0.5, where the lattice oxygen is involved in charge compensation, which leads to a hole transfer to the lattice oxygen. Depending on the preparation of LCO cathode, the oxygen involvement might occur at x>0.5. It is written in Ref. 36:…. Li CoO reveals that holes with strong O 2p character (the abstract) and… the Co4+ state has more O 2p holes than the Co3+ state… (p.4).\nThus, “the quick collapse of Li CoO ” is not “the presence of massive Co4+ in the LiPF -based LCO cathode”, stated by the authors of the manuscript, but a hole transfer to the oxygen site, which is abundant in “LiPF -based LCO cathode”.\nReviewer does not concur with the statement “…we cannot differentiate CoO and LiCoO based on the O K-edge spectra”. The shape of the O K XAS of these oxides is different. If the authors mean CoO instead CoO, one notes that CoO is unstable.\nIn addition, Reviewer has the same issue raised by his comment 1: were the samples exposed to air before the XAS measurements? There is no description about the experimental setup of XAS measurements, as well as the XAS modes. Were the O K and Co L XAS measured after the Bi ion bombardment? The XAS measured in the partial electron- and total electron- yield modes (PEY and TEY, respectively) are the surface sensitive techniques.\n\nComment 9. Did XRD confirm the spinel structure after cycling? To make these contradiction statements clear, Co L-edge XAS of LCO before cycling and after cycling (in accordance with O K-edge XAS) need to be shown.\nResponse: Thank you for this insightful comment. For a general XRD, the detection limit is about 5%. Due to the poor crystallinity of the spinel phase in the layered LCO cathode, XRD is difficult to determine the presence of trace and amorphous spinel phases. Due to the limited resources for XAS testing, we are unable to obtain sufficient data of the Co L-edge XAS within a limited time. Instead, a high-resolution transmission electron microscope (HRTEM) was employed to check the crystal structures of the charged LCO cathodes. As shown in Fig. R11 (Fig. S22), the LCO cathode after being charged in LiNFBS-based electrolyte can retain the layered structure, while that in LiPF6-based electrolyte turns into a spinel phase, indicating the LiNFBS-derived LiF-rich CEI can well retain the structural integrity of the highly delithiated LCO cathode.\nFig. R11 has been placed on page 24 in Supplementary Information, and the corresponding discussions have been summarized on page 8 in the main text.\nReviewer comment R9: What the authors mean “amorphous spinel phases”? Reviewer supposes that spinel possesses by crystal structure. Why HRTEM was not used to check crystal structure before cycling of the cathodes? It would gain an additional insight into the reason of the cathode stability by analyzing the changes in the atomic arrangement before and after cycling.\n\nComment 10. All photoelectron peaks should be identified in the fitted spectra (see for example, Fig.4a,4b,4c). Fitting of the photoelectron spectra should be done in a proper way: the FHWM of the C-O component (Fig. 4d) is more broad compared to other peaks in the C1s spectra. Also, some fitted components of the photoelectron spectra show very close binding energies. To make clear for the readers, the binding energies of the photoelectron peaks (Fig.4a-d, Figs.S17, S20 should be specified/plotted whether across the manuscript, or in SI.\nResponse: We appreciate the reviewer's kind suggestions. The fitting data of XPS have been revised carefully with comparable FHWM, and the revised results don't affect our conclusions. The re-fitted results of C 1s XPS spectra can be seen in Table R1 (Table S4) and Fig. R12 (Fig. 4a and Fig. R22a), while the other fitting peaks have been placed in Supplementary Information (Table S3,S5,S6).\nReviewer comment R10: the response is accepted.\n\nComment 11. p.9, lines 220- 224. Recent works reported on inhibition of the phase transition, oxygen release and transition metal dissolution via the surface treatment or coating of high-voltage cathode materials should be cited:\nJoule https://doi.org/10.1016/j.joule.2021.06.014;\nAdv. Sci. https://doi.org/10.1002/advs.202413054;\nElectrochim. Acta. https://doi.org/10.1016/j.electacta.2024.143862;\nACS Appl. Energy Mater. https://doi.org/10.1021/acsaem.4c02930\nResponse: We thank the reviewer for this good suggestion. The above works have been cited in the revised manuscript to broaden our research interest.\nReviewer comment R11: the response is accepted.\n\nComment 12. p.9, lines 214-216. It is not clear from the manuscript and Fig.S15: Why NFBS should be decomposed? The authors probably need to consider HOMO and LUMO of NFBS together with the energy levels of LCO to make the conclusion about the electrolyte decomposition at the LCO surface. How does it follow from Figure S15 that LiF is specifically formed due to the NFBS decomposition? What about other elements that constitute the molecule?\nResponse: We greatly appreciate the reviewer's insightful comments and suggestions. Though the energy levels of HOMO and LUMO are highly associated with the oxidation and reduction potentials under vacuum, respectively, there are nonnegligible gaps between the practical redox potentials in solvent and the energy levels calculated from frontier energy theory (Ref: J. Electrochem. Soc., 2007, 154, A810-A815; Energy & Environ. Sci., 2018, 11, 2306-2309; J. Phys. Chem. C, 2013, 117, 8661-8682). To get more accurate redox potentials of the electrolyte components, additional DFT calculations of the reduced and oxidized states were carried out to get the Gibbs free energy changes, and then we can get the redox potentials from the Nernst equation (ΔG = -nFEabs). Eabs is the potential versus absolute vacuum scale, which can be converted into the Li+/Li scale by a conversion factor of 1.4 V (Ref: J. Phys. Chem. C, 2013, 117, 8661-8682; Pure Appl. Chem., 1986, 58, 955-966). As the cycling voltage range of the Li||LCO batteries is restricted to 3.0-4.7 V, the Fermi level of the LCO cathode can be 3.0-4.7 V vs. Li+/Li.\nAs shown in Table R2 (Table S7) and Fig. R13 (Fig. 4d), NFBS- possesses the highest reduction potential (1.50 V) and the lowest oxidation potential (5.08 V vs. Li+/Li), which aligns well with the LSV tests (Fig. R13 or Fig. S2a&S17). Therefore, electrons in Li metal with high energy can spontaneously transfer to NFBS-. Although the oxidation potential of NFBS-, Eox (NFBS-), is slightly higher than the electrochemical potential (Fermi energy) of LCO (3.0-4.7 V vs. Li+/Li), the low energy barrier of 0.38 eV allows it to be easily surmounted, indicating that NFBS- is preferentially oxidized. Based on the DFT calculations and the XPS and TOF-SIMS experiments (Fig. 4a-c and Fig. S23), we can deduce the decomposition pathways of NFBS-, which are detailed in Table R3 (Table S8). Then the Ox- from lattice oxygen or decomposed electrolyte can react with SO3 and C4F9 to form Li SO , C F , LiF, Li CO , CO , etc. Finally, C F can be further degraded into a lower homologue (C F ) or completely decomposed in a manner similar to C F . The downtrend of Gibbs free energy during the decomposition means the decomposition reactions are thermodynamically feasible, and the decomposition products were evidenced by XPS spectra and TOF-SIMS results (Fig. 4a-c and Fig. S23). Combining the DFT calculations with experimental observations, we can firmly believe that LiNFBS-derived LiF-rich CEI endows the ultra-high LCO cathode with superior stability.\nAll of the data and notes have been supplemented into the main text (Fig. 4d,e) and Supplemental Information (Table S7,8).\nReviewer comment R12: The answer related to DFT calculations is accepted. Reviewer supposes that the CEI was measured after the Bi ion bombardment according to the authors answers on the Reviewer´s comments. However, in this case the ratio of the LiF intensities from the CEI of LiNFBS and LiPF electrolytes (Fig. R12) cannot be reliable, while the conclusion about LiF-rich CEI is speculative (see my comment R2).\n\nComment 13. p.3, line 42. The sense should be improved: a) the authors probably mean \"working lifetime of batteries\", because \"working lifetime of electronics\" might depend on various factors; b) \"improving the charge voltage\" -> the authors probably mean \"improving the stability (or performance) at high charge voltage\".\nResponse: We thank the reviewer for the kind reminder. \"working lifetime of electronics\" has been corrected to \"working lifetime of batteries\", and \"improving the charge voltage\" has been revised to \"improving the stability at high charge voltage\".\nReviewer comment R13: the response is accepted.\n\nComment 14. p. 4, lines 81-82. \"fluorine-rich salt anion is only composed of covalent bonds\" -> something wrong with the determination. Reviewer suggests the authors probably mean \"atoms form covalent bonds in the salt\", or something like that.\nResponse: We thank the reviewer for the insightful suggestion. Since lithium salt consists of Li+ and anion, changing the sentence \"fluorine-rich salt anion is only composed of covalent bonds\" to \"atoms only form covalent bonds in the salt anion\" may appear more rigorous.\nReviewer comment R14: the response is accepted.\n\nComment 15. p.4, line 83. \"ultrahigh voltage\".\nResponse: We thank the reviewer for the kind reminder. \"ultrahigh voltage\" has been specified as the voltage exceeding 4.6 V.\nReviewer comment R15: the response is accepted.\n\nComment 16. p.4, lines 83-84. \"Owing to the fluorine-rich nature of LiNFBS, it can endow the CEI with abundant LiF components\". This statement is not obvious. The authors should cite the relevant articles.\nResponse: We thank the reviewer for the comment. DFT calculations and the detailed decomposition pathway have been discussed in the response to comment 12, and we have also revised related contents in the main text (page 10) and Supplemental Information (pages 37,38).\nReviewer comment R16: Reviewer is not convinced with the conclusion (see the Reviewer comments R2 and R12).\n\nComment 17. p.4, lines 88-89. \"decomposition\". Do the authors mean \"thermal decomposition\"?\nResponse: We thank the reviewer for the comment. Decomposition can be thermal decomposition or chemical decomposition, and the \"decomposition\" means \"thermal decomposition\" here, so we have revised it.\nReviewer comment R17: the response is accepted.\n\nComment 18. p.4, line 89. \"empower LiNFBS to coexist with water peacefully\". How was this confirmed?\nResponse: We thank the reviewer for the comment. As shown in Fig. 1e, after storing the LiNFBS-based electrolytes with 1000 ppm H O for 4 days, no colour change has been observed. However, the LiPF6-based electrolytes turned dark brown. Then, 19F nuclear magnetic resonance (19F-NMR) spectra were employed to track the evolution of the lithium salts in electrolytes. From Fig. 1f, we can clearly see that the signals of PF6- were weakened and nearly disappeared after thermolysis and hydrolysis, respectively. Besides, the peaks of O=PFxOyz- were intensified, wherein the strong peaks at -73.6 ppm belonging to O=PF(OH)2 emerged, and some small peaks at -76~-82 ppm can be assigned to the signals of decomposed solvents, such as O=PF(OC2H5)2 and OPF2(OC2H5) (Ref: Nat. Mater. 2022, 21, 455-462; J. Electrochem. Soc. 2005, 152, A2327). However, nearly no difference can be observed in 19F-NMR spectra for the LiNFBS-based electrolyte after hydrolysis (Fig. R16 or Fig. 1g), unambiguously proving the hydrolysis stability of the LiNFBS.\nReviewer comment R18: This explanation and cited articles should be added to the manuscript.\n\nComment 19. p.5, line 112. \"two lithium salts\". -> the salts should be specified.\nResponse: We thank the reviewer for this positive comment. The two salts have been specified.\nReviewer comment R19: the response is accepted.\n\nComment 20. p.5, line 115. \"1M lithium salt\" -> the kind of salt should be specified.\nResponse: We thank the reviewer for this kind comment; the two salts have been specified.\nReviewer comment R20: the response is accepted.\n\nComment 21. p.6, line 126. \"proving the superiority of the LiNFBS\" -> the authors probably mean superior both thermal stability of LiNFBS and its stability with respect to water residuals.\nResponse: We thank the reviewer for this kind comment; these words have been amended to \"proving the thermolytic and hydrolytic superiority of the LiNFBS\".\nReviewer comment R21: the response is accepted.\n\nComment 22. p.6, line 137. \"ultrahigh-voltage \"-> the value of voltage should be specified.\nResponse: We thank the reviewer for this kind suggestion. For the LCO cathode, the high voltage range is greater than 4.2 V but not exceeding 4.6 V, and ultrahigh voltage usually exceeds 4.6 V. Additional comments have been supplemented in the main text.\nReviewer comment R22: the response is accepted.\n\nComment 22. p.6, line 139. The reason of challenging to fix on 4.7 V should be explained.\nResponse: Thanks for your insightful question. Pushing the charging voltage to 4.8 V can further release the capacity of LCO, but the tremendous O1 phase above 4.7 V can irreversibly ruin the cathode structure, which is still a great challenge even now. As shown in Fig. R17, the 4.8 V Li||LCO battery expires within 50 cycles, which is not suitable for practical utilization. So far, realizing a stable 4.8 V Li||LCO battery is a big challenge.\nReviewer comment R22: This explanation should be added to the main text or to Supplementary Information, because this helps to understand for a broad community of the readers of Nature Communications the challenges in commercial realization of high-voltage batteries.\n\nComment 23. p.7, line 162. \"about 50 cycles (Fig. 2e). -> The maximal amount of cycles plotted in Fig.2, is 30 cycles. 50 cycles is shown in Fig. 2f.\nResponse: Thanks for your kind reminder. The discharge-charge profiles of 40th and 50th cycles have been added in Fig. 2e, you can also see Fig. R18.\nReviewer comment R23: the response is accepted.\n\nComment 24. p.9, lines 216-218. \"the CEI in LiPF6 electrolyte\". The CEI is formed between the LCO surface and electrolyte, but not in LiPF6. Therefore, the sentence should be rewritten in a proper way.\nResponse: We thank the reviewer for the careful review. \"the CEI in LiPF6 electrolyte\" has been amended to \"the CEI of LCO in LiPF6 electrolyte\".\nReviewer comment R24: A more appropriate term is “in contact with”, i.e. “the CEI of LCO in contact with LiPF electrolyte”.\n\nComment 25. Minor remarks: Abstract (line 28): LCO should be decoded.\nResponse: We thank the reviewer for the kind reminder. LCO has been decoded into LiCoO2.\nReviewer comment R25: the response is accepted.\n\nReviewer comments in conclusion: although Reviewer agrees with the authors that the main reason in the improvement of the battery performance is the CEI properties, the main claim of Reviewer is not related to the excellent performance of the LCO batteries cycled to 4.7 V, but rather to the interpretation of the experimental results, on the basis of which the explanation of the battery performance is deduced. Therefore, the manuscript cannot be published in Nature Communications, until the authors will improve the manuscript according to the Reviewer comments."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The manuscript can be published in Nature Communications because the authors have addressed my concerns."}, "author_response": ""}]

Decision letter