Electrolytic construction of nanosphere‐assembled protective layer toward stable lithium metal anode

The uncontrolled dendrite growth and electrolyte consumption in lithium metal batteries result from a heterogeneous and unstable solid electrolyte interphase (SEI). Here, a high‐voltage forced electrolysis strategy is proposed to stabilize the lithium metal via electrodepositing a spherical protective layer. This peculiar SEI is composed of a nanosized Li sphere that is encased with adjustable composition, as proved by cryo‐transmission electron microscopy and multiple surface‐sensitive spectroscopies. Such a three‐dimensional nanosphere‐assembled protective layer has homogeneous components, mechanical strength, and rapid Li‐ion conductivity, enabling it to alleviate the volume expansion and prevent dendrite growth during Li deposition. The symmetric cell can be stably operated for ultralong‐term cycling time of 2000 and 800 h even at high current densities of 1 and 10 mA cm−2, respectively. Using this interface permits stable cycling of full cells paired with LiFePO4 and LiNi0.8Co0.1Mn0.1O2 cathodes with low negative/positive capacity ratio, high current density, and limited Li excess. This tactic also fosters a novel insight into interface design in the battery community and encourages the practical implementation of lithium metal batteries.


| INTRODUCTION
2][3][4][5] Lithium (Li) metal is designated as a promising anode material for building high-energydensity rechargeable batteries because of its high specific capacity (3860 mAh g −1 ) and low redox potential (−3.04 V vs. the standard hydrogen electrode). 6,7][10] Due to the inherent chemical reactivity and thermodynamic instability of metallic Li, the passivation solid electrolyte interphase (SEI) spontaneously reacted between Li metal and the organic electrolyte. 11,12Nevertheless, the nonuniformity and weakness of the electrolyte-derived SEI could promote inhomogeneous ion diffusion and Li dendrite growth.][15] Making feasible lithium metal anodes (LMAs) for practical batteries requires designing a homogenous and stable SEI layer during the Li plating/stripping process to prevent interfacial side reactions and Li dendrite formation. 16,17Enormous researchers have been directed toward the creation of fluorinated SEI that featured with a high lithium fluoride (LiF) concentration to modulate Li-deposition behaviors and boost the long-term stability of LMBs. 18,199][30] Due to the superior chemical stability, high mechanical strength, and low Li-ions (Li + ) diffusion barrier of LiF constituent, [31][32][33] the fluorinated SEI significantly improves the physiochemical properties of electrode/ electrolyte interface, such as ion transport, interfacial energy, spatial resistance, and mechanical stability. 34,35hese reported fluorinated SEIs support superior mechanical strength and fast Li + transferability for achieving stable LMAs.Nevertheless, these as-formed two-dimensional SEI layers still have a virtually infinite volume change that can lead to cracking, especially at high local current densities, which deteriorates the long-term cycle stability. 36,379][40] The undesirable volumetric fluctuations of a host-less LMA during battery operation may be efficiently reduced when LMA is accommodated into appropriate skeletons. 41,42Additionally, 3D structure with high surface area or abundant pores can lower the local current density and homogenize the distribution of Li + flux. 43,44This is achieved through features such as Li-affinity surface groups that concentrate Li + and channel-like structures that guide Li + transfer. 45,46While increasing the lithiophilicity of 3D porous substrates is crucial to obtaining uniform Li deposition, the fragile SEI formed from the electrolyte still results in continuous Li and electrolyte consumption. 30Hence, designing a 3D artificial SEI with controllable components, high homogeneity, strong mechanical strength, and good flexibility may be an attractive strategy in this case to achieve spatially confined Li deposition and excellent cycling performance of LMA, which is yet to be developed.
Here, we propose an effective strategy for creating a special hybrid nanosphere interface on the LMA surface via forced electrolysis of the commercial carbonate-based electrolyte under high-voltage conditions.The properties of the nanosphere hybrid interface can be manipulated by changing the electrochemical parameters (including electrolyte component, voltage, and time) in terms of composition, structure, practical size, and thickness.Benefiting from the 3D structure of the nanosphere hybrid interface, the localized current density and volume fluctuations that occur in the Li plating/stripping process can be efficiently reduced.In addition, the interface also supports high mechanical strength and rapid Li + conductivity to realize high-performance LMAs.Consequently, uniform Li deposition and ultralong-term reversible Li plating/stripping under rigorous working conditions (e.g., large current density, low negative/positive capacity ratio (N/P ratio), and limited Li excess) can be promised.

| RESULTS AND DISCUSSION
The preparation of the nanosphere hybrid interface is based on a high-voltage forced electrolysis electrodeposition process, and the detailed process is shown in Figure 1A.The electrolyte was 1 M lithium hexafluorophosphate (LiPF 6 ) dissolved in ethylene carbonate (EC)/diethyl carbonate (DEC)/FEC, with graphite serving as the anode and Li metal operating as the cathode.Firstly, 10 V voltage is applied to the electrolytic cell system, and the electric field causes the high concentration of Li + in the electrolyte to reduce on the Li metal surface as well as the decomposition of anion and solvents.After continuous electrolysis for 60 min in step 1, a significant amount of Li metal is deposited on the surface of the Li electrode, and the electrolyte's color immediately changes to a deep red (Supporting Information: Figure S1).The disappearance of the characteristic peaks of solvent and anion (EC/DEC/FEC and PF 6 − ) in the Raman spectra confirms the existence of the oxidative decomposition process of the electrolyte components under high-voltage electrolysis (Supporting Information: Figure S2). 47The 1 H and 13 C NMR results demonstrate the emergence of numerous additional peaks, which is evidence that the electrolyte's breakdown products have been further dissolved in the electrolyte (Supporting Information: Figure S3). 48,49In step 2, the fresh Li electrodes were replaced as cathode under continuous electrodeposition in an electrolyte system that already existed and had pre-electrolysis with an ultralow Li + concentration.It is feasible to uniformly nucleate the low concentration of Li + at the contact and deposit them into microscopic Li spheres.Furthermore, the deposited Li sphere was covered by the electrolyte decomposition products to form the special fluorinecontained spherical hybrid layer (FSHL) on the Li metal surface (FSHL-Li), which consists of multiple inorganic components (LiF, Li 2 O, Li x PO y F z , etc.) and fluorinecontained organic species.
Due to the low mechanical strength and inferior flexibility of the electrolyte-derived SEI for B-Li, irregular Li + diffusion channels are created, leading to interface cracks and the subsequent exposure to fresh Li metal following Li plating. 50,51Following that, the fracture regions create preferential deposition sites for Li + , creating problems with dendrite growth and continuous electrolyte consumption after several plating/stripping cycles.On the contrary, the 3D structure of FSHL may successfully lower the current density and volume change locally during Li plating/stripping, retaining the overall electrode's structural integrity (Figure 1B).
Meanwhile, the FSHL has homogeneously distributed LiF and F-riched organic components, enabling it to support high mechanical strength and fast Li + conductivity, 52,53 supporting the even Li + diffusion channels and avoiding dendritic formation.
To investigate the morphology and element distribution of the FSHL-Li electrode, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed.The B-Li exhibits a flat and smooth surface (Supporting Information: Figure S4).After undergoing the forced electrolysis deposition process, Li metal becomes uniformly covered with a amount of small micron-sized particles (Figure 2A), which change the color to dark yellow.The localized EDS elemental content comparisons show that the FSHL has a high F content (53.2%) and relatively low intensity of C (15%), O (26.8%), and P (5.1%) components, which originated from the breakdown of electrolyte components (Figure 2B and Supporting Information: Figure S5).The thickness of FSHL on Li metal can be measured as approximately 15 μm as proved by cross-sectional SEM images (Figure 2C).For comparisons with bare Li (B-Li) and FSHL-Li, we further investigated the interfacial properties of the spherical hybrid layer (SHL) that deposited on LMA in the electrolyte (LiPF 6 -EC/DEC) without FEC additive under a high-voltage forced electrolysis process.SEM images confirmed that the SHL deposited on LMA consisted of aggregated spherical particles but were mainly composed of C and O elements with low F content (Supporting Information: Figure S6).
To accurately visualize the sensitive structure and composition of FSHL-Li with atomic precision, we transplanted some particles of FSHL to the copper mesh and observed it using the cryogenic transmission electron microscopy (cryo-TEM) technique.The deposited FSHL has multiple surface irregularities of different nanoparticles that have aggregated into micrometer spheres, as shown by TEM pictures (Figure 2D), with a diameter of roughly 2-3 μm.Moreover, the inset selected area electron diffraction (SAED) pattern proves these particles consist of LiF and Li 2 O crystals with the inside Li seed.These results prove that the Li + could preferentially develop along the interface during the electrolysis process because the electrolyte has a low concentration of Li + .The Li seeds are subsequently coated with a variety of chemicals produced by the reduction of solvent and anion, which suppress the growth of the Li metal, forcing it to continue to grow and aggregate to form the distinctive structure of the SEI layer.Additionally, scanning transmission electron microscopy STEM and associated elemental mapping images verify that the outer shell of FSHL exhibits dense deposition and even distribution of various elements (Figure 2E).The highresolution transmission electron microscopy result demonstrated a large amount of uniformly dispersed crystalline inorganic nanoparticles (Figure 2F), especially with Li 2 O and LiF nanocrystals, consistent with the corresponding fast Fourier transform (FFT) pattern (Figure 2G). 46Additionally, the high-quality enlarged photos of several inorganic nanocrystals provide proof of LiF's existence (Figure 2H).It should be emphasized that multiple grain boundaries can be established by the organic matrixes that fill the space between the inorganic nanoparticles, which would act synergistically to promote Li + migration.
Next, the chemical composition of the FSHL was examined using X-ray photoelectron spectroscopy (XPS), which provided information on its chemical characteristics.The XPS spectra showed a distinct difference between SEIs according to B-Li and FSHL-Li, in which the former (SEI of B-Li) contained clear and high atomic ratio of O-contained components (including C-O, C═O, Li 2 O, and Li 2 CO 3 species) signal while the latter (FSHL) showed relatively lower intensity for the O-contained organic species (Figure 2I,J and Supporting Information: Figure S7).The higher concentrations of LiF and P-F signal with the discernible peaks of C-O, C═O, COO − , and C-F groups in the XPS F 1 s and C 1 s spectra provide evidence for the existence of LiF and F-riched organic component disperse in the SEI (Figure 2K).According to the obtained mass content of different elements, the FSHL SEI exhibits a higher percentage of the F and P elements (Figure 2I and Supporting Information: Figure S7D), demonstrating that the interface is fluorine-riched due to the breakdown of LiPF 6 and FEC.The B-Li SEI, on the other hand, has a lower amount of F and a greater content of O-containing compounds, which is mostly insufficient to sustain the mechanical strength and Li + diffusion capability of the SEI.In particular, SHL SEI also primarily consists of oxygen-containing components, while the amount of LiF is lower than that of the FSHL SEI (Supporting Information: Figure S8).This discrepancy is caused by the fact that the FEC solvent promotes the formation of high-quality LiF at the interface, which is advantageous for the development of high-efficiency SEIs. 54,55In addition, the strong intensity of the C═O, COO, C-O, and C−H peaks in the Fourier transform infrared (FTIR) spectra provide more evidence that the FSHL formed on the LMA (Supporting Information: Figure S9).All of these distinctive peaks are the result of anions and solvent molecules being forcedly broken down during the high-voltage electrolysis process.Therefore, the aforementioned findings offer strong support for the successful creation of the distinctive FSHL-Li structure, which is composed of nanoscale spherical particles covered with a LiF-enriched component.
Galvanostatic plating/stripping studies in Li symmetric cells employing ether-based electrolytes were initially performed to discover the SEI property's effect on Li + diffusion and electrodeposition behavior.The symmetric cell with the FSHL-Li anode displayed stable and low overpotentials of 42, 73, 81, 144, 82, and 83 mV at different current densities of 1, 2, 3, and 5 mA cm −2 , with a capacity of 1 mAh cm −2 , which demonstrated a fast Li + diffusion kinetics (Figure 3A).Nevertheless, the B-Li anode showed much greater overpotentials and extremely erratic plating/ stripping voltage profiles, especially at a high current density of 5 mA cm −2 .Further comparison with SHL-Li demonstrates that steady deposition behavior at the SHL interface can be accomplished in the first stage, but that it is challenging to stabilize the interface during extended cycling with a large rise in polarization voltage (Supporting Information: Figure S10).Due to the previous characterization and electrochemical performance comparison of the SHL-Li and FSHL-Li, all subsequent characterizations and performance measurements, unless otherwise noted, were based on the B-Li and FSHL-Li.
Moreover, even after extremely long-duration cycling for over 2000 h at a controlled Li plating/stripping capacity of 1 mAh cm −2 under the current density of 1 mA cm −2 , the voltage plateau of the FSHL-Li anode remained remarkably stable (Figure 3B).The B-Li anode cell, on the other hand, has worse electrochemical cycling properties, a progressive increase in overpotential, and fails after only 600 h.Because of the repeated formation of dendritic Li, electrical disconnection and electrolyte consumption are responsible for increased charge transfer resistance.Nevertheless, the FSHL-Li symmetric cell still shows stable cycling over 800 h with a lower overpotential of ∼31 mV (Figure 3C) even at an ultrahigh current density of 10 mA cm −2 with a plating/stripping capacity of 1 mA cm −2 .Moreover, the FSHL-Li anode significantly outperformed the B-Li anode, displaying long-term stability for 900 h with a reduced overpotential of 36 mV when the capacity of cycling was significantly raised to 10 mAh cm −2 (Supporting Information: Figure S11).The apparent variations in cycle performance clearly support the argument that the FSHL that has been deposited on Li metal is preferable for enabling advantageous dendritefree Li plating/stripping behavior.In a further test, it was additionally demonstrated that symmetric cells with ultrathin 50 μm Li metal electrodes could still sustain steady discharging/charging processes for longer than 600 h without undergoing abrupt variations in overpotential (Supporting Information: Figure S12).Moreover, even using the ester-based electrolyte, the symmetric cell with FSHL-Li anode displayed stable cycling over 400 h (Supporting Information: Figure S13), which is notably superior to the performance with B-Li.According to findings in reported experiments, extraordinarily high current densities and capacities of FSHL-Li anodes beat those of Li anodes that have been reported in the past and have been shielded by various protective layers (Figure 3D and Supporting Information: Table S1).The remarkable electrochemical stability of FSHL-Li is attributable to the integrated merits of the spherical structure's promotes the homogeneous dispersion and diffusion of Li + , as well as the ability of the Li nanosphere to act as lithophilic sites to encourage uniform Li deposition.
Using symmetrical cells, the electrochemical kinetics of Li + transport and charge transfer in the Li anode with various SEI were evaluated.Based on the cyclic voltammetry (CV) curve and Tafel plots (Figure 3E and Supporting Information: Figure S14), the corresponding exchange current density (j 0 ) of FSHL-Li has a higher j 0 (0.66 mA cm −2 ) than B-Li (0.28 mA cm −2 ), indicating a more significant oxidation and reduction reaction occurring through the stable SEI. 56,57While a low value denotes slow Li + transport kinetics on the interface.We further investigated the electrochemical impedance spectra (EIS) of the Li symmetric cell using B-Li, SHL-Li, and FSHL-Li anodes to illustrate the underlying mechanism.At an uncycled state, the symmetric cell of FSHL-Li shows lower SEI resistance (R SEI ) and charge transfer resistance (R ct ) than the B-Li and SHL-Li, indicating that the FSHL's presence significantly aided the Li + migration across the electrode/electrolyte interface (Figure 3F and Supporting Information: Figure S15).By comparing the EIS results after different times of FSHL deposition, it was confirmed that excessive deposition leads to an increase in interfacial impedance (Supporting Information: Figure S16).The extremely low R SEI (7.6 Ω) and R ct (12.6 Ω) of FSHL-Li after 100 cycles demonstrated that the presence of FSHL allowed for stable SEI even after cycling while supporting rapid Li + transfer (Supporting Information: Figure S17 and Table S2). 58,59EM observation was used to examine the morphology evolution of different electrodes following Li plating/ stripping.After the initial deposition of 1 mAh cm −2 , B-Li revealed the packing of irregular porous Li and Li dendrites on the surface, causing an increase in the tortuosity of the Li + diffusion channel and electrolyte consumption (Supporting Information: Figure S18).In contrast, the FSHL-Li electrode showed a dense surface morphology without any visible dendrites and cracks during the plating and stripping process (Supporting Information: Figure S19-20).The deposition behavior on B-Li was further monitored with an areal capacity of 5 mAh cm −2 , showing accelerated Li dendrite growth and a porous morphology (Figure 4A,B).In sharp contrast, FSHL-Li displayed dense and uniform deposition, indicating successful restraint of Li dendrite growth (Figure 4C,D).This improvement in Li deposition behavior is attributed to the F-riched component and 3D spherical structure of FSHL, supporting homogeneous Li + transport and reducing local current density.After 100 cycles, the electrolyte-derived SEI layer on B-Li began to break down and repair itself, forming a porous structural fractured layer, while the surface of FSHL-Li remained flat and smooth with no discernible dendrites (Figure 4E,F).This smooth and uniform interface helps prevent further crack expansion and dendrite growth (Figure 4G,H).In contrast, the surface of SHL-Li electrode exhibits lots of cracks and dendrites after cycling, demonstrating the inferior interfacial properties of SHL-Li due to the absence of LiF in SEI.Furthermore, even after 100 cycles, the cross-sectional images of FSHL-Li show a compact and uniform SEI layer, but the porous and thick "dead Li" layer (around 110 μm) occurred on B-Li (Figure 4F,H).The FSHL-Li establishes a strong interfacial bonding to the Li metal surface with no delamination or localized dendrite formation observed.
The SEI composition of FSHL-Li after cycling was investigated using in-depth XPS characterization.The XPS spectra display similar composition peaks throughout FSHL SEI layers before and after cycling (Figure 4I,J 4I). 22,60Most of the organic-related components (COO, C═O, and C-C) are found during the early stages of sputter etching (0-5 min), but as sputtering duration increases (5-15 min), the inorganic-related components (LiF and Li 2 O) eventually take over as the dominant compositions in the SEI.The presence of Li 0 , along with the strong intensity of the LiF peak, suggests that the fluorine-based interfacial layer offers adequate diffusion channels.Additionally, the three-dimensional pores of the spherical structure offer a wealth of Li deposition space for uniform deposition.The element atomic concentration variation after the different sputtering times shows that the SEI structures can exhibit high F content (~10%-20%), proving that the FSHL is effectively distributed on Li metal and efficiently promotes transport (Figure 4J).However, without the protection of FSHL, the SEI on B-Li surfaces after cycling predominantly consists of oxygen-containing organic species and inorganic components (Li 2 O and Li 2 CO 3 ) with minimal LiF components (Supporting Information: Figure S22).These components are derived from the decomposed electrolyte products.These findings show that the B-Li SEI undergoes breakdown and reform during cycling, depleting the electrolyte and Li metal.This is in conjunction with the shape of the B-Li anode during cycling. 15o evaluate the potential application and viability of the deposited FSHL in practical batteries, the FSHL-Li anode (~10 mAh cm −2 ) was paired with a LiFePO 4 (LFP) cathode (~2.3 mAh cm −2 ) and LiNi 0.8 Co 0.1 Mn 0.1 O 4 (NCM811) cathode (1.6 mAh cm −2 ) to assemble Li metal full cells.Benefiting from the fast and homogeneous Li + migration kinetics through the interface, the LFP||FSHL-Li also exhibited enhanced rate performance with lower polarization potential and higher discharge capacity, which was superior to the LFP||B-Li full cell.Specifically, LFP||FSHL-Li cells deliver enhanced capacities of 161.3, 148.6, 137.4,123.5, and 100.7 mAh g −1 at a discharge rate of 0.2, 0.5, 1, 2C, and 3 C, respectively (Figure 5A).When the discharge rate is switched back to 1 C, the capacity is back to 136.7 mAh g −1 (99.5% retention), indicating exceptional rate capability.When compared with discharge/charge voltage profiles of LFP||B-Li, the LFP||FSHL-Li profiles show lower polarization potential and a greater discharge capacity with higher Coulombic efficiency (Figure 5B,C).Subsequently, the LFP||FSHL-Li exhibited a high discharge capacity of ~132 mAh g −1 without obvious capacity decay with an average CE of 99.8% over 300 cycles at current density of 1C (Figure 5D).The LFP||B-Li cell, on the other hand, showed inferior capacity of 55 mAh g −1 after 150 cycles, followed by a quick deterioration of capacity, which was likely caused by a significantly elevated total overpotential.When the current density is increased to 2C, LFP||FSHL-Li cells exhibit excellent cycling stability with higher capacity retention of ~93% during 100 cycles (Supporting Information: Figure S23).The superiority of the FSHL-Li anode is further investigated by coupling with a high-voltage NCM811 cathode.Benefiting from the outstanding interfacial stability, the voltage curves of NCM811||FSHL-Li remained stable over prolonged cycling with no discernible increase in overpotential (Figure 5E and Supporting Information: Figure S24).Furthermore, the NCM811||FSHL-Li showed a high discharge capacity of 181 mAh g −1 and a stable cycling life span with a capacity retention rate of over 80% (Figure 5F).The LFP/B-Li cell, in comparison, had poor stability and quick capacity decline after just 65 cycles.The B-Li anode and electrolyte interface's instability is suggested by the rapidly declining capacity that has been observed.

| CONCLUSIONS
In summary, we first developed a unique spherical SEI structure consisting of LiF nanoparticles evenly distributed in a fluorine-contained organic matrix to realize dendritefree Li metal anodes via a high-voltage forced electrolysis technique.The optimized structure and chemical composition enhances the mechanical strength and interfacial stability of the SEI by managing the electrolyte component, and it also promotes uniform fast Li + diffusion through the interface.As a result, the FSHL-Li anodes can deliver stable Li plating/stripping for more than 2000 and 800 h under high current densities of 1 and 10 mA cm −2 , respectively.When paired with high mass loading LFP and NCM811 cathodes, particularly under lean electrolyte and low N/P ratio conditions, the FSHL-Li can also obtain stable cycling performance.This finding may encourage scientists to create SEI films with exceptional structural and functional characteristics based on a novel forced electrolysis method for the creation of stable LMBs.

| Materials
The Li foil (400 μm) and Li-Cu hybrid foil (50 μm) were bought from China Energy Lithium Co. Ltd.The LiFePO 4 (LFP) laminates (mass loading ~13.5 mg cm -2 ) were purchased from MTI Kejing Group.NCM811 cathodes with loadings of ~8 mg cm -2 were supplied by Guangdong Canrd New Energy Technology Co. Ltd. and also punched into 12 mm discs for use.All the electrolytes were bought from Dodochem Group.

| Synthesis of the FSHL-Li
An electrolytic cell was used to carry out the electrodeposition examinations in an Ar-filled glovebox.The work electrode was made of bare lithium metal with dimensions of 16 mm in diameter.The electrolytic cell with 20 wt % FEC additive) was used as the electrolyte for the electrodeposition process.First, a 1-h electrodeposition procedure at a voltage of 10 V was performed.The working electrode in the electrolytic cell was then replaced with fresh lithium metal foil or Li-Cu hybrid foil, while the electrodeposition process was continued for 10 min.Thereafter, the FSHL was successfully obtained on Li metal.

| Cell assembly and electrochemical measurements
In a glove box filled with Ar atmosphere (H 2 O < 0.1 ppm, O 2 < 0.1 ppm), CR2032-type coin cells were assembled.To assess the electrochemical performance, the Neware multichannel battery test system was used.The symmetric cells were measured at 1-10 mA cm −2 with a capacity of 1 or 10 mAh cm −2 , respectively.The Li symmetric cells were tested in an ether-based electrolyte (1.0 M lithium bistrifluoromethane sulfonimide (LiTFSI) in 1,3-dioxolane/1,2-dimethoxyethane with a 1:1 volume ratio and 2% LiNO 3 ).The LFP||Li and NCM811||Li full cells were used in the carbonate-based electrolytes (1 M LiPF 6 in EC/DEC + 5 wt % FEC).The B-Li or FSHL-Li anodes (Li disks or Li/Cu hybrid foils), a piece of Celgard 2500 separator, and LFP/NCM811 cathodes were used to build the LFP/NCM811||Li full cells.It should be noted that each cell's electrolyte usage was regulated at 40 μL.The working potential window of the LFP||Li and NCM811||Li full cells were 2.5-3.8V and 3-4.3 V, respectively.The diameter of all cathodes and anodes was 12 mm.CV and EIS were tested on CHI 760D electrochemical workstations.CV curves were tested in Li symmetric cells at scan rates of 0.5 mV s −1 .EIS was tested using Li||Li cells with 10 mV amplitude and frequency ranging from 0.01 Hz to 1 MHz.

| Material characterization
With the use of field emission scanning electron microscopy (FE-SEM, Nova Nano SEM 450), the morphology and microstructure of electrodes were examined.FTIR analysis was performed on a Nicolet 6700 infrared spectrophotometer.Raman spectra were collected on a HORIBA HR Evolution with a 532 nm laser source. 13C and 1 H NMR were performed on a Bruker Avance Ill HD600 MHz at room temperature using CDCl 3 as a solvent.X-ray photoelectron spectroscopy (XPS, Thermo Scientific Nexsa) with a monochromatic Al-K α X-ray source was performed to detect the chemistry of the sample.The samples were transferred using a vacuum transfer tank to prevent air exposure.The cryo-TEM characterizations were carried out using a Gatan 698 cryo-transfer holder and an FEI Talos-S TEM operating at 200 kV.All cryo-TEM images were taken at cryogenic temperature (−180°C).
illustration of the construction of the FSHL on the surface of LMA under a high-voltage electric field.(B) Schematic illustration of Li plating behaviors for the FSHL-Li.FSHL, fluorine-contained spherical hybrid layer.

F
I G U R E 2 (A) Top view scanning electron microscopy (SEM) images of FSHL-Li.Insets show digital photo of FSHL-Li.(B) The relevant mass fraction of varied elements of the FSHL-Li.(C) Cross-sectional SEM images of FSHL-Li.(D) Cryo-transmission electron microscopy (TEM) image of FSHL-Li.(E) STEM images and corresponding energy dispersive spectroscopy (EDS) mapping results of FSHL-Li.(F) The high-resolution TEM images of the FSHL.(G) The corresponding fast Fourier transform (FFT) result of the FSHL from (F). (H) HRTEM images of the marked area from (F). (I-K) X-ray photoelectron spectroscopy (XPS) characterization of the surface of FSHL-Li.(L) The mass fraction of various elements of B-Li and FSHL-Li was obtained from the XPS result.FSHL, fluorine-contained spherical hybrid layer; HRTEM, high-resolution transmission electron microscopy; STEM, scanning transmission electron microscopy.

F
I G U R E 3 (A) Rate performance of Li symmetric cells with B-Li and FSHL-Li electrodes at various current densities.(B) Cycling stability measurement of Li symmetrical cells with B-Li and FSHL-Li at 1 mA cm −2 and 1 mAh cm −2 .The insets are magnified views of the voltage profile during cycles.(C) Cycling stability measurement of Li symmetrical cells with B-Li and FSHL-Li at 10 mA cm −2 and 1 mAh cm −2 .(D) Comparison of the cycling time of symmetric cells with FSHL-Li anodes and those of previously reported excellent protective layer on LMAs.(E) Tafel plot of symmetrical cells with B-Li and FSHL-Li.(F) Nyquist plot of Li symmetrical cells with FSHL-Li before and after 100 cycles at 1 mA cm −2 .FSHL, fluorine-contained spherical hybrid layer; LAMs, lithium metal anodes.
), including the P−F at 686.8.0 eV and LiF at 684.8 eV in F 1 s, the COO − at 532.6 eV, C−O at 531.6 eV, C═O at 530.4 eV, and Li 2 O at 528.5 eV in O 1 s, the COO − at 289.7 eV, C═O at 288.0 eV, C−O at 286.5 eV, and C−C at 284.8 eV in C 1 s, and the LiF at 56.6 eV, Li 2 O/ROLi at 54.8 eV, and Li 0 at 53.8 eV in Li 1 s, respectively (Figure

F I G U R E 4
Top-view and cross-sectional scanning electron microscopy (SEM) images of (A, B) B-Li and (C, D) FSHL-Li after Li plating with a deposition capacity of 5 mAh cm −2 , respectively.Top view and cross-sectional SEM images of (E, F) B-Li and (G, H) FSHL-Li after 100 cycles, respectively.(I) X-ray photoelectron spectroscopy (XPS) spectra at various etching times of FSHL-Li.(J) The element atomic concentration of the FSHL was obtained in the XPS depth profiles.FSHL, fluorine-contained spherical hybrid layer.

F
I G U R E 5 (A) Rate performance of LFP||Li full cells with B-Li and FSHL-Li anodes at various current densities from 0.2 to 3C. (B) Voltage profiles of LFP||B-Li full cell at 0.5 C. (C) Voltage profiles of LFP||FSHL-Li full cells at 0.5C.(D) Cycling stability of LFP||Li full cells with B-Li and FSHL-Li anodes at 0.5 C. (E) Voltage profiles of the NCM811||FSHL-Li full cells of 0.5 C after different cycles.(F) Cycling stability of NCM811 ||Li full cells with B-Li and FSHL-Li anodes at 0.5 C. FSHL, fluorine-contained spherical hybrid layer.was positioned 7 mm away from the graphite electrode in a Teflon holder.The carbonate-based electrolyte (LiPF 6 (1.0 M) in EC, DEC (EC/DEC, 1:1 by volume)