Synthesis and characterization
Li-metal foil was first polished to take away impurities after which heated to 250 °C on titanium foil in an argon-filled glovebox (O2 and H2O < 1 ppm) to acquire molten Li. GF powder was added slowly into the molten Li with fixed stirring till the combination was homogeneous, after which left to face for three h with out stirring. In a static state and at excessive temperature, the GF powder floats to the floor to type a uniform GF layer. The Li slowly reacted with the GF to provide LiF on the stable–liquid interface that finally fashioned a GF–LiF protecting layer. As soon as cooled, the GF–LiF–Li composite was faraway from the glove field and reduce into disks for meeting in coin cells. The anode membrane is versatile. Determine 1 offers a schematic illustration of the synthesis response.
Schematic illustration of GF–LiF–Li preparation and its protecting impact for Li-metal anodes. Within the fashions, the carbon (C), fluorine (F), lithium (Li), oxygen (O), and hydrogen (H) atoms are displayed as spheres in orange, cyan, blue, purple, and white, respectively
An in depth density useful principle (DFT) calculation utilizing GGA-PPE computation was made to find out whether or not a spontaneous response between Li and GF is feasible. For simplicity, graphene fluoride, graphene, and the unit cells of Li and LiF served because the modeling substrate to signify the reactants and merchandise (Fig. 2a). The lattice parameters of every modeled substrate are proven in Fig. 2b. The calculations point out that graphene fluoride is decreased to LiF and graphene spontaneously on contact with metallic Li; the Li atoms are dynamically inserted and four.133 eV is launched with each inserted Li atom. XPS evaluation with Ar-ion sputtering (Fig. 2c) was performed on the electrode to discover the floor chemistry and component spatial distribution within the GF–LiF layer. The depth profiles of the F 1 s XPS spectra are proven in Fig. second, e. The XPS survey spectrum of the GF–LiF–Li (Supplementary Fig. 1) confirms the chemical composition of the GF–LiF coating with the weather F, O, C, and Li. Excessive-resolution XPS F 1 s spectra of the primary and final sputtered layers of the pattern are introduced in Fig. second. In each spectra, the peaks centered at ~687.eight eV correspond to C–F bonding and the peaks at ~685.zero eV are attributed to LiF30, indicating that GF is the main ingredient of the GF–LiF layer. Determine 2e reveals an XPS depth profile of the F 1 s spectra by means of all the sputtering vary. Because the sputtering depth elevated, the depth of the height similar to LiF elevated, revealing the rising quantity of LiF within the GF–LiF coating because it approaches the Li-metal interface. So as to decide whether or not any contemporary Li metallic is uncovered on the floor, an XPS measurement (space scan mode, 1.5 × 2.5 mm) was employed to analyze the GF–LiF–Li composite. The high-resolution Li 1 s spectra (Supplementary Fig. 2) didn’t present a peak similar to pristine Li metallic, indicating that there isn’t any publicity of contemporary Li metallic on the floor of the GF–LiF–Li composite and that the floor is properly lined by the protecting GF–LiF layer. To verify the high-quality construction of GF–LiF–Li composite, focused-ion-beam scanning electron microscopy (FIB-SEM) tomography evaluation was employed to carry out a chemical characterization of the anode membrane. Supplementary Fig. 3a reveals the imaged cross-section of the GF–LiF–Li composite which was trenched to a depth of 10 μm by FIB. It may be seen that GF–LiF–Li composite consists of three layers, together with the highest GF–LiF layer, adopted by a transitional zone consisting of GF, LiF, and Li metallic and a backside layer consisting solely of Li metallic. As well as, Supplementary Fig. 3b–c reveals energy-dispersive X-ray (EDX) maps of C and F components on the FIB-ablated cross-sections that exhibit the distribution of C and F components within the GF–LiF–Li composite which have a downward development from high to backside, which is according to the imaged construction.
DFT calculations and XPS spectra of GF–LiF layer. a Optimized constructions of lithium, graphite fluoride, lithium fluoride, and graphene molecular fashions by DFT calculations, Within the fashions, the fluorine (F), carbon (C), and lithium (Li) atoms are displayed as spheres in inexperienced, yellow, and grey, respectively. b The lattice parameter of every modeling substrate. c The schematic diagram for the etching detection of Ar-ion sputtering. d Excessive-resolution XPS F 1 s spectra of GF–LiF coating for the primary and final etching degree. e XPS depth profile of F 1 s
Dendrite-free research of the GF–LiF–Li anode
Supplementary Fig. four reveals a schematic diagram of Li deposition within the plating/stripping course of on naked Li anodes the place useless Li is generated and on a GF–LiF layer coated with Li anode the place the formation of dendrites is suppressed. This schematic was verified with field-emission scanning electron microscopy (FESEM) characterization. The thickness change of the GF–LiF–Li anodes was measured by SEM earlier than and after Li plating (Supplementary Fig. 5a and d). The contemporary GF–LiF–Li anode confirmed a thickness of round 300 μm previous to the Li plating course of and a thickness of about 337 μm after the Li plating course of with none indication of Li dendrite formation. The corresponding EDX elemental maps (Supplementary Fig. 5b–f) point out that F and C components are uniformly distributed on the anode floor earlier than and after Li plating, which reveals that the Li-metal anode stays well-protected by the GF–LiF layer because the metallic Li is plated beneath it. Moreover, an in situ optical microscopy visualization was performed. A symmetric cuvette-type optical cell was fabricated to analyze the morphology of the floor of naked Li and GF–LiF–Li electrodes through the Li deposition course of, which was considered by means of in situ optical microscopy (see supplementary video 1 and a couple of). The photographs which might be recorded at completely different occasions are displayed in Fig. 3a. The symmetric cell was topic to a excessive mounted present density of three mA cm−2. At first, the pristine Li electrode on which Li was plated was clean and flat, however Li dendrites appeared instantly upon imposition of the present and quite a few moss-like dendrites fashioned on the naked Li anode as time went on. As compared, the GF–LiF–Li electrode was seen to electrodeposit Li uniformly at excessive present density with a flat floor with virtually no dendritic construction. This remark means that the GF–LiF layer can suppress the formation of Li dendrites successfully in a Li-metal rechargeable battery. Extra electrochemical efficiency of the GF–LiF–Li symmetric cells was studied with 1 M LiPF6 in 1:1 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1 M LiPF6/EC/DEC) electrolyte at completely different present densities with respect to the geometric space of the working electrodes. The GF–LiF–Li cells have been discharged for 1 h adopted by 1 h of cost at a present density of 1 mA cm–2, which delivers a capability of 1 mAh cm–2 with a low overpotential of round 70 mV (Fig. 3b). The voltage hysteresis (a sum of the overpotentials for Li stripping/plating31) reveals glorious stability with a negligible voltage fluctuation throughout repeated biking. In distinction, cells with naked Li foils have a a lot bigger overpotential (~100 mV) and confirmed random voltage oscillations that enhance throughout biking. A magnified view of the voltage profiles of cells with naked Li electrodes (black) and GF–LiF–Li electrodes (purple) is offered in Supplementary Fig. 6a, b, which reveals that each naked Li and GF–LiF–Li anodes give a gradual voltage plateau with a comparatively low overpotential on the 2nd cycle. With futher biking, the plateau of naked Li anodes turns into much less clean till a sudden voltage change occurrs on the 50th cycle, indicating that the SEI on the naked Li is regularly being damaged/reformed and accumulates on the floor of the Li electrodes. Biking at elevated present densities (5and 10 mA cm−2), proven in Fig. 3c, d, reveals a rise in voltage hysteresis for the symmetric Li-metal cells, whereas the symmetric GF–LiF–Li cells exhibit a way more secure voltage profile with a smaller voltage hysteresis. The corresponding partially enlarged view of the voltage profiles (2nd and 50th cycle) at present densities of 5 mA cm−2 and 10 mA cm−2 are displayed in Supplementary Fig. 7 and Supplementary Fig. eight, respectively. Deep Li stripping/plating exams have been additionally performed; it was discovered that the symmetric GF–LiF–Li cells might operate stably beneath a capability as excessive as 6 mAh cm−2 with low overpotentials (Fig. 3e). These knowledge point out that the unreal GF–LiF coating is secure and successfully inhibits facet reactions and suppresses the era of Li dendrites beneath extraordinarily quick Li plating/stripping. Subsequently, electrochemical impedance spectroscopy (EIS) analyses have been carried out on symmetric cells with naked Li and GF–LiF–Li electrodes with the outcomes displayed in Supplementary Fig. 9a and b, respectively. With extended occasions of the Li plating/stripping course of at a present density of 1 mA cm−2 and a capability of 1 mAh cm−2, the impedance of the naked Li cell elevated considerably, which will be ascribed to the formation of an SEI on the floor of the Li electrode. There may be virtually no important change after eight h of biking within the batteries with GF–LiF–Li electrodes, indicating the soundness of GF–LiF–Li interphase.
In situ optical microscopy visualization of Li electrodeposition and long-term biking on symmetric cells. a The photographs from microscopy of the naked Li (left column) and GF–LiF–Li (proper column) electrolyte interface at zero, 10, 15, and 20 min at a present price of three mA cm–2. The dimensions bars are 200 μm. Corresponding films are offered within the Supplementary info. The Li deposition of naked Li (left) and GF–LiF–Li (proper) in Li plating/stripping course of. Naked Li foil symmetric cells (black) and GF–LiF–Li symmetric cells (purple) at varied present densities of (b) 1 mA cm–2, (c) 5 mA cm–2, and (d) 10 mA cm–2. The capability is 1 mAh cm–2. e Biking efficiency of symmetric GF–LiF–Li cells cycled beneath 2 mA cm−2 with 2–6 mAh cm−2 circumstances
Atomic power microscopy (AFM) evaluation was used to indicate additional the inhibition of Li dendrite development by the GF–LiF–Li anodes based mostly on a probing tip at one finish of a cantilever to work together with the pattern32. Because the schematic diagram reveals (Fig. 4a), each enticing and repulsive forces give rise to an interplay between the tip and the pattern to offer details about the topography and mechanical properties of the floor of the pattern. Determine 4b reveals such a topographic picture of a naked Li electrode floor after biking in 1 M LiPF6/EC/DEC electrolyte. The SEI layer on the Li floor is fashioned from discount of the electrolyte; it offers a tough floor pitted with holes and enormous granular options that may present preferential websites for the formation of dendrites and useless Li. In distinction, the floor of GF–LiF–Li is comparatively clean with the uniform coating depicted in Fig. 4c. Additional research of the morphological evolution of naked Li and GF–LiF–Li have been performed by observing the floor of the 2 completely different anodes earlier than and after 30 cost/discharge cycles. Supplementary Fig. 10a reveals the floor topography of pristine Li metallic earlier than biking, revealing a clean and flat floor. After 30 cycles (Supplementary Fig. 10b), Li dendrites develop on the floor of the pure Li metallic, which give it a tough texture with mossy and rugged hilly websites. Supplementary Fig. 11a is the floor of a GF–LiF–Li anode with uniformly distributed nanoparticles. The graceful floor doesn’t present any proof of Li dendrites after biking and the morphology of the GF–LiF–Li is maintained (Supplementary Fig. 11b), indicating that the GF–LiF coating is a perfect protecting layer to suppress Li dendrites. Determine 4d, e offers the power curves as a operate of tip–floor distance through the indentation loading and unloading cycle; the slopes of the load and unload curves are attributed to the stiffness of the fabric being probed with the AFM tip. The measured largest damaging power is the adhesion power between the probed floor and tip33. In Fig. 4d, the loading and unloading curves of the SEI on naked Li virtually overlap. The excessive slope of the curve and negligible viscoelasticity counsel that the SEI layers are stiff and brittle. This remark is in stark distinction to the curves of the GF–LiF–Li layer (Fig. 4e), which present a big hysteresis between the load and unload with an extended pull-off or meniscus earlier than the AFM tip utterly separates from the GF–LiF–Li floor and returns to its regular place. The height power and slope obtained from the loading curves are displayed in Fig. 4f for comfort. These knowledge point out that the GF–LiF layer is extra elastic than its naked Li-metal counterpart. The decreased modulus of the SEI and GF–LiF floor layers was estimated with indentation testing. The modulus of the SEI layers on naked Li is examined as 600 MPa, however the GF–LiF layer delivers a low modulus of roughly 130 MPa. The decreased modulus of the GF–LiF layer counsel that it’s way more versatile and never as simply damaged because the SEI layers that develop on naked Li metallic through the plating/stripping course of.
AFM and in situ XRD measurements. a A schematic diagram for the working precept of AFM. AFM topography photos of (b) SEI on naked Li foil after the charging–discharging cycle and (c) GF–LiF–Li at room temperature. The dimensions bars are 400 nm. Indentation curves of (d) the SEI layer on naked Li and (e) GF–LiF layer. f The slope and peak power of loading curves in addition to the decreased modulus. g The colour plots represented in situ XRD patterns of naked Li anodes (high) and GF–LiF–Li anodes (backside) upon the primary cost–discharge course of
In situ biking XRD research of the avoidance of electrolyte decomposition
In situ galvanostatic biking XRD was employed to observe the GF–LiF–Li electrodes through the first cost–discharge course of in actual time to find out the section composition based mostly on X-ray scattering34. Determine 4g reveals that the diffraction sample of the GF–LiF–Li electrode is secure with out important change to any diffraction peaks upon cost/discharge. Nonetheless, the diffraction peaks ascribed to the LiPF6 (003) and (zero12) planes (PDF no. 52–0488) regularly weaken within the shade plots of the XRD patterns of the naked Li anode used for comparability beneath the identical testing circumstances as with the GF–LiF–Li anode. The corresponding detailed XRD patterns are offered in Supplementary Fig. 12, which shows a constant phenomenon with the colour plots of the XRD patterns (Fig. 4g). The one first-scan XRD patterns of naked Li and GF–LiF–Li anodes are introduced within the inset of Supplementary Fig. 12a and b, respectively. This consequence demonstrates the decomposition of the electrolyte through the first cost/discharge cycle of naked Li because of the formation of an SEI layer and Li dendrites. The findings of the in situ XRD research reinforce our earlier assertions of the avoidance of electrolyte decomposition on the anode, in addition to the shortage of facet reactions between Li metallic and an organic-liquid electrolyte when the GF–LiF layer is current.
A excessive reversible capability and good biking stability are obligatory for a battery. The electrochemical efficiency of GF–LiF–Li electrodes was evaluated with a number of completely different cathode and electrolyte mixtures by means of galvanostatic biking. Full cells with a GF–LiF–Li anode have been examined at room temperature with LiFePO4 because the cathode and 1 M LiPF6/EC/DEC as an electrolyte. Determine 5a presents the voltage profiles of the fifth cycle within the voltage window of two.5–four.zero V at varied present densities. Typical discharge plateaus of LiFePO4 at round three.four V versus Li+/Li have a capability of 160 mAh g−1, 150 mAh g−1, and 140 mAh g−1 at present charges of zero.1 C, zero.5 C, and 1 C (1 C = 170 mA g−1), respectively. Determine 5b reveals the electrochemical efficiency of GF–LiF–Li/LiFePO4 cells with a liquid electrolyte at a present price of 1 C; a particular capability of 140 mAh g−1 with ~100% coulombic effectivity was secure for 200 cycles. Determine 5c shows the biking efficiency of a GF–LiF–Li/LiFePO4 cell with a solid-state polymer electrolyte. The all-solid-state cell demonstrated a reversible capability of 150 mAh g−1 and a excessive coulombic effectivity of 99.eight% at zero.2 C. At 2 C (Fig. 5d), the cells with GF–LiF–Li electrodes exhibited a capability of 102 mAh g−1 and excellent cyclability, remaining secure for 300 cycles. Additional cost–discharge measurements of GF–LiF–Li electrodes have been taken with LiNiCoMnO2 as a cathode (GF–LiF–Li/LiNiCoMnO2). The attribute fifth cost–discharge voltage profiles at completely different present densities within the voltage window of three.zero–four.three V are proven in Supplementary Fig. 13. The place discharge plateaus round ~three.6–three.7 V are noticed whereas the cells ship a capability of roughly 160 mAh g−1, 150 mAh g−1, and 140 mAh g−1 similar to present densities of 27.eight mA g−1, 139 mA g−1, and 278 mA g−1, respectively. The lengthy cycle life efficiency of GF–LiF–Li/LiNiCoMnO2 cells at a excessive present price of 278 mA g−1 is proven in Supplementary Fig. 14. The cell confirmed a discharge capability of 140 mAh g−1 with a coulombic effectivity of round 100% similar to a capability fade price of solely zero.06% per cycle for 300 cycles. The complete cells with secure electrochemical efficiency verify the practicability of GF–LiF–Li electrodes in a Li-metal battery system with a liquid or solid-state electrolyte.
Galvanostatic biking efficiency of GF–LiF–Li anodes. Attribute cost–discharge voltage profiles of (a) liquid-state GF–LiF–Li/LiFePO4 cells at completely different present densities. The biking efficiency of (b) liquid-state GF–LiF–Li/LiFePO4 cells. Biking stability of all-solid-state GF–LiF–Li/LiFePO4 cells at a present price of (c) zero.2 C and (d) 2 C
Air stability of GF–LiF–Li anodes
We studied the soundness of the anodes in ambient air; Fig. 6a reveals optical images of the as-obtained GF–LiF–Li and naked Li foil uncovered to air with RH of 20–35% for various quantities of time. In an inert ambiance, the pristine Li initially displays a silver-like shade with a flat floor, however shade change happens instantly as quickly because the Li foil is uncovered to ambient circumstances. After publicity to ambient air for 1 h, the colour of Li metallic turns utterly ash black with a tough texture. GF–LiF–Li anodes present no important change in shade, form, or texture throughout its publicity to humid air, revealing that the GF–LiF coating can function a superb hydrophobic safety layer to stabilize Li metallic in ambient air. The hydrophobicity of the GF–LiF coating was additional investigated with in situ XRD measurements to be able to analyze all the oxidation course of. The floor of the Li foil was not polished, and the RH of the check chamber was round 10%, which slowed down the speed of corrosion. The primary XRD scan patterns between the naked Li and GF–LiF–Li are given in Supplementary Fig. 15 after they have been simply uncovered to air. Naked Li and GF–LiF–Li each exhibit sharp diffraction peaks similar to metallic Li (PDF no. 15–0401), however the GF–LiF–Li sample reveals further diffraction peaks of LiF (PDF no. 89–3610) that derive from the GF–LiF coating and different diffraction peaks attributed to LiC (PDF no. 14–0649), LiC24(PDF no. 35–1047), and Li2O2 (PDF no. 09–0355). Supplementary Fig. 16 reveals the final XRD patterns performed after 5 h of publicity. New diffraction peaks listed to LiOH (PDF no. 32–0564) will be discovered within the naked Li sample, whereas the XRD patterns of GF–LiF–Li present no apparent change. In situ XRD measurements reveal real-time observations of a steady variation of LiOH era as time goes on, which will be ascribed to the response between ambient H2O and Li metallic (Fig. 6c for naked Li). All peaks of naked Li shifted completely different levels and enhance in depth abruptly as a consequence of a form change of Li foil that was brought on by response with O2 and H2O in air. In distinction, there isn’t any apparent change within the GF–LiF–Li XRD sample over the course of its publicity to ambient circumstances (Fig. 6b). The corresponding contour plots of in situ XRD patterns are displayed in Fig. 6e. The highest contour represents GF–LiF–Li, and the underside contour is for naked Li. XRD peaks related to a LiOH section seem within the naked Li plot and regularly enhance in depth. These findings point out that the GF–LiF layer successfully inhibits the response between H2O and Li metallic and might function a strong protecting coating on the floor of metallic Li. Furthermore, the floor chemical composition of GF–LiF–Li composites after publicity to air for 1 and 5 h was investigated to check with contemporary composites by XPS measurements. From Supplementary Fig. 17a–d, no important adjustments are noticed in a survey of Li 1 s, F 1 s, and C 1 s spectra, revealing the soundness of GF–LiF–Li composite in air.
Characterization of GF–LiF–Li air stability. a Images of pristine Li and GF–LiF–Li uncovered to air with relative humidity of 20–35% for varied occasions. All in situ XRD patterns of (b) pristine Li metallic and (c) GF–LiF–Li at each scan. d The biking efficiency of anodes after an publicity in air for 12 h (high) and 24 h (backside). e The colour plots of in situ XRD patterns of GF–LiF–Li (high) and naked Li (backside) in air for five h
The feasibility of the air-stable GF–LiF–Li anodes to be used in sensible purposes was demonstrated by galvanostatic cost/discharge biking. Analysis of the charge-storage capabilities of GF–LiF–Li anodes that have been uncovered to ambient air for 12 h, and 24 h, respectively, was performed with cells having LiFePO4 because the cathode (Fig. 6d). Each of the cells with GF–LiF–Li electrodes after 12 and 24 h of air publicity introduced virtually the identical electrochemical efficiency at 1 C (i.e., ~140 mAh g−1) because the contemporary GF–LiF–Li anode (Fig. 5b). Cells with naked Li metallic positioned in air for 12 h have been additionally examined in the identical circumstances because the GF–LiF–Li anodes for comparability. The naked Li-metal cells exhibit poor biking stability that falls quickly to a particular capability of solely 40 mAh g−1 after 70 cycles. After 24 h of publicity to ambient air, the Li metallic is totally corroded and doesn’t ship any capability after cell meeting within the cost/discharge course of. Symmetric cells have been assembled with GF–LiF–Li anodes after publicity to air to additional check their electrochemical efficiency. Supplementary Fig. 18 presents typical electrochemical stripping curves for naked Li and GF–LiF–Li anodes; the cells have been all measured at a present density of 1 mA cm−2. The naked Li anode delivers a particular capability of 3616 mAh g−2, and GF–LiF–Li anodes earlier than and after publicity to air for 12 h and 24 h are capable of present particular capacities of 2924, 2903, and 2889 mAh g–1, respectively. This consequence signifies that ∼81 wt% of Li within the contemporary GF–LiF–Li composite is energetic and that little or no Li metallic is sacrificed within the humid air. Equally, from the voltage profiles and EIS measurements at 1 mA cm−2, the biking efficiency and interphase stability of GF–LiF–Li anodes uncovered to air for 12 and 24 h carry out in addition to unexposed composites (Supplementary Fig. 19 and Supplementary Fig. 20). These observations point out that the GF–LiF–Li anodes provide an anode for Li-metal liquid-electrolyte batteries that doesn’t require an especially inert ambiance for cell meeting; the practicability of this anode can successfully scale back the price of cell fabrication.