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Dendrite-Free Fluorinated Graphene/Lithium Anode Enabling InSitu LiF Formation for High-Performance Lithium#Oxygen Cells Hao Cheng, Yangjun Mao, Jian Xie, Yunhao Lu, and Xin-Bing Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10055 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Dendrite-Free Fluorinated Graphene/Lithium Anode Enabling In-Situ LiF Formation for High-Performance Lithium‒Oxygen Cells Hao Cheng,† Yangjun Mao,† Jian Xie,*,† Yunhao Lu,§ and Xinbing Zhao†, ‡ †State

Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University,

Hangzhou 310027, China §Department ‡Key

of Physics, Zhejiang University, Hangzhou 310027, China

Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, Hangzhou 310027,

China

* Corresponding author. Tel./Fax: +86-571-87951451 E-mail: [email protected] 1

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ABSTRACT: Metallic Li is considered as the ultimate choice of negative electrode for Li batteries due to its largest theoretical specific capacity. However, formidable issues such as poor safety and cyclability caused by lithium dendrites growth and tremendous interfacial side reactions have strictly hindered its practical applications. Here, we report fluorinated graphene (FG)-modified Li negative electrode (LFG) for high-performance lithium‒oxygen (Li‒O2) cells. The results show that only 3 wt.% FG introduction leads to a significant enhancement on rate capability and cycling life of Li electrodes. Compared with the half cells with bare Li, the cells with LFG exhibits much more stable voltage profiles even at a large areal capacity up to 5 mAh cm–2 or a large current density up to 5 mA cm–2. Li‒O2 cell with the LFG anode shows a longer cycle life than the cell with the pristine lithium anode. It was found that a LiF-rich layer could be in-situ built upon cycling when FG was used, which ensures uniform Li stripping/plating and effectively suppresses Li dendrites growth. Density functional theory calculations confirm the possibility of conversion from FG to graphene and LiF after Li intercalation into LFG during cycling. In-situ optical microscopy observation vividly exhibits the obvious inhibition effect of FG for Li dendrites growth. KEYWORDS: fluorinated graphene, lithium anode, Li–O2 cell, density functional theory, in-situ optical microscopy

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■Introduction A fast development of electric vehicles and portable electronics is putting stringent requirements for rechargeable battery systems.1,2 The current commercial Li-ion batteries using graphite anodes have a theoretical limit of specific energy density (~350 Wh kg‒1) which can hardly satisfy the increasing needs for high energy density. Therefore, it has been now generally acknowledged that new material systems should be explored.3‒5 Among various anodes, metallic Li is regarded as final choice for the anode of Li batteries owing to the highest theoretical capacity of 3860 mAh g‒1 and the lowest potential of ‒3.04 V (versus the standard hydrogen electrode).6 Furthermore, metallic Li anode is essential for Li‒O2 batteries and Li‒S batteries, which are promising as the next-generation battery systems.7 Nevertheless, rechargeable Li batteries have not realized commercialization yet due to formidable issues such as poor safety and cyclability caused by lithium dendrites growth and tremendous interfacial side reactions. The uncontrollable formation of lithium dendrite arises from the uneven lithium stripping/deposition during repeated discharge/charge processes, and the incessant lithium-dendrite growth will pierce the separator, and the Li dendrite will finally reach the cathode, leading to cell short circuit.8 The interfacial instability caused by the reactions between metallic lithium and organic electrolyte will thus result in building of a solid electrolyte interface (SEI) layer. Li dendrites growth and volumetric changes of the Li anode during repeated cycling induce cracking and collapsing of the SEI layer, rendering more exposed fresh Li surface and more parasitic reactions with the electrolyte, causing low Coulombic efficiency (CE) and short cell life.9,10 Li dendrites could also be detached from the anode by the side reactions of the SEI formation and become electrochemically inert “dead Li”. In the past few decades, a great effort has been made to addressing the challenges of metallic 3



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lithium anodes. Introducing additives in the electrolyte is considered as a potential solution to enhance interface stability of Li metal, forming more robust SEI films to prevent further Li degradation and Li dendrites formation. Various additives were investigated such as LiNO3,11 and fluoroethylene carbonate.12 Nevertheless, long-time cycling of metallic Li is still impracticable because of poor electrochemical/chemical stability of liquid electrolytes. Solid state electrolytes (SSE) with a large Young’s modulus and a wide electrochemical window against Li may restrain lithium dendrites growth and prevent the failure of metallic Li.13 However, the poor interface contact between electrode and SSE along with low ionic conductivity of SSE severely limit its applications.14 Another method is to design an artificial structure for Li anode. There are mainly two kinds of artificial structures investigated, one is the three-dimensional (3D) porous matrix to host Li metal, and the other is artificial SEI to regulate lithium dendrite growth and isolate Li from the electrolyte. Typical hosts, such as 3D metallic current collector15‒17 and 3D carbonaceous framework,18‒20 exhibit homogeneous stripping/plating of Li during cycling. The uniform local current density of matrix structure restrains nucleation of lithium and thereby inhibits growth of dendritic lithium. Although 3D matrix is effective to enhance stability of the Li anode to some extent, interfacial contact of Li with electrolyte is still unstable with continuous consumption of the electrolyte. Building an engineered SEI layer with large ionic conductivity, high electronic insulation and high mechanical strength provides an ideal way to solve this problem. Recently, LiF-rich artificial SEI layer was found to be more stable and to enable homogeneous Li stripping/plating.21,22 Besides, graphene and its derivatives with a high Young’s modulus are promising materials for artificial SEI.23 SEI film with a high shear modulus relative to lithium anode (~109 Pa) can partly inhibit Li dendrites growth.24 Zhang et al. confirmed that reduced graphene oxide and graphene oxide are effective in inhibiting Li dendrites growth.25,26 As a 4


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derivative of graphene, fluorinated graphene (FG) shows a high Young’s modulus of 0.3 TPa and a high resistance of over 10 GΩ at room temperature.27 In addition, fluorinated reduced graphene oxide used as interlayer additive has confirmed to exert a positive influence on the reversibility and stabilization of Li‒S batteries.28 Herein, we report the fabrication of FG-protected Li anode (LFG), enabling in-situ fabrication of a robust LiF-rich SEI film during electrochemical Li stripping/plating. The LFG anode possesses several advantages. First, the LiF-rich SEI layer restrains the electron transport and increases the Li ions diffusion, thus ensuring the even lithium plating and refraining dendritic Li growth. Second, the robust mechanical stability of the LiF-rich SEI film prevents the penetration of dendritic lithium and minimizes the parasitic reaction between the electrolyte and Li. Thus, the LFG composite anode exhibits stable cycling even at a large areal capacity (5 mAh cm–2) or a large current density (5 mA cm–2). Li–O2 cell with the LFG anode demonstrates a long cycling life. The formation mechanism for the LiF-rich layer and its contribution to the electrochemical performance were investigated by density functional theory (DFT) calculation and in-situ optical microscopic observation. This study would shed light on optimizing composite Li anodes for high-performance metallic Li batteries.

■ EXPERIMENT SECTION Synthesis of the LFG Composite Anode. FG (≥99.998%, XFNANO, Co., Ltd.) was first dried under vacuum at 80 °C for 5 h before transferring into a glove box filled with Ar. Li foil (≥99.9% purity, Sinopharm Chemical Reagent Co., Ltd.) was polished by a sharp blade to remove the surface oxide layer. FG powder and Li foil were first placed on a stainless-steel plate. The plate was then heated to 350°C to melt the lithium. The FG powder was evenly mixed with the molten Li under vigorous stirring for 5 min. After the melt has cooled down, the solid was reheated to melt and stirred for another 5 min. Such procedure was repeated 3 times to obtain the homogeneous LFG. 5



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The mass fraction of FG in the composite is around 3wt.%. To make the LFG electrodes, the LFG was mechanically pressed and cut into 0.785 cm‒2 disks (1 cm diameter). All the above operations were conducted in the argon-filled glove box. Material Characterization. X-ray photoelectron spectroscopy (XPS) measurements were performed using a KRATOS AXIS ULTRA-DLD system (Shimadzu, Japan) equipped with a monochromated Al Kα radiation source (hν= 1486.6 eV). The morphologies and microstructures of the pristine and cycled electrodes (or powder) were analyzed by field-emission scanning electron microscopy (FE-SEM) on an SU8010 microscope (Hitachi, Japan). The cycled symmetric cells were dismantled, and the electrodes were washed several times with 1,2-dimethoxyethane (DME) before characterization in the glove box. The in-situ optical microscopic observations on bare Li and LFG electrodes were carried out on a BETICAL XTL microscope. Electrochemical Measurements. CR2025-type symmetric cells were fabricated in the Ar-filled glove box to evaluate the electrochemical performance of bare lithium electrodes and LFG electrodes. 1 mol L‒1 LiPF6 in a mixed solvent of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (1:1:1 in volume) was used as electrolyte. Li–O2 coin cells (CR2025) were fabricated using bare Li or LFG as anode, δ-MnO2 on carbon cloth as cathode, and Celgard C480 membranes as separator. 1 mol L‒1 LiClO4 (≥99.99% purity, Sigma Aldrich) dissolved in tetraethylene glycol dimethyl ether (TEGDME, ≥99.0% purity, Sigma Aldrich) was used as electrolyte. The mass loading of δ-MnO2 is 0.5 mg cm‒2 and the δ-MnO2 catalyst was synthesized according to the previous work.29 The cathodes were dried under vacuum at 80 °C for 5 h prior to cell assembly. The fabricated cells were placed in bottles filled with pure oxygen gas for 20 min. Prior to the electrochemical tests, the cells were rested at the open voltage circuit for 5 h. The cycling performance of the cells was evaluated by galvanostatic cycling using a battery tester (Neware, China) at a 2.0–4.5 V cut-off voltage. The current density (mA g–1) and specific capacity (mAh g–1) of the cells were normalized to the mass of δ-MnO2. Electrochemical impedance 6


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spectroscopy (EIS) measurements were performed on an electrochemistry workstation (VersaSTAT3), where an AC signal is 5 mV amplitude was applied and a frequency range of 10–2 to 105 Hz was used. ZViewVer. 3.1 (Scribner Ass. Inc.) software was employed to analyze the spectra. All of the above electrochemical tests were conducted at room temperature. Theoretical Methods. All the calculations have been performed based on DFT using the generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE)30 executed in the Vienna ab-initio Simulation Package.31 We employed the projector augmented wave pseudo-potential technique to modelize the ionic potentials.32 The cutoff of kinetic energy was set over 400 eV for calculations. In the first Brillouin zone, the Monkhorst–Pack scheme k-points sampling method was applied for integration.33 All atoms were full relaxed as far as the force on them was below 0.01eV/Å. A 3×1×1 FG supercell (8 C and 8 F) was used for the simulation of the chair configuration of FG. The FG layer model was acquired from the bulk model of fluorinated graphite and the layer was separated by the vacuum layer of a 20 Å thickness, in order to avoid the interaction between neighboring images.

■RESULTS AND DISCUSSION Typical preparation process of the LFG is presented in Figure 1a. The molten Li metal and FG powder were blended uniformly in a stainless-steel plate with stirring under heating. After cooling, the composite was mechanically pressed and cut into disks of 10 mm diameter to make the LFG anodes. The original morphology of the FG powder is given in Figure S1 (Supporting Information, SI), which exhibits a sheet-like morphology with a size of around 5~10 μm. Figure 1b‒g shows the surface and cross-sectional SEM views of bare lithium electrode and LFG electrode. For the bare Li electrode, both cross section and surface are smooth and the Li plate has silver color. For the LFG electrode, both cross section and surface are coarse. The surface SEM images of LFG (Figure 1c,d) show that the FG is well dispersed on the Li surface of and the color of electrode becomes grey with FG introduced. From the cross-sectional image, the presence of FG in the LFG electrode is evident 7



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and it seems that the amount of FG in the interior is smaller than that on the surface. It can be seen that adding only 3 wt.% FG greatly modifies the morphology of the Li electrode and the FG on the surface of LFG will play a major role in forming a robust SEI layer, inhibiting Li dendrites growth. As a result, a LFG composite electrode can be fabricated by a facile melting and mixing method using the commercial raw materials. It is therefore anticipated that presence of FG will alter the stripping/plating behaviors of metallic Li as discussed later.

Figure 1. (a) Preparation process of the LFG electrode, surface SEM views of (b) bare Li and (c, d) LFG electrodes (inset is the digital photograph of the electrodes), and cross-sectional SEM views of (e) bare Li and (f, g) LFG electrodes To check the composition of the as-obtained LFG, XPS analyses were performed. As exhibited in Figure 2a, compared to that of bare FG, an additional Li 1s peak appears in the XPS of LFG, proving the formation of the LFG. The small O1s peak of FG in Figure 2a is due to the organic 8


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pollutants absorbed on the surface of FG during XPS measurement, and the increase in O1s peak intensity in LFG is caused by the reaction between air and Li metal inside LFG before and/or during XPS tests. The presence of the LiF peak in Figure 2b indicates that part of the FG has reacted with Li during the mixing process of Li and FG under heating owing to the large change in Gibbs free energy for LiF formation. The reaction is simply expressed as: nLi + (CF)n → nLiF + nC.34,35 Thus, it is expected that graphene sheets will be generated and dispersed in LFG after F-removal from FG. It is generally accepted the charge transfer resistance of lithium metal increases because of the unavoidable SEI film formation with the reactions between electrolyte and Li.36 EIS measurements were performed to investigate the difference in resistance variation with time between LFG and bare Li using the symmetric cells. As shown in Figure 2c, the Nyquist plots start with the Ohmic resistance (Re) which is determined by the ionic conductivity in electrolyte, and the semicircle at lower frequencies corresponds to the surface SEI film resistance (Rf) and the charge transfer resistance (Rct) of lithium electrodes, which are summed as “Li electrode surface resistance” (Rsurface).37,38 The fitting values using the equivalent circuit (Figure S2) are given in Table S1. The resistance of symmetric cell with bare Li after 45 h is around 607 Ω, and it increases to 916 Ω after 135 h with an increment of about 300 Ω. While for the LFG electrode, the resistance increment is only 81Ω (from 242 Ω to 323 Ω) in the same time interval. The results reveal that Re is far lower than Rsurface, indicating that the overpotential of Li stripping/plating is mainly depends on the SEI layer and the Li/electrolyte interface. The lower value and small increment of resistance of LFG could be due to the presence of graphene and LiF layer in LFG. LiF is a strong electronic insulator and plays a crucial role in boosting overall ionic conductivity by forming a heterogeneous structure with Li2CO3.39,40 The calculation on the surface diffusion barrier for ion transfer at LiF/Li surface by Ozhabes et al. showed that the surface diffusion barrier of lithium-halides is small that ranges 9



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from 0.03 to 0.15 eV.41 The small diffusion barrier is favorable for the smooth Li-ion diffusion on the surface of LiF. The SEI layer with LiF could facilitate Li-ion conductivity and improve the ability to block electron transfer, suppressing consumption of Li and electrolyte over a long period of time. Based on the above results, it could be concluded that the introduction of FG in Li could effectively alleviate the Li degradation and maintain a low interfacial resistance.

Figure 2. (a) XPS survey spectrum and (b) F1s spectrum of FG and LFG, and impedance evolution of the symmetric cells with the (c) bare lithium electrode and (d) LFG electrode at open circuit voltage. Figure 3a exhibits voltage hysteresis of symmetric LFG and Li cells at a stripping/plating capacity of 1 mAh cm‒2 under a current density of 1 mA cm‒2. For the symmetric cell with bare Li, 10


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the voltage hysteresis increases rapidly after 100 cycles and increases to above 200 mV after 150 cycles. A sudden drop of voltage along with anomalous voltage fluctuation in following cycles is observed after 200 cycles, indicative of the formation of Li dendrites and unstable SEI film on the bare lithium electrode. By contrast, the LFG electrode has a much smaller voltage hysteresis of below 60 mV at the initial 200 cycles and maintains a long cycle life (over 600 h) without an obvious voltage fluctuation. The voltage profiles of LFG electrodes at different cycles further prove the relatively flat voltage plateau and lower voltage hysteresis of LFG than that of the bare lithium (Figure S3). Figure S4 and Figure 3b exhibit the cycling performance of bare Li electrodes and LFG electrodes with large capacities of 3 mAh cm–2 and 5 mAh cm–2, respectively. The voltage profiles of bare Li show sharp fluctuations at the initial several cycles, while the cell with LFG could be operated stably for over 300 h with a low voltage hysteresis. However, the fluctuating overpotential of bare-Li symmetric cell before 240 h in Fig 3b indicates large kinetic hindrances for lithium plating/stripping process compared with that with LFG anode, which depends on the nature of SEI and the processes at the Li/electrolyte interface. Nevertheless, the symmetric cell with bare Li exhibits a lower and more stable overpotential during the 240 h⁓330 h period. The major reason may be attributed to the rupture of SEI due to drastic volume change of Li anode and local short circuit caused by overgrowth of Li dendrites, which can result in both a large rise in the surface area of fresh Li and decrease in the overpotentials. After several cycles, the fresh Li is coated with SEI which leads to a large consumption of lithium and electrolyte. Therefore, the kinetic hindrances for lithium plating/stripping process show a sharp increase and symmetric cell with bare Li even exhibits a larger overpotential after 330 h. The superior stability and small voltage hysteresis of the LFG electrode could be attributed to the LiF-rich SEI film and graphene sheets formed after chemical/electrochemical lithiation which will be discussed later. The rate capability of the LFG electrodes was evaluated by cycling the symmetric cells under various current densities (0.5 to 5 mA cm‒2) at a capacity of 1 mAh cm‒2. As shown in Figure 3c, the LFG electrode shows a stable voltage polarization of 20, 30, 60, and 80 mV at current densities 11



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of 0.5, 1.0, 3.0, and 5.0 mA cm‒2, respectively. By contrast, the bare lithium electrode has a much larger voltage polarization of 50, 100, 200, and 300 mV at a same current density. The mild polarization of LFG indicates that the LFG could effectively adjust the local current density and facilitate Li-ion transport, suppressing the uneven Li stripping/plating at a large current density. Figure S5 exhibits the performance of bare lithium and LFG at a large current density (3 mA cm–2), where the better cycling stability of LFG is also evident. Figure 3d shows the polarized potential profile of bare Li, where the fluctuation and large hysteresis of voltage profiles indicate formation of lithium dendrites. On the contrary, LFG exhibits a flat and smooth polarized potential profile (Figure 3e). The LFG electrode can exhibit a capacity of over 96.8 mAh at a current density of 1 mA cm‒2 with a voltage hysteresis below 500 mV, which is close to the theoretical value of the Li plate (99.6 mAh). The enhanced cycling performance of LFG can be ascribed to the robust LiF-rich SEI layer which inhibits the growth of Li dendrites. Nevertheless, the cell shows high overpotentials in the late cycles in Figure 3a and Figure S5, this is because long-term volume change during Li plating/stripping process will cause fatigue rupture of the SEI layer and Li dendrites will grow along the cracks and eventually cause an increase in overpotential and the death of the cell. To solve the problem, the uniformity of protective FG layer needs to be further improved and the thickness of the layer should be further optimized. To address the volumetric changes of Li anode during cycling, a 3D matrix may be combined with our artificial SEI layer to obtain a Li composite anode with outstanding performance.

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Figure 3. Discharge/charge profiles of bare lithium electrode and LFG electrode in symmetric cells at a current density of 1 mA cm–2 with capacities of (a) 1 mAh cm–2, (b) 5 mAh cm–2, (c) rate capability of symmetric cells at 1 mAh cm–2, and polarized potentials of (d) bare Li and (e) LFG at a current density of 1 mA cm–2.

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To further investigate the influence of FG introduction on the electrochemical performance of Li, morphology changes of bare Li and LFG were observed after 50 cycles at a current density of 1 mA cm‒2 with an areal capacity of 1 mAh cm‒2. As shown in Figure 4a, the surface of bare Li becomes rough with the formation of cracks and aggregates after 50 cycles, which may be due to the uneven Li stripping/plating and the formation of “dead lithium”. The significant morphology variation of bare Li after cycling agrees well with the large voltage fluctuation and poor cycling stability. In contrast, as seen in Figure 4d and 4e, the LFG electrode still maintains a flat, crack-free surface after 50 cycles, indicating uniform Li stripping/plating and inhibited Li dendrites growth and “dead Li” formation. The stable interface of LFG compared to bare Li is attributed to the robust LiF-rich SEI layer and embedded graphene, which refrains growth of dendritic Li, ensuring excellent rate performance and a long cycling life of the LFG electrode. XPS characterizations were conducted to check the electrode composition of LFG after 50 cycles. As seen in Figure 4c, the peak intensity of C-F bond is weak and decreases obviously compared to that in the as-prepared LFG electrode (Figure 2b), while the intensity of LiF peak is significantly larger than that of the C-F bond. The result suggests that the FG in the LFG electrode has reacted with Li during cycling with the conversion of FG into LiF and graphene. The Li 1s spectrum in Figure 4f also confirms the existence of LiF in the LFG electrode after cycling. The LiF-rich SEI layer enables a robust interface with a high ionic conductivity for rapid Li-ion transport, inhibited Li dendrites formation and reduced interfacial reactions between Li and electrolyte. The generated graphene sheets are uniformly dispersed inside the LFG electrode and could regulate the local current density to promote the homogeneous Li deposition. The Li-plating process of bare Li and LFG was observed by in-situ optical microscopy (Figure 4g, Figure S6). From Figure 4g, it is clear that numerous Li dendrites form on the bare Li surface during lithium deposition, which will result in the rupture of 14


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SEI layer, degradation of Li electrode and even short circuit of the cell. While for the LFG electrode, minor surface variation was observed, indicating the uniform Li deposition with suppressed Li dendrites growth. It thus can be concluded that the presence of FG and its structural/compositional evolution upon cycling play a crucial role in determining the stripping/plating behaviors of metallic Li and thereby electrochemical performance of the Li electrode. Videos were recorded during the in-situ optical microscopy observation to see the dynamic changes of the electrodes with or without FG (see Video 1 and Video 2 in the supporting information). The videos are played at a speed 100 times that of the original one to more clearly show the changes. From the videos, it is evident that the inhibition of Li dendrites growth is effective with only 3 wt.% FG added.

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Figure 4. SEM images of (a, b) bare Li electrode and (d, e) LFG electrode after 50 cycles (inset is the digital photograph of the electrodes), (c) F 1s spectrum and (f) Li 1s spectrum of the LFG electrode after 50 cycles, and (g) in-situ optical photos of plating processes of bare Li and LFG at a current density of 1 mA cm–2 after 0, 15 and 30 min. To obtain further insight into the mechanism for enhanced electrochemical properties of Li by FG introduction, DFT calculation was performed to simulate the situation of Li intercalation into LFG. Eight Li atoms are intercalated among the FG sheet and the atom model is shown in Figure 5a. In the optimized structure, all the F atoms in the FG sheet are extracted, forming a graphene layer 16


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instead. As the C-F bonds are broken, the hybridization structure of the carbon changes from the initial sp3 to sp2 with the structure of FG converted to flat graphene. As shown in Figure 5b, the calculations indicate that the reaction is spontaneous and an energy of 4.081 eV is released with the insertion of each Li atom. The strong interactions between F atoms in the FG and Li atoms result in fracture of C‒F bonds with the formation of F vacancy sites. The generation of F vacancies will accelerate the degradation of the FG structure, as fewer charges are transferred to F layer from C layer. The Coulomb attractions between F layer and C layer thus become weakened, further promoting the breaking of the C‒F bonds. Namely, the presence of more F vacancies causes weaker C‒F bonds. Therefore, when F atoms leave from FG layers by a strong attraction of lithium atoms, rupture of the remaining C‒F bonds in the FG layers will become easier, and eventually all the F atoms are stripped from the FG layers, which can explain the conversion from FG to graphene after Li intercalation. Such conversion provides FG with a specific capacity of 617 mAh g–1 in the first cycle (Figure S8), which is rather lower compared with the whole capacity of the LFG anode. Hence, the capacity of FG will not be taken into consideration when we calculate the capacity of the anode. Combined with the experimental results, the difference in morphological changes between bare Li and LFG is compared in Figure 5c. For the bare Li electrode, the uneven deposition of Li, rupture of SEI layer during cycling, and Li dendrites growth seriously damage the electrode and arouse safety issue. For the LFG electrode, the robust LiF-rich SEI film can effectively inhibit the growth and penetration of lithium dendrites owing to the robust mechanical stability of generated graphene and high Li-ion conduction of LiF.42 In addition, conductive graphene inside the SEI film can regulate local current density and promote even distribution of Li nucleation sites that is also helpful to suppress the formation of Li dendrite.43 As a result, the LFG electrode demonstrates much improved rate capability and cycle stability. 17



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Figure 5. (a) Model of Li intercalation into LFG andthe optimized structure, (b) proposed major chemical reaction verified by DFT method, and (c) schematic illustration of morphological changes of the bare Li electrode and LFG electrode.

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Li‒O2 full cells are assembled to evaluate the potential applications of the LFG anode by using a δ-MnO2 catalytic cathode, where the specific capacity and current density of the cells were normalized to the weight of δ-MnO2 catalyst. The discharge plots of Li‒O2 cells with the LFG and bare Li anodes at 100 mA g‒1 are presented in Figure 6a. Note that the cell with LFG anode yields a high discharge capacity of 6520 mAh g‒1, while the cell with the bare lithium anode delivers a much lower discharge capacity of 2965 mAh g‒1. As shown in Figure S8, FG paired with Li metal anode shows a specific capacity of 617 mAh g–1 in the first cycle, but its capacity shows a rapid decline in the following cycles. This is because the conversion from FG to LiF and graphene is irreversible. Hence, based on the irreversible conversion and low content of FG in LFG (3 wt.%), the capacity of FG will not be taken into consideration for the calculation of capacities of the anode and the full cell. As shown in Figure S9a, the discharge capacity of Li/MnO2 cell in Ar is only 357 mAh g–1, compared with 1350 mAh g–1 in O2, indicating that Li-ion intercalation effect in MnO2 only contributes a small part to the total capacity of the cell. The significant increase in capacity for the cell with LFG anode is attributed to the homogeneous stripping of Li from LFG and the relatively low overpotential. EIS tests were performed to investigate the merit of LFG anode compared with bare Li. It reveals that the use of LFG anode efficiently reduces the charge transfer resistance of the Li‒O2 cell (Table S2, Figure S7). The cycling performance of Li‒O2 cells with LFG and bare Li anode was evaluated by cycling the cells at 400 mA g‒1 between 2.0 V and 4.5 V at a limited specific capacity of 1000 mAh g‒1. As indicated in Figure 6b, cell with the bare lithium anode can last only 22 cycles, while cell with the LFG anode could keep stable cycling of 91 cycles. Furthermore, Li‒O2 cell with LFG anode has a higher terminal voltage of discharge and lower terminal voltage of charge compared with that with bare Li, indicating a relatively low polarization which is a key factor to ensure a long cycle life. The Columbic efficiency comparison of Li‒O2 cells 19



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with the bare lithium and LFG anodes is presented in Figure S9b. As shown in Figure 6c, a rapid increase in polarization occurs for the cell with bare Li during cycle, while for the cell with LFG anode, the polarization increases slowly (Figure 6d). The sloping feature of voltage plateau in Figure 6c and 6d is caused by the sluggish kinetics of deposition of poorly conductive Li2O2 and Li-insertion into MnO2. To confirm the assumption, we charged/discharged the cell using longer rest time after the each charge step and discharge step, lower current density and higher limited capacity. As displayed in Figure S10, the initial voltage slope is due to the capacity of MnO2, and the sloping degree of voltage curves after 300 mAh g–1 decreases progressively with a longer rest time, decreased current density and larger limited capacity. A significant reduction in polarization voltage can be observed in Figure S10d during the rest time, while the cells presented in Figure 6c and 6d did not undergo such a rest time after the each charge step and discharge step. Hence, the voltage plateau cannot be clearly seen. Figure S11 and Table S3 also suggest that cycling of Li‒O2 cell with the LFG is highly reversible. The outstanding electrochemical properties of LFG is ascribed to unique structure of the LiF-rich SEI film which can facilitate rapid Li-ion transportation, inhibit Li dendrites growth, and alleviate the deterioration of Li from side reactions. The results demonstrate that the utilization of LFG anode increases the full-discharge specific capacity of the Li‒O2 cell and enhances its cycling stability considerably. To highlight the good cycling stability of Li‒O2 cell with the LFG anode, cycle life of the Li‒O2 cells using various Li anodes is compared in Table S4. The electrochemical performance of our cell using LFG anode is among the best when we comprehensively consider the limited capacity, the current density, and the cost-effective MnO2 catalyst used. It should be noted that the current price of FG is relatively high. However, with the mass production of FG, its price should be much reduced in the future. Therefore, the application of this

method

in

the

future

is 20


promising

and

practical.

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Figure 6. (a) Discharge plots of Li‒O2 cells with the bare Li electrode and LFG electrode at 100 mA g‒1, (b) cycling stability and (c, d) voltage profiles of Li‒O2 cells with the bare Li electrode and LFG electrode at 400 mA g‒1 at a limited specific capacity of 1000 mAh g–1.

■ CONCLUSIONS In summary, we fabricated a lithium composite anode composed of metallic Li and fluorinated graphene. The composite anode exhibits a low resistance with effectively alleviated electrode degradation over a long time. Compared with bare Li, LFG anode shows improved cycling stability with a low polarization for over 300 cycles at 1 mA cm–2, and realizes stable cycling even at a high areal capacity (5 mAh cm–2) or a high current density (5 mA cm–2). SEM and in-situ optical microscopy observations confirm dendrite-free Li stripping/plating of LFG. The excellent property of LFG is ascribed to the robust LiF-rich SEI layer formed in situ during cycling. The SEI layer could facilitate Li-ion conductivity, block electron transfer, suppress Li dendrites growth, and inhibit reaction between Li and electrolyte. DFT calculation confirms the conversion from FG to LiF and graphene after Li intercalation, and the in-situ generated graphene can regulate local 21



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current density and further alleviate Li-dendrite formation. Li‒O2 cell with LFG anode shows obviously increased full-discharge capacity and better cycling stability than that with bare lithium anode. This work will provide a practical method to design stable Li anodes for lithium metal batteries with a high energy density.

■ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications Website at DOI: 10.1021/ SEM image of FG, equivalent circuit for EIS, EIS fitting of the half cells, voltage profiles of Li and LFG half cells at 1 mAh g‒1 under 1 mA cm‒2, discharge/charge profiles of lithium and LFG half cells at 3 mAh g‒1 under 1 mA cm‒2, discharge/charge profiles of lithium and LFG half cells at 1 mAh g‒1 under 3 mA cm‒2, devices for in-situ optical microscopy observation, EIS and fitting of Li–O2 cells with Li and LFG anodes, discharge profile and cycling performance of Li/FG cell, discharge profiles of the Li/MnO2 cells in Ar and O2 and the Coulombic efficiency of Li–O2 cells, charge/discharge profiles of the Li‒O2 cells with LFG anode with rest time and large limited capacity, EIS and fitting of Li–O2 cells with LFG anode at different states, and a comparison of cycling stability of the Li–O2 cells using different Li anodes

■AUTHOR INFORMATION Corresponding Author *Tel/Fax: +86-571-87951451. E-mail: [email protected]. (J. X.) Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS 22


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This work was supported by the National Natural Science Foundation of China (No. 51572238), and Zhejiang Provincial Natural Science Foundation of China under Grant no. LY19E020013.

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