Facile construction of a hybrid artificial protective layer for stable lithium metal anode

Facile construction of a hybrid artificial protective layer for stable lithium metal anode

Journal Pre-proofs Facile Construction of a Hybrid Artificial Protective Layer for Stable Lithium Metal Anode Guangmei Hou, Caleb Ci, Huanhuan Guo, Xi...

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Journal Pre-proofs Facile Construction of a Hybrid Artificial Protective Layer for Stable Lithium Metal Anode Guangmei Hou, Caleb Ci, Huanhuan Guo, Xiang Zhang, Qidi Sun, Jun Cheng, Devashish Salpekar, Qing Ai, Long Chen, Anand B. Puthirath, Keiko Kato, Samuel Castro Pardo, Robert Vajtai, Ganguli Babu, Lijie Ci, Pulickel M. Ajayan PII: DOI: Reference:

S1385-8947(19)32957-2 https://doi.org/10.1016/j.cej.2019.123542 CEJ 123542

To appear in:

Chemical Engineering Journal

Received Date: Accepted Date:

12 November 2019 17 November 2019

Please cite this article as: G. Hou, C. Ci, H. Guo, X. Zhang, Q. Sun, J. Cheng, D. Salpekar, Q. Ai, L. Chen, A.B. Puthirath, K. Kato, S. Castro Pardo, R. Vajtai, G. Babu, L. Ci, P.M. Ajayan, Facile Construction of a Hybrid Artificial Protective Layer for Stable Lithium Metal Anode, Chemical Engineering Journal (2019), doi: https://doi.org/ 10.1016/j.cej.2019.123542

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© 2019 Published by Elsevier B.V.

Facile Construction of a Hybrid Artificial Protective Layer for Stable Lithium Metal Anode Guangmei Houa, b, Caleb Ci

c, a,

Huanhuan Guob, Xiang Zhang a, Qidi Sunb, Jun

Chengb, Devashish Salpekar a, Qing Aia, b, Long Chena, b, Anand B. Puthiratha, Keiko Kato a, Samuel Castro Pardo a, Robert Vajtai a, Ganguli Babu a *, Lijie Cib*, Pulickel M. Ajayan a * aDepartment

of Materials Science and NanoEngineering, Rice University, Houston, TX 77005,


Rice Joint Center for Carbon Nanomaterials, Key Laboratory for Liquid-Solid Structural

Evolution & Processing of Materials (Ministry of Education), School of Materials Science and En gineering, Shandong University, Jinan 250061, China cGlenda

Dawson High School, Houston, TX 77584, USA


lithium anode, dendrite growth, hybrid protective layer, in-situ reaction

Abstract Lithium metal is considered as the ultimate anode for next-generation rechargeable batteries due to its high theoretical specific capacity and low electrochemical potential. However, the commercial application of lithium anode is hampered by its dendritic growth during the charging process resulted from the unstable lithium/electrolyte interface. Herein, we demonstrate the formation of a hybrid protective layer consists of LixAl, LiCl and organics on the lithium anode surface. This stable hybrid layer facilitates uniform distribution of Li-ion to eliminate the

surface inhomogeneity and thus suppress the dendrite formation. As a result, the modified metallic lithium anode realizes long-term stable cycling with a minimal polarization at 1mA cm-2 for 1000 h. Furthermore, cells with protected lithium as anode and LiFePO4 as cathode were cycled up to 600 cycles at a 1C rate with higher capacity retention. This work presents an effective way to regulate a stable lithium anode-electrolyte interface with the formation of a hybrid artificial protective layer to advance lithium metal batteries, which may provide good references for the practical application of Li metal batteries. 1. Introduction With high theoretical specific capacity (3860 mA h g−1) and the lowest electrode potential (3.04 V vs. standard hydrogen electrode), lithium metal anode exhibits enormous potential for next-generation lithium battery systems with higher energy density. Unfortunately, it suffers from short cycle life and poor Coulombic efficiency, which severely hinders the practical application of rechargeable lithium metal batteries (LMBs). This arises from the highly active nature of metallic Li, which generates solid electrolyte interphase (SEI) layer on the surface once the lithium electrode is exposed to the non-aqueous electrolyte. The intrinsic nonuniformity and poor mechanical properties of the spontaneously formed SEI lead to inhomogeneous lithium deposition (dendrite growth) and accelerated consumption of active Li metal and electrolyte during the repeated charge/discharge cycles. What’s worse, the notorious lithium dendrite growth may trigger short-circuiting and can even result in disastrous explosions and fire. Therefore, the eventual success realization of a safe

and stable metallic lithium anode relies on a chemically and mechanically stable SEI film.[1, 2] For decades, researchers have made considerable efforts to tackle this issue. For instance, lithium dendrite suppression by greatly reduced local current density through the design of 3D current collectors with increased specific surface area.[3-8]Surface chemistry modification of the conductive scaffolds with lithiophilic decoration has been reported to further regulate the lithium deposition behavior especially in the initial stage.[9-11] Despite the improvements in lithium anode morphology and performance, metallic lithium/framework composite will greatly weaken the specific capacity advantage of lithium metal anode. Therefore, interfacial issue










always been emphasized.[12-16] A routine strategy is engineering the electrolytes, such as using additives and developing new electrolytes, to remodel and strengthen the naturally formed unfavorable SEI film.[17-28] However, the sacrificial film-forming constituents are consumed upon cycling, leading to continuous compositional and structural evolution of the passivation layers formed using this method. Moreover, some of the added compounds have detrimental effects (such as gas generation and corrosion of the cathode or current collector) on the full cell.[29] The in situ-formed SEI on the thermodynamically unstable Li in organic electrolyte still does not satisfy the demands for practical application. Thus, recently, more attention has been paid to replace the electrolyte-derived SEI with ex-situ-fabricated artificial SEI protective layers.[30-33] For instance, ~10 nm-thick two-dimensional

MoS2 was directly coated on the surface of Li metal via sputtering, the lithiated MoS2 layer benefits a homogeneous flow of Li ions onto and out of the electrode.[34] Kozen et al reported that Al2O3 layers fabricated directly on Li metal by atomic layer deposition (ALD) with exquisite thickness control could effectively protect the active metallic Li from being corroded by the electrolyte.[35] Although much improvement in Li anode performance has been achieved, scaling up these methods in practical batteries is hindered by their complexity or high cost. Simple methods of spin-coating and doctor-blade casting have been reported to be used for preparation of ex-situ organic films such as poly (dimethylsiloxane) (PDMS) and “solid-liquid” Silly Putty.[36, 37] However, these kinds of polymeric layers suffer from inferior lithium ion transport capability and will reduce the specific capacity of a Li anode. Therefore, constructing an ion-conducting surface layer capable of solving these multifaceted problems is essential to achieve high-performance Li anodes. Herein, we propose a hybrid artificial protective layer by a facile in situ chemical reaction between Li metal and AlCl3 in solution. The reduction of the aluminum chloride and subsequent alloying reaction with Li gives a film comprised of lithium-enriched LixAl alloy, insulating LiCl with lower energy barrier for surface diffusion and flexible organics. Modified lithium anode shows much-improved stability and low reactivity against metallic lithium, which is capable of reducing the occurrence of side reactions and inhibit the formation of Li dendrites (Figure 1). As a result, the modified metallic Li exhibits a stable electrode structure and good cycle performance in symmetric cells. The fabricated batteries using the modified lithium

anode pairing with LiFePO4 cathode exhibits significantly improved cycling stability and rate performance.

Figure 1. Schematic illustration of the hybrid artificial protective layer fabrication process and Li plating/stripping behavior for (a) pristine Li and (b) modified Li anode during cycling.

2. Experimental section 2.1. Materials and Electrode Preparation Anhydrous AlCl3 (99.999%, Sigma-Aldrich) and tetrahydrofuran (THF, anhydrous, 99.9%, inhibitor-free, Sigma-Aldrich) were used as received without further purification. The modified lithium foil was prepared as follows in an argon-filled glovebox with <0.1 ppm moisture and oxygen. Firstly, original Li foil (99.9%, Alfa Aesar) was slightly pressed onto a glass plate and polished using a nylon brush to remove the surface contaminants. Then a certain amount of 0.05 mol Kg-1 AlCl3 solution in THF was spread out on the shiny surface of Li by drop casting. Subsequently, Li foils coated with the black protective layers were rinsed with THF and kept in the glovebox for further use after total volatilization of the THF. The LFP cathodes were fabricated by casting the uniform N-Methyl-2-pyrrolidone (NMP, anhydrous, 99.9%, Sigma-Aldrich) slurry composed of the commercial LFP, super P

and poly(vinylidene difluoride) with a weight ratio of 8:1:1 onto the aluminum foil with Automatic Film coater. The cathodes were then punched into disks after vacuum-drying at 80 °C for 12 h. 2.2. Characterization The morphology of the Li electrode before and after cycling was characterized by a scanning electron microscope (SEM, JSM-6500F, JEOL). The surface smoothness was analyzed under the atomic force microscopy (AFM, Park NX20). Surface composition of the protective layer was confirmed by X-ray photoelectron spectroscopy (XPS) analysis on PHI Quantera Scanning X-ray Microprobe. All the samples were sealed in a container to avoid exposure to air before being quickly transferred to the vacuum chamber of testing equipment. 2.3. Electrochemical Measurement Electrochemical studies were carried out in coin cells (CR2032-type, MTI). All the cells were assembled in an argon-filled glovebox with <0.1ppm moisture and oxygen. For impedance and Li dissolution/deposition processes studies, symmetric Li | Li cells with pristine Li or modified Li electrodes were assembled using 60 µl electrolyte






dioxolane/dimethoxyethane (DOL/DME) (1:1 vol) with 2 wt% LiNO3 or 1 M lithium hexafluorophosphate (LiPF6) in a mixture of ethylene carbonate/dimethyl carbonate (EC/DMC,1:1 v/v) and Celgard 2325 or Whatman glass-fiber as separator. The symmetric cells were then operated in galvanostatic mode at different current densities with different plating/stripping capacities. Electrochemical impedance

measurements were performed using Metrohm-Autolab B.V. with a frequency range of 1M Hz to 1 Hz at room temperature. To investigate the practical application of the hybrid protective layer, LFP | Li full cells were assembled with LFP cathode, Celgard 2325 separator and pristine or modified Li anode employing 60 µl carbonate electrolyte of 1.0 M lithium hexafluorophosphate (LiPF6) in a mixture of EC/DMC. After three activation cycles at 0.1 C, LFP | Li full cell was cycled at 1 C within a voltage window of 2.0-4.0 V based on the theoretical capacity of LFP cathode. The cycling of all the coin cells was conducted on a Land 2001A multi-channel battery tester. 2.4. In Situ Optical Microscope Studies A home-made transparent cell was used for the in situ optical microscope (OM) experiments. A piece of bare Li foil was employed on one side of the slot as the reference and counter electrodes, and pristine or the modified Li foil was placed on the other side as the working electrode. A piece of glass-fiber separator was placed between them before adequate amount of electrolyte (1 M LiPF6 in EC/DMC) was injected in the slot. Cu foil strips were placed under both electrodes as current collector. The morphology evolution during the Li metal deposition process at a current density of 2.0 mA cm−2 was in situ recorded by the optical microscope (LB-616, Labomed, USA) combined with a CCD camera. 3. Results and discussions

Figure 2. SEM image of the pristine Li surface (a); SEM image (b), the corresponding EDX maps (c, d) and cross-section SEM image (g) of the modified Li anode; AFM topographic images of the pristine Li (e) and modified (f) Li anode.

The hybrid protective film is constructed onto the surface of Li metal by a facile and scalable method through in situ reaction between aluminium chlorides (AlCl3) and Li metal at room temperature. When the AlCl3 precursor solution is dropped on Li foil surface, the silvery Li foil turns into black within seconds (Figure S1, Supporting Information). The formation process of the hybrid protective layer can be supposedly summarized to be multiple synergistic chemical reactions: AlCl3+3Li →Al+3LiCl (1); xLi+M→LixAl (2); Open-ring polymerization of THF promoted by LiCl and AlCl3 (3). AlCl3 is directly reduced by Li metal and the generated metallic Al (co-exists with LiCl) immediately undergoes alloying reaction with the underlying lithium to form the LixAl alloy. Scanning electron microscopy (SEM) is used to observe the morphology of the Li anode with and without the hybrid protective layer (hereafter denoted as modified Li and pristine Li, respectively). Compared to the rugged surface of the pristine Li, the modified Li is adequately smoothed covered by a uniform

protective layer (Figure 2a, b). The topographic AFM images (Figure 2e, f) also exhibit a much more conformal morphology of the modified Li. Energy dispersive spectroscopy (EDS) confirms a uniform distribution of Al and Cl in the hybrid layer (Figure 2 c, d). Note that the morphology of the protective layer depends on the dropping amount of the precursor solution. The insufficient or excessive reaction leads to inadequate cover or rupture of the composite film (Figure S2, Supporting Information). The thickness of the optimal hybrid protective layer is measured to be ~2.5 μm, as shown in Figure 2g, which is treated by dropping an appropriate amount of 20 µl cm-2 AlCl3 solution.

Figure 3. XPS analysis of the hybrid protective layer on modified Li anode before and after sputtering etching (Raw data: open circle; Cumulative fit peak: solid gray line; Fit peaks: solid line in other color).

The compositions of the hybrid layer are investigated by X-ray photoelectron spectroscopy (XPS) depth profile (Figure 3). As shown in Figure 3a, the Li 1s spectrum before sputtering is fitted into three peaks centered at 55.89, 55.18 and 54.39 eV, which can be assigned to LiCl, Li2CO3 and LiOH, respectively. Only signals of LiCl and LixAl are visible in Li 1s after Ar sputtering for 5 min and 10 min. High-resolution Al2p XPS spectra was analyzed to verify the existence form of Al. As shown in Figure 3b, the XPS spectra of Al 2p of the very uppermost layer can be fitted into LixAl alloy (72.06 eV), Al (72.89 eV) and Al2O3 (73.74 eV) peaks. The disappeared signal of Al2O3 after Ar sputtering shows that it only exists on the uppermost surface. Therefore, Al2O3 may be formed by oxidation of Al during the sample transfer process. The obvious increase in fraction of LixAl alloy after Ar-ion sputtering is an evidence of an alloy concentration gradient, which is resulted from the kinetically limited self-alloying process. Besides, the XPS spectra of Cl 2p is fitted into characteristic peaks of LiCl centered at 198.08 eV and 199.62 eV before and after Ar-ion sputtering (Figure 3c), which suggests its uniform distribution in the thickness.[38] As a contrast, no signals of Al and Cl were detected for pristine Li foil (Figure S3, Supporting Information). High-resolution XPS C1s spectra can be divided into two peaks at 286.3 and 284.6 eV, respectively representing C-O and C-C from organic products of THF ring-opening polymerization reaction (Figure 3d)[39, 40]. Need to note that only CO32- peak without evidence of organic composition was detected for Li metal treated by pure THF (see Figure S4 in Supporting Information for more details). Therefore, the protective layer is composed of LixAl, LiCl and

organics with a minor contribution of insufficiently lithiated Al, of which the components penetrate well with each other. LixAl alloy exhibits great promise to stabilize metallic Li anode and inhibit dendrite formation as a Li-ion conductor.[41] This insulating LiCl can increase the resistance of the hybrid layer, which is important to avoid Li deposition on the top of this layer. In addition, LiCl shows lower energy barrier for surface diffusion that of LiF based on recent joint density functional theoretical analysis, which means it can facilitate fast Li ions transport in the plane and benefit for a compact Li deposition.[42] The organic components at the outer layer of the protective layer can improve its mechanical property. The invisibility of LixAl alloy and LiCl signal in the XRD results suggests their amorphous or nanocrystalline nature (Figure S5, Supporting Information). The stability of the hybrid protective film was studied by Electrochemical Impedance Spectroscopy (EIS) tests of Li | Li symmetric cells composed of pristine Li or modified Li. The impedance is recorded after different rest times, which indicates the extent of lithium corrosion by reacting with the electrolyte. As shown in Figure S6, impedance of the pristine Li | Li cell underwent almost a sevenfold increase during the first 24 h rest and then kept stable. On the contrary, the modified Li | Li cell exhibits stable impedances with a negligible increase over 48 h. The later possibly benefits from the high stability of hybrid film, which prevents direct contact and reaction between metallic Li and electrolyte.

Figure 4. Operando optical microscopy images of continuous Li deposition on pristine (a-d) and modified (e-h) Li anodes Li at a current density of 2.0 mA cm−2.

In situ optical microscopy observation of Li deposition on pristine and modified Li anode was conducted in a homemade sealed transparent cell (Figure S7, Supporting Information) to investigate the dendrite-suppression behavior of the hybrid protective layer. Photos taken at different plating time at a current density of 2 mA cm-2 are shown in Figure 4. Before Li deposition, the surfaces of both pristine and the modified Li were smooth. Uneven mossy Li arose only after 10 min on pristine Li surface, and then they continuously grew and evolved into highly developed dendrite clusters with high specific surface area (Figure 4a-d). On the contrary, a shiny compact Li layer without dendrites were observed on the surface of modified Li even after Li plating for 40 min (Figure 4e-h), indicating a strong capability of the hybrid protective film to effectively suppress dendritic and mossy Li growth.

Figure 5. Voltage–time profiles of the galvanostatic Li plating/stripping process for symmetric cells in 1M LiTFSI 1:1 DOL:DME with 2 wt% LiNO3: (a) current density: 1mA cm-2, capacity: 1mA h cm-2;(b) current density: 4 mA cm-2, capacity: 2 mA h cm-2; (c) 0.5~5 mA cm-2, capacity: 1mA h cm-2; (d) cycling performance of pristine Li | LFP modified Li | LFP full cells at 1C; (e, f) rate performance and corresponding voltage-capacity profiles of the Li | LFP full cell.

Galvanostatic dissolution/deposition of Li at different current densities was carried out in symmetric Li | Li cells to evaluate the electrochemical performance of the hybrid film protected Li anode. Stable voltage profiles with small voltage hysteresis (~20 mV) were achieved for the modified Li | Li cell even after cycling for 1000 h at a current density of 1 mA cm-2 with a capacity of 1 mA h cm-2 (Figure 5a). However, the over-potential of the pristine Li | Li cell is quite unstable, of which the obvious decrease after a period of increase is an indicator of micro short-circuits. The

cycling performance of the symmetric cells composed of Li anodes treated by other amounts of AlCl3 solution is shown in Figure S8-9. The separators of the cells after cycling were harvested and observed under SEM. Many rips in the same direction are observed in the enlarged image of the separator from the pristine Li | Li cell, which is caused by nonuniform Li deposition (Figure S10, Supporting Information). The Li bulges formed at the weak tensile strength direction of the separator may be “pushed” through the membrane by the localized stress caused by volume expansion during cycling, causing soft short circuits of the cell.[13, 43] In order to further illustrate the dendrite-induced short circuits, a glass-fiber membrane with a wide range of pore sizes, has also been used here to induce dendrite growth. The voltage profiles as a function of time for the Li | Li cells tested at exactly the same conditions in Figure 5a except that the Celgard is replaced by glass-fiber membrane are shown in Figure S11. The pristine Li | Li cell experienced a sudden voltage drop after cycling only for 200 h, while the modified Li | Li cell shows stable cycling with a small over-potential even after 800 h. From the images of the disassembled electrodes after plating/stripping for 100 cycles, a thick glass-fiber layer with numerous entangled Li dendrites was observed on the pristine Li anode. This is because Li dendrites grew through the porous membrane (Figure S12a, Supporting Information). However, the surface of the modified Li is much cleaner with very few fibers stick on it (Figure S12b, Supporting Information), indicating much-suppressed dendrite growth. Surprisingly, the modified Li symmetric cells also exhibit an exceptionally stable voltage profiles even after cycling for 900 h at a high current density of 4 mA cm−2

with a capacity of 2 mA h cm−2 (Figure 5b). However, pristine Li | Li cell experienced obvious voltage oscillation and much higher over-potential, reflecting an unstable interface and serious Li corrosion process in which insulating SEI layers are continuously generated with rapid consumption of the electrolyte (Figure 1a). Cycling performance of the symmetric cells in 1 M LiTFSI 1:1 DOL:DME without LiNO3 was also evaluated (Figure S13,Supporting Information). XPS spectra were collected to investigate the in-situ formed SEI on the surface after cycling (Figure S14, Supporting Information). The cycling stability of symmetrical cells in carbonate electrolyte was also evaluated, which shows that the modified Li has a nearly threefold cycle life to that of the pristine Li (Figure S15, Supporting Information).The modified Li | Li cell also exhibits much better rate capability than the pristine Li | Li cell when the current density varied from 0.5 to 5 mA cm–2 as shown in Figure 5c. All these results indicate that the hybrid protective film contributes to achieving a stable and homogeneous Li deposition/dissolution behavior. Low voltage hysteresis results from the sufficient interpenetration of Li-ion conductive LixAl and LiCl with lower energy barrier for surface Li+ diffusion. Lithium-metal batteries using modified Li anode matched with LiFePO4 cathode were made to study the potential application of the hybrid protective film. As shown in Figure 5d, the modified Li | LFP cell delivers an initial discharge capacity of 126.3 mAh g−1 at 1C and still maintains a high value of 94.5 mA h g−1 after 300 cycles (74.8% capacity retention after 300 cycles). Although the pristine Li | LFP cell shows a similar initial discharge capacity, it experienced obvious capacity decrease from

124.4 mA h g−1to only 28.4 mA h g-1 after 600 cycles. The rate performance of the Li | LFP full cell is displayed in Figure 5e. The modified Li | LFP cell realized high discharge capacities of about 148, 134, 122, 100 mA h g−1 at 0.5, 1, 2, 5 C, respectively. After cycling at 5 C, the discharge specific capacities recovered to 154 mA h g−1 at 0.1 C. The pristine Li | LFP offered discharge capacities of 141, 132, 117, 82 mA h g−1 from 0.5 C to 5C. The higher capacities, especially at a higher rate, indicate fast charging/ discharging capabilities of modified Li anode, which is further demonstrated by the charge/discharge voltage-capacity curves at 0.5 to 5C shown in Figure 5f. The reduced voltage hysteresis for modified Li | LFP suggests a more uniform Li plating/striping processes for the metallic Li protected by the hybrid layer. EIS and SEM results further validated the improved performance of modified Li anode (Figure S16-17, Supporting Information). In order to further understand the cycling stability of the modified Li anode, electrodes after different cycles in symmetric cells were obtained and characterized. As shown in Figure 6a, the pristine Li anode shows uneven surface after the 30th cycle. A porous lithium layer with a maximum thickness of 405 µm was formed after 400 cycles (Figure 6e), which is easily broken and peeled off from the bulk lithium anode, exposing massive mossy lithium dendrites (Figure 6c). On the contrary, a relatively smooth and uniform surface was observed for the modified Li after 30 and 400 cycles (Figure 6b, d). From the cross-section SEM for modified Li after 400 cycles, the top lithium layer is much more compact and in good contact with the substrate with a thickness of 152 µm (Figure 6f). Photograph of separators and Li

anodes after plating/stripping for 30 cycles were shown in Figure S18, displaying more uniform Li reaction for modified Li anode intuitively. All of the data above revealed that the hybrid protective film could mitigate side reactions of metallic lithium with the electrolyte and effectively suppress dendrite growth, and thus improve the cycling stability of LMBs.

Figure 6. SEM images of surface morphologies for the Li anodes obtained from (a, c, e) pristine Li | Li and (b, d, f) modified Li | Li symmetric cells after (a, b) 30 and (c-f) 400 cycles at 1 mA cm-2 with a capacity of 1 mA h cm-2 in 1M LiTFSI 1:1 DOL:DME with 2 wt% LiNO3.

4. Conclusions In summary, we demonstrate a stable lithium metal anode enabled by a hybrid protective layer with a thickness of 2.5 µm formed by facile in-situ reactions. The hybrid layer composed of highly interpenetrated Li-ion conductive LixAl, LiCl with lower energy barrier for surface Li+ diffusion and organics can protect the Li-metal anode, stabilize the lithium–electrolyte interface and thus enables homogeneous

lithium deposition during repeated lithium deposition/dissolution. Compared to pristine lithium anode, improved cycling stability and rate capability have been achieved benefit from uniform utilization of metallic lithium. As a result, symmetric modified Li | Li cell exhibits stable cycling with small overpotential over 1000 h at a current density of 1 mA cm−2 with a capacity of 1 mA h cm−2. A much more compact lithium layer was obtained after 400 cycles for modified lithium anode compared to the loose structure of cycled pristine lithium. An exceptionally stable voltage profile over 900 h cycling is obtained even at a high current density of 4 mA cm−2 with a capacity of 2 mA h cm−2. It also demonstrates the prospect for practical application in modified Li | LFP full cell, which delivers a discharge capacity of 94.5 mA h g−1 after 300 cycles at 1 C. In a word, this strategy demonstrates new possibilities of designing artificial protective films to settle the Li dendritic problem and yield more practical lithium metal batteries with prolonged lifespan. ACKNOWLEDGMENTS The authors acknowledge funding support from High-level Talents' Discipline Constr uction Fund of Shandong University (31370089963078), Shandong Provincial Scienc e and Technology Major Project (2018JMRH0211 and 2017CXGC1010), the Fundam ental Research Funds of Shandong University (2017JC042 and 2017JC010), and the Natural Science Foundation of Shandong Province (ZR2019MEM052 and ZR2017M EM002). NOTES The authors declare no competing financial interests.

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Highlights ⚫ The hybrid artificial protective layer is fabricated by a facile method. ⚫ Uniform plating of modified Li is confirmed by in situ optical microscopy study. ⚫ The modified Li anode realizes long-term stable cycling with minimal polarization. ⚫ The modified anode exhibits much improved performance in full battery.