High Current Enabled Stable Lithium Anode for Ultralong Cycling Life

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Energy, Environmental, and Catalysis Applications

High Current Enabled Stable Lithium Anode for Ultra-Long Cycling Life of Lithium-Oxygen Batteries Huanhuan Guo, Guangmei Hou, Deping Li, Qidi Sun, Qing Ai, Pengchao Si, Guanghui Min, Jun Lou, Jinkui Feng, and Lijie Ci ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08153 • Publication Date (Web): 06 Aug 2019 Downloaded from pubs.acs.org on August 6, 2019

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ACS Applied Materials & Interfaces

High Current Enabled Stable Lithium Anode for Ultra-Long Cycling Life of Lithium-Oxygen Batteries Huanhuan Guo,1 Guangmei Hou,1 Deping Li,1 Qidi Sun,1 Qing Ai,1 Pengchao Si,*,1 Guanghui Min,1 Jun Lou,2 Jinkui Feng,1 Lijie Ci,*,1 1SDU

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

Structural Evolution & Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, China 2

Department of Materials Science and NanoEngineering, Rice University, Houston,

Texas 77005, United States *Correspondence: [email protected] (L.C.). [email protected] (P.S.)

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The rechargeable lithium-oxygen batteries (Li-O2 batteries, LOBs) with extremely high theoretical energy density have been regarded as a promising next-generation energy storage technology. However, the limited cycle life, undesirable corrosion and safety hazards are seriously limiting the practical application of lithium metal anode in LOBs. Here, we demonstrate a rational design of Li-Al alloy (LiAlx) anode that successfully achieves ultra-long cycling life of LOBs with stable Li cycling. Through in-situ high current pre-treatment technology, Al atoms accumulates and a stable Al2O3-containing solid electrolyte interphase (SEI) protective film formed on the LiAlx anode surface to suppress side reactions and O2 crossover. The cycling life of LOB with protected LiAlx anode increases to 667 cycles under a fixed capacity of 1000 mA h g-1, as compared to 17 cycles without pre-treatment. We believe that this in-situ high current pre-treatment strategy presents a new vision to protect lithium-containing alloy anode, such as Li-Al, Li-Mg, Li-Sn and Li-In alloy for stable and safe lithium metal batteries (Li-O2 batteries and Li-S batteries).

KEYWORDS: Li-O2 battery, Li-Al alloy, Stable Li anode, High current pretreatment, Al2O3 protective film

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INTRODUCTION Rechargeable batteries have attracted great attention for their applications in electric vehicles, wearable electronic devices and energy storage systems. Among various electrochemical battery systems, lithium oxygen battery (LOB) has been widely supposed as a promising next-generation energy storage technologies for its extra high theoretical energy density.1-3 However, a few challenges restrain the development of practical LOBs, such as low energy efficiency, limited practical capacity and capacities fading during cycling, short lifespan and safety hazards. These defects of LOBs are caused by multiple factors including sluggish electrochemical oxidation kinetic of Li2O2, instability of air electrode and Li metal electrode, electrolyte decomposition and undesired gas contamination.4-7 Developing efficient oxygen evolution reaction/oxygen reduction reaction (OER/ORR) catalysts8-10, designing appropriate cathode structures and electrolyte systems to accelerate electrochemical kinetics and inhibit side-effect on cathode have received extensive research attention for LOBs.11-18 Despite the rapid development of air electrode and electrolyte system, several remaining hurdles caused by lithium metal anode should be overcome prior to the practical application of lithium air batteries: 1) limited service life caused by thermodynamic instability and continuous reaction of Li metal with electrolyte due to its high Fermi energy level; 2) severe safety hazards resulted from the uncontrollable lithium dendrite growth;

3, 19-22

3) corrosion

with dissolved O2, H2O and discharge intermediates, which leads to seriously irreversible consumption of active lithium during cycling.23-27 For the past four decades, masses of researches have been done on Li metal stripping/plating behavior, but the

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problems faced on lithium metal batteries have not been solved effectively.28 Therefore, protecting lithium anode in LOBs is an extremely challenging issue due to its semiopen nature. In response, few strategies have been employed to increase the cycling stability of lithium metal anode in LOB, including introducing organic or inorganic SEI protective film23,

25, 26, 29-31,

polymer electrolytes,32 electrolyte additives13,

33

and

composite anode34. These approaches have made much progress in restraining lithium corrosion and lithium dendrite growth, but there are still issues of the complexity of processing conditions, high cost, instability of the lithium-protective layer and limited lifespan. Therefore, it’s of great importance to develop stable lithium anode materials for practical LOBs. Ding and Aurbach et al. have found that trace Al doped in lithium metal would not radically change the charge/discharge characteristic of the anode, such as ultrahigh theoretical specific capacity and very low redox potential. Improved interfacial stability between electrolyte and Li-Al alloy is confirmed to improve the performance of batteries.35, 36 In this work, we report a simple in-situ high current pre-treatment method for long-term cycling LOBs. Al atoms accumulated and Al2O3-containing SEI layer formed on the LiAlx anode surface in O2 atmosphere under high current cycling. EDX line scanning technology is employed on the cross-section of LiAlx anode to illustrate the distribution of elements. Meanwhile, the optimized pre-treating time is explored to generate stable and thin Al2O3-containing SEI layer. The pre-treated LOB with LiAlx anode exhibits ultra-long cycling life (667 cycles), which is 38 times than that of the pristine LOB without pre-treatment. For comparision, the pre-treated LOB with pure

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Li anode without Al element died early due to the seriously broken Li anode after high current pre-treatment. Moreover, the morphology of the protective LiAlx anode before and after long term cycling in LOBs is presented through ex-situ SEM characterizations. EXPERIMENTAL SECTION Preparation of Cathode with N-doped Graphene Aerogel (NGA) Catalyst The NGA was synthesized as reported in our previous work.37 Briefly, 2.4 g urea was dissolved in 40 mL graphene oxide aqueous suspension (2 mg mL-1). A hydrogel was prepared through a hydrothermal reaction of the above mixture at 180 oC for 12 h. After freeze-dried and annealed, NGA was synthesized as catalyst material. The air electrode was prepared by dropping a mixture of NGA and PTFE with a weight ratio of 9:1 on a carbon paper. And the final NGA loadings were about 0.20 mg after being vacuum-dried at 110 oC for 12 h. Assembling of LOBs and the Pre-treatment Method Coin-type porous LOBs (CR-2032) were assembled in a glove box filled with Ar gas, where both H2O and O2 concentrations were ≤ 0.1 ppm. Typically, a LOB composed of a LiAlx metal anode (11 mm, with 0.2 wt % Al content, China Energy Lithium Co., LTD, Type B001), electrolyte, a glass fiber filter and a cathode. The electrolyte was 1 M bis(trifluoromethane)sulfonimide lithium (LiTFSI) salt in tetraethylene glycol dimethyl ether (TEGDME). The electrolyte was purchased from DoDoChem Company and its H2O content was lower than 20 ppm. LOBs were tested in a sealed bottle with high purity oxygen (1 atm). Firstly, the assembled LOBs with LiAlx metal anodes were cycled at a high current of 800 μA (equivalent to 0.8 mA cm-2 relative to the anodes)

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for several times (from 5 times to 50 times) to a capacity of 1000 mA h g-1 for pretreatment.

And

the

pre-treated

LOBs

are

named

as

LiAlx-800-N

cells

(N=5/10/20/30/50). When the pre-treating current varied from 300 μA to 2000 μA for 30 times, LOBs are mentioned as LiAlx-C-30 cells (C=300/600/800/1000/1500/2000). Then, galvanostatic discharge/charge tests of the pre-treated LOBs were performed at a relative low current of 100 μA (equivalent to 0.08 mA cm-2) with a capacity of 1000 mA h g-1 for regular cycling. The voltage window for the galvanostatic discharge/charge measurements is 2.0～4.9 V to eliminate the restriction of catalyst on the air electrode. For comparison, the electrochemical performance of LOBs with LiAlx anode and Li anode were conducted without pre-treatment, as marked as pristine LiAlx cell and pristine Li cell. Li-800-30 cell was obtained through pre-treating at 800 μA for 30 times. Assembling of the Symmetric Cells Symmetric LiAlx/electrolyte/LiAlx cells were assembled into porous CR-2032 coin structure. LiAlx-800-30 anodes were obtained through disassembling the pretreatedLiAlx-800-30 cells, washing the anodes by dimethylacetamide (DMA) solution for twice and drying them in the Ar-filled glovebox. A typical symmetric LiAlx-800-30 ￜLiAlx-800-30 cell contains two LiAlx-800-30 anodes of 11 mm, a glass fiber separator and 1M LiTFSI/TEGDME electrolyte. And symmetric LiAlx ￜ LiAlx cell, symmetric LiAlx-800-20ￜLiAlx-800-20 cell and symmetric LiAlx-800-50ￜLiAlx-800-50 cell was prepared for comparison. Electrochemical Measurements

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Galvanostatic discharge/charge measurements of the LOBs and symmetric cells were conducted by a battery cycler (Land CT 2001) at room temperature. EIS measurement was carried out on the Autolab with frequency range from 1 MHz to 0.1 Hz at 10 mV. Characterization Ex-situ field emission scanning electron microscope (FE-SEM, Hitachi SU-70) was performed to characterize the surface of anodes and cathodes after taking out of the cycled cells. Energy dispersive spectroscopy (EDS) and energy dispersive X-ray (EDX) line scanning were employed to analyze the elements distribution on the topsection and cross-section of anodes. X-ray photoelectron spectroscopy (XPS) spectra of anodes were collected with Al Kα radiation.

RESULT AND DISCUSSION In-situ Pre-treatment Process and Composition Characterizations

Figure 1. (a) Schematic illustration of the LOB with LiAlx anode subjected to in-situ pretreatment process at high current density before regularly cycling at low current density. (b) Ultra-long electrochemical cycling performance of pre-treated LOB (LiAlx-1500-30 cell)

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compared with pristine LOB without pre-treatment.

Figure 1 illustrates the pre-treatment and regular cycling process of LOB at different current densities. Firstly, a LOB with LiAlx anode experiences an electrochemical discharge/charge process to a fixed capacity of 1000 mA h g-1 for 30 cycles at high current density (0.8 to 2.0 mA cm-2 relative to the anode). As a reference, the discharge/charge curves for the pre-treatment process of the LiAlx-800-30 cell (pretreated at 0.8 mA cm-2 for 30 times) are shown in Figure S1a. Followed by high current pre-treatment to form a stable protective layer on LiAlx anode, the LOB experiences a regular discharge/charge cycling at 0.1 mA cm-2 to a capacity of 1000 mA h g-1 and a voltage range of 2.0-5.0 V. Moreover, we employed electrochemical impedance spectroscopy (EIS) to examine the charge transfer resistance of the anode-protection layer. As shown in Figure S1b, the impedance of LiAlx-800-30 cell decreases significantly than that of pristine LiAlx cell without pre-treatment after the first regular cycling, confirming that the protective film inhibits parasitic reactions at the anodeelectrolyte interface. At last, an extra-long cycling performance of LiAlx-1500-30 cell (pre-treated at 1.5 mA cm-2 for 30 times) is achieved through this pre-treatment process, which is up to 667 cycles and 38 times than that of the pristine LOB without pretreatment, as evident in Figure 1b and Figure S2. This facile in-situ pre-treatment method shows outstanding advantages to increase the cycling stability of LOB compared with recently reported anode protection strategies (shown in Table S1).

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Figure 2. Discharge/charge voltage profiles of (a) pristine LiAlx cell and (b) LiAlx-800-30 cell with pre-treatment. Voltage profiles of the Li stripping/plating of symmetric LiAlx cell and symmetric LiAlx-800-30 cell (c) at 0.1 mA cm-2 and a capacity of 0.1 mA h cm-2 under O2, (d) at 0.2 mA cm-2 and a capacity of 0.2 mA h cm-2 under O2. The inset in (c) and (d) are highresolution voltage profiles at special times.

The detailed discharge/charge voltage curves of LOBs are shown in Figure 2a-b. The LiAlx-800-30 cell runs up to 400 cycles without obvious voltage decrease. In contrast, the terminal discharge voltage of the pristine LiAlx cell decreases below 2.0 V at the 20th cycle (Figure 2a). Scanning electron microscope (SEM) images of air electrodes in LiAlx-800-30 cell are shown in Figure S4. It is clearly shown that a thick layer get deposited on the surface of NGA catalyst after pre-treatment (Figure S3c-d), which are mainly contained of Li2O2 (54.5 eV in Li 1s and 531.2 eV in O 1s), LiOH (55.5 eV in Li 1s and 531.9 eV in O 1s)38 and little LiF (684.9 eV in F, no signal in Li 1s), as shown in Figure S4. However, most of Li2O2 and LiF disappeared after the first discharge/charge cycling under low current density. Thus the surface of NGA catalyst got clean again with little LiOH (Figure S3e-f, S4). Moreover, to evaluate the 9



capability of the pre-treated LiAlx anode, symmetric cells with two pre-treated LiAlx800-30 electrodes are employed. Obviously, fluctuating voltage profile is shown for the pristine LiAlx symmetric cell and reaches about 40 mV after 500 h at a current density of 0.1 mA cm-2. By contrast, the voltage hysteresis of the pre-treated LiAlx-800-30 symmetric cell keeps stable at about 18 mV throughout the test over 580 h, as shown in Figure 2c. The voltage difference becomes more apparent during Li plating/stripping process when the current density increases to 0.2 mA cm-2 as evident in Figure 2d. The pristine LiAlx symmetric cell shows larger polarization after 200 h and comes to short circuits at about 380 h, which is possibly due to serious corrosion caused by unstable native SEI film on pristine LiAlx anode. In comparision, the pre-treated LiAlx-800-30 symmetric cell cycles for 580 h during 240 plating/stripping cycles and the polarization is still below 30 mV. To further identify the composition and element distribution on the surface of LiAlx anodes, X-ray photoelectron spectroscopy (XPS) and SEM are carried out. The pristine LiAlx surface is very smooth (Figure 1a) and contains mainly Li2CO3 (55.3 eV in Li 1s spectra, 531.5 and 532 eV in O 1s and 289.8 eV in C 1s, Figure 3a-d and LiOH (55.3 eV in Li 1s spectra, 531.5 and 532 eV in O 1s, Figure 3a-d.39 No Al peak is found for the pristine LiAlx (Figure 2d), due to low Al content (0.2 wt %) in LiAlx. After pretreatment, the LiAlx-800-30 anode turns dark, as the photo shown in Figure 1a. The detailed analysis of Li 1s, O 1s, Al 2p and F 1s spectra (Figure 3a-f) indicates that the surface of pre-treated LiAlx-800-30 anode mainly consists of organic component of ROCO2Li (532.5 and 533 eV in O 1s and 55.3 eV in Li 1s spectra), Li2CO3, LiOH, LiF

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(56.6 eV in Li 1s and 685.5 eV in F 1s spectra) and Al2O3 (531.1 eV in O 1s and 74.3 eV in Al 2p spectra).40, 41 The appearance of LiTFSI in Li 1s and F 1s spectra derives from the lithium salt in electrolyte. The aluminum and fluorine elements distribute uniformly (Figure S5h-i) on the pre-treated anode surface. However, the carbon and oxygen elements distribute unevenly (Figure S5f-g), due to the break of native SEI film during the pre-treatment process.

Figure 3. XPS spectra of pristine LiAlx surface and pre-treated LiAlx-800-30 anode surface for (a) survey spectrum, (b) Li 1s, (c) C 1s, (d) O 1s, (e) Al 2p, and (f) F 1s.

Electrochemical Performance with Different Pre-treatment Conditions The pre-treatment number and the pre-treating current density effects the cycling performance of LOBs greatly. Firstly, LOBs were pre-treated at 0.8 mA cm-2 for different times (0/5/10/20/30/50). The stable cycling performance of as marked LiAlx800-N cells is shown in Figure 4a and Figure S6. It is clear to find little positive effects on the cycling performance of cells when the pre-treating number is less than 10 cycles. As the pre-treating cycling number increases from 20 to 50, the lifespan of cells are 88, 11



477 and 247 cycles, respectively. According to Figure 4a, we believe that a pronounced change happens on the LiAlx anode during the pre-treating process, especially from 20 to 50 pre-treatment times. And this change is most likely caused by the layer thickness of the formed Al2O3. Here, we utilize symmetric cell performance to confirm effect of pre-treatment process on the LiAlx anode stability, as shown in Figure S7. The symmetric LiAlx cell without pre-treatment exhibits obvious over-potential tips and bumps at the beginning and end of each stripping and plating cycle (Figure S7a) due to the high specific kinetic hindrance for non-uniform lithium dissolution and deposition.42-44 This phenomenon decreases after high current pre-treating for 20 times. As shown in Figure S7b, the over-potential of symmetric LiAlx-800-20 cell keeps stable at about 28 mV. And the symmetric LiAlx-800-30 cell (Figure S7b) displays a more flat voltage platform than the symmetric LiAlx-800-20 cell throughout the entire cycle for 250 h, indicating a more stable Al2O3-containing SEI layer for easier lithium plating/stripping. However, when LiAlx anodes were pre-treated for 50 times under high current density, the voltage platform enhances a little than that of pre-treated for 30 times, as evidenced at Figure S3d. We assume that this phenomenon is possibly caused by the increased thickness of the as-formed Al2O3 insulating layer on the surface of LiAlx-800-50 anode, indicating a harder Li plating/stripping process than that of symmetric LiAlx-800-30 cell. Hence, the excellent cycling performance of LiAlx-80030 cell shown in Figure 4a could be explained by the stable, uniform and thin Al2O3containing SEI layer on the surface of anode.

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Figure 4. (a) The dependence of the cycle number of LiAlx-800-N cells on the pre-treating number (N=0/5/10/20/30/50) to protect the LiAlx anode. (b) The discharge terminal voltage in 300

cycles

of

LiAlx-C-30

cells

pre-treated

at

different

current

densities

(C=300/600/800/1000/1500/2000). All the cells are conducted with a capacity of 1000 mAh g-1. SEM image and EDX line scanning profile for the cross-section of (c, f) pristine LiAlx, (d, g) LiAlx-300-30 anode and (e, h) LiAlx-800-30 anode after the pre-treatment process.

The pre-treating current density is another factor for the cycling performance of LOBs. The as named LiAlx-C-30 cells are pre-treated at different current (300/600/800/1000/1500/2000 μA) for 30 times. As shown in Figure 4b, the lifespan of LOBs (LiAlx-300-30 cell and LiAlx-600-30 cell) gets worse when pre-treated at low current density (300 μA and 600 μA) . As the pre-treatment current increases from 800 μA to 2000 μA, LOBs exhibit hardly any discharge voltage drop in 300 cycles, 13



demonstrating superior advantage of this high current pre-treatment method. The crosssectional SEM andEDX line scanning of the pristine LiAlx, LiAlx-300-30 anode and LiAlx-800-30 anode interface are carried out to explore the composition change of SEI film formed through pre-treatment at different current densities, as shown in Figure 4ch. Carbon and fluorine elements experience substantial increase from the interior (point A) to the interface (point B) of LiAlx-300-30 anode and LiAlx-800-30 anode, indicating the formation of Li2CO3 and LiF film on the pre-treated anode surface combining with the XPS results discussed above. For the aluminum element, it distributes homogeneously in pristine LiAlx (Figure 4f), and keeps relatively uniform distribution in LiAlx-300-30 anode (Figure 4g). In contrast, aluminum element experiences obvious surface enrichment for the LiAlx-800-30 anode from 7 μm to 8.5 μm (Figure 4h), which is possibly caused by the aggregation of Al element and the formation of Al2O3. So the thickness of Al2O3 protective layer on the pretreated LiAlx-800-30 anode may be about 1.5 μm. As the pre-treatment current goes up to 2000 μA , Al 2p peak of Al2O3 becomes strong on the smooth surface of LiAlx-1500-30 anode (Figure S8c-e), which contributes to the extra-long cycle performance of LiAlx-1500-30 cell (Figure 1b). And the premature death of LiAlx-300-30 cell may be caused by the rough anode surface and uneven Li deposition without a protective film containing enough Al2O3 (Figure S8a-b, e) after pre-treatment at low current density. Morphology Characterization of Cycled Anode and Mechanism Analysis

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Figure 5. SEM images of anode in LiAlx cell: (a) pristine LiAlx, (b) after 15 cycles, and (c) corss-section of anode after 15 cycles. SEM images of anode in LiAlx-800-30 cell: (d) LiAlx800-30 anode after pre-treatment, (e) after 100 cycles, and (f) after 200 cycles. The inset figures are magnifying SEM images.

Accordingly, the morphology of LiAlx anodes was further characterized by SEM. Pristine LiAlx anode owns a relatively smooth surface (Figure 5a), however, it is covered with loose lithium after 15 cycles in LOB without pre-treatment, as shown in Figure 5b, 5c. The loose dead lithium layer deteriorates the cycling performance and leads to the death of LiAlx cell only after 17 cycles (Figure 1b). While the pre-treated LiAlx-800-30 anode keeps smooth surface without loose lithium even after 100 and 200 cycles (Figure 5e-f). Our SEM observation indicates that the uniform deposition of Li during cycling and the enhanced cycle life of LOB is mainly attributed to the as formed stable Al2O3-containing SEI layer after high current pre-treatment.

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Figure 6. Schematic diagrams of lithium dissolution/deposition. (a) LiAlx anode without pretreatment. (b) LiAlx anode with pre-treatment at high current density. (c) Li metal anode with pre-treatment at high current density.

The mechanism of protecting LiAlx anode in LOBs via in-situ pre-treatment method can be illustrated in Figure 6. An assembled Li-O2 coin cell with LiAlx anode experiences a pre-treatment process with high current density for several cycles in order to promote the accumulation of aluminum atoms, which leads to the formation of Al2O3 layer later at the interface of anode (Figure 6b). The composite SEI film (including Al2O3, LiF, ROCO2Li, LiOH and Li2CO3) formed after the pre-treatment process facilitates the uniform Li+ shuttling during the following Li plating/stripping process and stabilizes LiAlx anode interface even after hundreds of cycles. Thus, the LiAlx anode is protected against side reactions by this Al2O3-containing SEI film, which can be employed as ultra-long cycling metal anode in LOB. In addition, we applied this pre-treatment method in LOB with pure Li anode. As shown in Figure S9 the pristine Li metal surface is smooth and contains mainly Li2CO3 (531.3 and 532.2 eV in O 1s, and 55.5 eV in Li 1s spectra) and LiOH (532.2 eV in O 1s, and 55.5 eV in Li 1s spectra). And the pre-treated Li-800-30 anode surface mainly consists of organic components of ROCO2Li, Li2CO3, LiOH and LiF (686.0 eV in F 1s, 16


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and 56.0 eV in Li 1s spectra), which is similar to that of LiAlx-800-30 anode except for Al2O3. The cycling performance of Li cell and Li-800-30 cell is evaluated in Figure S10e. Masses dead Li appear on the Li anode after 22 cycles (Figure S10b), leading to the loss of active lithium and the end of Li cell life. After pre-treatment, the discharge terminal voltage of Li-800-30 cell drops below 2.0 V only after 5 regular cycles at 0.1 mA cm-2, which may be caused by the damage of Li-800-30 anode during the pretreatment process at 0.8 mA cm-2 for 30 times (Figure S10c). After regular cycling, obvious dead lithium is presented on the side-view of Li-800-30 anode (Figure S10d). The cycling stability of symmetric Li cell and symmetric pretreated Li-800-30 cell is illustrated in Figure S11. An obvious short circuit occurred at about 120 h in the symmetric Li-800-30 cell, indicating the unstability of pretreated Li-800-30 anode, which is cooresponding to its poor cycling performance in LOB shown in Figure S10. The failure of pre-treatment method on Li-800-30 cell can be ascribed to the lack of Al2O3 on the surface of Li-800-30 anode. Without the protection of Al2O3-containing SEI film, pristine LiAlx cell and Li-800-30 cell with pre-treatment fails rapidly due to the formation of serious dead lithium on anode surface during cycling (Figure 6a, 6c).

CONCLUTIONS In summary, we report an in situ pre-treatment method at high current density to spontaneously form a uniform Al2O3-containing SEI protective film on the surface of LiAlx anode for LOB. The formed SEI film can contribute to the uniform stripping/plating of Li and restrain the formation of dead lithium during cycling at regular low current density. With suitable pre-treating current density and pre-treatment 17



time, the cycle performance of LOBs with LiAlx anode improved dramatically than the pristine LOBs without pre-treatment. This novel approach is a promising step towards engineering lithium metal batteries with ultra-long cycling stability and enhanced safety through the use of Li-Al alloy metal anode. We believe that this work is a promising step towards engineering lithium metal batteries (Li-O2, Li-S batteries), and also provides us an in-situ strategy to protect lithium alloy with sable SEI layer, such as LiAl, Li-Mg, Li-Sn and Li-In alloy, as excellent anode substitutes.

ACKNOWLEDGMENTS The authors acknowledge funding support from High-level Talents' Discipline Construction Fund of Shandong University (31370089963078), research projects from Shandong Province (ZR2019MEM052, 2018JMRH0211, ZR2017MEM002 and 2017CXGC1010), and the Fundamental Research Funds of Shandong University (201810422046, 2017JC010, and 2017JC042).

Supporting Information The Suporting Information is available free of charge on the ACS Publications website at DOI: XXX. The discharge/charge curves of the pre-treatment process, EIS for LOBs before and after pre-teatment, discharge/chage voltage profiles of different cells, comarison of long-term cycling performance of LOBs or LABs with previously reported anod protection, SEM images of air electrode in the LiAlx-800-30 cell in different condition, XPS spectra of NGA after pre-treatment and regular cycling, top-view SEM images and corresponding EDS mapping of LiAlx metal before and after pre-treatment, voltage 18


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profiles of Li stripping/plating process of symmetric cells with different pre-treating numbers, SEM images and Al 2p XPS spectra of LiAlx-300-30 anode and LiAlx-150030 anode, XPS spectra of the pristine Li metal surface and the pre-treated Li-800-30 anode surface, SEM images of Li anodes before and after pre-treatment, the discharge terminal voltage of Li cell and Li-800-30 cell, voltage profiles of the Li stripping/plating of symmetric Li cell and symmetric Li-800-30 cell.

DECLARATION OF INTERESTS The authors declare no competing interests.

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