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Oxide film efficiently suppresses dendrite growth in aluminum-ion battery Hao Chen, Hanyan Xu, Bingna Zheng, Siyao Wang, Tieqi Huang, Fan Guo, Weiwei Gao, and Chao Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07024 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Oxide film efficiently suppresses dendrite growth in aluminum-ion battery Hao Chen, Hanyan Xu, Bingna Zheng, Siyao Wang, Tieqi Huang, Fan Guo, Weiwei Gao, and Chao Gao* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China KEYWORDS: aluminum dendrite; aluminum oxide film; aluminum metal anode; aluminum-ion battery; aluminum anode reaction mechanism

ABSTRACT: Aluminum metal foil is the optimal choice as an anode material for aluminum-ion battery for its key advantages such as high theoretical capacity, safety and low cost. However, the metallic nature of aluminum foil is very likely to induce severe dendrite growth with further electrode disintegration and cell failure, which is inconsistent with the previous reports. Here, we discover that it is aluminum oxide film that efficiently restricts the growth of crystalline Al dendrite and thus improves the cycling stability of Al anode. The key role of surficial aluminum oxide film in protecting Al metal anode lies in decreasing the nucleation sites, controlling the

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metalic dendrite growth and preventing the electrode disintegration. The defect sites in aluminum

oxide

film

provide

channels

for

electrolyte

infiltration

and

further

stripping/depositing. Attributed to such a protective aluminum oxide film, the Al-graphene full cells can attain up to 45,000 stable cycles.

Introduction Rechargeable aluminum-ion battery (AIB) is a promising candidate for future energy storage technologies due to its impressive advantages: high anode capacity, cost-effectiveness and safety.1-3 Its key advantages mostly benefit from unique properties of ideal aluminum metal anode: low-cost, abundance, incombustibility and air-stability, high gravimetric capacity of 2980 mAh g-1 and volumetric capacity of 8040 mAh cm-3. However, the progress of AIB over past 30 years was impeded by critical problems such as lack of suitable electrolyte or cathode materials.4 A recent breakthrough, reported by Dai et al.,5 opens a new avenue for this stagnant field by employing an AlCl3/[EMim]Cl ionic liquid electrolyte and graphitic cathode. This pioneering work stimulates various research enthusiasms in cathode materials of AIB: graphite,6-8 graphene,9-12 carbon13-15 and sulphur-based materials.16-18 In contrast to the significant progresses in cathode materials3 and ionic liquid electrolyte,19,20 the in-depth study on aluminum anode of AIB has not been done. Importantly, the metallic nature of aluminum is very likely to induce dendrite formation during reversible aluminum plating/stripping,21,22 which is analogous to the well-known problem of lithium dendrite hindering the practical application of ultimate lithium metal anode of Li-ion battery.23 Those fatal dendrites may pierce the separator to cause safety hazard, leading to anode disintegration

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and further cell failure. However, most of recent reports on AIB exhibited absence of dendrite during cycling and even dendrite-free behaviour of cycled aluminum metal anode, which is inconsistent with the metallic nature of aluminum foil anode and various reports on electrodeposition of aluminum species.21,22,24 Accordingly, it is of significance to study such a dendrite puzzle associated with aluminum metal anode. Here, we show the detailed insight into the behaviour of aluminum metal anode of AIB during cycling. Our study reports the first observation of an ultrathin yet uniform protective aluminum oxide layer covering the aluminum metal anode to effectively suppress dendrite growth and prevent electrode disintegration. Constituents of this first-discovered protective layer and detailed reaction pathway through this protective layer are also revealed. Benefiting from this protective layer, the aluminum metal anode can be reversibly and stably cycled to support the long cycle life of Al-graphene full battery up to 45,000 cycles, far surpassing that without this protective layer.

Experimental methods Materials preparation Normal aluminum foil (Al-n, purchased from MTI,thickness of 20µm) was used as received. The aluminum anode control sample, whose surficial oxidation film was removed beforehand, was named as Al-r. The oxidation film was removed by a widely used method: Al foil was immersed in mixture of 6% H3PO4 and 1.8% H2CrO4 solution at 60 oC for 30 minutes,25 fast washed with argon-saturated anhydrous ethanol and then transferred into the glove box. The defect-few graphene aerogel cathode was prepared by freeze-drying of graphene oxide solution,26 followed by thermal annealing at 2000 oC.11 The electrolyte was made by mixing 1ethyl-3-methylimidazolium chloride ([EMIm]Cl, 97%, Acros Chemicals, previously heated in

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vacuum at 130 oC for 24 h) and 1.3 mole equ. anhydrous aluminum chloride (AlCl3, 99.999%, Sigma-Aldrich, use as bought) in glove box for 12 hours to obtain transparent yellow liquid. The cycled aluminum foils from dissembled cells were washed with argon-saturated anhydrous dichloromethane27 in glove box to get rid of residual electrolyte, and then vacuumed in glove box to remove dichloromethane. Electrochemistry The cells were assembled in glove box in both coin cell type and soft pack cell type. The aluminum symmetric cell was fabricated into soft pack cell by using either Al-n or Al-r as both working electrode and counter electrode, with glassy fiber as separator and AlCl3/[EMIm]Cl ionic liquid as electrolyte. Cycling stability and cycle life of the anodes was evaluated employing symmetric cell, and the cells were then charged and discharged at 1 mA cm-2 for 0.5 h or 5 mA cm-2 for 0.25 h in each half cycle until cell failure. Cyclic voltammetry (CV) measurements were performed in two-electrode mode at different scan rate. The Al-graphene full cell was prepared by using defect-few graphene aerogel as cathode, Al-n or Al-r foil as anode with glassy fiber as separator and AlCl3/[EMIm]Cl ionic liquid as electrolyte. The long-term galvanostatic discharge/charge of the full cell was tested by cycling under voltage range of 0.7~2.51 V. Characterization The morphologies of the samples were investigated through a scanning electron microscope (SEM, Hitachi S-3000N). The elemental mapping results were examined through an energydispersive spectrometer (EDS) attached to Hitachi S-3000N. Powder XRD data were collected on a Bruker D8 X-ray diffractometer with Cu Kα1 radiation (1.5405 Å) in the scan range of 10o– 80o. X-ray photoelectron spectroscopy (XPS) analysis results were obtained on an Escalab250Xi spectrometer. The discharge and charge measurements at room temperature were carried out on a

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Land BT2000 Battery Test System (Wuhan, China). Cyclic voltammetry (CV) was performed on a CHI600D Electrochemical Workshop (Shanghai, China) at a scan rate of 1~10 mV s−1.

Results and discussion Figure 1a exhibits the SEM image of Al-n foil anode obtained from the dissembled Algraphene cell11 after 1,000 cycles, showing a plain surface without obvious corruption site or aluminum dendrite observed. This phenomenon is consistent with many recently published reports on AIB, that an apparent dendrite-free flat surface of the cycled aluminum foil anode was observed.5,13,16 Notably, when the cycled Al-n foil was folded, a mossy interlayer was observed at the crack site (Figure 1b). The magnified SEM image of this cracked site (Figure 1c) shows numerous nano-sized wire-shaped crystalline fragments hiding under a uniform membrane. This first observed, skin-cover-sands like morphology accounts for why the cycled aluminum foil anode appears a plain surface and absence of dendrites. According to the reversible anodic reaction of dissolution/deposition of metallic and crystalline Al species (equation (1)), we speculate these fragments are aluminum dendrites based on many research efforts on aluminum electrodeposition:21,24 4Al Cl + 3e ⇌ Al + 7AlCl (1) To further study this interesting and first witnessed phenomenon, we investigate the crosssection SEM images of the uniform Al-n foils before being cycled (Figure 2a) and after different cycles (Figure S1). As the cycling proceeded, the uniformly dense Al-n foil gradually transferred into a porous structure exactly from the direction facing the cathode and electrolyte (Figure S1),28 and the proportion of dendritic area gradually increased. After 10,000 cycles in the aluminum-graphene full cell, the aluminum foil anode completely turned into a sandwich structure as shown in Figure 2b: two ultrathin protective films with thickness lower than 50 nm

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(Figure 2c; Figure S2) homogeneously covering the highly porous thick filled-layer. Such a filled-layer was consisted of excessive wire-shaped aluminum dendrite with diameter around 100 nm as exhibited in Figure 2c. This form of aluminum dendrite is very similar to that of wireshaped lithium dendrite formed during reversible stripping/plating.29 In the gently sloping crosssection SEM images of the cycled Al-n foil anode, this ultrathin protective film exhibited a flat, clean and continuous surface uniformly covering the porous interlayer and dendrites (Figure 2d), which consolidates the explanations for the plain and dendrite-free surface aforementioned. This gradual transformation of interlayer of aluminum metal anode during cycling is very similar to that morphology of cycled lithium metal anode in Li-ion battery,23,29 affording dendrites that compromise the safety of battery operation. However, unlike the brittle solid electrolyte interphase (SEI) that derives from decomposition of electrolyte and then compactly coats on lithium metal anode,30 there exist nano-sized interspaces between the protective film and porous interlayer of aluminum metal anode. This phenomenon reveals that this unique protective film may natively exist in the commercial aluminum foil, rather than like that SEI of lithium metal anode coming from electrolyte decomposition in the metal-electrolyte interphase.30 To further study the component of this protective film covered the cycled aluminum foil, we conducted semiquantitative EDS mapping on the cross section of cycled Al-n foil (Figure 2e-g; Figure S3). Only four elements were observed in this cycled Al-n anode: 78 wt% aluminum, 10.33 wt% oxygen and only 7 wt% carbon with 3 wt% chlorine which may come from residual electrolyte and dichloromethane. Although the aluminum species (Figure 2f) with other neglectable species (Figure S3) are uniformly distributed across the aluminum foil, the oxygen species are mostly concentrated on the side location corresponding to the protective layer (Figure 2g). This element distribution directly demonstrates the constituent elements of such a cycled Al-

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n foil: pure aluminum in the filled-layer (Figure S4), and aluminum oxide at the protective layer. The element component of surficial oxide film is supported by XPS spectra, indicating existence of several elements: C, O, Al, Si, Cl, N. These elements just correspond well with the element species of oxide film, residue electrolyte, dichloromethane and glass-fiber separator. Impressively, the deconvoluted Al2p spectrum is mainly composed of the 2p3/2 peak of Al2O3 at 74.2 eV. Meanwhile, peaks at 72.8 eV and 72 eV corresponding to Al0 become almost undetectable especially compared with Al-n foil before being cycled (Figure S4a). This supports that the surficial protective film is mostly composed of Al2O3, which was assumed to be derived from the native oxide film that coated on aluminum foil. Thus also explains that the uneven distributed oxygen species in cycled Al-n (as revealed by EDS and XPS) should come from the surficial oxide film. To demonstrate the effect and origination of such a protective aluminum oxide film, we inspect CV curves (Figure 3c) , voltage profiles (Figure S5) of the symmetric aluminum-aluminum cell and cycle stability of aluminum-graphene full cell. Two kinds of aluminum metal anode were utilized in cell tests: as-received Al-n that was exposed to air, and Al-r whose oxide film was removed beforehand. The CV curves of symmetric cell exhibit reversible anodic peak at around 0.25 V (dissolution of aluminum) and cathodic peak at -0.25 V (deposition of aluminum) at scan rate of 1 mV s-1, demonstrating the stable and reversible aluminum plating/stripping in the ionic liquid electrolyte.5,16,20,27 When the scan rate was enhanced to 10 mV s-1, these peaks exhibited small change in position yet high increase in corresponding peak current, suggesting an ideal high electronic-conducting anode material in principle. Stable voltage profiles of the Al-n symmetric cell in Figure S5a exhibited a very small yet constant hysteresis at a current density of 1 mA cm-2. This small hysteresis with flat voltage plateau can be well attained throughout 400

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hours (400 cycles). However, the Al-r symmetric cell displayed a 100% higher hysteresis compared with Al-n symmetric cell. After 240 hours, a sudden voltage drop was observed for Al-r foil, suggesting an internal short-circuit caused by aluminum dendrite penetration. Given that the unique high power density advantage of aluminum-graphene battery is based on performances of electrodes under higher current densities, the areal current density tested on aluminum anode was increased to 5 mA cm-2 to detect the aluminum foil's cycling behaviour at higher current density. Impressively, stable cycling beyond 1100 cycles was attained for the Al-n foil (Figure S5b) with flat and similar voltage plateau displayed in 100th, 400th and 1,000th cycle (Figure S5c). In comparison, the Al-r foil counterparts displayed varying hysteresis and multiple internal soft short-circuit (Figure S5b) with fluctuating voltage profiles through these cycles (Figure S5d). Such a higher overpotential can lead to worse dendrite growth as reported by recent reference, which will cause a vicious circle: more dendrites lead to higher overpotential, and higher overpotential lead to more dendrites.24 In the full cell fabricated with Al-n anode and graphene cathode, a stable capacity of ~50 mAh g-1 based on defective graphene cathode is delivered over 45,000 cycles (Figure 3d). Even though the full cell using Al-r anode and same graphene cathode afforded similar specific capacity and stability within initial 10,000 cycles, fast decay was observed in the following 10,000 cycles with total cell failure at around 20000th cycles. Notably, this phenomenon of fast decay from 10,000th cycle and cell total failure in around 20000th cycle is commonly observed in multiple full cells using Al-r anode. These differences in electrochemical performance strongly support the vital importance of the surficial oxide film. The critical role of surficial oxide film can be further supported by post-mortem SEM images of cycled Al-r foil anode. In comparison to those of Al-n foil, numerous aluminum dendrites can

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be directly observed from the top view of Al-r anode only after 1,000 cycles (Figure 4a; Figure S6) without any appearance of protective film. Such a difference inmorphology directly proves that the surficial protective film originates from the native aluminum oxide film which was removed in Al-r foil beforehand, rather than from the decomposition of electrolyte shown in SEI case of lithium metal anode.30 The absence of innate surficial oxide film can be also confirmed by difference in Al2p spectra of Al-n and Al-r anodes (Figure S4). Cross-section SEM image of the Al-r anode after 1,000 cycles (Figure 4b) exhibited severe mossy dendrite shooting out from the surface without any appearance of protective film aforementioned. Without the protection of surficial protective film, the dendrites can keep growing perpendicular to the anode surface without any suppression, and may pierce the seperator to cause cell short-circuit safety hazard. Magnified SEM images on the dendrite clearly revealed the branching shape of an aluminum dendrite (Figure 4e),21 which is quite similar to the lithium dendrite.29 The thermodynamic and dynamics of aluminum dendrite growth have been well reported by previous reports on electrodeposition of aluminum species, revealing a visible perpendicular growth that is dependent on the overpotential, electrode material and kinetic parameters.24, 31 As the cycling proceeds, these dendrite growth will be more and more severe to induce higher overpotential,24 and eventually cause the cell short circuit just as exhibitied in Figure S5. The detailed dendrite growth mechanism is an open question, which needs more theoratical and experimental studies in the future. Element mapping on the cross-section of cycled Al-r foil displayed homogeneous element distribution (especially oxygen) across aluminum metal (Figure S7), which differs from that of Al-n foil with oxygen concentrated on the surficial film. Significantly, the Al-n anode maintains its integrity after 45,000 cycles with less than 5% weight loss (Figure 4c). It is worth noticing that the total amount of charge/discharge capacity of this cell reached more than 3 Ah,

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which was 100 times higher than that theoretic capacity of aluminum foil (10 mg). This comparison consolidates the reversibility of aluminum metal anode. The cycled Al-n foil can even react with anhydrous methanol and water to create suspected hydrogen bubbles (Figure 4d; Figure S8; Video S1; Video S2), which is a unique property of highly reactive nano-sized aluminum dendrite (Figure S8).32 By contrast, the Al-r anode seriously collapsed into powderform aluminum debris with only few left after 10,000 cycles (Figure 4f), due to the missing oxide film which can suppress dendrite formation and maintain its integrity. Notably after 20,000 cycles, the Al-r anode totally collapsed into black powder that cannot be collected at all. These results demonstrate that the ultrathin native oxidation film33,34 leads to this distinction of stability between Al-n and Al-r anode and corresponding full cells. Even though the critical roles of aluminum oxide film in suppressing dendrite growth and maintaining the integrity of anode are confirmed above, a vital issue arises: How does the ionic liquid electrolyte immerse through this dense, uniform yet none ionic-conducting aluminum oxidation film to react with inner aluminum metal? Hence, we inspect the SEM images of Al-n anode after 10,000 cycles, displaying abundant cavities with semitransparent film covering the inner holes (Figure 5a). It is worth noticing that these holes only exist under fissures in the protective semitransparent film. The plain inner surface of filled-interlayer, which demonstrates absence of aluminum stripping/platting reaction, was observed under the continuous unbroken protective semitransparent film as outlined by squares. Magnified SEM image of the fissure is shown in Figure 5b. Nano-sized aluminum dendrite can be clearly observed in the hole beneath the fissure, proving that the aluminum dendrite can be formed below the defect sites of protective semitransparent film. Elemental mapping was casted on this defect site, showing uniform distribution of aluminum (Figure 5c) and other species (Figure S9). However, the oxygen species

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were mostly concentrated in the region covered with such protective semitransparent film (Figure 5d). This uneven element distribution is in consistence with previous results shown in Figure 2, suggesting that the semitransparent layer is just the protective aluminum oxidation film aforementioned. Notably, as shown in Figure 5a aluminum dendrites were only observed under the defect sites of protective aluminum oxidation film, which were the evidence for reaction sites of aluminum stripping/plating.35 These phenomena demonstrate that only when defects existed in the surficial protective aluminum oxidation film can the internal aluminum beneath this fissure become reactive, which is in accordance with the enhancement of electroactivity in the defect sites of native aluminum oxide film reported.36 Accordingly, a modelling of aluminum metal anode can be built as exhibited in Figure 5e to speculate the exact role of aluminum oxide film and reaction mechanism of Al-n anode:35,37 the electrolyte can only infiltrate through defect sites in the native aluminum oxide film to react with internal aluminum metal at the metal/oxide interface, affording less nucleation sites than Al-r without such surficial oxide film. Owing to the defects in oxide film, Al-n anode can deliver decent electrochemical performance under surface coating of such an electron and ion insulative oxide film (Figure S10). This inert aluminum oxide film with high Young's modulus (>100 GPa)23,38 can effectively suppress the dendrite growth and inhibit further disintegration of the anode by confined reaction space, restricted nucleation site and nearly full interfacial protection. In contrast, the Al-r foil anode as being modelled in Figure 5f does not possess such an effective protective film. Therefore, aluminum dendrites are developed all over the surface without limitation, leading to a rugged surface of Al metal anode and further cell short-circuit. After several cycles, dead aluminum dendrite can be easily detached from original aluminum foil due to unrestricted volume variation and inhomogeneous dissolution of dendrites, resulting in active material loss and further electrode disintegration

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(Figure 4d). This unique model of aluminum metal anode accords with the experiment data aforementioned and can rationalize such a stable cycling of metallic aluminum anode well.

Conclusions In summary, we find and demonstrate Al dendrite in the aluminum metal anode of AIB, demonstrate the critical role of native surficial aluminum oxide film in suppressing dendrites and stabilizing anode. Rather than suspected dendrite-free behaviour, the aluminum dendrites do exist, yet they are confined beneath the protective aluminum oxide layer. Through restricted nucleation sites and nearly full interfacial protection, the native aluminum oxide film can effectively suppress the growth of aluminum dendrite and prevent the disintegration of aluminum foil anode during repeated anodic aluminum plating/stripping reaction, affording an almost intact aluminum foil with flat surface after cycling. In addition, the defect sites in the aluminum oxide film enable the penetration of electrolyte and the internal reaction of aluminum plating/stripping in the oxide/metal interface, which accounts for how the aluminum metal acts as a reactive anode under such a nearly full interfacial protection by the native aluminum oxide film. Benefiting from this protective film, the Al-n metal anode can attain stable cycling without short circuit or fluctuating voltage profiles, and the aluminum-graphene full cell achieved a stable cycling over 45,000 cycles. This unique dendrite issue of aluminum metal anode indicates a fundamental insight into the aluminum foil anode of AIB, prompting both practical applications and further study on aluminum metal anode and AIB.

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Figure 1. Morphology of Al foil after 1,000 cycles. SEM images of the (a) plain surface of cycled aluminum foil and (b) the cracked site of the folded cycled aluminum foil. (c) Magnified SEM images of the cracked site, exhibiting the aluminum dendrite hidden under the protective layer.

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Figure 2. Cross-section SEM images of normal aluminum foil before or after cycling. SEM images of (a) normal aluminum foil before cycle and (b) after 10,000 cycles. Magnified crosssection SEM images of the normal aluminum foil after 10,000 cycles, exhibiting (c) an ultrathin protective layer with cubic dendrite, and (d) the continuous plain surface of the protective layer. (e) Cross-section images of the cycled aluminum foil with element mapping of (f) aluminum and (g) oxygen.

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b C

N Cl Si Al

600 400 200 0 Binding energy (eV)

Al2O3 2p3/2 Al 2p3/2 Al 2p

Current (mA)

O

c

Intensity (a. u.)

Intensity (a. u.)

a

20 10 0 -10 -20

78 76 74 72 Binding energy (eV)

1 mV s-1 10 mV s-1 -1 -0.5 0 0.5 1 Voltage (V vs Al)

d Capacity (mAh g-1)

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

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120 100 80 60 40 20 0

fail

0

10000

20000 30000 Cycle number

Al-r Al-n 40000

120 100 80 60 40 20 0

Coulombic efficiency (%)

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Figure 3. Analysis of the protective layer component of cycled aluminum foil and electrochemical behaviours. (a) Total XPS spectra and (b) Al 2p spectra. (c) CV curves of the aluminum-aluminum symmetric cell at different scan rate. (d) Cycling performance of aluminum-graphene full cells using Al-n or Al-r anode, the capacity is based on mass of graphene cathode.

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Figure 4. Analysis of the difference behaviour of Al-n and Al-r anode. (a) surficial SEM image of cycled Al-r after 1,000 cycles. (b) Cross-section SEM image of Al-r anode after 1,000 cycles. Comparison of the (c) Al-n anode and (f) Al-r anode after 10,000 cycles. (d) Reaction of cycled Al-n anode (after 10,000 cycles) with methanol. (e) Magnified SEM image of aluminum dendrite.

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Figure 5. Analysis of the mechanism of the aluminum foil protected by the oxide film. (a) SEM images of Al-n anode after 10,000 cycles. (b) Magnified SEM images of Al-n anode after 10,000 cycles, exhibiting a hole in the protective layer with most dendrite appeared beneath this hole with element mapping of (c) aluminum and (d) oxygen. (e) Models of cycled Al-n metal and (f) cycled Al-r metal.

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ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org.” Cross-section SEM images of Al-n foil after different cycles; Magnified cross-section SEM images of the ultrathin protective layer covering Al-n foil anode after 10000 cycles; Element distribution of cycled Al-n foil anode; Deconvoluted Al2p spectrum and XRD spectrum of Al-n (Al-r) metal before or after being cycled; Galvanostatic cycling of symmetric aluminum foil; SEM images of Al-r metal anode after 1000 cycles; Element distribution of cycled Al-r foil anode; Photograph of cycled Al-n metal reacting with methanol and water; Element mapping of carbon and chlorine corresponding to Figure 5b. (PDF) Reaction of cycled aluminum metal with methanol (avi) Reaction of cycled aluminum metal with water (avi) AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Natural Science Foundation of China (No.s 21325417, 51533008 and 51603183), National Key R&D Program of China (No. 2016YFA0200200), and

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Fundamental Research Funds for the Central Universities (Nos. 2017QNA4036, 2017XZZX008006). Notes The authors declare no competing financial interest. REFERENCES (1) Elia, G. A.; Marquardt, K.; Hoeppner, K.; Fantini, S.; Lin, R.; Knipping, E.; Peters, W.; Drillet, J.-F.; Passerini, S.; Hahn, R. An Overview and Future Perspectives of Aluminum Batteries. Adv. Mater. 2016, 28, 7564-7579. (2)

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