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Revisiting the Corrosion of Aluminum Current Collector in Lithium-ion Batteries Tianyuan Ma, Gui-Liang Xu, Yan Li, Li Wang, Xiangming He, Jianming Zheng, Jun Liu, Mark H. Engelhard, Peter Zapol, Larry A Curtiss, Jacob Jorne, Khalil Amine, and Zonghai Chen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02933 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 17, 2017

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The Journal of Physical Chemistry Letters

Revisiting the Corrosion of Aluminum Current Collector in Lithium-Ion Batteries

Tianyuan Ma1,2, Gui-Liang Xu1, Yan Li1, Li Wang3, Xiangming He3, Jianming Zheng4, Jun Liu4, Mark H. Engelhard5, Peter Zapol6, Larry A. Curtiss6, Jacob Jorne2,7*, Khalil Amine1, and Zonghai Chen1*

1) Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA 2) Materials Science Program, University of Rochester, Rochester, NY 14627, USA 3) Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China 4) Energy and Environmental Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99354, USA 5) Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99354, USA 6) Material Science Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, IL 60439, USA 7) Department of Chemical Engineering, University of Rochester, Rochester, NY 14627, USA

Corresponding author: [email protected]; [email protected] 1

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Abstract The corrosion of aluminum current collectors and the oxidation of solvents at a relatively high potential have been widely investigated with an aim to stabilize the electrochemical performance of lithium-ion batteries using such components. The corrosion behavior of aluminum current collectors was revisited using a home-build high-precision electrochemical measurement system, and the impact of electrolyte components and the surface protection layer on aluminum foil was systematically studied. The electrochemical results showed that the corrosion of aluminum foil was triggered by the electrochemical oxidation of solvent molecules, like ethylene carbonate, at a relative high potential. The organic radical cations generated from the electrochemical oxidation are energetically unstable, and readily undergo a deprotonation reaction that generates protons and promote the dissolution of Al3+ from the aluminum foil. This new reaction mechanism can also shed light on the dissolution of transitional metal at high potentials.

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The corrosion of aluminum foils, which are generally used as the current collectors for state-of-the-art lithium-ion technologies, at a relatively high potential has been the prime consideration for optimizing non-aqueous electrolytes to ensure the longterm and stable operation of lithium-ion batteries. It has been widely accepted that the naturally generated Al2O3 on aluminum foils is not resistive enough against the corrosion in the normal operation environment of lithium-ion batteries, and that the decomposition of LiPF6 with the presence of moisture, which generates HF, helps to deposit a layer of AlF3 that physically passivates the aluminum foils in the non-aqueous electrolytes. Severe corrosion of aluminum foil has been widely reported in electrolyte using lithium salts, either with the lack of fluorine (like lithium perchlorate, LiClO41-2) or showing high compatibility with moisture3 (lithium bistrifluoromethylsulfonyl imide, LiTFSI), while electrolyte containing unstable salts like LiPF64-5 and LiBF46 significantly suppresses the corrosion of aluminum foils. In addition, some fluorinated solvents like methyl difluoroacetate7 are also reported to suppress the corrosion of aluminum foils with the speculation that those fluorinated solvents can also facilitate the generation of AlF3 to passivate the aluminum surface. An alternative hypothetical mechanism is related to the high dielectric constant of the carbonate solvents, particularly ethylene carbonate, that prevent the precipitation of ionic compounds such as AlF3. Hence, a solvent with low dielectric constant is reported beneficial to suppress the corrosion process8. A third hypothesis is that the corrosion process is related to the high concentration of free solvent molecules in the traditional electrolytes that helps to lift up Al3+ from the aluminum foils, and hence, superconcentrated electrolytes with no free solvent molecule were reported to

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prevent the corrosion of aluminum foils9-10. Clearly, a consensus on the corrosion mechanism hasn’t reached yet even with the help of the modern analytical instruments. Besides the debate on the corrosion mechanism, some research efforts have been devoted to establishing the correlation between the corrosion of aluminum foil and the electrochemical performance of the positive electrodes for lithium-ion batteries. Even with the presence of LiPF6 as the main salt, the corrosion of aluminum foil was observed in the cycled lithium-ion cells using LiFePO4 as the positive electrode, which has a relative low working potential at about 3.5 V vs. Li+/Li11. More corrosion issues were reported for batteries using 4 V class positive electrode materials, like LiMn2O411 and LiNi0.8Co0.15Al0.05O2 (NCA)

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. Devine et. al observed substantial amount of corrosion

pits and crack in the aluminum foils in graphite/NCA cells cycled at 25 oC for 36 weeks. They also observed significant increase of aluminum-based compounds (~2700 ppm) in the recovered electrolyte. The authors also believed that capacity fade and power fade of these cells should be at least partially attributed to the negative impact of aluminum corrosion 12. Similarly, Zhang et. al reported that the self-discharge behavior of LiMn2O4 cathode could be dramatically suppressed by proper protection of aluminum foils using an artificially introduced passivation layer2. With the help of on-line inductive coupled plasma-optical emission spectroscopy (ICP-OES), Winter et. al observed both the dissolution of aluminum foil and the oxidation of organic solvent at a high potential, and concluded that the reaction at aluminum current collector at high potential in LiTFSIbased electrolytes should scientifically assigned as anodic dissolution, not a corrosion reaction as in LiPF6-based electrolytes13.

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Bearing with above unsettled debates, we revisit the corrosion (or anodic dissolution) process of aluminum foils in non-aqueous electrolytes using our newly developed highprecision electrochemical measuring system. It was found that the degradation of aluminum foil at high potentials is a coupled electrochemical-chemical reaction; the electrochemical oxidation of organic solvents at high potentials generates proton and trigger the chemical corrosion of aluminum foils. It is of our great interest to investigate the parasitic reactions occurring inside lithium-ion batteries. Hence, a high precision electrochemical measurement system was built for this purpose

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. As a benchmark, Figure 1 shows typical potential-dependent

results for widely used electrode materials for lithium-ion batteries. Figure 1a shows the dependence of the measured static leakage current of lithiated mesocarbon microbeads (MCMB) as a function of the working potential. Two dataset for two individual cells in Figure 1a show good reproducibility between different cells. In both cells, a typical nonaqueous electrolyte of 1.2 M LiPF6 in a mixture solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used; and the nominal reversible capacity was about 3.6 mAh per disc (~1.5 cm2). It can be clearly seen that the static leakage current was very small when the cell was held at 1.0 V vs. Li+/Li since lithium was barely inserted into the graphite at this potential. As the holding potential decreased, the static leakage current increased monotonically; a leakage current of about 0.6 µA was observed when the potential was reduced to 1 mV. This current-potential relationship can be phenomenally explained with the Tafel equation for a typical electrochemical reduction reaction. On the other hand, the electrochemical behavior of the positive electrode (see Figure 1b), comprising LiCoO2 as the active electrode material, looks much more

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complicated than the negative electrode. A fairly large static leakage current, about 0.5 µA, was observed within the potential window between 4.1 V and 4.3 V. Since LiCoO2 has been widely reported to work well when the potential was not higher than 4.2 V vs. Li+/Li, It is speculated that the large static leakage current measured at 4.1 V vs. Li+/Li could be related to the reaction between the electrolyte components and the de-lithiated cathode material.

(a)

Graphite at 25oC using carbonate electrolyte.

Current, µA

0.6

0.4

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0.0 0.0

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Potential, V vs. Li+/Li

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LiCoO2 at 25oC using carbonate electrolyte. 8

Current, µA

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4

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0 4.1

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Potential, V vs. Li+/Li

Figure 1 Evolution of the parasitic current as a function of the holding potential for (a) MCMB/Li and (b) LiCoO2/Li cells. The electrolyte used was 1.2 M LiPF6 in EC/EMC (3:7 by weight).

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Figure 2 Evolution of the static parasitic current as a function of the holding potential for (a) carbon black cast on aluminum foil, and (b) the bared aluminum foil; evolution of the measured current as a function of the holding time for Al/Li cell at (c) 3.8 V, (d) 3.9 V, and (e) 4.5 V. The electrolyte used was 1.2 M LiPF6 in EC/EMC (3:7 by weight).

To validate our speculation, similar experiment was carried out by coating carbon black/PVdF composite on the aluminum foil, without the presence of LiCoO2. The loading of the carbon black is about 0.97 mg/cm2. Figure 2a shows the measured static leakage current as a function of the applied potential; the data can be well fitted with the Tafel equation (see eq. 1) as shown in Figure 2a.

∈

(1)

In above equation, i is the measured static leakage current, i0 is the exchange current density, n is the number of transferred charge for the elemental electrochemical reaction,

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E is the applied potential, ε is the standard redox potential of the elemental electrochemical reaction, R is the gas constant, and T is the temperature for the experiment. For any given elemental electrochemical reaction, i0, n, and ε should remain constant regardless of applied potential (E). Brought to our attention is the small peak at about 3.9 V vs. Li+/Li that shows slight deviation from the typical Tafel equation. To minimize the potential impact of high specific surface area of carbon black, an identical experiment was carried out on bare aluminum foil, whose data is shown in Figure 2b. Comparing to Figure 2a, the value of the static leakage current shown in Figure 2b was substantially reduced due to the lack of the active electrochemical surface area provided by carbon black. A clearly abnormal electrochemical behavior was observed within a narrow potential window at about 3.9 V vs. Li+/Li, which leads to an open gap in Figure 2b. For a better illustration, the time-dependent current profiles for the constant-potential holding period, up to 10 hours, are shown in Figures 2c, 2d and 2e, respectively. At the beginning of the experiment, the current, decayed exponentially, was mostly contributed from the double layer capacitance. After a short period of time, the static current was measured, indicating a slow parasitic reaction going on at a potential as low as 3.8 V vs. Li+/Li (see Figure 2c). When the holding potential was slightly increased to 3.9 V vs. Li+/Li, the detected current initially decreased with the holding time as expected for charging double layer capacitor. After a short period of time (about 50 minutes), the current surprisingly increased with the holding time (see Figure 2d). The variation of the current with time cannot be simply explained with the capacitance behavior; a reasonable explanation is that the a certain reaction occurred at about 3.9 V vs. Li+/Li that modified the surface chemistry of the aluminum foil, leading to the increase of the exchange

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current density (i0). When the holding potential was raised to a higher value, said 4.5 V vs. Li+/Li, the current-time relationship returned to normal again (see Figure 2e), presuming that the surface chemistry on aluminum foil was stabilized after the reaction at about 3.9 V vs. Li+/Li. Figure 2b shows that the measured parasitic current increased exponentially with the holding potential, indicating an electrochemical oxidation of a certain species on the aluminum surface. It is our speculation that this reaction is related to the oxidation of ethylene carbonate. Density functional theory calculations revealed that EC has a fairly high redox potential at about 7.2 V vs. Li+/Li (see supplementary Table S1), in reasonable agreement with previous calculations15. The proposed mechanism of EC oxidation in previous studies involved proton transfer to the carboxyl group of another EC molecule in solution, decreasing the oxidation potential to 5.94 V. Calculations using models representative of current collector’s surface basic groups indicate that redox potential can decrease even more than that when the proton is transferred to either hydroxyl (by 1.4 to 2.5 V) or thiol group (by 1 to 2 V) coordinated to aluminum (see Table S1 and Figure S2). Larger calculations on either hydroxyl group or a bridging oxygen site on amorphous alumina models shown in Figure S2 similarly indicate that the alumina promotes the deprotonation reaction of the radical cation (EC+), leading to an increase in the parasitic current at a low potential as predicted by the Tafel equation (eq. 1). It is believed that the proton generated from the deprotonation reaction of EC+ slowly reacted with the natural passivation layer, Al2O3 on aluminum, leading to an increase on the charge transfer reaction kinetics on the aluminum surface (see Figure 2d). At the same time, it is proposed that the presence of protons on aluminum surface can also

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substantially promote the decomposition of PF6- anion, forming proton-resistive AlF3 on the surface of aluminum. After accumulation of enough protons from the oxidation of EC, the surface of the aluminum foil was stabilized by AlF3 coating, and a stable exchange current density was then established again (see Figure 2e).

Figure 3 (a) XPS profiles of aluminum foils before and after the anodic treatment in 1.2 M LiPF6 in EC/EMC (3:7 by mass); XPS depth profile analysis of aluminum foils (b) bare Al foil, (c) being anodic treated at 3.9 V, and (d) being anodic treated at 4.8 V. Electrolyte used here was GenII (1.2 M LiPF6 in solvent of Ethylene Carbonate (EC) and Ethyl Methyl Carbonate (EMC) with a ratio of 3:7 by weight.) Figure 3 shows the XPS analysis results for aluminum foils before and after the anodic treatment. The control experiment was carried out with bare Al foil. A weak absorption peak was observed at about 73 eV, corresponding to the binding energy of metallic aluminum. The strong peak at 75.6 eV was attributed to the Al2O3 thin film on 11



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the surface of aluminum foil. It clearly shows that as-used non-aqueous electrolyte with the presence of a trace amount of moisture was not enough to convert the Al2O3 layer into a more stable AlF3 passivation layer. After the anodic treatment at 3.9 V for 20 hours, the absorption peak for Al2O3 at 75.6 eV was replaced with a new peak at 77.2 eV, which is corresponding to the binding energy of AlF3. Similar result was observed for sample anodized at 4.8 V for 20 hours (see Figure 3a). The depth profile analysis also revealed that the sample without anodic treatment was mostly covered by 15-25 nm of Al2O3 (see Figure 3b); the anodic treatment at 3.9 V led to a deposition of about 5 nm AlF3 on the top of the thinned Al2O3 layer (10-20 nm) (see Figure 3c). Although a substantially higher parasitic current was measured at 4.8 V, no significant change on the thickness of AlF3 layer and embedded Al2O3 was observed (see Figure 3d). This indicates that the interface of aluminum foil was stabilized by AlF3 coating that was electrochemically triggered by the oxidation of solvent to generate protons on the surface. After repeated cycling between 3.4 V and 4.9 V for 5 cycles, it was observed that the open gap at about 3.9 V disappeared, primarily due to the stabilization of the aluminum surface (see supplementary Figure S3). To further demonstrate the importance of AlF3 layer, similar anodic treatment was done with 1.2 lithium bis(fluorosulfonyl)imide (LiFSI) in EC/EMC (3:7 by weight), in which LiFSI is compatible with proton. Without surprise, no conversion of Al2O3 to AlF3 was observed (see supplementary Figure S4), and the parasitic current kept increasing with the time when the holding potential was higher than 3.9 V (see supplementary Figure S5). In addition, the open gap associated with the corrosion of Al2O3 was pushed to a higher potential when ethylene carbonate was replaced with a fluorinated ethylene carbonate (see supplementary Figure S6a). And the

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gap almost disappeared when both ethylene carbonate and ethyl methyl carbonate were replaced with a mixture of fluorinated carbonate and fluorinated ether (see supplementary Figure S6b), indicating that the EMC also play a major role on the Al corrosion. This may be owing to its similar oxidation potential with EC as reported by Xing et al. using DFT calculation16. Given that carbon is mostly resistive to chemical corrosion in an acidic environment, a layer of carbon black was deposited on the surface of the aluminum foil. However, the electrochemical characterization showed that the static parasitic current measured was increased almost 10 folds (see Figure 4a), comparing to that for the non-coated aluminum foil (see Figure 2a). This increase of parasitic current can potentially be resulted from either the increase in active electrochemical surface area or the decrease of the charge transfer impedance at the interface of aluminum foil. Considering the high loading of carbon black physically cast on the sample shown in Figure 2a, the high parasitic current shown in Figure 4a must be dominated by the reduction of charge transfer impedance across the interface due to the carbon coating, which is intended for protecting the aluminum foil from the chemical corrosion in acidic environment. In addition, graphene that has low out-of-plane electronic conductivity was also deposited on the aluminum foil. The results showed that the measured parasitic current was substantially lower than that for the carbon coated samples, but not better than that for the uncoated sample (see Figure 4b). Alternative, AlPO4 coating that is an electronic insulator and that is also resistive to corrosion in acidic environment was coated on the surface of aluminum foil, and a dramatic change on the parasitic current was observed with the AlPO4 coating (see Figure 4c). First of all, the corrosion window showing at

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about 3.9 V completely disappeared because the coating layer is really resistive towards reaction with protons, resulting in a constant charge transfer impedance throughout the whole course of the potential sweep. In addition, the maximum parasitic current measured for AlPO4 coated sample was about 50% of that for the non-coated sample, implying that a significantly smaller amount of proton is expected at the similar operation condition.

Figure 4 Potential-dependent of the static parasitic current for aluminum foil with surface coating of (a) carbon, (b) graphene, and (c) AlPO4. Thus, it is believed here that the corrosion reaction of aluminum foil at a relatively high potential is actually a coupled electrochemical-chemical reaction as illustrated in Scheme 1. The old wisdom believes that the trace amount of moisture in the electrolyte can trigger the decomposition of LiPF6 to generate HF that covert Al2O3 into AlF3 that stabilizes the interface. It was demonstrated here that this is not the case; the passivation is only triggered during the anodic treatment of the aluminum foil at 3.9 V or higher. In principle, it is the slow electrochemical oxidation reaction of solvent molecules, most possibly ethylene carbonate that generates high concentration of proton on the surface of the working electrode. The generated proton takes dual roles in this process. First of all, the proton can chemically dissolve Al2O3 layer, lead to a reduction of charge transfer impedance, and result in a substantial increase in the rate of electrochemical oxidation 14


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reaction of the solvent molecules. In addition, the proton can also react with PF6- anion to generate high concentration of HF at the interface that participates into converting Al2O3 into AlF3 to stabilize the interface. Although some surface coating has been demonstrated effective to suppress the corrosion reaction of the aluminum foil in non-aqueous, a more broaden implication of this work is that more scientific attention needs to be given to the electrochemical stability of the electrolyte components that act as the massive proton source. Even if the aluminum foil can be protected from the chemical corrosion, the generated proton will alternatively pose detrimental impact on the electrochemical performance of the cathode materials, including the dissolution of the transition metal ions from the cathode materials at a high potential17 and the hypothetic insertion of proton into the cathode framework18.

Scheme 1 Schematics showing the proposed coupled electrochemical chemical reaction occurred on the aluminum surface at a relatively high potential. The corrosion of aluminum foil in non-aqueous electrolytes was revisited using a home-built high-precision electrochemical measurement system. It was found that the

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corrosion and passivation of aluminum foil in LiPF6-based electrolytes occurs simultaneously during the anodic treatment of aluminum foil. This is consistent with DFT calculations indicating lower oxidation potential for the electrolyte near alumina surface. It is concluded that the corrosion reaction of the aluminum foil is a coupled electrochemical-chemical reaction; the proton generated from the electrochemical oxidation of electrolyte components leads to thinning of the Al2O3 layer, and the proton also promote the conversion of Al2O3 in AlF3 for more active passivation. It also implies that to increase the electrochemical stability of electrolyte components is more important than to protect the aluminum foil from corrosion by surface coating.

Acknowledgement Research at the Argonne National Laboratory was funded by U.S. Department of Energy, Vehicle Technologies Office. Support from Tien Duong of the U.S. DOE‫׳‬s Office of Vehicle Technologies Program is gratefully acknowledged. Argonne National Laboratory is operated for the U.S. Department of Energy by UChicago Argonne, LLC, under contract DE-AC02-06CH11357. Part of the research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. The authors also thank Dr. Sanja Tepavcevic and Dr. Nenad M. Markovic of Argonne National Laboratory for valuable discussion on XPS experiments.

Supporting Information Available: Experimental and computational details; Typical decay of current being held at certain potential for 10 hours; Reaction energies and oxidation potentials of EC in the presence of different functional groups on alumina; Structure of EC at different state, alumina models (Alumina, Alumina1H and Alumina 2H) represented by clusters derived from a periodic model of amorphous alumina; Potential-dependent of the static parasitic current for conditioned aluminum foil by

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repeated cycling between 3.4 V and 4.9 V for 5 cycles; XPS and depth profile of aluminum foils before and after the anodic treatment at different potential in 1.2 M LiFSI in EC/EMC (3:7 by weight); Decay of current for Al, carbon coated Al, graphene coated Al, and AlPO4 coated Al in LiFSI based electrolyte at 4.2 V; Potential-dependent of the static parasitic current for aluminum foil in two different kinds of solvent mixture: Fluorinated Ethylene Carbonate and Ethyl Methyl Carbonate; FEC and fluorinated ether.

References (1) Zhang, S. S.; Jow, T. R. Aluminum Corrosion in Electrolyte of Li-Ion Battery. J. Power Sources 2002, 109, 458-464. (2) Zhang, S. S.; Ding, M. S.; Jow, T. R. Self-discharge of Li/LixMn2O4 Batteries in Relation to Corrosion of Aluminum Cathode Substrates. J. Power Sources 2001, 102, 1620. (3) Dahbi, M.; Ghamouss, F.; Tran-Van, F.; Lemordant, D.; Anouti, M. Comparative Study of EC/DMC LiTFSI and LiPF6 Electrolytes for Electrochemical Storage. J. Power Sources 2011, 196, 9743-9750. (4) Zhang, X. Y.; Devine, T. M. Factors that Influence Formation of AlF3 Passive Film On Aluminum in Li-Ion Battery Electrolytes with LiPF6. J. Electrochem. Soc. 2006, 153, B375-B383. (5) Zhang, X. Y.; Devine, T. M. Identity of Passive Film Formed on Aluminum in Li-Ion battery electrolytes with LiPF6. J. Electrochem. Soc. 2006, 153, B344-B351. (6) Zhang, S. S.; Xu, K.; Jow, T. R. Study of LiBF4 as an Electrolyte Salt for a Li-Ion Battery. J. Electrochem. Soc. 2002, 149, A586-A590. (7) Kawamura, T.; Tanaka, T.; Egashira, M.; Watanabe, I.; Okada, S.; Yamaki, J. Methyl Difluoroacetate Inhibits Corrosion of Aluminum Cathode Current Collector for Lithium Ion Cells. Electrochem. Solid St. 2005, 8, A459-A463. (8) Wang, X. M.; Yasukawa, E.; Mori, S. Inhibition of Anodic Corrosion of Aluminum Cathode Current Collector on Recharging in Lithium Imide Electrolytes. Electrochim. Acta 2000, 45, 2677-2684. (9) McOwen, D. W.; Seo, D. M.; Borodin, O.; Vatamanu, J.; Boyle, P. D.; Henderson, W. A. Concentrated Electrolytes: Decrypting Electrolyte Properties and Reassessing Al Corrosion Mechanisms. Energy Environ. Sci. 2014, 7, 416-426. (10) Yamada, Y.; Chiang, C. H.; Sodeyama, K.; Wang, J. H.; Tateyama, Y.; Yamada, A. Corrosion Prevention Mechanism of Aluminum Metal in Superconcentrated Electrolytes. Chemelectrochem 2015, 2, 1687-1694. (11) Zhang, X. Y.; Winget, B.; Doeff, M.; Evans, J. W.; Devine, T. M. Corrosion of Aluminum Current Collectors in Lithium-Ion Batteries with Electrolytes Containing LiPF6. J. Electrochem. Soc. 2005, 152, B448-B454.

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Revisiting the Corrosion of the Aluminum Current ... - ACS Publications

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