Article pubs.acs.org/JPCC
Allotropic Control: How Certain Fluorinated Carbonate Electrolytes Protect Aluminum Current Collectors by Promoting the Formation of Insoluble Coordination Polymers Ilya A. Shkrob,*,† Krzysztof Z. Pupek,‡ and Daniel P. Abraham*,† †
Chemical Sciences and Engineering Division and ‡Energy Systems Division, Materials Engineering Research Facility, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Presently, there is a strong incentive for increasing the operation voltage of Li-ion cells above 4.5 V in order to increase the density of stored energy. Aluminum is an inexpensive, lightweight metal that is commonly used as a positive electrode current collector in these cells. Imide LiX salts, such as lithium bis(trifluoromethylsulfonyl)imide (X = TFSI), and lithium bis(fluorosulfonyl)imide (X = FSI), are chemically stable on the energized lithiated transition metal oxide electrodes, but their presence in the electrolyte causes rapid anodic dissolution and pitting of Al current collectors at potentials exceeding 4.0 V versus Li/Li+. For LiBF4 and LiPF6, the release of HF near the energized surfaces passivates the exposed Al metal, inhibiting this pitting corrosion, but it also causes the gradual degradation of the cathode active material, negating this important advantage. Here we report that in certain electrolytes containing fluorinated carbonate solvents and LiX salts, the threshold voltage for safe operation of Al current collectors can be increased to 5.5 V versus Li/Li+. Interestingly, the most efficient solvent also facilitates the formation of an insoluble gel when AlX3 is introduced into this solvent. We suggest that this solvent promotes the aggregation of coordination polymers of AlX3 at the exposed Al surface that isolate this surface from the electrolyte, thereby preventing further Al dissolution and corrosion. Other examples of Al collector protection may also involve this mechanism. Our study suggests that such “allotropic control” could be a way of widening the operation window of Li-ion cells without electrode deterioration, Al current collector corrosion, and electrolyte breakdown. organic carbonates shown in Scheme 1, such as fluoroethylene carbonate (FEC) and bis(2,2,2-trifluoroethyl)carbonate (BFEC), which have higher oxidation potentials; see refs 1 and 2. However, taking full advantage of this wider electrochemical window is still hindered by the current collector dissolution problem In conventional Li-ion batteries (LIBs), the electric charge at the positive electrode is collected by a thin foil of aluminum, which is lightweight, inexpensive, easy-to-process, and a wellconducting metal.3,4 However, in many electrolyte systems, including the organic carbonates shown in Scheme 1, the Al collector becomes pitted once the voltage exceeds a critical value. This well-known phenomenon is a prime reason why so few kinds of lithium salts are used in LIB electrolytes. The commonly used salt, LiPF6, is undesirable in many respects, as it readily hydrolyzes releasing corrosive hydrogen fluoride that degrades the cathode active material.5,6
1. INTRODUCTION There is a strong incentive for increasing the operation voltage of Li-ion cells above 4.5 V to increase the density of stored energy. Electrolytes containing carbonate solvents, such as ethyl methyl carbonate (EMC, Scheme 1) and ethylene carbonate (EC, Scheme 1), that are presently used in commercial Li-ion batteries (LIBs) are not optimal as they oxidize and decompose at the positive electrode at elevated voltages. These oxidation reactions can be minimized by using fluorinated analogs of the Scheme 1. Structural Formulas for the Organic Carbonates and Imide Anions
Received: May 24, 2016 Revised: July 27, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.jpcc.6b05241 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C The examination of Al current collectors from failed cells reveals multiple pits,7 suggesting that the oxide layer (that typically covers and protects aluminum surfaces) becomes compromised, causing dissolution of the metal. The rate of this process is anion and solvent dependent.8 The dissolution is least facile for lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), LiPF6, and LiBF44 and most aggressive for lithium bis(trifluoromethanesulfonyl)imide, LiTFSI (NTf2, Scheme 1), and LiOTf (where Tf stands for the triflyl group CF3SO2), while LiFSI (lithium bis(fluorosulfonyl)imide, Scheme 1),9−11 LiBETI (Scheme 1),12−14 and LiClO4 fall in between.15,16 Combining LiPF6 with lithium salts, such as LiTFSI, prevents Al dissolution in proportion to the fraction of LiPF6 in the electrolyte.11,12 Addition of certain retarding agents, such as C4F9SO2F15 and fumed silica,17 impedes this corrosion/dissolution but does not prevent it entirely. Growing a thick oxide layer on aluminum also slows down the corrosion but avoiding cracks in such coating during cell assembly becomes a technological challenge.15 Apart from the organic carbonates, relatively few other solvents have been studied in this regard. Pitting corrosion becomes even more problematic in lactones (such as γbutyrolactone), while it appears to be negligible in nitriles (such as acetonitrile and adiponitrile) and cyclic ethers (tetrahydrofuran).8,18 Likewise, when the same lithium salts (LiX, where X = FSI−, TFSI−, or BETI−, Scheme 1) are introduced in ionic liquids (C+X−) consisting of the same anion X− as these salts, Al corrosion/dissolution is reduced. Unfortunately, these ionic liquid electrolytes are too viscous at room temperature for practical use. Thinning them with the carbonate solvents (even adding as little as 10 vol %) reintroduces Al dissolution, and the same is observed in mixtures of (di)nitriles with the carbonate solvents.19−22 Over the years, several models for Al current collector damage have been put forward; see review8 for an introduction. The central assumption4,23 made in all of these models is that due to mechanical contact of thin protective coating of Al2O3 with cathode particles during cell assembly, microscopic defects are introduced, exposing Al metal to electrolyte and allowing the subsequent anodic oxidation and dissolution. Indeed, even lightly scratched Al surfaces exhibit rapid and severe corrosion.8 Ironically, it is the poor thermal, oxidation, and hydrolytic stability of PF6− and BF4− anions that are the likely cause for their beneficial action. The hexafluorophosphate undergoes decomposition (to form fluoride and PF5) and/or hydrolysis at the cathode/collector, releasing HF near the surface.24 This acid reacts with exposed aluminum and etches through aluminum oxide yielding insoluble aluminum trifluoride (and/ or mixed aluminum oxide-fluoride). As this material coats the surface, it prevents further Al dissolution. Other researchers suggest direct reaction of Al2O3 with PF5 (see above) with the formation of AlF3 and PO2F2−.6 In contradistinction to PF6−, the adverse effect of TFSI− and other sulfonylimide anions (X−) shown in Scheme 1 has been accounted for by their chemical and electrochemical stability. Kanamura et al.14 showed that addition of HF to electrolyte containing LiTFSI retards pitting, suggesting that the protective action of LiPF6 may indeed be due to HF release alone. When HF was not introduced or generated, a protective AlFxOy coating did not form.12 Thus, it has been speculated that for such (electro)chemically stable anions the protection of exposed Al metal depends on (in)solubility of the corresponding AlX3 salts and/or the products of their further oxidation.4
For this rationale to work, this solubility must be high in carbonate electrolytes, while in ionic liquids and other “benign” solvents it should be much lower.21 The experiments described in ref 20 support such a possibility but only indirectly. On the other hand, studies of Kramer et al.18 indicate that F− is actually one of the products generated through electrochemical cycling of LiTFSI solution as observed by sampling of the electrolyte. Furthermore, some faradaic currents attributed to Al dissolution did not correlate with weight losses, that is, they were due to electrolyte oxidation as opposed to Al dissolution. It appears that the former is frequently confused with anodic dissolution in previously reported data.18 The peculiarity of LiTFSI containing electrolytes is said to be in causing anodic dissolution despite the possible presence of fluoride in the electrolyte bulk. As seen from this discussion, the properties of AlX3 salts (which are the tentative products of anodic oxidation), especially their solubility, play a central role in these scenarios. It is, therefore, striking that Al(TFSI)3 was not obtained until 2009,25,26 which is a decade after these scenarios were put forward initially, while Al(FSI)3 and Al(BETI)3 still remain hypothetical compounds. As Al(TFSI)3 is volatile, single crystals can be grown from vapor.25 The TFSI− anions preferentially assume anti-configuration 1a shown in Scheme 1 and Figure 1a (while syn-configuration 1s is close in energy,
Figure 1. (a) All-anti (1a) and (b) all-syn (1s) tautomers of the C3 symmetrical, bidentate Al(TFSI)3 molecular complex and (c) Z = 4 molecular crystal 2.
see Figure 1b), with the pendant trifluoromethyl groups pointing into the opposite directions away from the S−N−S plane. The triclinic unit cell (containing Z = 4 complexes) consists of octahedral Al3+ ions chelated by three TFSI− anions in a bidentate fashion through their sulfonyl oxygens (Figure 1c). Al(TFSI)3 readily dissolves in polar solvents, such as chloroform, C+TFSI− ionic liquids, acetone, dimethoxyethane, tetrahydrofuran, and acetonitrile.25,26 In Section 3.2, we demonstrate that it also readily dissolves in fluorinated and nonfluorinated carbonate solvents. In fact, of all the solvents discussed above only adiponitrile does not dissolve Al(TFSI)3 readily. These observations do not accord with the currently held opinion that the suppression of Al dissolution in “benign” solvents (such as ionic liquids and acetonitrile) is due to the poor solubility of Al(TFSI)3. Furthermore, for this common rationale to apply at all, this solubility needs to be not just low B
DOI: 10.1021/acs.jpcc.6b05241 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C but extremely poor, as the amount of electrochemically generated Al(TFSI)3 at the interface is tiny, while there is a large volume of electrolyte in contact with it, shifting the equilibrium toward dissolution. These inconsistencies hint that there is a facet of chemistry here that is not captured by simple models. There is sparse information on Al current collector stability in fluorinated carbonate electrolytes1,2,27−29 that are designed specifically for high-voltage applications (as nonfluorinated carbonates all oxidize above 4.5−5 V), and we sought to address this knowledge gap. During our exploration of such systems, we serendipitously discovered that a particular solvents system, FEC/BFEC 2:8 w/w (Scheme 1) delayed Al dissolution for LiX salts (X = FSI and TFSI). It was through studies of this electrolyte that we realized that AlX3 solids actually exist in two forms: the previously recognized form shown in Figure 1c, which is a molecular crystal, and a more thermodynamically stable polymeric allotrope. The “benign” solvent mixture promotes formation of the insoluble polymer that protects the exposed Al metal. Recently the formation of insoluble [Li(FEC)3]PF6 solvate crystals from the same fluorinated solvents system was reported,30 and it was demonstrated that such crystals contain Li+ ion coordination polymers. Apparently, different kinds of metal ions have the increased tendency to form such polymers in fluorinated electrolytes. The supporting schemes, tables, figures, and the lists of abbreviations and reactions have been placed in the Supporting Information. When referenced in the text, these materials have the designator “S”, as in Figure S1.
Figure 2. Scaled and background corrected 27Al NMR spectra of Al(TFSI)3 in different solvents: (i) CDCl3, (ii) PC (trace iii is for a solution containing Al(OTf)3 instead), (iv) FEMC, (v) BFEC, and (vi) 2:8 w/w FEC/BFEC mixture.
added to the solutions for frequency locking. For analytical NMR, the solutions were diluted 1:15 v/v in dimethyl sulfoxide-d6 (DMSO). Long acquisition times (8−10 s) and adequate intervals between the radiofrequency pulses were required to avoid saturation of spin transitions to ensure accurate integration. We are reminded that 27Al is a quadrupolar (spin-5/2) nucleus, so Fourier transform NMR spectra are obtained only if the complex has a highly uniform electric field (almost perfect octahedral symmetry) around the Al3+ ion. It is noteworthy that for the same reason (strong line broadening, causing the free induction to decay within the dead time of the spectrometer) only species that rotate rapidly in solution are observed, even when large aggregates are known to be present in the solvent. To measure solubility of LiX salts in BFEC at saturation (required for electrochemical measurements in Section 3.1), 1 mL of this solvent was vortexed for 15 min with excess salt, and then this solution was stirred for another 2 h at 25 °C. The solid residue was separated by centrifuging, and the supernatant was centrifuged once again. The density of the liquid sample was determined, and it was dissolved in DMSO-d6. The mole ratio of LiX to BFEC was estimated by 19F NMR and used to calculate the molar concentration. The powder X-ray diffraction data were obtained at ambient temperature using a Bruker D8 Advance diffractometer (Cu Kα radiation of 40 mA, 40 kV, λ = 0.154 nm). The data were analyzed using DIFFRAC.SUITE EVA software (Bruker). The powder sample was placed inside an airtight plastic dome specimen holder (Bruker A100B36) mounted on a homemade assembly allowing purging of dry Ar through the assembly during signal acquisition. 2.2. Electrochemical Measurements. All potentials reported in this study are given versus Li/Li+. For the electrochemical measurements, aluminum clad CR2032 kits (Hohsen) were purchased from Pred Materials International. In these cells, the case, which is the positive electrode terminal exposed to high voltages, is made of 304-stainless steel that is Ni-plated with aluminum cladding. Unusually high corrosion currents are obtained if conventional CR2032 kits (not Al-clad) are used with LiTFSI and LiFSI salts presumably because of galvanic reactions between the stainless steel and Al foil. In addition to the electrolyte of interest, the cells contained an Al foil positive electrode (1.6 cm2 area), Celgard 2325 separator
2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1. Materials, Synthesis, and Characterization. Unless specified otherwise, all chemicals were obtained from SigmaAldrich. Battery grade fluoroethylene carbonate was obtained from Solvay. The LiPF6 was obtained from Strem Chemicals, LiTFSI from Aldrich, and LiFSI from Sarchem Laboratories and Suzhou Fluolyte Co. (China). Battery grade bis(2,2,2trifluoroethyl)carbonate (BFEC) and methyl 2,2,2-trifluoroethyl carbonate (FEMC) were synthesized at Argonne’s Materials Engineering Research Facility (MERF). The compounds were determined to be pure by gas chromatography−mass spectrometry and 1H, 13C, and 19F NMR (see Table S1 for spectroscopic characterization). Synthesis of Al(TFSI)3 followed a modified method of Rocher et al.26 Fine powder of anhydrous aluminum trichloride (260 mg, 1.95 mmol) was added to a solution of HTFSI superacid (1.72 g, 6.1 mmol) in dry toluene (5 mL) placed in a 15 mL centrifuge tube. The solution was stirred vigorously with a magnetic bar. Immediately after addition, the suspended AlCl3 turns yellow and there is evolution of HCl. The gas was removed by blowing dry nitrogen over the liquid surface. After 15 min, the solution becomes clear and oily liquid separates at the bottom and solidifies. The tube was centrifuged and the liquid was decanted. The solid residue was washed with toluene (3 × 5 mL) by suspending it by vortexing and centrifuging. The solid residue was ground into a fine white powder and the residual toluene was removed in vacuum (70% yield). Figure 2, trace i shows 27Al nuclear magnetic resonance (NMR) spectra (see also Table S2) of this material in chloroform, which are nearly identical to the ones reported in the literature.25,26 The 19F and 27Al NMR spectra were obtained using a Bruker Avance III HD 300 MHz spectrometer. CDCl3 (10 vol %) was C
DOI: 10.1021/acs.jpcc.6b05241 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C (1.98 cm2 area), and Li-metal counter electrode (1.89 cm2 area). The cells were assembled in an argon-atmosphere glovebox and held within a constant temperature chamber at 55 °C. The electrochemical tests were conducted using a MACCOR Series 4000 Test unit. The cells were held for an hour at the open circuit voltage before applying a 0.1 V increment step from 3.5 V to either 5.0 or 6.0 V, as illustrated in Figure 3a. The cells were held at each voltage step for an hour and the corresponding currents were measured during that time.
nonfluorinated electrolytes are consistent with observations previously reported in the literature.34 The effect of using fluorinated solvents and solvent mixtures is displayed in Figures 4 and 5. For FEC containing 1 M
Figure 4. Current versus potential plots for coin cells containing (a) 1.0 M LiTFSI in FEC and FEC/BFEC 2:8 w/w solvents, and (b) 1.2 M LiFSI in several FEC/BFEC mixtures (the weight ratios of the two components are given in the legend).
LiTFSI, a rapid current rise is seen at ∼4.4 V (see Figure 4a), which is 0.35 V higher than for Gen 2 solvent (Figure 3a). On the other hand, for the FEC/BFEC 2:8 w/w electrolyte containing 1.0 M LiTFSI the current displays a relatively Figure 3. (a) Potential and current as a function of time for an Al/Li coin cell. (b) Current versus potential plots for cells containing 1.2 M (i) LiTFSI, (ii) LiFSI, and (iii) LiPF6 in EC/EMC 3:7 w/w solvent. Aluminum electrode area was 1.6 cm2.
2.3. Molecular Modeling. For molecular modeling, semiempirical PM7 method31,32 from MOPAC2016 program suite33 was used in all of the calculations. The method choice was dictated by the complexity of the system that does not allow using more rigorous approaches without excessive computational resources. We argue that our findings are sufficiently robust for the conclusions to be model free.
3. RESULTS AND DISCUSSION 3.1. Electrochemical Measurements. Potential and current response profiles during a typical electrochemical test are shown in Figure 3a,b. In this test, negligible currents were observed until ∼16 h, when a rapid rise in current was observed (as indicated with the arrow). Figure 3b demonstrates that for 1.2 M salt concentrations in EC/EMC (3:7 w/w), also known as Gen 2 solvent, the rapid rise in current is observed at ∼4.05 and 4.25 V for LiTFSI and LiFSI, respectively. Note that smaller currents (
