Curious Case of Positive Current Collectors: Corrosion and

Subscriber access provided by READING UNIV

Article

The curious case of positive current collectors: corrosion and passivation at high temperature Farheen Sayed, Marco-Tulio F Rodrigues, Kaushik Kalaga, Hemtej Gullapalli, and Pulickel M. Ajayan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12675 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on December 5, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 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

ACS Applied Materials & Interfaces

The Curious Case of Positive Current Collectors: Corrosion and Passivation at High Temperature Farheen N. Sayed*, Marco-Tulio F. Rodrigues, Kaushik Kalaga, Hemtej Gullapalli, P. M. Ajayan* Department of Material Science and NanoEngineering, Rice University, Houston, Texas 77005, USA, E-mail: [email protected] ; [email protected]

Abstract: In evaluation of compatibility of different components of cell for high energy and extreme conditions applications, the highly focused are positive and negative electrodes and their interaction with electrolyte. However, for high temperature application, the other components are also of significant influence and contributes towards the total health of battery. In present study we have investigated the behavior of aluminum, the most common current collector for positive electrode materials for its electrochemical and temperature stability. For electrochemical stability different electrolyte, organic and RTILs with varying Li-salts (LiTFSI, LiFSI) are investigated. The combination of electrochemical and spectroscopic investigations reflects the varying mechanism of passivation at room and high temperature, as different compositions of decomposed complexes are found at the surface of metals.

Keywords: Current collector, high temperature, high voltage, passivation, corrosion,

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

Introduction: A colossal amount of efforts is directed at achieving near-theoretical energy density in Li-ion batteries (LIBs), with emphasis on the design and development of high performance materials for the anode, cathode and electrolyte. There is an exciting pool of high-voltage cathodes, robust anodes and supporting electrolytes suitable for extreme voltages under development, with deployment perspectives in the long term. For applications beyond consumer electronics, as in space exploration, medical devices and mining tools, enduring exposition to elevated temperatures constitutes an additional challenge. Under this demanding condition, unsolicited processes arising either at interphases (electrode/electrolyte, current collector/electrolyte) or in the bulk of the electrodes are accelerated, contributing to performance decay. Although there exists a reasonable understanding of environmental effects on the electrode/electrolyte interphase1, the physicochemical events involving current collectors remain largely unexplored 2. Current collectors (CC) owing to their irreplaceable function as electrical loop between electrode materials and external circuits, account for more than 90% of the electronic conductivity and 90% of the mechanical strength of the electrode3. The commonly employed CCs are copper and aluminum, for anode and cathode, respectively. In addition to these metals, nickel, titanium and stainless steel have also been tested for CCs in lithium-ion cells4. The pure surfaces of these metals may not be intrinsically stable in contact with the electrolyte under operating conditions. However, interaction with electrolyte may lead to formation of a passivating film, which artificially extends the electrochemical voltage stability of these metals. Under mechanical stress and extreme conditions (such as elevated temperatures), loss of adhesion of active materials from current collector leads to localized corrosion, such as pitting, and general corrosion5. The latter phenomena can be destructive in nature and may increase the internal impedance of the LIB during cycling, leading to capacity and power losses. At the positive electrode of LIBs, where demand for survival at high voltage is encouraged to achieve maximum energy density, suppressing the corrosion of current collectors becomes of utmost importance. Temperature increase is expected to incur in loss of the basic passivation properties of current collectors, both due to cracking of the surface film and enhanced solubility of its components into the electrolyte. Intrinsic stability limitations of the commonly used electrolytes have made it


Page 2 of 21

Page 3 of 21 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


difficult to probe specific environmental effects on the positive CC. While common salts display a tendency to hydrolyze at elevated temperatures6, the thermally stable LiTFSI is typically unable to protect aluminum beyond 3.7 V vs Li+/Li 7-8. The initial step for aluminum corrosion in the presence of this salt has been proposed to involve the formation of Al-TFSI complexes following the attack to the native protective Al2O3 layer. These complexes are fairly soluble in organic carbonate solvents, and do not exhibit passivating activity9. Suppression of dissolution, and thus protection of the current collector, can nevertheless be achieved when the electrolyte contains unusually high salt concentrations10. By a similar mechanism, aluminum corrosion does not seem to be substantial when the solvent is replaced by certain ionic liquids (ILs), as the high anionic concentrations would inhibit the dissolution of Al-TFSI complexes11. The thermal resilience of ionic liquids offers an attractive platform for the investigation of electrochemical processes at extreme environments. Previous studies from our group had demonstrated that IL-based electrolytes are able to achieve high cyclic stability at temperatures as high as 150oC

12 13

. The effective stability of the current collectors at high temperature in

combination with high voltages is, nevertheless, unknown. In the present study, we investigate the evolution of the current collector/electrolyte interphase at high voltages, both at room temperature and 120oC. Electrochemical and spectroscopic experiments rationalize the surface behaviors of several metal/electrolyte pairs. Experimental: Materials – The materials investigated as current collectors in this work were battery grade Al, Ni and Ti foils (Alfa Easar), stainless steel discs (Hohsen Corporation), and the Ni-based alloy Hastelloy C276. The reported composition of Hastelloy C276 is Mo (15.0-17.0%), Cr (14.516.5%), Fe (4.0-7.0%), W (3.0-4.5%), Co (2.5% max), Mn (1.0% max), C (0.01% max), V (0.35% max), P (0.04% max), S (0.03% max), Si (0.08% max). For Al, Ni and Ti metal foils, discs with 12 mm of diameter were punched, thoroughly cleaned with acetone and IPA, and finally dried at 80 oC overnight under vacuum. Hastelloy (HA) and stainless steel (SS) were cut into 10x10 mm squares and used after similar cleaning steps. The dry metallic substrates were then transferred to an Argon-filled glovebox for cell fabrication. The conventional electrolyte 1M LiPF6 in EC:DMC, 1:1 vol/vol from Solvionic was used without any treatment. For RTIL electrolytes (from Iolitec), 1-propyl-1-methylpiperidinium



bis(trifluoromethylsulfonyl)imide

(PIP-TFSI)

and

Page 4 of 21

(N-methyl-N-propylpyrrolidinium

bis(trifluoromethylsulfonyl)imide) (PY13-TFSI) were selected and used after drying at 100 oC in the glove box before mixing with salts. The Li-salts used were LiTFSI (Sigma Aldrich) and LiFSI (Oakwood Products, Inc.). To maintain consistency with our previous works13

12

,

concentration of LITFSI in PIP-TFSI was 1M. In tests involving concentrated electrolytes, 2M and 3M LiTFSI were prepared with PIP-TFSI, respectively. Solutions of PY13-TFSI with LiTFSI and LiFSI were prepared with salt concentration of 0.4 mol/kg. Electrochemical measurements – The tests were conducted in a 2-electrode setup, using different metal foils as working electrode and Li foil as both counter and reference electrodes, with quartz microfiber membranes as separator (WHATMAN) soaked with fixed volume of the electrolytes. Coin cell cases from Hohsen Corporation were used in the tests, as they were found to remain well-sealed under the experimental conditions. The cells typically exhibited stable OCP values ranging from 2.2 to 3.0 V immediately following the assembly. The characteristic cyclic voltammetry (CVs, from OCP to 4.3 V) were recorded for 5 cycles using AUTOLAB PGSTAT 302 N ECOCHEMIE potentiostat/galvanostat, with the scan rate of 10 mV/s. For the high temperature measurements, the coin cells were equilibrated in an oven for 4 h at the desired temperature before the electrochemical tests. For chronoamperometry measurements, cells were first polarized to high voltage (4.3 V) by linear sweep voltammetry (LSV) with the scan rate of 10 mV/s, and subsequently held at this potential for 15 h while monitoring the current variation. X-ray photoelectron spectroscopy (XPS) analysis - XPS measurements were performed with a Physical Electronics Quantera Scanning X-ray Microprobe. This system uses a focused monochromatic Al Kα X-ray (1486.7 eV) source for excitation and a spherical section analyzer. High energy resolution spectra were collected using a pass-energy of 26.0 eV. Ar+ etching was undertaken with 2kV 3x3 mm2 energy for 1 minute. Morphology was identified using scanning electron microscope images recorded from FEI Quanta 400 ESEM FEG at an accelerating voltage of 15 keV.


Page 5 of 21 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


Results and discussion: 1: Aluminum with organic and RTIL electrolytes The passivation behavior of aluminum with conventional organic electrolyte at room temperature was investigated to set a baseline for our experiments, and the results are displayed in Figure 1. In view of the practical voltage limit of commonly employed positive materials (4.2 V), as LiCoO2, LiNixMnyCozO2, and LiNixCoyAlzO2 (x + y + z ≤ 1), the electrochemical tests were conducted with 4.3 V as a cut-off voltage. Chronoamperometry was performed by first polarizing the electrode up to 4.3 V by LSV, to ensure the homogenous current increase under similar condition to the cyclic voltammetry, as described below. During chronoamperometry, the working aluminum electrode was held at 4.3 V for 15 h, while changes in current were monitored. Immediately after the terminal voltage was reached, the potentiostatic hold begins, involving multiple phenomena that can contribute to the measured current and giving rise to a complex curve (Figure 1a). Within this curve, three separate behaviors/processes could be assigned as P(I) (300-20,000 s), P(II) (20,000-40,000 s) and P(III) (from 40,000 s onwards), as detailed in Figure 1a(inset). The flow of assigned process may or may not be in same order, however for distinction as separate processes are defined here in terms of their time interval. Immediately after the aluminum reached 4.3 V, there was a strong current decay associated with the disruption of the double layer formed while the electrode was completely polarized (P(I)). Once this process had sufficiently relaxed, the oxidative decomposition current dominated the signal. It is important to note that the measured current constantly decreased in P(II), which could be attributed to a number of processes, including depletion of reacting species, diminished active surface upon deposition of decomposed species, or a lingering discharge of the double layer. As our interest resided in analyzing the anodic processes, chronoamperometric experiments were analyzed by considering only the final fraction of the curves, P(III). During CV experiments (Figure 1b), the first voltammogram exhibited a peak at ~3.6 V with onset at 3.22 V, after which the current showed steady rise with increasing potentials. The fist peak was completely irreversible, and vanished after the first cycle. Additionally, subsequent voltammograms achieved lower currents at the high potential region. At potentials higher than 3.5 V vs Li+/Li, electrolyte instabilities have a growing contribution to the interphasial behavior, when the native oxide passivation may prove to be unable to sustain oxidation of solvents and



salts14. In Figure 1b, the peaking behavior in the initial anodic sweep arises from the oxidation of electrolyte at the aluminum surface, where a reduction process occurs through the etching effect of fluoride species generated from the electrolyte decay. This results into the exposure of fresh aluminum to further chemical reactions, leading to the formation of fluorine complexes15. The passivation mechanism by AlF3, nevertheless, has not been fully elucidated, as conflicting reports imply presence of both AlF3 and Al2O3 at the surface, with the subsequent breakage of alumina to form Al-OF compounds16. In subsequent cycles, the current density decreases, as well as the peak becomes insignificant, suggesting a saturation of the surface reactions. In order to get further insights into the composition of surface features of fresh aluminum and after the chronoamperometric experiments, XPS data were recorded. Spectral acquisition initiated after etching off the surface layer for 1min, to minimize the interference of adsorbed electrolyte. In Figure 1c, Al2p spectrum of bare untreated aluminum shows the presence of peaks at 72.36, 72.77, 73.16 and 74.66 eV, where the first two peaks correspond to metallic aluminum, and the next two peaks belonging to the alumina layer. It is interesting to observe that the oxide content was prominent (80.19 at%) even after the etching, suggesting the formation of a thick interphase until passivation is finally achieved. After the chronoamperometric measurement with conventional electrolyte, four distinct aluminum peaks could be fitted. The peaks at 72.69, 74.35, 74.96 and 75.52 eV can be assigned to metallic aluminum, aluminum oxide, lithium aluminum hydride, and aluminum fluoride, respectively17 . A closer inspection of the F1s spectrum of the sample held at 4.3 V for 15 hours reveals the occurrence of peaks at 684.91, 686.17, 688.46, and 689.28 eV, corresponding to Li salt, LiP-F-O, AlF3 and C-F elements (Figure 1d) 18. No fluoride species were detected at the surface of bare aluminum. For the investigations at HT, room temperature ionic liquids were selected. Comparative studies of current collector with 1MLiTFSI-PIP at both room (RT) and high temperatures (HT) were carried out. For this purpose, five-consecutive cyclic voltammograms were recorded to elucidate the oxidative electrochemistry of the electrolyte under each environmental condition (Figure 2). In Figure 2a, the first CV at RT shows one weak anodic peak with onset potential lower than 3.22 V, which is irreversible, as suggested by the absence of peaks in the following cathodic scan. The large current density of aluminum could be attributed to the reduction of the surface oxide layer by chemically active species, with concomitant oxidation of the CF3SO2−, generated


Page 6 of 21

Page 7 of 21 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


from TFSI decomposition, into C–F active species. The majority of CF2 anions produced by the disproportionation reaction of CF3 anions would easily reduce the surface oxide layer of aluminum, also yielding COF2, CO and/or CO2 19. Thus, irreversible reactions occur at the newly exposed aluminum metal, which can create a new surface film, Al-TFSI, with the anions remaining in the electrolyte. In subsequent cycles, the peak diminishes and becomes untraceable, also showing lower charge accumulation in form of lower total current under the CV curve. The passivating behavior can be assumed to be completed in the first cycle. The increase in current is marginally less than that observed for the conventional carbonate electrolytes. The subsequent cycles also exhibited a continuous decrease in the maximum current density, although more timidly than observed in Figure 1. At 120 oC, as seen in Figure 2b, along with the prominent feature of electrolyte and alumina layer decomposition at 3.22 V, a 2nd peak related to possible formation of Al-TFSI at 3.7 V is also observed which was not observed for RT (Figure 2a). The current density associated with these interfacial processes at high temperatures are higher than the ones observed for the RT CVs, which can be attributed to the combination of increased mass transport and enhanced kinetics. Also, in the subsequent cycles, the suppression of oxidative (peaks) processes at the electrode surface seems to be as effective as in the samples measured at room temperature, suggesting that the electrolyte and surface alumina layer may undergo similar processes under both conditions. For the HT case, in subsequent cycles the peak at ~3.7 V is still visible. This can be due to two reasons: I) formation of Al-TFSI in the first cycle, which is the product of galvanic pitting corrosion of pre-occurring alumina layer, as described for the experiments at RT, followed by dissolution at higher temperature exposing fresh metal for further formation of AlTFSI; and/or II) the passivation by formation of Al-TFSI was not completely achieved, as the reaction time at the optimum potential could be insufficient and still continues in subsequent cycles, even though the electrode was cycled multiple times. This second hypothesis is, however, unlikely, given the accelerated reaction rates at 120 oC. Here, it is interesting to note that, in combination with this galvanic corrosion of aluminum, the high temperature (non-galvanic) corrosion also comes into picture. As soon as initial oxidation of electrolyte occurs at the current collector surface, the electrolyte will become a pool of different decomposition species, which can contribute to further deterioration of the current collector20 .



For these cells with LiTFSI in PIP, there is a marked difference in the chronoamperometric behavior at RT and 120 oC. After the cells were polarized to a fixed potential of 4.3 V (Figure 2c), chronoamperometry measurements were performed. The initial stages of the potentiostatic hold, i.e. P(I), are similar, but afterwards there are different trends in the current (Figure 2d). The source of varying behavior in P(II) may be the thermal activation of a different oxidation reaction mechanism which is absent at lower temperatures. An alternative explanation is the time-dependency of physical process associated with interphasial decay, as dissolution of surface film components. Instead of the asymptotic current decay expected for a properly passivated surface, the current density further increases as a function of temperature in P(III), as both the double layer and anodic components can be affected by the surface chemistry, evolving from their initial state due to electrolyte degradation products over the course of the test. At any arbitrary period during the test, the higher temperature environment results in a surface that has experienced more reactions, as evidenced by a greater average current, and subsequently more degradation products are present in the system. This data indicates the accelerating effect of temperature on oxidation rate, and that any decrease in the reactivity of the surface brought about by oxidative decomposition products has a smaller effect than the temperature. The high resolution C1s spectra of these two samples include the peak at 285.4 eV, which belongs to C-S bonds. Clear peaks could also be seen at higher binding energies (292.9 eV), and are associated with -C-F (CF3) bonds (Figure 3a). As mentioned earlier, these spectra were recorded after etching the sample to eliminate the adsorbed ions from electrolyte. In the high resolution Al2p spectra, the intensity of the pure metallic peak decreases from 11.8 at% (RT) to 0.92 at% (HT sample) (Figure 3b). This decrease could suggest a possible monotonic increase in the thickness of surface structures with test temperature. An interesting behavior is observed for the F1s peak, where ratios of peaks corresponding to AlF3 and LiF are exact reverse for RT and HT experiments with RTILs (Figure 3c). For LiTFSI-PIP at room temperature, the peak intensity at 688.55 eV, corresponding to AlF3, is higher, whereas the LiF peak at 685.08 eV is modest. Formation of LiF have been reported earlier for RTILs, even at room temperature, contributing to CC passivation with organic electrolytes21. This could suggest that, along with the formation of Al-TFSI, at high temperature the anion itself decomposes to CF3 anions and helps in formation of LiF on further disproportionation reaction. This decomposition of TFSI anion is evident by the decreased intensity of carbon peak corresponding to -C-F (-CF3). Even


Page 8 of 21

Page 9 of 21 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


though small amounts of nitrogen and sulfur were also found to be present at the surface (not shown here), it can be assumed that the relative fluorine content is too high to be just due to the adsorbed anions from electrolyte. As seen from both electrochemical as well as spectroscopic data, the passivating behavior is different at RT and HT. The probable increased solubility of decomposition products may render the passivation insufficient for long term usage at high temperature. Also, the role of non-conducting LiF in passivation is still inconclusive. As a conclusion after this initial study, the aluminum/1M LiTFSI-PIP pair did not display an ideal passivating behavior at 4.3 V and 120 oC. As the long-term stability of this electrolyte has been verified in our previous works12-13 we also carried the investigation of this RTIL-based system with other current collectors, hoping to identify a more suitable combination. To monitor the effect of temperature on surface of aluminum metal surface, samples were also subjected to SEM characterization. The Figure SI represents the micrographs recorded for the metals surface. It is clear that the treatment with conventional carbonate electrolyte at the constant potential of 4.3 the corrosion is very apparent Figure S1a. For high temperature, aluminum surface shows more pit and is rougher Figure S1b as compared to RT treated with conventional electrolyte. 2: Al, Ni, Ti, SS and HA with RTIL at high temperature In order to evaluate different current collectors, Ni, Ti, SS and corrosion resistive Hastelloy C276 (HA) were investigated through chronoamperometric studies at HT. Most of these current collectors have been investigated for practical LIBs as alternatives to aluminum16, 22

. Despite its electrochemical stability, nickel is known to promote the electrochemical

degradation of organic electrolyte, and is prone to oxidize and get easily dissolved by anodic polarization on charging the cell, suggesting its inefficiency to be used as positive current collectors with non-aqueous electrolytes

23 24

. However, changing the electrolyte chemistry can

have greater impact on interfacial reactions, and hence the electrochemical stability. In view of this, Ni was also evaluated with 1M LiTFSI-PIP at 120oC. During the LSV from OCP to 4.3 V, it was evident that the use of ionic liquids did little to stabilize Ni metal at high voltage and high temperature, as observed by the continuous increase in current density from voltages as low as 3.4 V (Figure 4a). Chronoamperometric studies of Ni and Ti showed no relaxation in P(II), observed in the abovementioned cases with aluminum (Figure 4b). In these cases, the current increase was enormous, due to which the half-cells could not survive during the long hold time at



4.3 V at 120oC, popping open as a consequence of pressure build-up from gases released during the electrolyte decay. For SS, the onset potential (Figure 4a) and time (Figure 4b) for the current increase was higher than for nickel and titanium, suggesting the delay of chemical events involving the electrolyte. This latter behavior is, however, diagnostic of surface instabilities, indicating that the electrolyte fails to effectively protect the stainless steel. Although previous works have identified that, at 40oC, SS’s interface with conventional electrolyte was found to be passivated by thin layers of oxide, hydroxide and fluorides25-26, its complex chemistry makes it difficult to clearly identify the reasons for breakdown at high temperatures. To the best of our knowledge there are no reports of the electrochemical behavior of Ni, Ti and SS with RTILs. For HA, however, after a starting fast relaxation, the current exhibited an initial increase, followed by a steady decrease. Although the increase in current after the initial equilibration is similar to that observed for aluminum and SS, the subsequent drop suggests that the electrochemical reaction contributing to the current is being suppressed. This behavior can arise from two sources: the formation of passive layer on the surface on HA, and/or the accumulation of other decomposed products from electrolyte. The SEM micrograph recoded for the HA after high temperature treatment with RTIL shows strong surface modification Figure S1c, where a thicker layer is formed. However, the composition of this layer could not be confirmed due to many elements involved. From this first approach to optimize the current collector for HT operability, it was evident that, despite the lack of metals displaying an ideal passivating behavior, aluminum still holds the promise for furtherance, with HA being a strong next contender. For an in-depth probe of the possibilities offered by these two best metallic current collectors, their performance in combination with different electrolytes, comprising electrochemically and thermally compatible solvents and salts, were also evaluated. 3: Aluminum and HA with different RTILs and additive combinations at HT The selection of different RTIL cations can affect the solubility of passivating layer components. In the present study, we have also investigated commonly used combinations of TFSI based PIP, PY13 solvents, with LiTFSI and LiFSI as Li salts (Figure 5a). For aluminum, in the case where PIP was replaced by the PY13-based IL, a peak is observed at 3.7 V in the LSV at 120oC with onset potential shifting towards lower values than for PIP solvent. Also, the LiTFSI PY13 combination showed the erroneous behavior in chronoamperometric measurement,


Page 10 of 21

Page 11 of 21 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


where current keeps elevating at the hold potential. For temperatures up to 60oC, the IL PY14TFSI, similar to the one we investigate here, has been reported to be electrochemically stable at 4.6 V, where the stability is linked to the insolubility of Al-TFSI in PY1427. In the present study, however, surface protection seems to be insufficient at 120oC, even at lower potentials than reported in ref. 22, highlighting the challenges imposed by higher temperatures. The use of LiFSI, which is known for the lack of release of HF under normal operation conditions

28

, in combination with PY13 did not lead to significant changes in the anodic

behavior. These observations suggest that even though high electrochemical stability is assumed, these compounds still staggered at high temperature. The hypothesis that elevated salt concentrations favor the occurrence of passivation was also put to test at extreme environments. Increasing concentration of LiTFSI salt to 3M at 120oC, though the characteristic anodic behavior in the LSV remains the same, the overall current increases. Contrarily to the 1MLiTFSI electrolyte, the chronoamperometry data (Figure 5b) of 3MLiTFSI electrolyte suggests that the equilibrium between electrode and electrolyte has been achieved faster (P(II)), as the next process of surface reaction (P(III)) starts sooner than that for low concentration. The reason could be due to accelerated reaction kinetic leading to the relaxation of electrolyte polarization resulting into quick equilibrium between electrode and electrolyte. After the equilibrium has been achieved, the current further starts to elevate, suggesting the continuous dissolution of the passivation layer along with the formation. This phenomenon of having both the reactions happening simultaneously can be able to protect the surface of current collector. The increased concentration of salt in 3M electrolyte composition could specifically increase the number of coordinating anions with Li and decrease the bulk uncoordinating anions in salt. This would effectively stabilize the viscous solvent's electron lone-pairs, thereby reducing their susceptibility to further oxidation, as shown to suppress the aluminum corrosion in concentrated glyme electrolyte at RT 29. It is evident that after initial disintegration of passive alumina layer, the solubility of newly fostered Al-TFSI complex is highly influenced by ILs and temperature. It is noteworthy that all the standard remedies to combat the corrosion problems fall short to support the system at higher temperature, including concentrated electrolyte and more stable cation-anion pairs. The permutation of Li-salts and ILs suggests that



even though Al-TFSI/FSI layer formation at 3.7 V is occurring in PIP and PY13, the anodic stability of TFSI complex is worse than FSI in PY13. Similar experiments were also carried with the corrosion resistant Hastelloy (HA). The LSV showed a dependency with salt nature and concentration, but not so much with the identity of the RTIL cation (Figure 6a). Also, it is to be noted that, similar to SS, the composition of this Ni based alloy is so complex that it is difficult to correlate the passivation behavior to individual products originated from reaction with the electrolyte. The linear sweep voltammetry data of 1M LiTFSI in PIP and PY13 display similar behavior, while the samples with increased concentration of LiTFSI (3M) behaves similar to LiFSI in PY13. The behavior of 1M and 3M LiTFSI-PIP are strikingly different from each other. Where first one shows a peak ~3.7V, the latter case shows no peak in LSV. The chronoamperometric studies revealed that HA behaves similar to aluminum with 1M and 3M LiTFSI in PIP. Also, the degradation of PY13 based electrolyte is consistent with the trend observed for aluminum at HT, Figure 6b. Even though Al–LiTFSI-PIP system’s passivating nature is not validated, the formation of LiF at high temperature can contribute towards passivation. Neither of the current collector-electrolyte interfaces investigated in this work was able to maintain proper passivation in face of the combination of extreme conditions of temperature and voltage. In such scenarios, long term endurance of the cells at HT might be achieved at the expense of operating voltage, and vice versa. Hence, in another attempt to define the effective electrochemical stability window at HT, the chronoamperometric measurements were performed after polarizing aluminum up to different potentials 4: Aluminum with 1MLiTFSI-PIP polarized at different potentials at HT: As discussed above, the trend in current density in chronoamperometric measurements spanning around three different processes is expected to be highly dependent on cell voltage. The holding potential, as well as the temperature, will influence the different decomposition reactions of the electrolyte, and hence will affect the passivating behavior. In another experiment, an attempt has been made to vary the polarization voltage and observe its influence on the electrochemical reactivity while monitoring current density. Figure 7a presents the aluminum metal being polarized up to different potentials from 3.6 to 4.5 V. The anodic behavior is similar to what was discussed in Figure 2d, where the peak at ~3.2 V is due to the oxidation of


Page 12 of 21

Page 13 of 21 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


electrolyte at the metal surface, followed by a peak at 3.6 V, which is supposedly of the aluminum complexation with the decomposed anion from the electrolyte. It is evident that, from 3.6 to 4.2 V, the current showed a monotonic increase. However, an increase in the final potential to 4.3 and 4.5 V lead to a decrease in current, suggesting that additional reactions happening at potentials > 4.2 V are capable of forming reaction products which could lower the additional charge transfer, and hence possibly contributes to improve the passivation

20

. The

chronoamperometry results show that even in P(I) there is a clear difference and influence of the hold potential. Towards the end of P(I) the current starts increasing for 4.2 and 4.5 V sample. However, as mentioned earlier, these processes can be overlapping to each other and difficult to differentiate clearly. As seen in Figure 7b, inset, in P(III) the current starts increasing for 4.2 and 4.3 V, and then decreases at hold potential of 4.5 V. This could suggest that the electrochemical reaction occurring at and beyond 4.2 V might assist in passivation. Conclusion: The electrochemical interaction at high temperature at the interphase of electrolyte and current collector is the key factor to determine overall stability of LIB. The reaction kinetic and nature of decomposed product alter the dynamic of current collector passivation and hence the positive electrode. Our experimental studies indicate that, for positive electrode materials operating at potentials up to 4 V, as LiFePO4 (lithiation potential ~3.6V), the use of aluminum current collectors with 1MLiTFSI in PIP-TFSI as electrolyte can minimize faradaic effects arising from corrosion at 120oC. The chemical composition of the new fostered surface layer through XPS suggested the decomposition of Al-TFSI complex and further formation of other fluorides. Although this same electrolyte demonstrated reasonable stability both with aluminum and Hastelloy at 4.3 V and high temperature, the corrosion current is not negligible under these circumstances, and might affect the coulombic efficiency, impedance, cycle life, and selfdischarge rates of Li-ion cells. In a practical sense, the overall stability of the cathode/electrolyte interface also needs to consider the electrolyte reactivity towards the delithiated cathode particles. This work sets a preliminary basis to investigate these complex interfaces, and the information discussed here will be further enriched by future work. Acknowledgement: This work was supported by the Advanced Energy Consortium (http://www.beg.utexas.edu/aec/) with BHP Billiton, Department of Energy, ExxonMobil,



Repsol, Shell, and Total as members. Author Farheen N. Sayed would like to acknowledge USIEF sponsored Nehru-Fulbright program and Institute of International Education (IIE) for postdoctoral fellowship. Authors would like to acknowledge Dr. G. B. Reddy for scientific and technical inputs. Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images of aluminum metal after chronoamperometric measurements with 1M conventional EC:DMC; 1:1 (vol/vol) electrolyte (at RT), and with 1M LITFSI-PIP electrolyte (at 120 oC). SEM image of Hastelloy after chronoamperometric measurements with 1M LITFSI-PIP electrolyte (at 120 oC). References: 1. Leng, F.; Tan, C. M.; Pecht, M., Effect of Temperature on the Aging rate of Li Ion Battery Operating above Room Temperature. Scientific Reports 2015, 5, 12967. 2. Rodrigues, M.-T. F.; Babu, G.; Gullapalli, H.; Kalaga, K.; Sayed, F. N.; Kato, K.; Joyner, J.; Ajayan, P. M., A materials perspective on Li-ion batteries at extreme temperatures. Nature Energy 2017, 2 (17108), 14. 3. Wang, M.; Le, A. V.; Shi, Y.; Noelle, D. J.; Qiao, Y., Heterogeneous current collector in lithium-ion battery for thermal-runaway mitigation. Applied Physics Letters 2017, 110 (8), 083902. 4. Whitehead, A. H.; Schreiber, M., Current Collectors for Positive Electrodes of Lithium-Based Batteries. Journal of The Electrochemical Society 2005, 152 (11), A2105-A2113. 5. Braithwaite, J. W.; Gonzales, A.; Nagasubramanian, G.; Lucero, S. J.; Peebles, D. E.; Ohlhausen, J. A.; Cieslak, W. R., Corrosion of Lithium-Ion Battery Current Collectors. Journal of The Electrochemical Society 1999, 146 (2), 448-456. 6. Lux, S. F.; Lucas, I. T.; Pollak, E.; Passerini, S.; Winter, M.; Kostecki, R., The mechanism of HF formation in LiPF6 based organic carbonate electrolytes. Electrochemistry Communications 2012, 14 (1), 47-50. 7. Matsumoto, K.; Inoue, K.; Nakahara, K.; Yuge, R.; Noguchi, T.; Utsugi, K., Suppression of aluminum corrosion by using high concentration LiTFSI electrolyte. Journal of Power Sources 2013, 231, 234-238. 8. Morita, M.; Shibata, T.; Yoshimoto, N.; Ishikawa, M., Anodic behavior of aluminum current collector in LiTFSI solutions with different solvent compositions. Journal of Power Sources 2003, 119– 121, 784-788. 9. Myung, S.-T.; Hitoshi, Y.; Sun, Y.-K., Electrochemical behavior and passivation of current collectors in lithium-ion batteries. Journal of Materials Chemistry 2011, 21 (27), 9891-9911. 10. Wang, J.; Yamada, Y.; Sodeyama, K.; Chiang, C. H.; Tateyama, Y.; Yamada, A., Superconcentrated electrolytes for a high-voltage lithium-ion battery. Nature Communications 2016, 7, 12032. 11. Garcia, B.; Armand, M., Aluminium corrosion in room temperature molten salt. Journal of Power Sources 2004, 132 (1–2), 206-208.


Page 14 of 21

Page 15 of 21 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


12. Rodrigues, M.-T. F.; Kalaga, K.; Gullapalli, H.; Babu, G.; Reddy, A. L. M.; Ajayan, P. M., Hexagonal Boron Nitride-Based Electrolyte Composite for Li-Ion Battery Operation from Room Temperature to 150 °C. Advanced Energy Materials 2016, 6 (12), 1600218-n/a. 13. Kalaga, K.; Rodrigues, M.-T. F.; Gullapalli, H.; Babu, G.; Arava, L. M. R.; Ajayan, P. M., Quasi-Solid Electrolytes for High Temperature Lithium Ion Batteries. ACS Applied Materials & Interfaces 2015, 7 (46), 25777-25783. 14. Krause, L. J.; Lamanna, W.; Summerfield, J.; Engle, M.; Korba, G.; Loch, R.; Atanasoski, R., Corrosion of aluminum at high voltages in non-aqueous electrolytes containing perfluoroalkylsulfonyl imides; new lithium salts for lithium-ion cells. Journal of Power Sources 1997, 68 (2), 320-325. 15. Zhang, X.; Devine, T. M., Identity of Passive Film Formed on Aluminum in Li-Ion Battery Electrolytes with LiPF6. Journal of The Electrochemical Society 2006, 153 (9), B344-B351. 16. Myung, S.-T.; Sasaki, Y.; Sakurada, S.; Sun, Y.-K.; Yashiro, H., Electrochemical behavior of current collectors for lithium batteries in non-aqueous alkyl carbonate solution and surface analysis by ToFSIMS. Electrochimica Acta 2009, 55 (1), 288-297. 17. Song, J.-W.; Nguyen, C. C.; Song, S.-W., Stabilized cycling performance of silicon oxide anode in ionic liquid electrolyte for rechargeable lithium batteries. RSC Advances 2012, 2 (5), 2003-2009. 18. Ziv, B.; Borgel, V.; Aurbach, D.; Kim, J.-H.; Xiao, X.; Powell, B. R., Investigation of the Reasons for Capacity Fading in Li-Ion Battery Cells. Journal of The Electrochemical Society 2014, 161 (10), A1672A1680. 19. Nakajima, T.; Mori, M.; Gupta, V.; Ohzawa, Y.; Iwata, H., Effect of fluoride additives on the corrosion of aluminum for lithium ion batteries. Solid State Sciences 2002, 4 (11–12), 1385-1394. 20. Ma, T.; Xu, G.-L.; Li, Y.; Wang, L.; He, X.; Zheng, J.; Liu, J.; Engelhard, M. H.; Zapol, P.; Curtiss, L. A.; Jorne, J.; Amine, K.; Chen, Z., Revisiting the Corrosion of the Aluminum Current Collector in LithiumIon Batteries. The Journal of Physical Chemistry Letters 2017, 8 (5), 1072-1077. 21. Younesi, R.; Veith, G. M.; Johansson, P.; Edstrom, K.; Vegge, T., Lithium salts for advanced lithium batteries: Li-metal, Li-O2, and Li-S. Energy & Environmental Science 2015, 8 (7), 1905-1922. 22. Iwakura, C.; Fukumoto, Y.; Inoue, H.; Ohashi, S.; Kobayashi, S.; Tada, H.; Abe, M., Electrochemical characterization of various metal foils as a current collector of positive electrode for rechargeable lithium batteries. Journal of Power Sources 1997, 68 (2), 301-303. 23. Veith, G. M.; Dudney, N. J., Current Collectors for Rechargeable Li-Air Batteries. Journal of The Electrochemical Society 2011, 158 (6), A658-A663. 24. Sotomura, T.; Tatsuma, T.; Oyama, N., An Organosulfur Polymer Cathode with a High Current Capability for Rechargeable Batteries. Journal of The Electrochemical Society 1996, 143 (10), 3152-3157. 25. Olsson, C. O. A.; Landolt, D., Passive films on stainless steels—chemistry, structure and growth. Electrochimica Acta 2003, 48 (9), 1093-1104. 26. Olefjord, I.; Wegrelius, L., Surface analysis of passive state. Corrosion Science 1990, 31, 89-98. 27. Kühnel, R.-S.; Lübke, M.; Winter, M.; Passerini, S.; Balducci, A., Suppression of aluminum current collector corrosion in ionic liquid containing electrolytes. Journal of Power Sources 2012, 214, 178-184. 28. Grugeon, S.; Eshetu, G. G.; Bertrand, J.-P.; Gachot, G.; Lecocq, A.; Forestier, C.; Sannier, L.; Armand, M.; Marlair, G.; Laruelle, S., Safety Appraisal of Lithium Bis(fluorosulfonyl) Imide (LiFSI) As Electrolyte Salt for Li-Ion Batteries. Meeting Abstracts 2016, MA2016-01 (5), 457. 29. 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 & Environmental Science 2014, 7 (1), 416-426.



Figure 1: a) Chronoamperometry at 4.3V, inset demonstrates the distribution of processes involved and b) Cyclic voltammogram of aluminum with 1M LiPF6 in EC:DMC (1:1 vol/vol) at RT; c) High resolution Al2p spectra of pure aluminum (i) and aluminum after chronoamperometry (ii); d) F1s

spectra of aluminum after chronoamperometry with

conventional electrolyte


Page 16 of 21

Page 17 of 21 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


Figure 2: Cyclic voltammetry of aluminum with 1M LiTFSI-PIP (a) at RT; (b) at 120 oC; (c) LSV polarizing till 4.3V; (d) Chronoamperometry at constant potential of 4.3 V for 15 h inset shows the distribution of processes involved, with 1M LiTFSI-PIP at RT and 120 oC,



Page 18 of 21

Figure 3: High resolution spectra of a) C1s, b) Al2p and c) F1s of aluminum with 1MLiTFSIPIP at (i) RT and (ii) HT (120 oC)

Figure

4:

a)

LSV,

different

metallic

current

collectors

polarized

till

4.3V,

Chronoamperometry at constant potential of 4.3 V for 15 h; 1M LiTFSI in PIP at 120 oC


b)

Page 19 of 21 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


Figure 5: a) LSV of Al till 4.3 V and b) Chronoamperometry at constant potential of 4.3 V with different electrolytes and additives at 120 oC

Figure 6: a) LSV of HA till 4.3 V and b) Chronoamperometry at constant potential of 4.3V with different electrolytes at 120 oC



Figure 7: a) LSV of Al till different potential and b) Chronoamperometry at constant potential with 1MLiTFSI-PIP at 120 oC


Page 20 of 21

Page 21 of 21 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


Table of Content:


Curious Case of Positive Current Collectors: Corrosion and

Recommend Documents