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Possible carbon-carbon bond formation during decomposition? Characterization and identification of new decomposition products in lithium ion battery electrolytes by means of SPME-GC-MS
Accepted Manuscript Possible carbon-carbon bond formation during decomposition? Characterization and identification of new decomposition products in lithium ion battery electrolytes by means of SPME-GC-MS Fabian Horsthemke, Alex Friesen, Lukas Ibing, Sven Klein, Martin Winter, Sascha Nowak PII:
S0013-4686(18)31906-6
DOI:
https://doi.org/10.1016/j.electacta.2018.08.159
Reference:
EA 32905
To appear in:
Electrochimica Acta
Received Date: 28 May 2018 Revised Date:
24 August 2018
Accepted Date: 24 August 2018
Please cite this article as: F. Horsthemke, A. Friesen, L. Ibing, S. Klein, M. Winter, S. Nowak, Possible carbon-carbon bond formation during decomposition? Characterization and identification of new decomposition products in lithium ion battery electrolytes by means of SPME-GC-MS, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.08.159. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
Possible Carbon-Carbon Bond Formation
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During Decomposition? Characterization
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and Identification of New Decomposition
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Products in Lithium Ion Battery
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Electrolytes by Means of SPME-GC-MS
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Fabian Horsthemke1, Alex Friesen1, Lukas Ibing1, Sven Klein1, Martin Winter1,2 and Sascha
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Nowak1*
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Helmholtz Institute Münster, IEK-12 FZ Jülich, Corrensstraße 46, 48149 Münster, Germany
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Corrensstraße 46, 48149 Münster, Germany
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University of Münster, MEET Battery Research Center, Institute of Physical Chemistry,
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*To whom correspondence should be addressed:
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E-mail: [email protected]
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Tel: +49-251-83-36735
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Fax: +49-251-83 36032
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ACCEPTED MANUSCRIPT Abstract
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Commercially available lithium ion batteries (LIBs) of the 18650 cell format were aged with
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different cycling protocols. After cell opening and electrolyte extraction, the obtained
27
electrolyte was characterized with respect to the occurring organic aging products.
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Therefore, gas chromatography - mass spectrometry (GC-MS) with standard liquid injection
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as well as solid phase microextraction (SPME) was applied. The composition of the pristine
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electrolyte and the main decomposition products have already been discussed in a
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preceding study. However, the SPME method provides access to study aging products,
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which are present in lower concentrations. Structural elucidation was done for several signals
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of interest. For ongoing validation of previously unknown compounds, selected standards
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were synthesized in order to compare fragment patterns and retention times. Three
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carbonates with butoxy-moieties were identified, although the longest carbon chain in the
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constituents of this specific electrolyte are the C2 chains of ethyl methyl carbonate (EMC)
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and ethylene carbonate (EC). Furthermore, the longest chain in standard carbonate based
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electrolytes in general is the C3-chain of propylene carbonate (PC). Thus, the formation of a
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carbon-carbon bond during cycling of LIB electrolytes has to be considered. In addition to the
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well-known transesterification reactions the occurrence of this type of reaction leads to a
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variety of new carbonate based decomposition products. Hence, their presence has to be
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taken into account with respect to possible influences on the interphases formed at the
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electrode/electrolyte interfaces.
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Keywords
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Solid phase microextraction (SPME); Organic carbonates; LIB electrolyte aging; GC-MS;
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ACCEPTED MANUSCRIPT 1. Introduction
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Lithium ion batteries (LIBs; a list of abbreviations is shown in the supplements) are the most
50
applied electrochemical energy storage systems in portable electric devices as well as in
51
electric vehicles.[1-4] Furthermore, the shift from energy production by fossil fuels or nuclear
52
power towards renewable energy sources requires stationary energy storage systems (=
53
“grid batteries”).[5] A better understanding of the different aging mechanisms occurring in
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lithium ion batteries (LIBs) is crucial with respect to the further improvement of these systems
55
as well as for recycling purposes. As the electrolyte is in contact with all parts of the LIB, its
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aging has a huge influence on the battery performance, safety and toxicity.[6-9] Regarding
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the recycling processes, especially the emerging hazardous compounds with high volatility
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are of interest.[10] Considering the high amounts of electrolytes, which will be present at
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recycling sites when gathering spent battery packs from portable electronic devices and
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automotive cells their possible danger has to be redefined.[9, 11, 12]
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LIB electrolytes usually consist of a conducting salt - typically LiPF6 - diluted in a mixture of
62
cyclic and linear carbonates.[7] Moreover, different additives might be present in order to
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improve distinct electrolyte properties.[13] For example, the properties of the solid electrolyte
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interphase (SEI)[14] can be affected by different film forming additives like vinylene
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carbonate or fluoroethylene carbonate (FEC).[7, 15] The SEI on the anode surface and the
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respective cathode electrolyte interphase[16] on the cathode are important to prevent an
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excessive reaction between the electrodes and the electrolyte. However, investigations in the
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last years suggest that the interphases are not fixed but their composition changes during
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charge and discharge[17, 18] - suggesting several equilibria between components of the
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interphases with electrolyte components. In this case, electrolyte aging and thereby
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compositional changes have a direct impact on the equilibria between the interphases and
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the electrolyte.
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The analysis of LIB electrolytes has been a major aspect of the LIB research for the last two
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decades.
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spectroscopy[19], ion chromatography[20], high performance liquid chromatography[18, 21]
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Several
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decomposition products; a) the potential toxic group of organo phosphates and organofluoro
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phosphates which includes ionic and non-ionic forms. This group emerges mainly from the
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hydrolysis of the conducting salt and consecutive alkoxylations of the OPF3 with moieties
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from the carbonates.[9, 20, 23] b) The second group contains compounds with ester and
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ether moieties. These decomposition products result from reactions at the oxygen sites of the
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carbonates and result in polymerizations and/or transesterifications.[6, 22]
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The solid phase microextraction (SPME) developed in the group of Prof. Pawliszyn provides
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the possibility of a preconcentration of volatile organic compounds from air, water and soil
85
samples.[24, 25] Despite the different sample matrix, a preceding publication showed that
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the SPME coupled to a GC-MS system is a suitable tool to investigate LIB electrolytes.[26]
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However, the method has not yet been investigated regarding its power to pre-concentrate
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low concentrated aging products.
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This work reports on the analysis of a commercially available 18650 type cell which was
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already discussed as proof of principle in the preceding study.[26] The focus is in this case
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on the decomposition products emerging during the applied cycling procedure. Therefore,
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the standard cycling protocol was prolonged to the point where the cells reached 70% of their
93
initial capacity while applying moderate discharge currents. Defined as state of health (SOH)
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of 70% compared to the pristine conditions. The electrolyte was recovered and the structures
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of emerging decomposition products were elucidated. Subsequently, standards were
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synthesized in house according to Jin et al.[27] to confirm the structural elucidations results.
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2. Experimental 2.1. Chemicals and Materials
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The 18650-type cells were purchased from BattEnergy (Germany). The organic carbonates
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dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) (99.0%), diethyl carbonate (DEC)
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(99.9%) ethylene carbonate (EC) (99.9%), isopropanol (Emsure) as well as dichloro methane
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(99.8%) were purchased from Merck (Germany). Succinonitrile (SN) (99%) was purchased
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from TCI (Germany) and DMDOHC (98%) from abcr chemicals (Germany). 1-Butanol (99%),
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2-butanol
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p-toluenesulfonic acid monohydrate (97%) from ACROS (Belgium). 1-propanol (99.7%) was
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purchased from Sigma Aldrich (USA).
isobutanol
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(Emsure)
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2.2. Cycling procedure
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The cycling of the 18650 cells (supplier 1.1) was performed with a Maccor Series 4000
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Battery Tester (Maccor, Inc., USA) in a Binder MK 240 or KB400 climate test chamber
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(BINDER GmbH, Germany) at constant chamber temperatures of 20 °C. The cycling protocol
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consisted of a constant current/constant voltage (CC/CV) charge followed by a CC discharge
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in the full voltage window between 4.2 V and 2.75 V according to the material safety data
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sheets. The CC step of the discharge was performed with 1C (2.2 A), 3 C (6.6 A) and 4.55 C
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(10 A); the CV step until the current fell below C/20. The end-of-life criterion was defined as a
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state-of-health (SOH) of less than or equal to 80% or 70%, depending on the applied cycling
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protocol.
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2.3. Sample Preparation
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The 18650 cells were opened in a glovebox (O2, H2O ≤ 0.1 ppm) by cutting off the caps on
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both ends of the cell housing with an in-house-made cutter similar to Aurbach et al.[28] In
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order to investigate the remaining liquid electrolyte the jelly rolls were transferred into 50 mL
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tubes and centrifuged for 30 min at 4200 rpm with a centrifuge from Sigma (Germany).
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The standards butyl methyl carbonate (BMC) and sec-butyl methyl carbonate (sBMC) were
129
synthesized according to Jin et al.[27] Butyl ethyl carbonate (BEC) and sec-butyl ethyl
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carbonate (sBEC) were synthesized by replacing the DMC used as solvent and reagent by
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DEC. Methyl isopropyl carbonate (MiPC) and ethyl isopropyl carbonate (EiPC) were
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synthesized using DMC or DEC and isopropanol as reactants. Methyl propyl carbonate
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(MPC), ethyl propyl carbonate (EPC), isobutyl methyl carbonate (iBMC) and isobutyl ethyl
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carbonate (iBEC) were synthesized from a mixture of 1-propanol/DMC, 1-propanol/DEC,
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isobutanol/DMC and isobutanol/DEC, respectively.
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2.5. Analytical Equipment
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Solid Phase Microextraction (SPME)
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All experiments were carried out at room temperature to prevent further aging of the
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electrolytes by thermal decomposition during the sampling procedure. The SPME setup from
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CTC Analytics (Switzerland) controlled by the cycle composer software of the AOC 5000
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autosampler (Shimadzu, Japan) was used. Acrylate fibers with 85 µm were obtained from
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Axel Semrau (Germany) and were exposed to the headspace above the samples for an
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extraction time of 10 min. The pure electrolyte (200 µL) was transferred to 20 mL headspace
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vials and stirred by a magnetic stirrer at 400 rpm 5 min before and during the extraction.
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ACCEPTED MANUSCRIPT Gas Chromatography - Mass Spectrometry (GC-MS)
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The experiments with liquid injections were done with an injection volume of 0.6 µL. The
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SPME fibers were exposed to the injection unit for 1 min.
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The GC-MS measurements were carried out with a Shimadzu GCMS-QP2010 Ultra single
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quadrupole equipped with an AOC 5000 Plus autosampler. A nonpolar Supelco SLBTM-5ms
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(30 m x 0.25 mm x 0.25 µm) column was used. The system was controlled by the GCMS
153
Real Time Analysis with implemented Cycle Composer for an AOC 5000 Plus autosampler
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(both Shimadzu). The chromatograms were analyzed with GCMS Postrun Analysis
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(Shimadzu). Compounds were validated with NIST 11 library. DMC, EMC, DEC, FEC, EC,
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SN and DMDOHC, were additionally confirmed by the comparison of their retention times
157
and fragment patterns with commercially available standards. The assignment of MPC,
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sBMC, EPC, BMC and sBEC was confirmed by in-house synthesized standards. Helium (6.0
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purity, Westfalen Gas, Germany) was used as carrier gas with 1.16 mL/min column flow and
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3 mL/min purge flow. The temperature program started at 40 °C which was held for 1 min.
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Afterwards, temperature ramps with 3 °C/min until 60 °C and 30 °C/min until 260 °C followed.
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The final temperature was held for 2 min.
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The overall measurement time was 16.32 min with a mass range from 20-350 m/z and an
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event time of 0.1 s in scan mode. The mass spectrometer was run in the electron impact
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ionization (EI) mode with the following parameters: the temperature of the ion source was set
166
to 200 °C; the interface was held at 250 °C and the filament was operated at a voltage of
167
70 V; the detector voltage was set relative to the respective tuning results.
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Experiments with positive chemical ionization (PCI) were run in a mass range from 60-250
169
m/z with an event time of 0.3 s in scan mode with the following parameters: the temperature
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of the ion source was set to 200 °C; the interface was held at 250 °C and the detect or voltage
171
was set to 1.5 kV.
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Experiments with negative chemical ionization (NCI) were run in a mass range from
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40-250 m/z with an event time of 0.3 s in scan mode with the following parameters: the
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temperature of the ion source was set to 170 °C; the interface was held at 220 °C and the
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detector voltage was set to 1.5 kV.
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ACCEPTED MANUSCRIPT 2.6. Results and Discussion
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Figure 1 shows the capacity retention of the commercially available 18650 cells (supplier 1.1)
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cycled with four different protocols. The cells were cycled with a 4.55 C, 3 C and 1 C
180
discharge current, respectively, to an end-of-life criterion of 80 % state of health (SOH) and
181
1 C with 70% SOH. The 3 C and 4.55 C cycled cells reach cycle numbers of ≈190 and ≈220,
182
respectively, whereas the cells cycled at 1 C reach more than 480 cycles, each. These cells
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started to show accelerated aging after ≈280 cycles. This behavior is reported to originate
184
from the almost complete depletion of the film forming additive - fluoroethylene carbonate
185
(FEC) in this case.[29] Thereby the characteristics of the freshly formed SEI are changed in
186
comparison to the FEC influenced SEI. Due to this accelerated aging effect, cells cycled with
187
1 C to 70% and 80% SOH do not differ drastically in their overall achieved cycle number
188
(480-580). Figure 2 shows the chromatograms of two electrolytes from cells cycled with
189
protocols applying 1 C discharge rates. The chromatograms are compared to an electrolyte
190
extracted from a pristine cell. The main components of the electrolytes were already
191
identified in a preceding study: Namely dimethyl carbonate (DMC), ethyl methyl carbonate
192
(EMC), FEC, ethylene carbonate (EC) and succinonitrile (SN) as main constituents and
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diethyl carbonate (DEC) dimethyl-2,5-dioxahexane dicarboxylate (DMDOHC) and ethyl
194
methyl-2,5-dioxahexane dicarboxylate (EMDOHC) as main decomposition products.[26] The
195
region between retention times of 5.5 and 9.5 min of the chromatograms is shown magnified.
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Beneath the decomposition of FEC, which is only present in the pristine electrolyte, there are
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various additional peaks visible in the aged electrolyte. Typically, the region with higher
198
retention time than EC provides more decomposition products than DMDOHC and
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EMDOHC. However, the presence of the alkane fraction interferes with a more precise
200
investigation. Unfortunately, further experiments to separate the alkanes did not improve the
201
overall separation.
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Figure 3 focusses on the retention times between 5.6 and 9.4 min and again shows an
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overlay of an electrolyte aged with 1 C and a pristine electrolyte. Samples of the same
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electrolyte were investigated with a standard liquid injection and a SPME pre-concentration.
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ACCEPTED MANUSCRIPT Besides the FEC peak of the pristine electrolyte all main peaks of the aged electrolyte are
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serially numbered. All compounds of interest show higher signals for the SPME method. The
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peaks 1, 4 and 6 show responses for both, the standard procedure and SPME. Whereas
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peaks 2, 3 and 5 are only visible in the SPME experiment. The main fragments of all peaks
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are shown in Table 1 and the fragment patterns of 4 and 5 are exemplarily displayed in
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Figure 4. More detailed information are summarized in Table S 1 in the supplements. The
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fragment patterns of 1, 2 and 6 are already known in literature. In combination with CI
212
experiments, different structures have been assigned.[30] For the fragment patterns of peak
213
3 and 5 obtained by electron impact ionization (EI) the NIST 11 library proposes ethyl propyl
214
carbonate (EPC) [90%] and sec-butyl ethyl carbonate (sBEC) [93%], respectively.
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Unfortunately, the concentration of compounds 1-6 was too low to obtain clear signals when
216
applying positive and negative chemical ionization (PCI/NCI). Thus, it was decided to
217
synthesize the compounds of interest to compare retention times and fragment patterns in a
218
direct manner. Furthermore, the presence of sBEC in an electrolyte containing DMC, EMC
219
and EC as main components leads to two questions:
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a) Is sec-butyl methyl carbonate (sBMC) present? sBMC should usually emerge as a matter
221
of probability in a DMC/EMC based electrolyte; b) What is the origin of the C4 chains? Is the
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formation of a carbon-carbon bond between electrolyte components feasible during cycling?
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Especially considering the fact that the longest carbon chain in standard carbonate based
224
electrolytes is the C3 chain of PC.
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Jin et al. recently reported the synthesis of the compounds sBMC and n-butyl methyl
226
carbonate (BMC) via transesterification reactions in presence of different acidic catalysts.[27]
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The supporting information gives EI spectra, which are in agreement with the spectra
228
reported by Schulze et al.[31] Moreover, these spectra match with the corresponding spectra
229
of peaks 2 and 4. To compare the GC retention times, these compounds were synthesized in
230
house applying the same synthesis protocol. Furthermore, the substitution of DMC by DEC
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should in theory lead to the formation of the compounds with one ethoxy side chain. Finally,
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the protocol was adapted in order to synthesize methyl propyl carbonate (MPC) and EPC.
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ACCEPTED MANUSCRIPT The reaction monitoring of the five reactions after 7 h led to the chromatogram shown in
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Figure 5. For straightforwardness, only the peaks of the products are depicted, while the
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chromatogram of the aged electrolyte is shown as comparison. Since MPC and EPC are only
236
obtained as side products in the isopropanol synthesis, the signals are in the range of the
237
limit of detection (LOD). The peak of butyl ethyl carbonate (BEC) provides a retention time of
238
10.01 min and elutes between EC and SN in the side area of the alkanes. Therefore, a
239
presence of the BEC could not be clarified. Furthermore, the methyl isopropyl carbonate
240
(MiPC) elutes during the filament outage associated with the EMC elution. The electrolyte
241
sample shows no corresponding peak to the elution of the synthesized ethyl isppropyl
242
carbonate (EiPC) standard. However, sBMC, BMC and sBEC elute with peaks 2, 4 and 5,
243
respectively. Additionally, the fragment patterns are in good agreement between the
244
decomposition and synthesis products (For detailed information the reader is referred to
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Table S 1 in the supplements). Furthermore, the side products MPC and EPC of the
246
isopropyl-based synthesis co-elute with the signals 1 and 3, respectively. Therefore, in case
247
of the C4 chain there is the linear and one branched chain present, while in the case of the
248
C3 chain only the linear specimen was found. Until now the isopropyl side chain was only
249
found in electrolytes containing PC and a reaction mechanism was already proposed.[30] A
250
comparison of the reactions, which occurred and did not occur in the electrolyte discussed in
251
this study, is shown in Figure 6. Furthermore, Table 2 lists the main fragments of the
252
synthesized standards obtained by PCI, NCI and EI experiments, respectively.
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The assignment of sBMC to peak 2 leads to the assumption, that the previously proposed
254
structure for the corresponding fragment pattern was falsely assigned.[30] Taking into
255
account that 1) the nominal mass of both structures is the same and thereby PCI
256
experiments with single quadrupole MS would show the same results for both structures and
257
2) the fact that the presence of a C4 chain was not estimated as a probability in LIB
258
electrolytes; therefore the structure was newly assigned.
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Carbonates with C4 sidechains were already discussed in the LIB electrolyte context.[32-34]
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However, in these cases, the carbonates were only used as solvents / co-solvents in order to
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ACCEPTED MANUSCRIPT improve the overall LIB performance or distinct parameters. Since the amount of the
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compounds 1-6 in the pristine electrolyte was at least below the LOD of both GC-MS
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methods, their deliberate presence e.g. as additives was excluded. Therefore, they have to
264
be formed or cleaved off the organic electrode parts during the applied cycling procedure.
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Since graphite and SBR do not contain oxygen and the standard carbon chain of CMC is C6
266
the probability of them being the origin of the C4 chains in our understanding negligible. To
267
our knowledge the only work proposing the formation of a C4 chain in a LIB electrolyte was
268
done by Kohs et al.[35], who discussed the origin of tetrahydrofuran (THF) found in their
269
experiments. The proposed mechanism works via a polarization of a carbonate moiety by
270
two lithium ions and as result a cleavage of lithium carbonate and the formation of a butylene
271
glycol. However, this reaction requires a two electron uptake. Furthermore, Ortiz et al. found
272
several long chained carbonates in a DEC solution which was aged by radiolysis
273
experiments.[36] In addition to lithium ions, several different metal ions can be present in the
274
LIB electrolyte. Depending on the electrode composition aluminum, cobalt, manganese or
275
nickel might be diluted from the cathode and copper/alumina from the current collectors.[37]
276
These metals are known to act as catalysts in organic chemistry.
277
In contrast to the separate electrolyte, the carbon-carbon bond formation is extensively
278
discussed in the SEI context. The reactions are generally started by an electron transferred
279
to a carbonate molecule, resulting in a negatively charged radical. These radicals can then
280
undergo different reactions, like H-abstraction from other organic molecules, further
281
decomposition or recombination of two radical species.[7, 38] However, favored reaction
282
routes and stable species are controversially discussed in literature.[38] Considering the
283
polymerization of FEC during SEI formation, this reaction could be the origin of the propyl
284
and butyl carbonates. However, some batteries characterized in the work of Friesen[39]
285
show the discussed decomposition products without containing FEC. A direct comparison of
286
the chromatograms from supplier 1.1 cells and these cells is displayed in Figure S 4. Thus,
287
FEC might contribute to the formation of the decomposition products, but can be excluded as
288
only origin.
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ACCEPTED MANUSCRIPT Shkrob et al. investigated the stability of radicals by matrix isolation electron paramagnetic
290
resonance spectroscopy. Among others, the radicals depicted in Scheme 1 were found for
291
EC, DMC and EMC samples. When EC abstracts an electron the ring can be opened, which
292
results in a terminal radical on the ethoxy moiety. This structure can be altered intramolecular
293
by a radical 1,2-migration. When a linear carbonate is reduced, an alkane radical can be
294
formed. Furthermore, carbonate based radicals can be formed by a radical abstracting a
295
hydrogen atom from any carbonate present (3 and 4). The recombination of the different
296
radicals from Scheme 1 leads to the carbonates or lithium alkyl carbonates with C3 and C4
297
moieties shown in Scheme 2. However, the displayed products are only a small part of the
298
possible combinations. For example, the whole group of dicarbonates is not included.
299
Nevertheless, except for the two synthesized carbonates with isobutyl moiety all C3 and C4
300
chains are accessible by recombination of two radicals. Taking into account that the
301
carbonate based radical in reaction 5 has either a methyl or an ethyl rest, MPC and EPC
302
should be accessible without further reaction. Since the carbonate based radicals of
303
reactions 6 and 7 provide no ethyl groups the corresponding products have at least to
304
undergo a substitution of the methyl group or the lithium. Considering the reaction route via
305
EC the presence of lithium alkyl carbonates with C4 chains is needed. At least as
306
intermediates on the electrode surface, if not as SEI component. This would be in
307
compliance to the work of Schafzahl et al. who investigated the influence of long chained
308
alkyl carbonates on the SEI performance.[40]
309
A comparison of the aging products in dependence of the cycling protocol is shown in Figure
310
6. All electrolytes show signals which relate to compounds 1-6. The 3 C and 4.55 C cells
311
show some remaining FEC, whereas, the FEC content of both 1 C cells is below the LOD.
312
Comparing the signals of the decomposition products, their intensity does not seem to be a
313
function of the corresponding SOH. The electrolytes gathered from cells, which were cycled
314
for more overall cycles (both 1 C cells), show comparable and increased signal heights in
315
contrast to the cells discharged at high C-rates. One possible reason could be the presence
316
of FEC residues in the latter case. This might have suppressed the formation of the
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ACCEPTED MANUSCRIPT decomposition products in higher amounts. Especially with respect to the FEC derived SEI
318
protecting the anode surface. Furthermore, the reactions could suffer from slow kinetics,
319
which leads to a slow and steady formation from cycle to cycle.
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ACCEPTED MANUSCRIPT 3. Conclusions
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The electrolyte of a commercially available 18650 cell was analyzed with respect to emerging
322
decomposition products. Therefore, the cells were aged with protocols applying different
323
C-rates during discharge. The gathered electrolyte was investigated by standard GC-MS with
324
liquid injection and SPME-GC-MS. The SPME-GC-MS system shows advantages regarding
325
the detection of several decomposition products. Thereby, compounds with unknown EI
326
spectra were obtained and their structures were elucidated. In addition, the compounds of
327
interest were synthesized according to literature and fragmentation patterns as well as
328
retention times were compared. Three of the assigned decomposition products contain
329
butoxy moieties - namely sec-butyl methyl carbonate (sBMC), butyl methyl carbonate (BMC)
330
and sec-butyl ethyl carbonate (sBEC) - and their possible origin was discussed. As
331
conclusion, a reaction of two C2 moieties of EC or EMC via a formation of a carbon-carbon
332
bond is the most probable way. However, this assumption has to be proven or disproven in
333
further experiments. E.g., some of these compounds should emerge in DEC/EC electrolytes
334
cycled for long times; DMC/EC electrolytes should in theory lead to increasing amounts of
335
compounds with C3 chains. Ortiz et al. detected long chained carbonates aged by radiolysis
336
experiments.[36] A more recent study of the same group treated carbonate mixtures with the
337
same setup to identify the primarily formed radicals.[41] Thus, the radiolysis demonstrated
338
the ability to generate aging products formed during cycling in LIB electrolytes.
339
Furthermore, the formation of these compounds indicate that the SEI has to be considered
340
as a dynamic interphase. Especially regarding the long chain lithium alkyl carbonates. Thus,
341
the addition of butyl carbonates or lithium butyl carbonates to electrolyte formulations could
342
have an impact on the equilibrium conditions of the SEI and thereby on the stability.
343
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Acknowledgment
345
The authors would like to thank the Federal Ministry of Education and Research for funding
346
the project EffiForm (03XP0034H) and the German Federal Ministry of Education and
347
Research (BMBF) for funding the project “Elektrolytlabor 4E” (03X4632).
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Figure 1: Discharge capacities versus cycle number of differently aged cells: 1 C, SOH 70,
352
purple diamonds; 1 C, SOH 80, black circles; 3 C, SOH 80, red squares; 4.55 C, SOH 80,
353
blue triangles. The cells were cycled at 20 °C.
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Figure 2: Chromatograms of the electrolyte gathered from an aged cell (black) and a pristine
357
cell (magenta) obtained by SPME-GC-MS. The excerpt between 5.5 and 9.5 min is shown as
358
magnified inset.
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Figure 3: Overlay of the chromatograms from an aged electrolyte obtained by GC-MS with
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liquid injection (olive) and SPME (black) - the chromatogram of the pristine electrolyte
363
(magenta) is shown as comparison. Signals of interest are numbered sequentially.
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Figure 4: Fragment patterns of compound 4 (left) and 5 (right), obtained with SPME-GC-MS
367
in EI mode.
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Figure 5: Comparison of the chromatograms obtained from the aged electrolyte (black, top)
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and the synthesized standards (green, bottom).
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Figure 6: Summary of the decomposition products present in the aged electrolyte (right).
375
Compounds, which were not detected (below LOD), but provide similar structures (left).
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Figure 7: Chromatograms of the differently aged electrolytes obtained with SPME-GC-MS:
379
1 C SOH 70 (purple, top); 1 C SOH 80 (black, upper middle); 3 C SOH 80 (red, lower
380
middle); 4.55 C SOH 80 (blue, bottom).
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Tables
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Table 1: Compounds 1-6, the assigned structures and their most prominent EI fragments.
385
(For more detailed information the reader is referred to Table S 1 in the supplements.)
Assigned Structure
77, 59, 45, 43
2
117, 103, 73, 59, 41
3
105, 91, 63, 59, 43
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7.07
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103, 77, 73, 56, 41
8.71
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9.09
104, 77, 59, 58, 45
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Retention Time / min
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Compound Nr.
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Table 2: Structures and corresponding fragments of the synthesized standards obtained by
390
SPME-GC-MS and SPME-GC-CI-MS. The compounds are listed in their retention order. (For
391
more detailed information the reader is referred to Table S 1 found in the supplements.) NCI
EI
MiPC
136, 119
103, 89, 75
117, 103, 89, 75, 59
MPC
136, 119
103, 89, 75, 59, 57
EiPC
150, 133
103, 89
sBMC
150, 133
117, 75, 73
iBMC
150, 133
117, 103, 75, 73
90, 77, 73, 59, 56
EPC
150, 133
103, 89, 59
105, 91/90, 63, 59
BMC
150, 133
117, 103, 75, 73
103, 77, 73, 59, 56
164, 147
117, 89, 73
117, 91, 90, 73, 63, 59, 57
164, 147
117, 89, 73, 71
104, 91, 63, 57/56
164, 147
117, 89, 73, 71
91, 90, 73, 63, 57, 56
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sBEC
77, 59, 45, 43
91, 90, 63, 59
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Structure
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Schemes
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Scheme 1: Proposed formation of carbonate based radicals in LIB electolytes. Redrawn for
398
DMC EMC EC based electrolytes from Shkrob et al.[42] and Xu et al.[38]
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Scheme 2: Recombination of radicals from scheme 1 leading to the formation of C3 and C4
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alkane chains.
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References
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[1] M. Dresselhaus, I. Thomas, Nature, 414 (2001) 332-337. [2] F. Schipper, D. Aurbach, Russian Journal of Electrochemistry, 52 (2016) 1095-1121. [3] R. Schmuch, R. Wagner, G. Hörpel, T. Placke, M. Winter, Nature Energy, 3 (2018) 267. [4] T. Placke, R. Kloepsch, S. Dühnen, M. Winter, Journal of Solid State Electrochemistry, 21 (2017) 1939-1964. [5] P. Meister, H. Jia, J. Li, R. Kloepsch, M. Winter, T. Placke, Chemistry of Materials, 28 (2016) 7203-7217. [6] G. Gachot, S. Grugeon, I. Jimenez-Gordon, G.G. Eshetu, S. Boyanov, A. Lecocq, G. Marlair, S. Pilard, S. Laruelle, Analytical Methods, 6 (2014) 6120-6124. [7] K. Xu, Chemical Reviews, 104 (2004) 4303-4418. [8] S. Wiemers-Meyer, S. Jeremias, M. Winter, S. Nowak, Electrochimica Acta, 222 (2016) 1267-1271. [9] S. Nowak, M. Winter, Journal of the Electrochemical Society, 162 (2015) A2500-A2508. [10] J. Diekmann, M. Grützke, T. Loellhoeffel, M. Petermann, S. Rothermel, M. Winter, S. Nowak, A. Kwade, Potential Dangers During the Handling of Lithium-Ion Batteries, in: A. Kwade, J. Diekmann (Eds.) Recycling of Lithium-Ion Batteries: The LithoRec Way, Springer International Publishing, Cham, 2018, pp. 39-51. [11] M. Grützke, S. Krüger, V. Kraft, B. Vortmann, S. Rothermel, M. Winter, S. Nowak, ChemSusChem, 8 (2015) 3433-3438. [12] S. Nowak, M. Winter, Molecules, 22 (2017) 403. [13] S.S. Zhang, Journal of Power Sources, 162 (2006) 1379-1394. [14] M. Winter, Zeitschrift für Physikalische Chemie; International Journal of Research in Physical Chemistry and Chemical Physics, 223 (2009) 1395-1406. [15] P. Verma, P. Maire, P. Novák, Electrochimica Acta, 55 (2010) 6332-6341. [16] M. Thomas, P. Bruce, J. Goodenough, Journal of the Electrochemical Society, 132 (1985) 1521-1528. [17] Z. Zhuo, P. Lu, C. Delacourt, R. Qiao, K. Xu, F. Pan, S.J. Harris, W. Yang, Chemical Communications, (2018). [18] C. Schultz, S. Vedder, M. Winter, S. Nowak, Analytical Chemistry, 88 (2016) 1116011168. [19] S. Wiemers-Meyer, M. Winter, S. Nowak, Physical Chemistry Chemical Physics, 18 (2016) 26595-26601. [20] V. Kraft, W. Weber, M. Grützke, M. Winter, S. Nowak, RSC Advances, 5 (2015) 8015080157. [21] C. Schultz, S. Vedder, B. Streipert, M. Winter, S. Nowak, RSC Advances, 7 (2017) 27853-27862. [22] M. Grützke, W. Weber, M. Winter, S. Nowak, RSC Advances, 6 (2016) 57253-57260. [23] V. Kraft, M. Gruetzke, W. Weber, M. Winter, S. Nowak, Journal of Chromatography A, 1354 (2014) 92-100. [24] C.L. Arthur, J. Pawliszyn, Analytical Chemistry, 62 (1990) 2145-2148. [25] Z. Zhang, J. Pawliszyn, Analytical Chemistry, 65 (1993) 1843-1852. [26] F. Horsthemke, A. Friesen, X. Mönnighoff, Y.P. Stenzel, M. Grützke, J.T. Andersson, M. Winter, S. Nowak, RSC Advances, 7 (2017) 46989-46998. [27] S. Jin, A.J. Hunt, J.H. Clark, C.R. McElroy, Green Chemistry, 18 (2016) 5839-5844. [28] D. Aurbach, B. Markovsky, A. Rodkin, M. Cojocaru, E. Levi, H.-J. Kim, Electrochimica Acta, 47 (2002) 1899-1911. [29] A. Friesen, F. Horsthemke, X. Mönnighoff, G. Brunklaus, R. Krafft, M. Börner, T. Risthaus, M. Winter, F.M. Schappacher, Journal of Power Sources, 334 (2016) 1-11. [30] X. Mönnighoff, A. Friesen, B. Konersmann, F. Horsthemke, M. Grützke, M. Winter, S. Nowak, Journal of Power Sources, 352 (2017) 56-63. [31] P. Schulze, W. Richter, International Journal of Mass Spectrometry and Ion Physics, 6 (1971) 131-151.
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[32] Y. Ein-Eli, R. Laura, Pat. U.S. 5,986,879, (1999). [33] J. Vetter, P. Novák, Journal of Power Sources, 119-121 (2003) 338-342. [34] J. Vetter, H. Buqa, M. Holzapfel, P. Novák, Journal of Power Sources, 146 (2005) 355359. [35] W. Kohs, J. Kahr, A. Ahniyaz, N. Zhang, A. Trifonova, Journal of Solid State Electrochemistry, (2017). [36] D. Ortiz, V. Steinmetz, D. Durand, S. Legand, V. Dauvois, P. Maître, S. Le Caër, Nature communications, 6 (2015). [37] M. Evertz, F. Horsthemke, J. Kasnatscheew, M. Börner, M. Winter, S. Nowak, Journal of Power Sources, 329 (2016) 364-371. [38] K. Xu, Chemical Reviews, 114 (2014) 11503-11618. [39] A. Friesen, Aging and performance of lithium ion cells at low temperatures and the impact on life time and cell safety, in: Institute of Physical Chemistry, Westfälische WilhelmsUniversität Münster, Universitäts- und Landesbibliothek Münster, 2017. [40] L. Schafzahl, H. Ehmann, M. Kriechbaum, J. Sattelkow, T. Ganner, H. Plank, M. Wilkening, S.A. Freunberger, Chemistry of Materials, (2018). [41] F. Wang, F. Varenne, D. Ortiz, V. Pinzio, M. Mostafavi, S. Le Caer, ChemPhysChem, 18 (2017) 2799-2806. [42] I.A. Shkrob, Y. Zhu, T.W. Marin, D. Abraham, The Journal of Physical Chemistry C, 117 (2013) 19255-19269.
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S0013-4686(18)31906-6
DOI:
https://doi.org/10.1016/j.electacta.2018.08.159
Reference:
EA 32905
To appear in:
Electrochimica Acta
Received Date: 28 May 2018 Revised Date:
24 August 2018
Accepted Date: 24 August 2018
Please cite this article as: F. Horsthemke, A. Friesen, L. Ibing, S. Klein, M. Winter, S. Nowak, Possible carbon-carbon bond formation during decomposition? Characterization and identification of new decomposition products in lithium ion battery electrolytes by means of SPME-GC-MS, Electrochimica Acta (2018), doi: https://doi.org/10.1016/j.electacta.2018.08.159. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
Possible Carbon-Carbon Bond Formation
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During Decomposition? Characterization
4
and Identification of New Decomposition
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Products in Lithium Ion Battery
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Electrolytes by Means of SPME-GC-MS
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Fabian Horsthemke1, Alex Friesen1, Lukas Ibing1, Sven Klein1, Martin Winter1,2 and Sascha
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Nowak1*
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Helmholtz Institute Münster, IEK-12 FZ Jülich, Corrensstraße 46, 48149 Münster, Germany
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Corrensstraße 46, 48149 Münster, Germany
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University of Münster, MEET Battery Research Center, Institute of Physical Chemistry,
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*To whom correspondence should be addressed:
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E-mail: [email protected]
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Tel: +49-251-83-36735
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Fax: +49-251-83 36032
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ACCEPTED MANUSCRIPT Abstract
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Commercially available lithium ion batteries (LIBs) of the 18650 cell format were aged with
26
different cycling protocols. After cell opening and electrolyte extraction, the obtained
27
electrolyte was characterized with respect to the occurring organic aging products.
28
Therefore, gas chromatography - mass spectrometry (GC-MS) with standard liquid injection
29
as well as solid phase microextraction (SPME) was applied. The composition of the pristine
30
electrolyte and the main decomposition products have already been discussed in a
31
preceding study. However, the SPME method provides access to study aging products,
32
which are present in lower concentrations. Structural elucidation was done for several signals
33
of interest. For ongoing validation of previously unknown compounds, selected standards
34
were synthesized in order to compare fragment patterns and retention times. Three
35
carbonates with butoxy-moieties were identified, although the longest carbon chain in the
36
constituents of this specific electrolyte are the C2 chains of ethyl methyl carbonate (EMC)
37
and ethylene carbonate (EC). Furthermore, the longest chain in standard carbonate based
38
electrolytes in general is the C3-chain of propylene carbonate (PC). Thus, the formation of a
39
carbon-carbon bond during cycling of LIB electrolytes has to be considered. In addition to the
40
well-known transesterification reactions the occurrence of this type of reaction leads to a
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variety of new carbonate based decomposition products. Hence, their presence has to be
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taken into account with respect to possible influences on the interphases formed at the
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electrode/electrolyte interfaces.
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Keywords
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Solid phase microextraction (SPME); Organic carbonates; LIB electrolyte aging; GC-MS;
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ACCEPTED MANUSCRIPT 1. Introduction
49
Lithium ion batteries (LIBs; a list of abbreviations is shown in the supplements) are the most
50
applied electrochemical energy storage systems in portable electric devices as well as in
51
electric vehicles.[1-4] Furthermore, the shift from energy production by fossil fuels or nuclear
52
power towards renewable energy sources requires stationary energy storage systems (=
53
“grid batteries”).[5] A better understanding of the different aging mechanisms occurring in
54
lithium ion batteries (LIBs) is crucial with respect to the further improvement of these systems
55
as well as for recycling purposes. As the electrolyte is in contact with all parts of the LIB, its
56
aging has a huge influence on the battery performance, safety and toxicity.[6-9] Regarding
57
the recycling processes, especially the emerging hazardous compounds with high volatility
58
are of interest.[10] Considering the high amounts of electrolytes, which will be present at
59
recycling sites when gathering spent battery packs from portable electronic devices and
60
automotive cells their possible danger has to be redefined.[9, 11, 12]
61
LIB electrolytes usually consist of a conducting salt - typically LiPF6 - diluted in a mixture of
62
cyclic and linear carbonates.[7] Moreover, different additives might be present in order to
63
improve distinct electrolyte properties.[13] For example, the properties of the solid electrolyte
64
interphase (SEI)[14] can be affected by different film forming additives like vinylene
65
carbonate or fluoroethylene carbonate (FEC).[7, 15] The SEI on the anode surface and the
66
respective cathode electrolyte interphase[16] on the cathode are important to prevent an
67
excessive reaction between the electrodes and the electrolyte. However, investigations in the
68
last years suggest that the interphases are not fixed but their composition changes during
69
charge and discharge[17, 18] - suggesting several equilibria between components of the
70
interphases with electrolyte components. In this case, electrolyte aging and thereby
71
compositional changes have a direct impact on the equilibria between the interphases and
72
the electrolyte.
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The analysis of LIB electrolytes has been a major aspect of the LIB research for the last two
74
decades.
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spectroscopy[19], ion chromatography[20], high performance liquid chromatography[18, 21]
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Several
different
analytical
methods
like
nuclear
magnetic
resonance
ACCEPTED MANUSCRIPT and GC[6, 22] with different detectors were applied. These methods identified two groups of
77
decomposition products; a) the potential toxic group of organo phosphates and organofluoro
78
phosphates which includes ionic and non-ionic forms. This group emerges mainly from the
79
hydrolysis of the conducting salt and consecutive alkoxylations of the OPF3 with moieties
80
from the carbonates.[9, 20, 23] b) The second group contains compounds with ester and
81
ether moieties. These decomposition products result from reactions at the oxygen sites of the
82
carbonates and result in polymerizations and/or transesterifications.[6, 22]
83
The solid phase microextraction (SPME) developed in the group of Prof. Pawliszyn provides
84
the possibility of a preconcentration of volatile organic compounds from air, water and soil
85
samples.[24, 25] Despite the different sample matrix, a preceding publication showed that
86
the SPME coupled to a GC-MS system is a suitable tool to investigate LIB electrolytes.[26]
87
However, the method has not yet been investigated regarding its power to pre-concentrate
88
low concentrated aging products.
89
This work reports on the analysis of a commercially available 18650 type cell which was
90
already discussed as proof of principle in the preceding study.[26] The focus is in this case
91
on the decomposition products emerging during the applied cycling procedure. Therefore,
92
the standard cycling protocol was prolonged to the point where the cells reached 70% of their
93
initial capacity while applying moderate discharge currents. Defined as state of health (SOH)
94
of 70% compared to the pristine conditions. The electrolyte was recovered and the structures
95
of emerging decomposition products were elucidated. Subsequently, standards were
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synthesized in house according to Jin et al.[27] to confirm the structural elucidations results.
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2. Experimental 2.1. Chemicals and Materials
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The 18650-type cells were purchased from BattEnergy (Germany). The organic carbonates
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dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) (99.0%), diethyl carbonate (DEC)
103
(99.9%) ethylene carbonate (EC) (99.9%), isopropanol (Emsure) as well as dichloro methane
104
(99.8%) were purchased from Merck (Germany). Succinonitrile (SN) (99%) was purchased
105
from TCI (Germany) and DMDOHC (98%) from abcr chemicals (Germany). 1-Butanol (99%),
106
2-butanol
107
p-toluenesulfonic acid monohydrate (97%) from ACROS (Belgium). 1-propanol (99.7%) was
108
purchased from Sigma Aldrich (USA).
isobutanol
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(Emsure)
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obtained
from
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2.2. Cycling procedure
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The cycling of the 18650 cells (supplier 1.1) was performed with a Maccor Series 4000
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Battery Tester (Maccor, Inc., USA) in a Binder MK 240 or KB400 climate test chamber
113
(BINDER GmbH, Germany) at constant chamber temperatures of 20 °C. The cycling protocol
114
consisted of a constant current/constant voltage (CC/CV) charge followed by a CC discharge
115
in the full voltage window between 4.2 V and 2.75 V according to the material safety data
116
sheets. The CC step of the discharge was performed with 1C (2.2 A), 3 C (6.6 A) and 4.55 C
117
(10 A); the CV step until the current fell below C/20. The end-of-life criterion was defined as a
118
state-of-health (SOH) of less than or equal to 80% or 70%, depending on the applied cycling
119
protocol.
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2.3. Sample Preparation
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The 18650 cells were opened in a glovebox (O2, H2O ≤ 0.1 ppm) by cutting off the caps on
123
both ends of the cell housing with an in-house-made cutter similar to Aurbach et al.[28] In
124
order to investigate the remaining liquid electrolyte the jelly rolls were transferred into 50 mL
125
tubes and centrifuged for 30 min at 4200 rpm with a centrifuge from Sigma (Germany).
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The standards butyl methyl carbonate (BMC) and sec-butyl methyl carbonate (sBMC) were
129
synthesized according to Jin et al.[27] Butyl ethyl carbonate (BEC) and sec-butyl ethyl
130
carbonate (sBEC) were synthesized by replacing the DMC used as solvent and reagent by
131
DEC. Methyl isopropyl carbonate (MiPC) and ethyl isopropyl carbonate (EiPC) were
132
synthesized using DMC or DEC and isopropanol as reactants. Methyl propyl carbonate
133
(MPC), ethyl propyl carbonate (EPC), isobutyl methyl carbonate (iBMC) and isobutyl ethyl
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carbonate (iBEC) were synthesized from a mixture of 1-propanol/DMC, 1-propanol/DEC,
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isobutanol/DMC and isobutanol/DEC, respectively.
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2.5. Analytical Equipment
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Solid Phase Microextraction (SPME)
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All experiments were carried out at room temperature to prevent further aging of the
140
electrolytes by thermal decomposition during the sampling procedure. The SPME setup from
141
CTC Analytics (Switzerland) controlled by the cycle composer software of the AOC 5000
142
autosampler (Shimadzu, Japan) was used. Acrylate fibers with 85 µm were obtained from
143
Axel Semrau (Germany) and were exposed to the headspace above the samples for an
144
extraction time of 10 min. The pure electrolyte (200 µL) was transferred to 20 mL headspace
145
vials and stirred by a magnetic stirrer at 400 rpm 5 min before and during the extraction.
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148
The experiments with liquid injections were done with an injection volume of 0.6 µL. The
149
SPME fibers were exposed to the injection unit for 1 min.
150
The GC-MS measurements were carried out with a Shimadzu GCMS-QP2010 Ultra single
151
quadrupole equipped with an AOC 5000 Plus autosampler. A nonpolar Supelco SLBTM-5ms
152
(30 m x 0.25 mm x 0.25 µm) column was used. The system was controlled by the GCMS
153
Real Time Analysis with implemented Cycle Composer for an AOC 5000 Plus autosampler
154
(both Shimadzu). The chromatograms were analyzed with GCMS Postrun Analysis
155
(Shimadzu). Compounds were validated with NIST 11 library. DMC, EMC, DEC, FEC, EC,
156
SN and DMDOHC, were additionally confirmed by the comparison of their retention times
157
and fragment patterns with commercially available standards. The assignment of MPC,
158
sBMC, EPC, BMC and sBEC was confirmed by in-house synthesized standards. Helium (6.0
159
purity, Westfalen Gas, Germany) was used as carrier gas with 1.16 mL/min column flow and
160
3 mL/min purge flow. The temperature program started at 40 °C which was held for 1 min.
161
Afterwards, temperature ramps with 3 °C/min until 60 °C and 30 °C/min until 260 °C followed.
162
The final temperature was held for 2 min.
163
The overall measurement time was 16.32 min with a mass range from 20-350 m/z and an
164
event time of 0.1 s in scan mode. The mass spectrometer was run in the electron impact
165
ionization (EI) mode with the following parameters: the temperature of the ion source was set
166
to 200 °C; the interface was held at 250 °C and the filament was operated at a voltage of
167
70 V; the detector voltage was set relative to the respective tuning results.
168
Experiments with positive chemical ionization (PCI) were run in a mass range from 60-250
169
m/z with an event time of 0.3 s in scan mode with the following parameters: the temperature
170
of the ion source was set to 200 °C; the interface was held at 250 °C and the detect or voltage
171
was set to 1.5 kV.
172
Experiments with negative chemical ionization (NCI) were run in a mass range from
173
40-250 m/z with an event time of 0.3 s in scan mode with the following parameters: the
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ACCEPTED MANUSCRIPT 174
temperature of the ion source was set to 170 °C; the interface was held at 220 °C and the
175
detector voltage was set to 1.5 kV.
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ACCEPTED MANUSCRIPT 2.6. Results and Discussion
178
Figure 1 shows the capacity retention of the commercially available 18650 cells (supplier 1.1)
179
cycled with four different protocols. The cells were cycled with a 4.55 C, 3 C and 1 C
180
discharge current, respectively, to an end-of-life criterion of 80 % state of health (SOH) and
181
1 C with 70% SOH. The 3 C and 4.55 C cycled cells reach cycle numbers of ≈190 and ≈220,
182
respectively, whereas the cells cycled at 1 C reach more than 480 cycles, each. These cells
183
started to show accelerated aging after ≈280 cycles. This behavior is reported to originate
184
from the almost complete depletion of the film forming additive - fluoroethylene carbonate
185
(FEC) in this case.[29] Thereby the characteristics of the freshly formed SEI are changed in
186
comparison to the FEC influenced SEI. Due to this accelerated aging effect, cells cycled with
187
1 C to 70% and 80% SOH do not differ drastically in their overall achieved cycle number
188
(480-580). Figure 2 shows the chromatograms of two electrolytes from cells cycled with
189
protocols applying 1 C discharge rates. The chromatograms are compared to an electrolyte
190
extracted from a pristine cell. The main components of the electrolytes were already
191
identified in a preceding study: Namely dimethyl carbonate (DMC), ethyl methyl carbonate
192
(EMC), FEC, ethylene carbonate (EC) and succinonitrile (SN) as main constituents and
193
diethyl carbonate (DEC) dimethyl-2,5-dioxahexane dicarboxylate (DMDOHC) and ethyl
194
methyl-2,5-dioxahexane dicarboxylate (EMDOHC) as main decomposition products.[26] The
195
region between retention times of 5.5 and 9.5 min of the chromatograms is shown magnified.
196
Beneath the decomposition of FEC, which is only present in the pristine electrolyte, there are
197
various additional peaks visible in the aged electrolyte. Typically, the region with higher
198
retention time than EC provides more decomposition products than DMDOHC and
199
EMDOHC. However, the presence of the alkane fraction interferes with a more precise
200
investigation. Unfortunately, further experiments to separate the alkanes did not improve the
201
overall separation.
202
Figure 3 focusses on the retention times between 5.6 and 9.4 min and again shows an
203
overlay of an electrolyte aged with 1 C and a pristine electrolyte. Samples of the same
204
electrolyte were investigated with a standard liquid injection and a SPME pre-concentration.
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ACCEPTED MANUSCRIPT Besides the FEC peak of the pristine electrolyte all main peaks of the aged electrolyte are
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serially numbered. All compounds of interest show higher signals for the SPME method. The
207
peaks 1, 4 and 6 show responses for both, the standard procedure and SPME. Whereas
208
peaks 2, 3 and 5 are only visible in the SPME experiment. The main fragments of all peaks
209
are shown in Table 1 and the fragment patterns of 4 and 5 are exemplarily displayed in
210
Figure 4. More detailed information are summarized in Table S 1 in the supplements. The
211
fragment patterns of 1, 2 and 6 are already known in literature. In combination with CI
212
experiments, different structures have been assigned.[30] For the fragment patterns of peak
213
3 and 5 obtained by electron impact ionization (EI) the NIST 11 library proposes ethyl propyl
214
carbonate (EPC) [90%] and sec-butyl ethyl carbonate (sBEC) [93%], respectively.
215
Unfortunately, the concentration of compounds 1-6 was too low to obtain clear signals when
216
applying positive and negative chemical ionization (PCI/NCI). Thus, it was decided to
217
synthesize the compounds of interest to compare retention times and fragment patterns in a
218
direct manner. Furthermore, the presence of sBEC in an electrolyte containing DMC, EMC
219
and EC as main components leads to two questions:
220
a) Is sec-butyl methyl carbonate (sBMC) present? sBMC should usually emerge as a matter
221
of probability in a DMC/EMC based electrolyte; b) What is the origin of the C4 chains? Is the
222
formation of a carbon-carbon bond between electrolyte components feasible during cycling?
223
Especially considering the fact that the longest carbon chain in standard carbonate based
224
electrolytes is the C3 chain of PC.
225
Jin et al. recently reported the synthesis of the compounds sBMC and n-butyl methyl
226
carbonate (BMC) via transesterification reactions in presence of different acidic catalysts.[27]
227
The supporting information gives EI spectra, which are in agreement with the spectra
228
reported by Schulze et al.[31] Moreover, these spectra match with the corresponding spectra
229
of peaks 2 and 4. To compare the GC retention times, these compounds were synthesized in
230
house applying the same synthesis protocol. Furthermore, the substitution of DMC by DEC
231
should in theory lead to the formation of the compounds with one ethoxy side chain. Finally,
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the protocol was adapted in order to synthesize methyl propyl carbonate (MPC) and EPC.
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ACCEPTED MANUSCRIPT The reaction monitoring of the five reactions after 7 h led to the chromatogram shown in
234
Figure 5. For straightforwardness, only the peaks of the products are depicted, while the
235
chromatogram of the aged electrolyte is shown as comparison. Since MPC and EPC are only
236
obtained as side products in the isopropanol synthesis, the signals are in the range of the
237
limit of detection (LOD). The peak of butyl ethyl carbonate (BEC) provides a retention time of
238
10.01 min and elutes between EC and SN in the side area of the alkanes. Therefore, a
239
presence of the BEC could not be clarified. Furthermore, the methyl isopropyl carbonate
240
(MiPC) elutes during the filament outage associated with the EMC elution. The electrolyte
241
sample shows no corresponding peak to the elution of the synthesized ethyl isppropyl
242
carbonate (EiPC) standard. However, sBMC, BMC and sBEC elute with peaks 2, 4 and 5,
243
respectively. Additionally, the fragment patterns are in good agreement between the
244
decomposition and synthesis products (For detailed information the reader is referred to
245
Table S 1 in the supplements). Furthermore, the side products MPC and EPC of the
246
isopropyl-based synthesis co-elute with the signals 1 and 3, respectively. Therefore, in case
247
of the C4 chain there is the linear and one branched chain present, while in the case of the
248
C3 chain only the linear specimen was found. Until now the isopropyl side chain was only
249
found in electrolytes containing PC and a reaction mechanism was already proposed.[30] A
250
comparison of the reactions, which occurred and did not occur in the electrolyte discussed in
251
this study, is shown in Figure 6. Furthermore, Table 2 lists the main fragments of the
252
synthesized standards obtained by PCI, NCI and EI experiments, respectively.
253
The assignment of sBMC to peak 2 leads to the assumption, that the previously proposed
254
structure for the corresponding fragment pattern was falsely assigned.[30] Taking into
255
account that 1) the nominal mass of both structures is the same and thereby PCI
256
experiments with single quadrupole MS would show the same results for both structures and
257
2) the fact that the presence of a C4 chain was not estimated as a probability in LIB
258
electrolytes; therefore the structure was newly assigned.
259
Carbonates with C4 sidechains were already discussed in the LIB electrolyte context.[32-34]
260
However, in these cases, the carbonates were only used as solvents / co-solvents in order to
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ACCEPTED MANUSCRIPT improve the overall LIB performance or distinct parameters. Since the amount of the
262
compounds 1-6 in the pristine electrolyte was at least below the LOD of both GC-MS
263
methods, their deliberate presence e.g. as additives was excluded. Therefore, they have to
264
be formed or cleaved off the organic electrode parts during the applied cycling procedure.
265
Since graphite and SBR do not contain oxygen and the standard carbon chain of CMC is C6
266
the probability of them being the origin of the C4 chains in our understanding negligible. To
267
our knowledge the only work proposing the formation of a C4 chain in a LIB electrolyte was
268
done by Kohs et al.[35], who discussed the origin of tetrahydrofuran (THF) found in their
269
experiments. The proposed mechanism works via a polarization of a carbonate moiety by
270
two lithium ions and as result a cleavage of lithium carbonate and the formation of a butylene
271
glycol. However, this reaction requires a two electron uptake. Furthermore, Ortiz et al. found
272
several long chained carbonates in a DEC solution which was aged by radiolysis
273
experiments.[36] In addition to lithium ions, several different metal ions can be present in the
274
LIB electrolyte. Depending on the electrode composition aluminum, cobalt, manganese or
275
nickel might be diluted from the cathode and copper/alumina from the current collectors.[37]
276
These metals are known to act as catalysts in organic chemistry.
277
In contrast to the separate electrolyte, the carbon-carbon bond formation is extensively
278
discussed in the SEI context. The reactions are generally started by an electron transferred
279
to a carbonate molecule, resulting in a negatively charged radical. These radicals can then
280
undergo different reactions, like H-abstraction from other organic molecules, further
281
decomposition or recombination of two radical species.[7, 38] However, favored reaction
282
routes and stable species are controversially discussed in literature.[38] Considering the
283
polymerization of FEC during SEI formation, this reaction could be the origin of the propyl
284
and butyl carbonates. However, some batteries characterized in the work of Friesen[39]
285
show the discussed decomposition products without containing FEC. A direct comparison of
286
the chromatograms from supplier 1.1 cells and these cells is displayed in Figure S 4. Thus,
287
FEC might contribute to the formation of the decomposition products, but can be excluded as
288
only origin.
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ACCEPTED MANUSCRIPT Shkrob et al. investigated the stability of radicals by matrix isolation electron paramagnetic
290
resonance spectroscopy. Among others, the radicals depicted in Scheme 1 were found for
291
EC, DMC and EMC samples. When EC abstracts an electron the ring can be opened, which
292
results in a terminal radical on the ethoxy moiety. This structure can be altered intramolecular
293
by a radical 1,2-migration. When a linear carbonate is reduced, an alkane radical can be
294
formed. Furthermore, carbonate based radicals can be formed by a radical abstracting a
295
hydrogen atom from any carbonate present (3 and 4). The recombination of the different
296
radicals from Scheme 1 leads to the carbonates or lithium alkyl carbonates with C3 and C4
297
moieties shown in Scheme 2. However, the displayed products are only a small part of the
298
possible combinations. For example, the whole group of dicarbonates is not included.
299
Nevertheless, except for the two synthesized carbonates with isobutyl moiety all C3 and C4
300
chains are accessible by recombination of two radicals. Taking into account that the
301
carbonate based radical in reaction 5 has either a methyl or an ethyl rest, MPC and EPC
302
should be accessible without further reaction. Since the carbonate based radicals of
303
reactions 6 and 7 provide no ethyl groups the corresponding products have at least to
304
undergo a substitution of the methyl group or the lithium. Considering the reaction route via
305
EC the presence of lithium alkyl carbonates with C4 chains is needed. At least as
306
intermediates on the electrode surface, if not as SEI component. This would be in
307
compliance to the work of Schafzahl et al. who investigated the influence of long chained
308
alkyl carbonates on the SEI performance.[40]
309
A comparison of the aging products in dependence of the cycling protocol is shown in Figure
310
6. All electrolytes show signals which relate to compounds 1-6. The 3 C and 4.55 C cells
311
show some remaining FEC, whereas, the FEC content of both 1 C cells is below the LOD.
312
Comparing the signals of the decomposition products, their intensity does not seem to be a
313
function of the corresponding SOH. The electrolytes gathered from cells, which were cycled
314
for more overall cycles (both 1 C cells), show comparable and increased signal heights in
315
contrast to the cells discharged at high C-rates. One possible reason could be the presence
316
of FEC residues in the latter case. This might have suppressed the formation of the
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ACCEPTED MANUSCRIPT decomposition products in higher amounts. Especially with respect to the FEC derived SEI
318
protecting the anode surface. Furthermore, the reactions could suffer from slow kinetics,
319
which leads to a slow and steady formation from cycle to cycle.
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ACCEPTED MANUSCRIPT 3. Conclusions
321
The electrolyte of a commercially available 18650 cell was analyzed with respect to emerging
322
decomposition products. Therefore, the cells were aged with protocols applying different
323
C-rates during discharge. The gathered electrolyte was investigated by standard GC-MS with
324
liquid injection and SPME-GC-MS. The SPME-GC-MS system shows advantages regarding
325
the detection of several decomposition products. Thereby, compounds with unknown EI
326
spectra were obtained and their structures were elucidated. In addition, the compounds of
327
interest were synthesized according to literature and fragmentation patterns as well as
328
retention times were compared. Three of the assigned decomposition products contain
329
butoxy moieties - namely sec-butyl methyl carbonate (sBMC), butyl methyl carbonate (BMC)
330
and sec-butyl ethyl carbonate (sBEC) - and their possible origin was discussed. As
331
conclusion, a reaction of two C2 moieties of EC or EMC via a formation of a carbon-carbon
332
bond is the most probable way. However, this assumption has to be proven or disproven in
333
further experiments. E.g., some of these compounds should emerge in DEC/EC electrolytes
334
cycled for long times; DMC/EC electrolytes should in theory lead to increasing amounts of
335
compounds with C3 chains. Ortiz et al. detected long chained carbonates aged by radiolysis
336
experiments.[36] A more recent study of the same group treated carbonate mixtures with the
337
same setup to identify the primarily formed radicals.[41] Thus, the radiolysis demonstrated
338
the ability to generate aging products formed during cycling in LIB electrolytes.
339
Furthermore, the formation of these compounds indicate that the SEI has to be considered
340
as a dynamic interphase. Especially regarding the long chain lithium alkyl carbonates. Thus,
341
the addition of butyl carbonates or lithium butyl carbonates to electrolyte formulations could
342
have an impact on the equilibrium conditions of the SEI and thereby on the stability.
343
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Acknowledgment
345
The authors would like to thank the Federal Ministry of Education and Research for funding
346
the project EffiForm (03XP0034H) and the German Federal Ministry of Education and
347
Research (BMBF) for funding the project “Elektrolytlabor 4E” (03X4632).
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ACCEPTED MANUSCRIPT Figures
350 351
Figure 1: Discharge capacities versus cycle number of differently aged cells: 1 C, SOH 70,
352
purple diamonds; 1 C, SOH 80, black circles; 3 C, SOH 80, red squares; 4.55 C, SOH 80,
353
blue triangles. The cells were cycled at 20 °C.
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Figure 2: Chromatograms of the electrolyte gathered from an aged cell (black) and a pristine
357
cell (magenta) obtained by SPME-GC-MS. The excerpt between 5.5 and 9.5 min is shown as
358
magnified inset.
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Figure 3: Overlay of the chromatograms from an aged electrolyte obtained by GC-MS with
362
liquid injection (olive) and SPME (black) - the chromatogram of the pristine electrolyte
363
(magenta) is shown as comparison. Signals of interest are numbered sequentially.
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365 366
Figure 4: Fragment patterns of compound 4 (left) and 5 (right), obtained with SPME-GC-MS
367
in EI mode.
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Figure 5: Comparison of the chromatograms obtained from the aged electrolyte (black, top)
371
and the synthesized standards (green, bottom).
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Figure 6: Summary of the decomposition products present in the aged electrolyte (right).
375
Compounds, which were not detected (below LOD), but provide similar structures (left).
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Figure 7: Chromatograms of the differently aged electrolytes obtained with SPME-GC-MS:
379
1 C SOH 70 (purple, top); 1 C SOH 80 (black, upper middle); 3 C SOH 80 (red, lower
380
middle); 4.55 C SOH 80 (blue, bottom).
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Tables
383 384
Table 1: Compounds 1-6, the assigned structures and their most prominent EI fragments.
385
(For more detailed information the reader is referred to Table S 1 in the supplements.)
Assigned Structure
77, 59, 45, 43
2
117, 103, 73, 59, 41
3
105, 91, 63, 59, 43
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7.07
8.24
103, 77, 73, 56, 41
8.71
117, 91, 73, 57, 45
9.09
104, 77, 59, 58, 45
9.19
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387
5.65
SC
1
4
386
Retention Time / min
EI
RI PT
Compound Nr.
ACCEPTED MANUSCRIPT 389
Table 2: Structures and corresponding fragments of the synthesized standards obtained by
390
SPME-GC-MS and SPME-GC-CI-MS. The compounds are listed in their retention order. (For
391
more detailed information the reader is referred to Table S 1 found in the supplements.) NCI
EI
MiPC
136, 119
103, 89, 75
117, 103, 89, 75, 59
MPC
136, 119
103, 89, 75, 59, 57
EiPC
150, 133
103, 89
sBMC
150, 133
117, 75, 73
iBMC
150, 133
117, 103, 75, 73
90, 77, 73, 59, 56
EPC
150, 133
103, 89, 59
105, 91/90, 63, 59
BMC
150, 133
117, 103, 75, 73
103, 77, 73, 59, 56
164, 147
117, 89, 73
117, 91, 90, 73, 63, 59, 57
164, 147
117, 89, 73, 71
104, 91, 63, 57/56
164, 147
117, 89, 73, 71
91, 90, 73, 63, 57, 56
392 393
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BEC
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iBEC
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PCI
sBEC
77, 59, 45, 43
91, 90, 63, 59
SC
Structure
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Name
117, 103, 77, 73, 59
ACCEPTED MANUSCRIPT 394
Schemes
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Scheme 1: Proposed formation of carbonate based radicals in LIB electolytes. Redrawn for
398
DMC EMC EC based electrolytes from Shkrob et al.[42] and Xu et al.[38]
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Scheme 2: Recombination of radicals from scheme 1 leading to the formation of C3 and C4
402
alkane chains.
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References
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