Article pubs.acs.org/ac
Rapid Characterization of Lithium Ion Battery Electrolytes and Thermal Aging Products by Low-Temperature Plasma Ambient Ionization High-Resolution Mass Spectrometry Britta Vortmann,†,§ Sascha Nowak,‡ and Carsten Engelhard*,† †
Institute of Inorganic and Analytical Chemistry, University of Muenster, Corrensstr. 30, D-48149 Muenster, Germany MEET Battery Research Center, Institute of Physical Chemistry, University of Muenster, Corrensstr. 46, D-48149 Muenster, Germany
‡
S Supporting Information *
ABSTRACT: Lithium ion batteries (LIBs) are key components for portable electronic devices that are used around the world. However, thermal decomposition products in the battery reduce its lifetime, and decomposition processes are still not understood. In this study, a rapid method for in situ analysis and reaction monitoring in LIB electrolytes is presented based on high-resolution mass spectrometry (HRMS) with low-temperature plasma probe (LTP) ambient desorption/ionization for the first time. This proof-of-principle study demonstrates the capabilities of ambient mass spectrometry in battery research. LTP-HR-MS is ideally suited for qualitative analysis in the ambient environment because it allows direct sample analysis independent of the sample size, geometry, and structure. Further, it is environmental friendly because it eliminates the need of organic solvents that are typically used in separation techniques coupled to mass spectrometry. Accurate mass measurements were used to identify the time-/ condition-dependent formation of electrolyte decomposition compounds. A LIB model electrolyte containing ethylene carbonate and dimethyl carbonate was analyzed before and after controlled thermal stress and over the course of several weeks. Major decomposition products identified include difluorophosphoric acid, monofluorophosphoric acid methyl ester, monofluorophosphoric acid dimethyl ester, and hexafluorophosphate. Solvents (i.e., dimethyl carbonate) were partly consumed via an esterification pathway. LTP-HR-MS is considered to be an attractive method for fundamental LIB studies.
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example, gas chromatography,7 ion chromatography,8 or other means of separation mechanism before detection. Others have used X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry, but these techniques require each sample to be transferred from the atmosphere into an ultrahigh-vacuum environment before LIB analysis.9 The unique benefits of the method discussed here include the ability to perform analysis directly in the ambient environment, rapid and sensitive detection with high mass accuracy, and no need for preceding separation methods and sample preparation. In turn, this results in very short analysis times of less than 30 s per sample (in contrast to conventional chromatographic separation methods with >15 min per sample), ideally suited for high-throughput applications and screening of, for example, large compound libraries. In the present study, we coupled a home-built LTP probe, which was first described by Harper et al.10 and employs a
ithium ion batteries (LIBs) are key components for portable electronic devices and have reached production quantities of billions of units per year.1 They have attractive properties that include high-output voltages, high-energy densities, long cycle times, and rate capabilities that outperform comparable battery technologies.2,3 Today, extensive work is being pursued for the development of next generation electric vehicles, with advanced high-energy lithium batteries4,5 and novel materials such as graphene-based nanosheets6 as a major focus of research. Despite the large production quantities, characterizing and understanding all fundamental processes in a battery, for example, at the electrode−electrolyte interface, still remain a principal challenge. In fact, sensitive analytical methods for the characterization of LIBs remain scarce. Here, we report a rapid method based on high-resolution mass spectrometry (HR-MS) with a low-temperature plasma (LTP) desorption/ionization source for direct analysis of complex LIB samples in the ambient environment with no sample preparation. Specifically, LIB electrolytes and decomposition products therein were characterized before and after controlled thermal aging. Previously, LIBs were the subject of very few mass spectrometric studies, typically employing, for © 2013 American Chemical Society
Received: January 17, 2013 Accepted: February 8, 2013 Published: February 9, 2013 3433
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respectively. Samples were pipetted (20 μL) into the sample well plate and directly analyzed under ambient conditions with LTP-Orbitrap-MS. No further sample preparation was necessary. LTP9,10 conditions were as follows: helium gas, 0.3 L/min (high purity grade 4.6, Westfalen AG, Muenster, Germany); a copper electrode (20 mm long, 20 mm wide) served as the outer electrode, a stainless steel electrode served as the inner electrode (o.d. 1 mm), and a quartz glass capillary (o.d. 6 mm, i.d. 2 mm) served as the dielectric barrier. For safety, both electrodes were shielded with Teflon covers, and a home-built ion source housing was used to cover the apparatus during analysis. Samples were placed on a custom-built sample holder designed for high-throughput sampling. The angle of the LTP probe to the sample holder was 60°, and the distance between the sample and the MS interface was 3 mm. An alternating current (AC) high voltage of 4 kV at a frequency of 31 kHz was provided by a gas tube sign power supply to power the LTP probe. Electrolyte LP30 SelectiLyte (1 M LiPF6 in a 1:1 mixture (w/w) of ethylene carbonate (EC) and dimethyl carbonate (DMC)) was purchased from Merck KGaA (Darmstadt, Germany). For the investigation of electrolyte aging products, three sample batches stored under different conditions were prepared and exposed to different levels of thermal stress. Electrolyte samples were stored in closed Al containers filled with air for five weeks at room temperature (20 °C) and at 60 °C. A third sample batch was stored for seven weeks at room temperature in the open atmosphere.
dielectric barrier discharge, to a Thermo Exactive HCD HR-MS spectrometer with an Orbitrap detector. The afterglow emanating out of the probe was used directly to sample analytes in the ambient environment, which are then desorbed, ionized, and transported into the mass spectrometer. In general, the LTP probe and the other dielectric-barrier discharge ionization sources are considered soft ionization sources and induce little to no fragmentation in normal operation mode.10−12 LTP’s fundamental properties as an ionization source13,14 and applicability in mass spectrometry11 compared to conventional ionization methods (electrospray ionization and atmospheric pressure chemical ionization) have been discussed in detail previously. Important for this study is the fact that LTP does not require the use of solvents or other chemicals. Therefore, no chemical contamination occurs during sampling that could adversely affect LIB sample composition. Further, LTP is considered a nondestructive plasma-based ion source that can be used, for example, for temperature-sensitive samples such as LIB compounds. For this field of study, characteristics discussed above are significant benefits over other ambient desorption/ionization methods such as plasmabased methods that operate at higher power/temperature and spray-based methods that require the use of solvents.
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EXPERIMENTAL SECTION Experiments were carried out using a home-built LTP ionization source coupled to a HR-MS spectrometer with an Orbitrap detector (model Exactive HCD, Thermo Scientific, Bremen, Germany). The experimental setup for LTP-OrbitrapMS is shown schematically in Figure 1. Operating parameters
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RESULTS AND DISCUSSION LIBs exist in various configurations but typically contain a graphite anode, a cathode formed by a lithium metal oxide, separators, and a liquid electrolyte. A widely used electrolyte composition is a solution of lithium salt LiPF6 in a mixed organic solvent of EC and DMC. First, the LTP-Orbitrap-MS method was optimized for ambient sampling of such battery compounds. Since DMC is frequently used in LIBs and gives a very simple mass spectrum, tuning was performed on the protonated molecular ion [M + H]+ of DMC at m/z 91. Here, the helium flow rate of the ionization source was identified as an important factor. Maximum signal intensities with the lowest standard deviation for triplicate analysis were obtained at a flow rate of 0.3 L/min (see the Supporting Information, Figure S-1). All experimental parameters after optimization are summarized in the Supporting Information, Table S-1. As a result, intraday relative standard deviation (RSD) values were typically found to be 4−7% (DMC detected at m/z 91, averaged signals of 10− 30 s). Interday RSDs were evaluated over a period of three days and found to be better than 15%. It is noteworthy that the ionization source itself produces few background ions at low abundance. Typical background mass spectra are given in the Supporting Information, see Figure S-2. These spectra were used to perform a background correction on each surface sampling measurement discussed below. Furthermore, LTP is considered a soft ionization source because no significant fragmentation of analyte ions was observed under the conditions used here. The dominant analyte ions were [M + H]+ and [M − H]− in positive and negative ion modes, respectively. Proton transfer was the main ionization mechanism in positive ion mode. In negative ion mode, proton abstraction was the main ionization mechanism, but other mechanisms have been reported as well (e.g., electron capture
Figure 1. Experimental setup for direct analysis with LTP-OrbitrapMS. Inset shows a full mass spectrum recorded after direct analysis of LIB electrolyte with LTP-Orbitrap-MS in negative ion mode.
were tuned for sensitivity in positive and negative ion modes. Capillary temperature, 200 °C/240 °C; capillary voltage, 25 V/ −60 V; skimmer voltage, 36 V/−20 V; tube lens voltage:, 90 V/ −90 V; m/z range, 50−350; max injection time, 100 ms; and three microscans/spectrum. Daily tuning (small mass calibration) was performed prior to the measurements. The Exactive was operated in full scan mode with the mass range set to m/z 50−350 and the resolution set to R = 100 000 (full width at half-maximum, fwhm, at m/z 200). Data were recorded and processed with Xcalibur software 2.1 (Thermo Scientific) and Origin Pro 8.5 (OriginLab, Northampton, MA, USA), 3434
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Figure 2. High-resolution mass spectra of lithium ion electrolyte system SelectiLyte LP30 in (a) positive ion mode and (b) negative ion mode. Signals were assigned by exact mass, see Table S-2 (Supporting Information).
Figure 3. LTP-Orbitrap-MS analysis of thermally aged electrolyte LP30: (a) after 5 weeks of storage at 60 °C, negative ion mode; (b) after 5 weeks of storage at 60 °C, positive ion mode; (c) after 7 weeks of storage at 20 °C in negative ion mode; (d) after 7 weeks of storage at 20 °C in positive ion mode.
and dissociative electron capture) depending on the studied compounds. Characterization of battery electrolytes and identification of thermal aging products formed therein was straightforward. For example, Figure 2 shows direct analysis of a freshly prepared electrolyte mixture (LP30, day 0, no thermal aging). Mass spectra were recorded in high-resolution mode of the Orbitrap,
which allows direct analyte identification at high mass accuracy. Solvents DMC (C3H7O3+, m/z 91.03916, cf. Figure 2a; C3H5O3−, m/z 89.02393, cf. Figure 2b) and EC (C3H5O3+, m/z 89.02351, cf. Figure 2a) were readily detected in either positive or negative ion modes, respectively. Difluorophosphoric acid ion (PO2F2−, m/z 100.96072, negative ion mode) was detected but at lower relative intensity compared to results 3435
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Table 1. Summary of Compounds Detected in Thermally Aged Electrolytes with LTP-Orbitrap-MS
Scheme 1. Proposed Decomposition Pathway: Hydrolysis of Lithium Hexafluorophosphate Followed by Esterification of Different Hydrolysis and Decomposition Productsa
a
Adapted and extended from the literature.8,15
life. Therefore, the commercially available electrolyte was exposed to different levels of thermal stress and characterized by LTP-Orbitrap-MS, see Figure 3 and Figure S-3 (Supporting Information). In this experiment, high mass accuracy was critical to elucidate the molecular formulas of the detected species. Four sets of peaks were identified to be characteristic for decomposition products formed in the electrolyte: difluorophosphoric acid, monofluorophosphoric acid methyl ester, monofluorophosphoric acid dimethyl ester, and hexafluorophosphate. After five weeks at 60 °C, hydrolysis product difluorophosphoric acid ion (PO2F2−, m/z 100.96048) and
later in the study (cf. Figure 3). This compound is already present here because it is known to arise from hydrolysis of the lithium salt LiPF66 in the electrolyte induced by residual moisture content in the electrolyte (LP30 contained 20 mg L−1 H2O as stated by the manufacturer). Further, hydrolysisinduced decomposition of LiPF6 continues over the course of several days and results in higher abundances of PO2F2− later in the study (discussed in detail below). Carbonate-based electrolytes for LIBs are promising for high power requirements. However, they suffer from thermal decomposition, which ultimately limits cycle time and battery 3436
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ultimate goal would be the composition of improved materials that show little or no degradation at all. Methods for fast analysis such as LTP-Orbitrap-MS will help to identify and understand such materials. In general, this experiment shows the great potential of ambient mass spectrometry for in situ analysis and reaction monitoring. It is noteworthy that these experiments give qualitative results and were performed without the use of an internal standard. For quantitative analysis, it will be important to establish calibration protocols and to use an internal standard, for example, to compensate for potential instrument drift.
degradation products monofluorophosphoric acid methyl ester (CH3PO3F−, m/z 112.98014) and monofluorophosphoric acid dimethyl ester ((CH3)2PO3FH+, m/z 129.01116) were identified as well. In addition to solvents EC and DMC, hexafluorophosphate ion (PF6−, m/z 144.96418) was detected at lower abundance. Mass errors of
