Capillary electrophoresis with contactless conductivity detection for the quantification of fluoride in lithium ion battery electrolytes and in ionic liquids-A comparison to the results gained with a fluoride ion-selective electrode.

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Capillary Electrophoresis with Contactless Conductivity Detection for the Quantification of Fluoride in Lithium Ion Battery Electrolytes and in Ionic Liquids - A Comparison to the Results Gained with a Fluoride Ion-Selective Electrode Marcelina Pyschik1, Marcel Klein-Hitpaß1, Sabrina Girod1, Martin Winter1, 2 and Sascha Nowak1, * 1

University of Muenster, MEET Battery Research Center, Institute of Physical Chemistry, Corrensstraße 46, 48149 Münster, Germany

2

Helmholtz Institute Münster, IEK-12, Forschungszentrum Jülich, Corrensstraße 46, 48149 Münster, Germany

Received: MONTH DD, YYY; Revised: MONTH DD, YYY; Accepted: MONTH DD, YYY This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/elps.201600361. This article is protected by copyright. All rights reserved.


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* Corresponding author: [email protected]

This article is protected by copyright. All rights reserved. 2


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ABSTRACT In this study, an optimized method using capillary electrophoresis (CE) with a direct contactless conductivity detector (C4D) for a new application field is presented for the quantification of fluoride in common used lithium - ion battery (LIB) electrolyte using LiPF6 in organic carbonate solvents and in ionic liquids (ILs) after contacted to Li metal. The method development for finding the right buffer and the suitable CE conditions for the quantification of fluoride was investigated. The results of the concentration of fluoride in different LIB electrolyte samples were compared to the results from the ion-selective electrode (ISE). The relative standard deviations (RSDs) and recovery rates for fluoride were obtained with a very high accuracy in both methods. The results of the fluoride concentration in the LIB electrolytes were in very good agreement for both methods. In addition, the limit of detection (LOD) and limit of quantification (LOQ) values were determined for the CE method. The CE method has been applied also for the quantification of fluoride in ILs. In the fresh IL sample, the concentration of fluoride was under the LOD. Another sample of the IL mixed with Li metal has been investigated as well. It was possible to quantify the fluoride concentration in this sample.

KEYWORDS Lithium – ion battery electrolytes, ionic liquids, capillary electrophoresis, fluoride quantification, contactless conductivity detector

1. Introduction Ion chromatography (IC) has become more and more important in lithium - ion battery (LIB) analysis. Specifically, there are many studies which report on the determination and identification of thermally aged decomposition products in the LIB electrolyte.[1-7] However, the use of IC was difficult for the quantification of fluoride in the aged electrolyte, since it was not possible to separate fluoride from organophosphates, which are also decomposition products of commonly used electrolytes in LIBs. [1] In contrast, capillary electrophoresis (CE) was not used until now in the LIB research, since the focus was primarily in pharmaceutical research [8, 9] and food [10, 11] analysis. For the determination of small ions, the ultraviolet/visible (UV/Vis) mode was used especially as indirect UV/Vis detector, since using direct detection the analyte needs to be UV/Vis active. In the year 1980, the first direct contactless conductivity detector (C4D) was introduced by Gaš et al. [12]. Years later, Zemann et al. published an axial arrangement of the C4D for CE. [13] In the year 2004, the first commercially available axial C4D was produced. The advantages to UV/Vis detection are high sensitivity, low cost and the simple electronic circuitry. With this technique, it was possible to detect alkali and alkaline earth cations and ammonium ions for different applications. [13-23] Da Silvia et al. have separated sodium, calcium and magnesium from each other in blood serum This article is protected by copyright. All rights reserved. 3


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although sodium was high concentrated. [24] In addition, Zemann et al. have reported firstly that using modifier in the buffer for dynamic coating of the capillary surface was compatible with the C4D for the detection of inorganic anions. [13] Because of this compatibility, a number of research attempts have used CE for the investigations of inorganic anions in drinking water [15] and rain water. [17, 18] Additionally, the separation of 21 anions and cations from each other in less than three minutes with CE and C4D was published by Kubán et al. [25]. With CE-C4D, as well larger organic ions can be separated from each other. In some studies, alkylammonium cations [14-16], alkylsulfonic anions [16] and fatty acids [26] were detected by indirect conductivity detection. The CE is an interesting method for the separation of halides and the quantification of fluoride, in particular. In contrast, the reported IC methods are not useful for the separation of fluoride and the organophosphates. [1, 4, 6] However, determination and quantification of the fluoride contents are important for aging of electrolytes and ILs. Bromide and chloride are impurities from the synthesis process of ILs. These impurities can have a negative influence: Villagrán et al. quantified chloride using IC and showed that chloride ions change the density and viscosity of the ILs. [27]

In this study, the quantification of fluoride in typically organic solvent based LIB electrolytes and in ionic liquids (ILs) after contact with Li metal was investigated by CE with a C4D. At first, the method development for finding the right buffer and the suitable CE conditions for the quantification of fluoride was carried out. The relative standard deviations (RSDs) and concentrations of fluoride with this newly developed CE method were compared to the results of an ion-selective electrode (ISE) for fluoride. The limit of detection (LOD) and limit of quantification (LOQ) were determined for fluoride in electrolytes for the CE method.

2. Experimental 2.1 Capillary Electrophoresis Conditions For the quantification of fluoride, the Agilent CE 7100 from Agilent Technologies (Santa Clara, CA, USA) has been used. The detection of the anions was carried out with a direct C4D from TraceDec® (Innovative Sensor Technologies GmbH, Austria). The software OpenLAB from Agilent Technologies (Santa Clara, CA, USA) was used for the control and the evaluation of the CE system. The standard bare fused silica capillary was obtained from Polymicro (Phoenix, AZ, USA) and had an inner diameter of 50 µm and a total length of 100 cm. The capillary was preconditioned by rinsing with the buffer for 5 min. Between runs, a rinsing of 5 min was applied as well. It was taken care that the system ran stable in each measurement and that the capillary had not got any contamination left from the run before. The capillary voltage was -20 kV and the temperature 24 °C. Samples were injected hydrodynamically by applying a pressure of 50 mbar for 10 s. Then the buffer was injected for 10 s under the same conditions.



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2.2 Materials for the Capillary Electrophoresis Measurements 2-(N-morpholio)ethanesulfonic acid (MES, 99.9%) and L-histidine (His, 99.9%) were purchased from MERCK KGAA (Darmstadt, Germany). Hexadimethrine bromide (99.0%) and lactic acid (99.9%) were obtained from Sigma Aldrich Chemie GmbH (Steinheim, Germany). Arginine (Arg) (99.0%) were used from PanReac AppliChem (Illinois, USA). All chemicals were of the highest quality available. Milli-Q water was used for all experiments. For the setting of the pH-value, ammonia (25.0%) obtained from Merck KGaA (Darmstadt, Germany) was used to adjust to an alkaline pH and formic acid (98-100%) from Merck KGaA (Darmstadt, Germany) to adjust to an acid pH. For the method development, fluoride, chloride, bromide, iodide and nitrate from Fluka (TraceCert IC Standard, 1000 mg/L ± 4 mg/L) were used.

2.3 Sample Preparation for the Capillary Electrophoresis Measurements The investigated typical LIB electrolyte consisted of 1 M LiPF6 as electrolyte salt [28] in ethyl methyl carbonate (EMC), diethyl carbonate (DEC), ethylene carbonate (EC) as electrolyte solvents [29] and vinylene carbonate (VC), 1, 3-propane sultone (PS) and biphenyl (BP) as electrolyte additives [30]. The electrolyte samples were diluted 1:1000 with the buffer consisted by 20 mmol/L His, 20 mmol/L MES and 1 mg/L hexadimethrin bromide. The samples 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMIM TFSI) before and after being brought in contact with Li metal were diluted 1:1000 with the buffer, as well. Li – metal is an anode that is typically used in combination with ILs [31-33] and was immersed into the ILs to accelerate their decomposition. Furthermore, in each sample iodide or nitrate were used as internal standard (IS) during the method development. For the quantification of fluoride in the LIB electrolytes samples, nitrate was used as IS. The concentration of the IS was 12.0 mg/kg in all samples. For receiving the right concentration of the IS in each sample, a calibration curve of the IS was used. The calibration range of the IS was 1.0 mg/kg to 25.0 mg/kg. Each sample of the IS calibration was measured three times. For the calculation of the concentration, all pipetted solutions were weighted. The concentration of fluoride in the electrolyte samples were determined by external standard calibration. The calibration range of the fluoride standards was in the range of 1.0 mg/kg to 25.0 mg/kg. Each sample of the external calibration was measured three times.

2.4 Ion-Selective Electrode (ISE) Conditions The ISE was used for the quantification of fluoride in the organic solvent based LIB electrolyte. As reference electrode, an Ag/AgCl electrode was used. Both electrodes were obtained from Metrohm AG (Herisau, Switzerland). ISEs are well-suited for potentiometric titrations. For this reason, the 905 Titrando apparatus from Metrohm AG (Herisau, Switzerland) was used. The 858 Professional IC Sample Processor was for automation of the sample injection. The electrolyte samples were diluted in a ratio of 1:1000 with milli-Q water. 2 mL of the diluted sample were mixed with 10 mL TISABsolution in a 20 mL 800 Dosino from Metrohm AG. The 1 L TISAB solution consisted of 58 g sodium chloride obtained from VWR (Darmstadt, Germany), 5 g trans-1,2-diamino-cyclohexane-N,N,N’,N’tetraacetic acid monohydrate from Sigma Aldrich Chemie GmbH (Steinheim, Germany) and 57 mL



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glacial acetic acid purchased from VWR (Darmstadt, Germany) solved in 500 mL milli-Q water and the pH was adjusted with 8 M sodium hydroxide solution to a pH-value of 5.5 and filled up with milliQ water. The mixture of the sample and total ionic strength adjustment buffer (TISAB) solution were put into the vessel of the 905 Titrando and stirred with the 801 Stirrer until the start of the measurement. After each measurement, the vessel was washed automatically. The system was controlled by the software MagIC Net 3.1 and Tiamo 2.4 from Metrohm AG (Herisau, Switzerland). The concentration of fluoride in the electrolyte samples was determined by external calibration. The calibration range of the fluoride standard was in the range of 0.5 mg/kg to 25.0 mg/kg. Each sample of the external calibration was measured three times.

3. Results and Discussion 3.1

Method Development for the Quantification of Fluoride in Lithium Ion Battery Electrolytes Separating the halides from each other, different buffers and combinations of His, MES, Arg and lactic acid with 1 mg/L hexadimethrine bromide were applied (Figure 1). At first, iodide was used as the IS during the method development. The investigation of all four halides showed that the chloride, bromide and iodide ions migrated at similar migration times, but fluoride was baselineseparated from the other halides. The buffer of choice should have the largest area of the fluoride peak. Additionally the best separation should be achieved for the halides chloride, bromide and iodide with the chosen buffer. The peaks with the buffers containing Arg eluted earlier than those containing His. Therefore, two of the three investigated halides were better separated with His than with Arg. Accordingly, the best peak separation was achieved when His was used in the buffer solution instead of Arg. The separation of the halides chloride, bromide and iodide did not show any differences using MES or lactic acid in combination with His. However, the area of the fluoride peak was larger using His and MES. Moreover, the migration times with MES were shorter than with lactic acid. Therefore, further investigations were done with 20 mM MES, 20 mM His and 1 mg/L hexadimethrin bromide.



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Figure 1: Electropherogram of fluoride, chloride, bromide and iodide with the puffer combinations His/MES, His/lactic acid, Arg/MES and Arg/lactic acid at a pH of 9.93. Hexadimethrine bromide was applied as modifier in all buffer combinations.

The pH influence of the chosen buffer was investigated as well (Figure 2). The buffer solution consisted of 20 mM His, 20 mM MES and 1 mg/L hexadimethrine bromide with a pH value of 5.87 was used. Additionally, one buffer in the acidic range (pH 3.14) and one in the alkaline range (pH 9.93) were used to investigate the influence on the migration times and halide separations. To achieve an acidic pH of 3.14 (black), the buffer was adjusted with formic acid. Chloride, bromide and iodide migrated at the same migration time and fluoride was separated from the other halides. This pH was not suitable for the separation and quantification of fluoride, since two peaks were detected during the migration time of fluoride. The buffer without addition of formic acid or ammonia had a pH of 5.87 (red). This buffer solution consisted of 20 mM His, 20 mM MES and 1 mg/L hexadimethrine bromide. However, at this pH value, the three halides chloride, bromide and iodide migrated at the same migration time. Fluoride was detected baseline separated from the other halides. Since chloride, bromide and iodide co-migrated, this pH range was also not suitable for the measurements. For achieving the pH of 9.93 (blue), the buffer was adjusted with ammonia. By using this buffer at this pH value, fluoride was baseline separated from the other halides. The halides chloride, bromide and iodide still co-migrated. Instead of three peaks for the three halides, two peaks were determined. Comparing the baselines of the buffer 20 mM His, 20 mM MES and 1 mg/L hexadimethrine bromide with the pH values of 3.14, 5.87 and 9.93, it was obvious that the lowest conductivity and the most stable baseline were achieved at the pH of 5.87 (see Figure 2). A pH value of around 6 is normally used for this buffer combination. However, the best separation results for our application were obtained at a pH value of 9.93. For further investigations, the buffer solution 20 mM His, 20 mM MES and 1 mg/L hexadimethrine bromide at the pH 9.93 was used. There are many studies in the literature which dealt with the separation of many ions – anions and cations – from other. But in none of these studies, fluoride, chloride, bromide and iodide were



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separated from each other. [13, 15, 18, 23, 25] Since, the four halides were complicated to separate, instead of iodide, another IS was investigated.

Figure 2: Electropherogram of fluoride, chloride, bromide and iodide with the buffer 20 mM His, 20 mM MES and 1 mg/L hexadimethrine bromide at the pH values 3.14, 5.87 and 9.93.

As alternative for iodide, nitrate was used as IS. The baseline separation of bromide, chloride, nitrate and fluoride is presented in Figure 3. Therefore, for the quantification of fluoride in electrolytes and ILs, as IS nitrate was used with the buffer consisted of 20 mM His, 20 mM MES and 1 mg/L hexadimethrine bromide and a pH of 9.93. Bromide migrates at 8.9 min, chloride at 9.1 min, nitrate at 9.7 min and fluoride at 11.5 min (Figure 3).



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Figure 3: Electropherogram of fluoride, chloride, bromide and nitrate with the buffer 20 mM His, 20 mM MES, 1 mg/L hexadimethrine bromide at a pH of 9.93.



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3.2

Comparison of Capillary Electrophoresis and Ion-Selective Electrode Measurements for the Quantification of Fluoride in Electrolytes and Ionic Liquids Qualitative analysis of electrolytes from commonly used LIBs was carried out by CE with a C4D. In Figure 4, the electropherogram of the investigated electrolyte sample is shown. Nitrate was detected at a migration time of 9.7 min and fluoride at 11.5 min. By comparison of standards, additionally to nitrate and fluoride, hexafluorophosphate was identified at a migration time of 11.3 min. Next to fluoride, there were two other peaks, which have not been determined because of lack of suitable standards and since the focus of this study was on fluoride determination. During the method development, the challenge was to separate the halides from each other. No bromide and chloride were detected in the electrolyte samples, because between the nitrate and hexafluorophosphate peaks, no other peaks were detected.

Figure 4: Electropherogram of a LIB electrolyte_1 with a fluoride concentration of 2269 mg/kg without dilution investigated with the buffer 20 mM His, 20 mM MES, 1 mg/L hexadimethrine bromide with a pH of 9.93.

The quantitative analyses of LIB electrolytes were carried out by CE with C4D and by ISE to determine the fluoride content. The coefficient of determination (R2) was higher than 0.999 for the internal calibration of nitrate as well as for external calibration of fluoride. The RSD and the recovery rates were calculated as well. For the determination of the LOD, the samples were diluted until the signal was three times the standard deviation of the noise. For the determination of the LOQ, the signal was nine times the standard deviation of the noise. The LOD for fluoride was 0.9 mg/kg and the LOQ was 2.7 mg/kg. In Table , the results of two electrolyte samples are shown, measured by CE with C4D and ISE. With the CE method, for the electrolyte_1, the concentration was 2269 mg/kg and 2367 mg/kg with ISE. For the electrolyte_2, the concentration was 2710 mg/kg with the CE method and 2679 mg/kg with the ISE. The recovery rate for the CE was 95.3% - 98.1 % and the RSD was in a range of 1.53% - 1.79%. The RSD for the ISE was in a range of 0.30% – 0.68% and the recovery rate was between 100.0%-100.9%. The results have shown that CE provided a very valuable opportunity compared to the ISE in order to investigate the concentration of fluoride in electrolytes.



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Table 1: Determination of fluoride in the LIB electrolyte and IL sample by CE and ISE

CE

ISE

[mg/kg]

[mg/kg]

Electrolyte_1

2269 ± 39

2367 ± 16

Electrolyte_2

2710 ± 24

2679 ± 8

-

EMIM TFSI

< LOD

EMIM TFSI + Li metal

712 ± 27

-

-

The quantitative analysis of the fresh ILs EMIM TFSI was carried out by CE with C4D. In Figure 5, the electropherogram of EMIM TFSI is shown. Nitrate eluted at 9.7 min and TFSI- at 16.4 min, which were identified by standards. Another peak was detected at a migration time of 18.9 min. It was not possible to identify this substance in this work. At the migration time of fluoride, chloride and bromide, no peaks were detected. Accordingly, the concentration of fluoride in the ILs was below the LOD of 1.32 mg/kg.



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Figure 5: Electropherogram of the ILs EMIM TFSI investigated with the buffer 20 mM His, 20 mM MES, 1 mg/L hexadimethrine bromide with a pH of 9.93. The fluoride concentration was under the LOD.

In Figure 6, the electropherogram of EMIM TFSI brought in contact with Li metal is shown. Li metal was contacted to the ILs to accelerate the decomposition of the ILs. In this sample, a fluoride-peak was detected. The fluoride concentration without dilution was 712 mg/kg in the EMIM TFSI sample mixed with Li metal (see Table ). For EMIM TFSI without Li metal, the fluoride concentration was under the LOD. The recovery rate for the ILs investigated by CE was 104.4% - 105.7.1%. The RSD of the CE method was 3.78%.

Figure 6: Electropherogram of the ILs EMIM TFSI contacted with Li metal and investigated with the buffer 20 mM His, 20 mM MES, 1 mg/L hexadimethrine bromide with a pH of 9.93. In thus IL sample, a fluoride concentration of 712 mg/kg without dilution was determined.



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4. Conclusion In this work, a newly developed method using CE for the analysis of LiPF6-based LIB electrolytes was presented. The quantification of fluoride in the chosen samples could be achieved. The obtained results of this method were compared with the results of the ISE measurements for fluoride. The RSD and recovery rates for fluoride were obtained with a very high accuracy in both methods. Accordingly, the CE with C4D has found a new application field in LIB research. Furthermore, since in standard laboratory cells only small amounts of electrolytes are applied (120 – 240 µL) the CE is the method of choice for these applications. Typically, only small amounts of the initial electrolyte can be extracted. The CE operated with an injection volume in the nL-range, in contrast to ISE where for each measurement 2 mL were needed of the diluted sample. Additionally, the developed CE method has been applied also for the quantification of fluoride in ILs for the first time. In the fresh IL sample, the concentration of fluoride was under the LOD, which is in good agreement with the stability characteristics reported in literature. To prove the applicability, IL samples were brought in contact with Li metal in order to accelerate the decomposition and to generate fluoride contents. Subsequently, the fluoride concentration could be accessed and determined with the CE method.

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