Sascha Nowak

Dual-graphite cells based on the reversible intercalation of bis(trifluoromethanesulfonyl)imide anions from an ionic liquid electrolyte

Recently, dual-ion cells based on the anion intercalation into a graphite positive electrode have been proposed as electrochemical energy storage devices. For this technology, in particular electrolytes which display a high stability vs. oxidation are required due to the very high operation potentials of the cathode, which may exceed 5 V vs. Li/Li+. In this work, we present highly promising results for the use of graphite as both the anode and cathode material in a so-called “dual-graphite” or “dual-carbon” cell. A major goal for this system is to find suitable electrolyte mixtures which exhibit not only a high oxidative stability at the cathode but also form a stable solid electrolyte interphase (SEI) at the graphite anode. As an electrolyte system, the ionic liquid-based electrolyte mixture Pyr14TFSI-LiTFSI is used in combination with the SEI-forming additive ethylene sulfite (ES) which allows stable and highly reversible Li+ ion and TFSI− anion intercalation/de-intercalation into/from the graphite anode and cathode, respectively. By addition of ES, also the discharge capacity for the anion intercalation can be remarkably increased from 50 mA h g−1 to 97 mA h g−1. X-ray diffraction studies of the anion intercalation into graphite are conducted in order to understand the influence of the electrolyte additive on the graphite structure and on the cell performance.

Ion chromatographic determination of hydrolysis products of hexafluorophosphate salts in aqueous solution.

In this work, hydrolysis of three different hexafluorophosphate salts in purified water was investigated. Aqueous samples of lithium hexafluorophosphate (LiPF(6)), sodium hexafluorophosphate (NaPF(6)) and potassium hexafluorophosphate (KPF(6)) were prepared and stored for different times. Ion chromatography (IC) with UV as well as non-suppressed and suppressed conductivity detection was used for the analysis of the reaction products. For the detection and identification of the formed decomposition products, an IC method using IonPac AS14A 250 mm × 4.0 mm i.d. column and 2.5 mM KHCO(3)-2.5 mM K(2)CO(3) eluent was established. Besides hexafluorophosphate, four other anionic species were detected in fresh and matured aqueous solutions. The hydrolysis products fluoride (F(-)), monofluorophosphate (HPO(3)F(-)), phosphate (HPO(4)(2-)) and difluorophosphate (PO(2)F(2)(-)) were found and were unambiguously identified by means of standards or electrospray ionization mass spectrometry (ESI-MS). It was shown that stability of hexafluorophosphate solutions depends on the nature of the counter ion and decreases in the order potassium>sodium>lithium.

Ion chromatography electrospray ionization mass spectrometry method development and investigation of lithium hexafluorophosphate-based organic electrolytes and their thermal decomposition products

A method based on the coupling of ion chromatography (IC) and electrospray ionization mass spectrometry (ESI-MS) for the separation and determination of thermal decomposition products of LiPF6-based organic electrolytes is presented. The utilized electrolytes, LP30 and LP50, are commercially available and consist of 1mol/l LiPF6 dissolved in ethylene carbonate/dimethyl carbonate and ethylene carbonate/ethyl methyl carbonate, respectively. For the separation method development three ion chromatographic columns with different capacity and stationary phase were used and compared. Besides the known hydrolysis products of lithium hexafluorophosphate, several new organophosphates were separated and identified with the developed IC-ESI-MS method during aging investigations of the electrolytes. The chemical structures were elucidated with IC-ESI-MS/MS.

Qualitative and quantitative investigation of organophosphates in an electrochemically and thermally treated lithium hexafluorophosphate- based lithium ion battery electrolyte by a developed liquid chromatography-tandem quadrupole mass spectrometry method†

The presented work was focused on the development of a new liquid chromatography-tandem quadrupole mass spectrometry method (LC-MS/MS) for the identification and quantification of organophosphates in lithium hexafluorophosphate (LiPF6)-based lithium ion battery electrolytes. The investigated electrolyte consists of 1 M LiPF6 dissolved in ethylene carbonate/ethyl methyl carbonate (50/50, wt%) and was treated electrochemically and thermally. For the electrochemical experiments, the cut-off potential in the half cells was held at 5.5 V for 72 h. The thermal degradation experiments were performed in aluminum vials at 95 °C for a period of 13 days. In the first part of this work, an already established gas chromatography-mass spectrometry (GC-MS) method for identification of dimethyl fluorophosphates (DMFP) and diethyl fluorophosphate (DEFP) was applied. In the second part, the LC-MS/MS method including determination of characteristic transitions in a product ion scan was developed. The developed method was applied for the identification of various analytes in the decomposed electrolytes. In addition, a possible formation of ionic and non-ionic OPs based on findings of this work and our previous reports is presented. In the third and final part, a quantification study of DMFP and DEFP was performed with a newly developed LC-MS/MS method and compared with results obtained by GC-MS. In addition, trimethyl phosphate (TMP) and triethyl phosphate (TEP) were quantified. These studies included the investigation of the suppression effects caused by the sample matrix during the application of the LC-MS/MS method.

Rapid characterization of lithium ion battery electrolytes and thermal aging products by low-temperature plasma ambient ionization high-resolution mass spectrometry.

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 (HR-MS) 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.

Identification of alkylated phosphates by gas chromatography–mass spectrometric investigations with different ionization principles of a thermally aged commercial lithium ion battery electrolyte

The thermal aging process of a commercial LiPF6 based lithium ion battery electrolyte has been investigated in view of the formation of volatile phosphorus-containing degradation products. Aging products were analyzed by GC-MS. Structure determination of the products was performed by support of chemical ionization MS in positive and negative modes. A fraction of the discovered compounds belongs to the group of fluorophosphates (phosphorofluoridates) which are in suspect of potential toxicity. This is well known for relative derivatives, e.g. diisopropyl fluorophosphate. Another fraction of the identified compounds belongs to the group of trialkyl phosphates. These compounds may provide a positive impact on the thermal and electrochemical performance of Li-based batteries as repeatedly described in the literature.

Two-dimensional ion chromatography for the separation of ionic organophosphates generated in thermally decomposed lithium hexafluorophosphate-based lithium ion battery electrolytes.

A two-dimensional ion chromatography (IC/IC) technique with heart-cutting mode for the separation of ionic organophosphates was developed. These analytes are generated during thermal degradation of three different commercially available Selectilyte™ lithium ion battery electrolytes. The composition of the investigated electrolytes is based on 1M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate/dimethyl carbonate (50:50wt%, LP30), ethylene carbonate/diethyl carbonate (50:50wt%, LP40) and ethylene carbonate/ethyl methyl carbonate (50:50wt%, LP50). The organophosphates were pre-separated from PF6(-) anion on the low capacity A Supp 4 column, which was eluted with a gradient step containing acetonitrile. The fraction containing analytes was retarded on a pre-concentration column and after that transferred to the high capacity columns, where the separation was performed isocratically. Different stationary phases and eluents were applied on the 2nd dimension for the investigation of retention times, whereas the highly promising results were obtained with a high capacitive A Supp 10 column. The organophosphates generated in LP30 and LP40 electrolytes could be separated by application of an aqueous NaOH eluent providing fast analysis time within 35min. For the separation of the organophosphates of LP50 electrolyte due to its complexity a NaOH eluent containing a mixture of methanol/H2O was necessary. In addition, the developed two dimensional IC method was hyphenated to an inductively coupled plasma mass spectrometer (ICP-MS) using aqueous NaOH without organic modifiers. This proof of principle measurement was carried out for future quantitative investigation regarding the concentration of the ionic organophosphates. Furthermore, the chemical stability of several ionic organophosphates in water and acetonitrile at room temperature over a period of 10h was investigated. In both solvents no decomposition of the investigated analytes was observed and therefore water as solvent for dilution of samples was proved as suitable.

Speciation Analysis of Gadolinium-Based MRI Contrast Agents in Blood Plasma by Hydrophilic Interaction Chromatography/Electrospray Mass Spectrometry

The first analytical method for simultaneous speciation analysis of five of the most important gadolinium-based magnetic resonance imaging (MRI) contrast agents in blood plasma samples was developed. Gd-DTPA (Magnevist), Gd-BT-DO3A (Gadovist), Gd-DOTA (Dotarem), Gd-DTPA-BMA (Omniscan), and Gd-BOPTA (Multihance) were separated by hydrophilic interaction liquid chromatography (HILIC) and detected with electrospray mass spectrometry (ESI-MS). Spiking experiments of blank plasma with Magnevist and Gadovist were performed to determine the analytical figures of merit and the recovery rates. The limits of detection ranged from 1 x 10 (-7) to 1 x 10 (-6) mol/L depending on the ionization properties of the individual compounds, and limits of quantification ranged from 5 x 10 (-7) to 5 x 10 (-6) mol/L. The linear concentration range comprised 2 orders of magnitude. With application of this method, blood plasma samples of 10 healthy volunteers, with Magnevist or Gadovist medication, were analyzed for Gd-DTPA and Gd-BT-DO3A, respectively. The obtained results were successfully validated with inductively coupled plasma-optical emission spectroscopy (ICP-OES).

Analysis of the contrast agent Magnevist and its transmetalation products in blood plasma by capillary electrophoresis/electrospray ionization time-of-flight mass spectrometry.

To study transmetalation effects of the gadolinium-based contrast agent Magnevist (Gd-DTPA), the first analytical method for the simultaneous determination of Gd-DTPA and its transmetalation products in complex clinical samples was developed. The high separation efficiency of capillary electrophoresis (CE) was employed to separate Gd-DTPA, Fe-DTPA, Cu-DTPA, Zn-DTPA, and the free DTPA (diethylenetriaminepentaacetic acid) ligand. The coupling of CE with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) provided the required sensitivity and excellent selectivity for the analysis of complex samples, such as blood plasma and whole blood. Separation and detection parameters were optimized, and crucial steps for CE/MS method development are pointed out. Limit of detection (LOD) is 5 x 10(-7) mol/L, limit of quantification (LOQ) is 1.7 x 10(-6) mol/L, and the linear range comprises 2 decades, starting at the limit of quantification. To determine recovery rates, precision, and accuracy of the method, blank plasma samples were spiked with Gd-DTPA in three different concentrations. Blood plasma samples from 10 patients with normal renal function, having received Magnevist, were analyzed for Gd-DTPA and possible transmetalation products by CE/ESI-TOF-MS. The method was validated by determination of the total Gd concentration using inductively coupled plasma optical emission spectroscopy (ICP-OES). Transmetalation assays of Magnevist with and without supplementary iron were carried out in incubated whole blood samples.

Study of decomposition products by gas chromatography-mass spectrometry and ion chromatography-electrospray ionization-mass spectrometry in thermally decomposed lithium hexafluorophosphate-based lithium ion battery electrolytes

In this work, the thermal decomposition of a lithium ion battery electrolyte (1 M LiPF6 in ethylene carbonate/ethyl methyl carbonate, 50/50 wt%) with a focus on the formation of organophosphates was systematically studied. The quantification of non-ionic dimethyl fluorophosphate and diethyl fluorophosphate was performed with synthesized standards by gas chromatography-mass spectrometry. Due to absence of commercially available or synthesized standards for the monitoring of ionic methyl fluorophosphate, ethyl fluorophosphate and ethylene phosphate a method working with ion chromatography-electrospray ionization-mass spectrometry was developed, where dibutyl phosphate was used as an internal standard. In addition, an ion chromatography conductivity detection method with short analysis time for simultaneous determination and quantification of F−, PF6− and BF4− was developed. The formation and degradation of analytes was studied to show the dependence of different temperatures, electrolyte volumes and separator materials. The thermal aging experiments were carried out in gas-tight aluminum vials at 80 °C for three weeks. After the storage time, the samples were diluted with the appropriate analysis solvents and investigated with gas chromatography-mass spectrometry, ion chromatography and ion chromatography-electrospray ionization-mass spectrometry. Finally, the thermal degradation of the electrolyte at 85 °C after five days in aluminum and glass vials was studied.