Analysis of electrolyte degradation products in cylindrical automotive lithium-ion cells during thermal aging

Analysis of electrolyte degradation products in cylindrical automotive lithium-ion cells during thermal aging | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Analysis of electrolyte degradation products in cylindrical automotive lithium-ion cells during thermal aging Sabrina Schönemeier, Verena Peters, Fabian Horsthemke, Frank Michael Matysik This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6339226/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 04 Aug, 2025 Read the published version in Monatshefte für Chemie - Chemical Monthly → Version 1 posted 4 You are reading this latest preprint version Abstract The impact of battery electric vehicles (BEVs) using lithium-ion cells (LICs) as energy storage systems is increasing and topics as cell performance and lifetime are becoming more important. The electrolyte composition can be a limiting factor for the lifetime of the cell and highly effects the performance of LICs. In this research the aging process of LIC electrolytes at increased temperatures (55°C) is investigated. First, the electrolyte is analyzed separately from the cell materials and afterwards the electrolyte is extracted from thermally aged LICs to investigate the aging effect on the electrolyte and the cell. By performing specific conductivity measurements of thermally aged electrolytes with different conductive salts and various concentrations, representative information for the entire electrolyte was obtained. High performance liquid chromatography-mass spectrometry measurements revealed that the selected conductive salt has a significant impact on the quantity of formed oligocarbonates. Furthermore, a correlation between the LiPF 6 concentration and the amount of formed oligocarbonates could be identified. Finally, large format cylindrical LICs containing LiPF 6 or LiFSI electrolytes were investigated for their electrolyte degradation products after the thermal aging process. Understanding the thermal aging process of different electrolyte compositions and transferring the knowledge to large format LICs is a key factor for the automotive industry aiming for the development of long-lasting, high-performance cells. Extraction Green Chemistry High pressure liquid chromatography Mass spectrometry Material Science Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction The transportation sector has a significant impact on the European CO 2 emissions, and its electrification is considered as the key factor to become the first climate neutral continent [ 1 ]. Lithium-ion cells (LICs) which are used for energy storage in electric vehicles play an essential role to reach these goals [ 2 ] [ 3 ]. Charging and discharging of the LIC leads towards degradation reactions in the electrolyte which are effecting the cell lifetime [ 4 ]. Electrolytes in commercial LICs consist of a conductive salt dissolved in a mixture of organic carbonates (Fig. 1 ) and electrolyte additives [ 5 ]. Lithium hexafluorophosphate (LiPF 6 ) is the most used conductive salt with high specific conductivity and high dissociation constant [ 6 ]. A drawback to this conductive salt is the formation of degradation reactions which occur at increased temperatures (60°C). The thermal instability of LiPF 6 leads towards the decomposition of the conductive salt to the highly reactive compound phosphorous pentaflouride (PF 5 ) which degrades to numerous further products [ 7 ], [ 8 ]. A second and often used lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI). LiFSI has some advantages compared to LiPF 6 ; it is more stable against moisture, has a higher conductivity resulting in a higher cell performance. LiFSI decomposes at higher temperatures which makes the electrolyte more resistant towards increased temperatures [ 9 ]. A variety of inorganic and organic degradation products have been reported in literature on electrolytes containing LiPF 6 resulting in a decrease of the cell performance [ 10 ]. Thermal aging of the conductive salt leads towards the formation of hydroflouric acid (HF) alkylflourids and organic phosphates [ 11 ]. Degradation reactions between the conductive salt and the solvent result in a variety of degradation products e.g. organofluorophosphates and oligocarbonates [ 12 ], [ 13 ]. LICs are subject to a variation of electrolyte degradation reactions effecting their cell lifetime and performance [ 14 ]. The electrolyte analysis of automotive cylindrical cells includes some challenges due to the low amount of excess electrolyte. For a quantitative electrolyte analysis, a representative sample is needed to be extracted from the cell materials. Therefore, the tightly wound cell materials are rewound in a specially designed extraction chamber, to increase the contact between the extraction solvent and the electrolyte [ 15 ]. The specific conductivity indicates changes in the electrolyte composition and is a measure for the entire electrolyte, which is afterwards supported with further analysis methods. Many electrolytes with a variety of conductive salts and solvents have been studied in literature regarding their specific conductivity [ 16 ]. Dudley et. al [ 17 ] investigated a variety of conductive salts and Landesfeind et. al . [ 18 ] studied the electrolytes at different temperatures using a two-electrode system. The analyses performed in literature have been conducted before the electrolyte was filled into a LIC. The effect of the electrolyte aging on the specific conductivity hasn’t been studied before. It is known that the specific conductivity is related to the concentrations of the lithium salts and their decomposition products, ion chromatography-mass spectrometry (IC-MS) and high performance liquid chromatography-mass spectrometry (HPLC-MS) are performed to investigate the formed degradation products [ 19 ], [ 20 ]. In this study fundamental investigations of how thermal aging changes the electrolyte composition is performed and what effect these degradation reactions have on the specific conductivity of the electrolyte. In addition, the changes of electrolyte components are studied by HPLC-MS to understand the effect of the conductive salt on the aging process. Finally, the degradation products of electrolytes in large format LICs during thermal aging are investigated. Results and Discussion 1. Specific conductivity during electrolyte storing at elevated temperatures Electrolytes in lithium-ion cells (LICs) in general consist of a conductive salt as lithium hexafluorophosphate (LiPF 6 ) or lithium bis(fluorosulfonyl)imid (LiFSI) dissolved in organic solvents. The amount of conductive salt highly effects the specific conductivity of the electrolyte and therefore the charging and discharging behavior of the cell [ 18 ]. The effect of increased temperatures on the specific conductivity of electrolytes with various conductive salt concentrations were investigated and are displayed in Fig. 2 . Therefore, the electrolytes were stored in pouch bags inside the drying oven at 55°C for up to 42 days. For verification of the conductivity results the samples were analyzed three times and the results, including the standard deviation are listed in tables S 1 – S 10 of the supplementary information. Electrolytes with the conductive salt LiPF 6 in EC/EMC (1:1) (Fig. 2 left) showed increasing specific conductivity up to a conductive salt concentration of 1.1 mol L − 1 with a maximum conductivity of 9.1 mS cm − 1 at 25°C and a decreasing conductivity for higher conductive salt concentrations, as reported in literature [ 6 ], [ 18 ]. At a conductive salt concentration of 1.1 M LiPF 6 the highest number of free ions is reached, resulting in the highest specific conductivity. By increasing the conductive salt concentration further ion pairs are formed, decreasing the ion mobility and therefore the specific conductivity of the electrolyte [ 21 ]. Storing the electrolyte at increased temperatures lead towards changes in the electrolyte composition, indicated by the reduction of the specific conductivity. In the investigated concentration range, it can be observed that low concentrations of conductive salt lead towards less conductivity loss during thermal aging. It can be concluded that the LiPF 6 concentration highly effects the thermal stability of the electrolyte during the aging at increased temperatures. Electrolytes based on LiFSI in EC/EMC (1:1) (Fig. 2 right) generally show higher conductivities compared to electrolytes containing LiPF 6 [ 17 ]. For electrolytes containing LiFSI the maximum specific conductivity of 9.7 mS cm − 1 is achieved at a concentration of 1.1 mol L − 1 [ 17 ]. Electrolytes containing 0.6 mol L − 1 LiFSI have a low conductivity decrease of less than 0.2 mS cm − 1 . Comparing this to conductive salt concentrations of 1.7 mol L − 1 a higher decrease in conductivity of 1.2 mS cm − 1 can be observed. By comparing the specific conductivities of the two conductive salts LiPF 6 and LiFSI similarities can be observed. Both conductive salts show a significant specific conductivity loss at high salt concentrations. Due to the generally higher conductivity of LiFSI the conductivity loss is less significant compared to LiPF 6 . The degradation process of LiPF 6 containing electrolytes starts at lower temperatures, compared to LiFSI [ 5 ]. The significant decrease in the specific conductivity indicates that degradation reactions occur within the LiPF 6 containing electrolytes, changing the composition of the electrolyte. The selected conductive salt highly effects the conductivity of the electrolyte and therefore the internal resistance of the cell. A maximum of specific conductivity leads towards a low internal resistance of the cell resulting in the optimized charging and discharging behavior for the cell. Correlations between the conductive salt concentration and the degradation process of the electrolyte at elevated temperatures could be observed. Therefore, the changes in the conductive salt concentrations are examined in more detail in the following chapter. 2. Effect of thermal aging on conductive salt concentration In the next step, the impact of the thermal aging process on the conductive salt concentration is investigated. To investigate changes in the conductive salts during the aging period of 42 days (Fig. 3 ) two replicates of each electrolyte were analyzed by means of IC-CD-MS. The conductive salt quantification of LiPF 6 electrolytes (Fig. 3 left) shows that the aging process at 55°C over the period of 42 days has no significant impact on the conductive salt concentration of the entire electrolyte. Degradation products such as difluorophosphate (PO 2 F 2 − ), dimethylphosphate (PO 4 C 2 H 6 − ) and methyl fluorophosphate (PO 3 FCH 3 − ) identified in previous studies could be detected in the aged electrolytes containing LiPF 6 [ 13 ], [ 20 ]. It can be concluded that the concentration range of the degradation products is at least an order of magnitude lower than the amount of conductive salt used in the electrolyte. The specific conductivity measurements (Fig. 2 ) clearly show a decrease in conductivity during the aging period, outlining the high impact of the degradation products on the electrolyte performance despite their low concentration. Quantification of LiFSI containing electrolytes (Fig. 3 right) displays, similar to LiPF 6 , no significant changes in the conductive salt during the investigated aging period. Furthermore, it can be observed that electrolytes with low and high conductive salt concentrations show similar aging behavior during the 42 days at 55°C. It can be concluded that the thermal aging of the electrolyte doesn’t significantly change the overall conductive salt concentration of the investigated electrolytes. Despite no visible decrease in the conductive salt concentration many ionic degradation products of LiPF 6 containing electrolytes could be observed in the thermally aged samples, which have been reported in literature [ 13 ], [ 20 ]. The analyzed degradation products occur in a different concentration range compared to the conductive salt. Even though only traces of the degradation products could be determined in the aged electrolytes the specific conductivity decreases during the aging process. 3. Effect of conductive salt on the formation of oligocarbonates In this chapter the impact of the selected conductive salt for the electrolyte and its effect on the formed degradation products is investigated. The specific conductivity measurements (Fig. 2 ) display that degradation reactions occur at increased conductive salt concentrations, indicating that degradation products are formed. Oligocarbonates represent one of the main electrolyte degradation products in commercially used LIC electrolytes and have been reported in previous studies [ 22 ]. Spotte-Smith et. al . [ 23 ] and Shi et. al. [ 24 ] proposed reaction mechanisms for linear and cyclic carbonates in lithium-ion cell electrolyte to degrade towards oligocarbonates (Fig. 4 ). In this reaction a nucleophilic species reacts with a linear carbonate to form an ethoxy anion (1). The ethoxy anion triggers the decomposition of the cyclic carbonates and by further reaction with a linear carbonate the oligocarbonates can be formed (2). The end groups of the linear carbonate determine the end groups of the oligocarbonate [ 22 ]. For the identification of various oligocarbonates high performance liquid chromatography-mass spectrometry (HPLC-MS) measurements of the electrolyte containing 1.5 M LiPF 6 or 1.5 M LiFSI were performed. Figure 5 shows the chemical structures of the investigated oligocarbonates and the correlation to the conductive salt. For most of the identified oligocarbonates no standards were commercially available. Therefore, instead of quantifying the oligocarbonates the identified analytes were compared by the detected area in the HPLC-MS. For validation of the presented results two identical sample preparations of the electrolyte were performed and analyzed. The HPLC-MS measurements show that electrolytes containing LiPF 6 have high concentrations of the oligocarbonates compared to electrolytes containing LiFSI (Fig. 5 ), indicating that LiPF 6 electrolytes catalyze the reaction of EC and EMC towards the oligocarbonates. The HPLC-MS measurements show that the choice of conductive salt has an effect on the degradation products formed at increased temperatures. The formation of oligocarbonates is triggered by a nucleophilic species reacting with the linear carbonates of the electrolyte, as shown in the mechanism in Fig. 4 . Degradation products of electrolytes containing LiPF 6 have been studied in literature and a frequently reported analyte is the difluorophosphoric anion [ 13 ]. The difluorophosphoric anion is formed during the decomposition of LiPF 6 in contact with water [ 11 ]. The formation of difluorophosphoric anion is analyzed by means of IC-CD-MS in the thermally aged electrolytes with conductive salt concentrations from 0.6 M – 1.8 M (Fig. 6 ). It can be observed that traces of the difluorophosphoric anion can be identified in electrolytes containing LiPF 6 after the thermal aging process. In the thermally aged LiFSI electrolytes the difluorophosphoric anion could not be identified. The IC-CD-MS analysis showed that electrolytes with higher LiPF 6 concentrations show larger amounts of the decomposition product after the aging process compared to lower salt concentrations. The formation of the difluorophosphoric anion and other anionic degradation products in electrolytes containing LiPF 6 could initiate the reaction mechanism towards the oligocarbonates (Fig. 4 ). Anionic degradation products after the decomposition of the conductive salt could be identified for LiPF 6 and not in LiFSI electrolytes. The formation of oligocarbonates due to anionic degradation products could explain the increased concentrations of oligocarbonates for electrolytes containing LiPF 6 . The formation of oligocarbonates could be confirmed by HPLC-MS and these analysis results underline the specific conductivity measurements. A significant difference in the oligocarbonates formation could be determined by variation of the conductive salt. LiPF 6 showed higher concentrations for all investigated oligocarbonates. The formation of the degradation product difluorophosphoric anion was confirmed in electrolytes containing LiPF 6 . Anionic decomposition products can induce the formation of oligocarbonates, resulting in increased oligocarbonates concentrations for LiPF 6 electrolytes. 4. Effect of conductive salt concentration on the formation of oligocarbonates The effect of the selected conductive salt on the formed oligocarbonates at elevated temperatures was shown. Electrolytes with LiPF 6 display an increased amount of oligocarbonates and electrolytes with a higher conductive salt concentration generally have a lower specific conductivity after the aging process at elevated temperatures. In this chapter the effect of the LiPF 6 concentration on the amount of formed oligocarbonates is investigated. Therefore, the electrolytes with a concentration variation of the conductive salt LiPF 6 from 0.6 M to 1.8 M were stored in the drying oven for 42 days and analyzed my means of HPLC-MS (Fig. 7 ). Figure 7 displays the formation of the oligocarbonate for several different conductive salt concentration. It can be observed that higher salt concentrations of LiPF 6 show increased amounts of oligocarbonates, underlining the specific conductivity results (Fig. 2 ). Furthermore, the HPLC-MS results in Fig. 7 align with the IC-CD-MS results of the previous chapter. For increasing conductive salt concentrations higher amounts of anionic degradation products can be identified after the aging at elevated temperatures. The anionic decomposition products can induce the reaction of the carbonate solvents to oligocarbonates, resulting in higher oligocarbonate concentrations at higher salt concentrations. The specific conductivity measurements show a decreasing conductivity and HPLC-MS results confirm an increasing amount of degradation products in the thermally aged electrolytes with high conductive salt concentrations. The HPLC-MS measurements (Fig. 7 ) of variating conductive salt concentrations confirm that the degradation products increase with higher conductive salt concentrations. These findings support the suggested mechanism by Spotter-Smith and Shi which suggests that the reaction is accelerated by high conductive salt concentrations [ 23 ], [ 24 ]. 5. Electrolyte degradation analysis of large format cylindrical LICs The performance and lifetime in large format LICs are highly important topics in the automotive industry. In the following chapter the electrolytes of automotive cells are analyzed and the effects of formed degradation products after the aging at 55°C are investigated. The cylindrical cells were filled with different electrolytes to investigate the effect of the conductive salt in thermal aging of the cell. Cells were filled with 1 M LiPF 6 in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1:1) or 1 M LiFSI in EC/EMC (1:1). The formation protocol was applied, and the cells were stored at 55°C for 42 days at 2.8 V. For authentication of the results the electrolyte was extracted from two identical large format LICs. A computer tomography (CT) image was conducted after the electrolyte was filled into the cell (Fig. 8 left) and the excess electrolyte was highlighted in blue by gray level analysis. A second CT image was taken after the formation and the aging process at elevated temperatures was performed (Fig. 8 right). The CT image shows no excess electrolyte in the cell, indicating that during the formation and aging process of the cell the electrolyte was soaked into the electrode materials. For the electrolyte analysis of the large format automotive LICs an electrolyte extraction from the cell materials is necessary. The cells were extracted using a previously developed method [ 15 ]. Here, the electrodes are extracted using dimethyl carbonate and rewinding them in a specially designed extraction chamber to increase the contact between the extraction solvent and the cell materials. After the electrolyte extraction the conductive salt concentration was analyzed by means of IC-CD-MS. The electrolyte was analyzed before filling it into the cell, after the formation process and after the aging period of 42 days (Fig. 9 ). For the electrolytes extracted from the formed and aged cells the recovery rate for the lithium concentration of the previous developed extraction method was included [ 15 ]. The quantification of the conductive salts LiPF 6 and LiFSI over the investigated aging period shows similar results for both salts. It can be observed that a conductive salt reduction occurs after the formation process which is mainly associated with the to the formation of the solid electrolyte interface building a protective layer on the anode material [ 5 ]. The measurements also show that no conductive salt reduction occurs over the aging period of 42 days at 55°C, confirming the results from Fig. 3 . In a next step the extracted electrolyte samples were analyzed by means of HPLC-MS. The effect of the cell formation and the thermal aging process of the cells on the formation of oligocarbonates in the electrolytes using LiPF 6 or LiFSI as the conductive salt (Fig. 10 ) was investigated. The HPLC-MS measurements show an increased concentration of oligocarbonates the electrolytes using LiPF 6 conductive salt compared to LiFSI. This confirms the results from the electrolyte storage experiments indicating that the oligocarbonates are formed intensively by electrolytes containing LiPF 6 . It can be observed that the oligocarbonate concentration of the extracted electrolytes is significantly higher after the formation of the cells for both conductive salts compared to the thermally aged electrolytes. Indicating that the formation of the oligocarbonates can be ignited chemically and electrochemically. Electrolytes containing the conductive salt LiPF 6 show a higher amount of chemically induced reactions towards the selected oligocarbonates compared to electrolytes containing only LiFSI. The amount of electrochemically induced reactions during the formation process of the cell towards the oligocarbonates are similar for both conductive salts. The results achieved in the electrolyte storage experiments could be confirmed on large format cylindrical LICs. Furthermore, it could be shown that the degradation from carbonates towards oligocarbonates can be chemically and electrochemically ignited. Understanding and improving electrolyte degradation reactions in LICs towards longer lifetime and higher performance can have a significant impact on the automotive industry. Conclusion and outlook In this research the thermal aging of electrolytes and its effect on changes in the composition were investigated. The impact of the conductive salt variation between LiPF 6 and LiFSI on the formed degradation products were studied. For the first time the effect of thermal aging on LIC electrolytes was investigated by means of a combination of specific conductivity measurements, and characterization of the formation of oligocarbonates and degradation products of lithium salts. It was found that thermal aging of electrolytes with higher conductive salt concentrations showed a significant decrease in specific conductivity. The conductive salt highly effects the formation of degradation products like oligocarbonates. Electrolytes containing LiPF 6 led to significantly higher concentrations of the formed oligocarbonates compared to electrolytes with LiFSI. Furthermore, by increasing the LiPF 6 concentration in the electrolyte the amount of formed oligocarbonates was increasing. Understanding the degradation reactions of LiPF 6 and LiFSI, as representatives of the most used conductive salts, paves the way to the development of new high performing electrolytes. The electrolyte effects the aging process of LICs and by improving the electrolyte performance regarding cell lifetime a significant impact on large format automotive cells can be achieved. Experimental Part 1. Chemicals The conductive salts, lithium hexafluorophosphate (LiPF 6 ), lithium bisfluorosulfonylimid (LiFSI) and the solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were obtained in battery grade. The standard solution potassium chloride (1413 µS cm − 1 ) from Hanna Instruments (Vöhringen, Germany) was used for conductivity measurements. Deionized water was produced by Millipore Milli-Q IQ7000 from Merck (Darmstadt, Germany). Deionized water and LC-MS grade acetonitrile from Merck (Darmstadt, Germany, 99.9%) were used with formic acid (Avantor, Radnor, USA, 99.0%) for HPLC-MS measurements. The chemicals ethylene carbonate (99.0%), ethyl methyl carbonate (98.0%), diethyl carbonate (99.0%) and dimethyl carbonate (99.0%) were obtained from Thermo Scientific (Dreieich, Germany) for HPLC-MS standard solutions. Samples were diluted in anhydrous acetonitrile (Merck, Darmstadt, Germany, 99.0%). The following chemicals were obtained from Merck (Darmstadt, Germany) for IC-CD-MS measurements: Sodium carbonate (99.9%), LC grade acetone (99.0%), sulforic acid (97.0%) and nitric acid (65.0%). 2. Electrolyte mixing and aging at elevated temperatures The electrolytes were mixed in an argon filled glovebox (H 2 O < 1 ppm; O 2 < 1 ppm). The produced concentrations can be identified in the supplementary information table S 11 – S 12. Ion chromatography-mass spectrometry was performed to verify the conductive salt concentration of the produced electrolytes. Inside the dry room with a dew point lower then − 60°C 4 mL of the sample was transferred into pouch bags (10 cm × 2 cm) for LICs from Showa Denko with an inner polypropylene foil and vacuum sealed. The electrolytes were stored in a Thermo Scientific HeraTherm oven at 55°C for 7, 14, 28 and 42 days. 3. Cell assembly In this study large format cylindrical LICs with a diameter of 46 mm and height of 95 mm were produced in the pilot plant at the BMW Group Munich, Germany. The cell is comprised of a nickel manganese cobalt (NMC) cathode material in a 8:1:1 ratio and a silicon-graphite anode material. The electrodes have a specific mass loading, defining the active material amount on the electrodes. The cathode has a specific mass loading of m NMC = 195 ± 5 g m − 2 and the anode of m Si−Gr = 100 ± 5 g m − 2 . The principle of the electrolyte filling process for cylindrical automotive cells was recently published [ 15 ]. The LICs were filled with 42 ± 5 g 1 M LiPF 6 or 1 M LiFSI in EC/EMC (1:1). 4. Cell formation and aging at elevated temperatures The formation was performed in a climate chamber (Li plus GmbH) at 25°C using a Maccor series 4000. Starting with the PreCharge the cylindrical automotive cells were charged with constant current (CC) and 0.1 C (battery capacity) to 3.3 V. After the PreCharge the cells were sealed in the dry room. During the formation the cells were charged to 4.2 V using constant current-constant voltage (CC-CV) with 0.1 C. The cell was hold at 4.2 V with constant voltage for 1 h and discharged with 0.1 C to 2.8 V. The cells were stored at 30% state of charge (2.8 V) in the climate chambers for 42 days at 55°C. 5. Cell disassembly and electrolyte extraction The automotive cell was disassembled in an argon filled glovebox. The cell can and isolation tapes were removed. The cell material was disconnected from the discs and the current collectors were removed. The electrolyte extraction of the entire cylindrical automotive cell was performed in the glovebox. The principle of the developed electrolyte extraction method was described previously [ 15 ]. For the extraction the jelly roll was transferred into the designed extraction chamber and completely rewound after the addition of the extraction solvent DMC. A homogeneous mixture of the electrolyte in the cell materials and the added solvent is achieved by an extraction period of 48 h while shaking at 10 osc min − 1 . An aliquot of the electrolyte extract was taken for analysis. 6. Conductivity measurements The conductivity studies were performed using an Autolab AUTM101.S from Metrohm (Filderstadt, Germany) controlled by NOVA 2.1.6. A TSC microcell 1600 closed GC (rhd instruments, Darmstadt, Germany) with a platinum beaker electrode and glassy carbon electrode was used for the conductivity measurements. The microcell was connected to the temperature controlled microcell HC basic package from rhd instruments (Darmstadt, Germany). Impedance measurements were performed outside of the glovebox in a frequency range of 200 kHz to 0.1 kHz with an amplitude of 10 mV. For the determination of the cell constant a potassium chloride standard solution with a conductivity of 1413 µS cm − 1 from Hanna Instruments (Vöhringen, Germany) was used at 25°C resulting in a cell constant of 1.27 ± 0.03 cm − 1 . Afterwards the microcell was cleaned using a water isopropanol mixture and dried in the vacuum chamber at 60°C. The microcell was transferred into a glovebox and injected with 900 µL of electrolyte sample and sealed airtight. Electrolytes with a different conductive salt concentration were analyzed at 25°C. The specific conductivity is measured three times for each electrolyte sample. The results, including the standard deviation are listed in table S 1 – S 10 in the supplementary information. 7. High performance liquid chromatography – mass spectrometry (HPLC-MS) The samples were separated by means of a Vanquish™ HPLC (Thermo Scientific, Dreieich, Germany) consisting of a Vanquish™ binary pump, an Vanquish™ autosampler, a Vanquish™ column compartment with a Vaquish™ diode array detector. The HPLC was connected to a Orbitrap Exploris 120™ (Thermo Scientific, Dreieich, Germany) using a heated electrospray ionization (H-ESI) at 2.0 kV for the ionizsation. Samples were analyzed in positive and negative mode in a mass range from m/z 40–1000. Data evaluation was performed with Trace Finder™ 5.1 General Quan (Thermo Scientific, Dreieich, Germany) and Compound Discoverer™ 3.3 SP2 (Thermo Scientific, Dreieich, Germany). Separation of the samples was performed by a pentafluorophenyl column (150 × 2.1 mm) with a pore diameter of 2.7 µm from Raptor™ (Restek, Bellefonte, USA) at 45°C. A gradient system of formic acid (0.02%) and acetonitrile (2%) in water (solvent A) and formic acid (0.02%) and water (5%) in acetonitrile (solvent B) was used at a flow rate of 0.4 mL min − 1 . The gradient system started with solvent A (100%) for 1 min. From 1–10 min solvent B was increased to 100% and hold for 1 min. Afterwards, solvent B was decreased to 0% and hold at solvent A (100%) for 4 min. Electrolyte samples were diluted 1:50 and extracted electrolyte samples were diluted 1:5 in anhydrous ACN inside the glovebox and sealed airtight. An aliquot of 1 µL was directly injected into the HPLC-MS system and a double determination was performed. 8. Ion chromatography – conductivity detection - mass spectrometry (IC-CD-MS) IC-CD-MS data was acquired using a Professional IC AnCat MSM-HC MCS by Metrohm (Filderstadt, Germany) connected to a 6130 single quadrupole MS from Agilent Technologies (Santa Clara, USA). The anionic species were analyzed in the mass spectrometer. Negative electrospray ionization mode (ESI) with 3 kV was performed in a mass range of m/z 50–300. The cationic species were separated using a Metrosep C4 100/4.0 column connected to a Metrosep C4 Guard 4.0 column. Separation of the anionic species was achieved on a Metrosep A Supp 5-250/4.0 column coupled to a Metrosep A Supp Guard 4.0 column. Cationic eluation was achieved by 2 mM HNO 3 in water with a flow rate of 0.9 mL min − 1 . Anionic eluation was performed using a 10 mM NaHCO 3 and acetone (30%) in water with a flow rate of 0.7 mL min − 1 . Electrolyte solutions of LiPF 6 (2 mol L − 1 ) and LiFSI (2 mol L − 1 ) in DMC were prepared and diluted in water. Quantification was performed in the range of 0.1 to 2.0 mol L − 1 using an external six-point calibration. Samples were diluted 1:1000 in water and an aliquot of 10 µL (cation) and 20 µL (anion) was injected into the IC-CD-MS. 9. Computer tomography (CT) CT scans were conducted on a FF35 CT X-ray system using a FXT 225.48 fine focus directional tube and a 4343CT digital flat image detector with CsI scintillator. A current of 200 µA and a voltage of 220 kV with a performance of 44 W was used to scan the automotive LICs. Data visualization was performed using VGStudio Max 3.5 by Volume Graphics and electrolyte illustration was conducted by gray-level analysis. Declarations Declaration of competing interest: The authors declare that they have no known competing interest or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements: The authors would like to thank Simon Krebs and Cornelius Hupbauer (BMW Group Munich) for conducting the CT measurements and the grey level analysis. Furthermore, the authors thank Oleksandr Larionov and Christian Waldleitner (BMW Group Munich) for the performance of the formation for large format cylindrical LICs. Funding: No founding was received for conducting this study. Data availability: Data will be made available on request. References Davenport J, Wayth N (2004) Statistical Review of World Energy 73rd Edition. Energy institute Yuan M, Thellufsen JZ, Lund H, Liang Y (2021) The electrification of transportation in energy transition. Energy. http://doi.org/10.1016/j.energy.2021.121564 Rottoli M, Dirnaichner A, Pietzcker R, Schreyer F, Luderer G (2021) Alternative electrification pathways for light-duty vehicles in the European transport sector. 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J Electrochem Soc. http://doi.org/10.1149/2.0121514jes Solchenbach S, Tacconis C, Gomez Martin A, Peters V, Wallisch L, Stanke A, Hofer J, Renz D, Lewerich B, Bauer G, Wichmann M, Goldbach D, Adam A, Spielbauer M, Lamp P, Wandt J (2024) Electrolyte Motion Induced Salt Inhomogeneity – A Novel Aging Mechanism in Large-Format Lithium-Ion Cells. Energy Environ Sci. https://doi.org/10.1039/D4EE03211J Schönemeier S, Peters V, Horsthemke F, Seo H, Matysik FM (2025) Challenges in extracting and characterizing electrolytes from automotive lithium-ion cells. Anal Chim Acta. https://doi.org/10.1016/j.aca.2024.343530 Ue M, Mori S (1995) Mobility and Ionic Association of Lithium Salts in a propylene Carbonate-Ethyl Methyl Carbonate Mixed Solvent. J Electrochem Soc. https://doi.org/10.1149/1.2050056 Dudley JT (1991) Conductivity of electrolytes for rechargeable lithium batteries. J Power Sources. https://doi.org/10.1016/0378-7753(91)80004-H Landesfeind J, Gasteiger HA (2019) Temperature and Concentration Dependance of the Ionic Transport Properties of Lithium-ion Battery Electrolytes. https://doi.org/10.1149/2.0571912jes Kraft V (2015) Two-dimensional ion chromatography for the separation of ionic organophosphates generated in thermally decomposed lithium hexafluorophosphate-based lithium ion battery electrolytes. Chromatogr A. https://doi.org/10.1016/j.chroma.2015.07.054 Stenzel Y, Horsthemke F, Winter M, Nowak S (2019) Chromatographic Techniques in the Research Area of Lithium Ion Batteries: Current State-of-the-Art. Separations. https://doi.org/10.3390/separations6020026 Xu K (2023) Electrolytes, Interfaces and Interphases – Fundamentals and Applications in Batteries. Royal Society of Chemistry, Croydon Schultz C, Vedder S, Streipert B, Winter M, Nowak S (2017) Quantitative investigation of the decomposition of organic lithium ion battery electrolytes with LC-MS/MS. RSC Adv. https://doi.org/10.1039/c7ra03839a Spotte-Smith EWC, Kam RL, Barter D, Xie X, Hou T, Dwaraknath S, Blau SM, Persson KA (2022) Toward a Mechanistic Model of Solid Electrolyte Interphase Formation and Evolution in Lithium-Ion Batteries. ACS Energy Lett. https://doi.org/10.1021/acsenergylett.2c00517 Shi F, Ross P, Somorjai G, Komvopoulos K (2017) The Chemistry of electrolyte reduction on silicon electrodes revealed by in-Situ. ATR-FTIR spectroscopy Supplementary Files ElectrolytedegradationSupplementaryinformation.docx Cite Share Download PDF Status: Published Journal Publication published 04 Aug, 2025 Read the published version in Monatshefte für Chemie - Chemical Monthly → Version 1 posted Reviewers agreed at journal 08 May, 2025 Reviewers invited by journal 01 Apr, 2025 Editor assigned by journal 01 Apr, 2025 First submitted to journal 30 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6339226","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":436981856,"identity":"294257d8-c610-4c2d-91cc-14bbf1fcb0fe","order_by":0,"name":"Sabrina Schönemeier","email":"","orcid":"","institution":"BMW Group, Battery Cell Competence Center","correspondingAuthor":false,"prefix":"","firstName":"Sabrina","middleName":"","lastName":"Schönemeier","suffix":""},{"id":436981857,"identity":"5ccfa8d4-b7e3-4ac6-926f-26d8806e25ed","order_by":1,"name":"Verena Peters","email":"","orcid":"","institution":"BMW Group, Battery Cell Competence Center","correspondingAuthor":false,"prefix":"","firstName":"Verena","middleName":"","lastName":"Peters","suffix":""},{"id":436981858,"identity":"dac6ad0a-7a52-4794-b51e-c379f8a723ad","order_by":2,"name":"Fabian Horsthemke","email":"","orcid":"","institution":"BMW Group, Battery Cell Competence Center","correspondingAuthor":false,"prefix":"","firstName":"Fabian","middleName":"","lastName":"Horsthemke","suffix":""},{"id":436981859,"identity":"61ab0351-a43e-448e-9120-aa8f23045e7d","order_by":3,"name":"Frank Michael Matysik","email":"data:image/png;base64,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","orcid":"https://orcid.org/0000-0001-7029-1382","institution":"Universitat Regensburg","correspondingAuthor":true,"prefix":"","firstName":"Frank","middleName":"Michael","lastName":"Matysik","suffix":""}],"badges":[],"createdAt":"2025-03-30 15:16:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6339226/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6339226/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00706-025-03358-w","type":"published","date":"2025-08-04T15:57:15+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":81189293,"identity":"d37742d3-5c9c-48bc-9666-a391d76dc68d","added_by":"auto","created_at":"2025-04-23 08:57:04","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":30727,"visible":true,"origin":"","legend":"\u003cp\u003eChemical structures of the electrolyte components. The two conductive salts, lithium hexafluorophosphate (LiPF\u003csub\u003e6\u003c/sub\u003e), lithium bis(fluorosulfonyl)imide (LiFSI), and the carbonate solvents ethyl methyl carbonate (EMC) and ethylene carbonate (EC).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/1eb02681481fa49737a3d4f2.png"},{"id":81189294,"identity":"627532a3-b1ca-47aa-a855-1e2622c8768b","added_by":"auto","created_at":"2025-04-23 08:57:04","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":122215,"visible":true,"origin":"","legend":"\u003cp\u003eSpecific conductivities of the electrolytes for different concentrations after storage at 55°C for 0\u0026nbsp;days, 7\u0026nbsp;days, 14\u0026nbsp;days, 28\u0026nbsp;days and 42\u0026nbsp;days in sealed pouch bags. Specific conductivity measurements were performed at 25\u0026nbsp;°C in a microcell with a platinum beaker and a glassy carbon electrode. \u003cstrong\u003eLeft:\u003c/strong\u003e Electrolytes containing the conductive salt LiPF\u003csub\u003e6\u003c/sub\u003e in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1:1). \u003cstrong\u003eRight:\u003c/strong\u003e Electrolytes containing the conductive salt LiFSI in EC and EMC (1:1).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/ad161306d0e198ef3debd84f.png"},{"id":81189296,"identity":"fa3e2427-a218-46df-a946-49260dabe1f6","added_by":"auto","created_at":"2025-04-23 08:57:04","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":103405,"visible":true,"origin":"","legend":"\u003cp\u003eIC-CD-MS electrolyte analysis for different conductive salt concentrations after storage at 55°C for 0\u0026nbsp;days, 7\u0026nbsp;days, 14\u0026nbsp;days, 28\u0026nbsp;days and 42\u0026nbsp;days in sealed pouch bags. \u003cstrong\u003eLeft: \u003c/strong\u003eElectrolytes containing the conductive salt LiPF\u003csub\u003e6\u003c/sub\u003e in EC/EMC.\u003cstrong\u003e Right:\u003c/strong\u003e Electrolytes containing the conductive salt LiFSI in EC/EMC.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/bc2642672fd2ed6b1f9d8a67.png"},{"id":81189298,"identity":"00077f25-93c8-4d44-b4c0-749699c49e4a","added_by":"auto","created_at":"2025-04-23 08:57:04","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":24648,"visible":true,"origin":"","legend":"\u003cp\u003ePossible chemical mechanism of the oligocarbonate formation suggested by Spotte-Smith and Shi [23], [24]. During the thermal and electrochemical aging of the electrolyte the solvent ethylene carbonate (EC) and ethyl methyl carbonate (EMC) can react to an oligocarbonates.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/0fac6cc4196038c3e74334ee.png"},{"id":81189304,"identity":"7c110312-ada4-4121-bb72-ebf0adc0bbda","added_by":"auto","created_at":"2025-04-23 08:57:04","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":156146,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eleft: \u003c/strong\u003eHPLC-MS results of aged electrolyte at 55\u0026nbsp;°C for 42 days stored in pouch bags. The electrolytes contain 1.5\u0026nbsp;M LiPF\u003csub\u003e6\u003c/sub\u003e in EC/EMC (1:1) or 1.5\u0026nbsp;M LiFSI in EC/EMC (1:1). \u003cstrong\u003eRight: \u003c/strong\u003echemical structures of the identified oligocarbonates (from top to bottom) dimethyl-2,5-dioxahexane dicarboxylate (m/z\u0026nbsp;=\u0026nbsp;178.048), ethyl methyl-2,5-dioxahexane dicarboxylate (m/z\u0026nbsp;=\u0026nbsp;192.063), diethyl-2,5-dioxahexane dicarboxylate (m/z\u0026nbsp;=\u0026nbsp;206.079), oligocarbonate 1 (m/z\u0026nbsp;=\u0026nbsp;266.064), oligocarbonate 2 (m/z\u0026nbsp;=\u0026nbsp;280.079) and oligocarbonate 3 (m/z\u0026nbsp;=\u0026nbsp;294.095).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/4f84158ca4d8ea65308ba9c0.png"},{"id":81190894,"identity":"ee39a535-babe-4efc-a4b7-37b20c2979c6","added_by":"auto","created_at":"2025-04-23 09:13:04","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":103055,"visible":true,"origin":"","legend":"\u003cp\u003eIC-CD-MS studies of the formed difluorophosphoric anion m/z\u0026nbsp;=\u0026nbsp;101.07 after the electrolyte was stored at 55\u0026nbsp;°C for 42\u0026nbsp;days in sealed pouch bags in the drying oven. The electrolytes contained different conductive salt concentrations in the range of 0.6\u0026nbsp;M to 1.8\u0026nbsp;M LiPF\u003csub\u003e6\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/5e2f5e2d56a2c59053bafd50.png"},{"id":81189610,"identity":"f80edf4a-59b0-4cd5-ae64-6baf00d01939","added_by":"auto","created_at":"2025-04-23 09:05:04","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":102569,"visible":true,"origin":"","legend":"\u003cp\u003eHPLC-MS studies of the formed oligocarbonates m/z = 280.079 after the electrolyte was stored at 55\u0026nbsp;°C for 42\u0026nbsp;days in sealed pouch bags in the drying oven. The electrolytes contained different conductive salt concentrations in the range of 0.6\u0026nbsp;M to 1.8\u0026nbsp;M LiPF\u003csub\u003e6\u003c/sub\u003e.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/0152b88905ec828828455fcb.png"},{"id":81189608,"identity":"249c65eb-f8a9-4b1a-af07-9e11aa50e6c7","added_by":"auto","created_at":"2025-04-23 09:05:04","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":102932,"visible":true,"origin":"","legend":"\u003cp\u003eComputer tomography (CT) images of large format cylindrical automotive cells. The excess electrolyte, which is not soaked into the electrodes is highlighted in light blue by gray-level analysis. \u003cstrong\u003eLeft:\u003c/strong\u003eCT image after the electrolyte is filled into the cell.\u003cstrong\u003e Right:\u003c/strong\u003e CT image after the formation of the cell and the aging at 55 °C for 42 days.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/c8ac16cc629e7e7ca1ab89b4.png"},{"id":81189606,"identity":"998a23f2-d056-449c-965e-02a6ad8aef05","added_by":"auto","created_at":"2025-04-23 09:05:04","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":55952,"visible":true,"origin":"","legend":"\u003cp\u003eIC-CD-MS measurements of the original electrolyte before the cell filling process and the extracted electrolyte form the cell after the formation and after the aging process. By extracting the electrolyte from the cylindrical cell, the recovery rate of the extraction method (shaded area) is included [15].\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/e84151e7ef4f0091bb76a7f9.png"},{"id":81189309,"identity":"a3ca1b42-bb82-45d4-94d1-21528ee57705","added_by":"auto","created_at":"2025-04-23 08:57:04","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":139188,"visible":true,"origin":"","legend":"\u003cp\u003eHPLC-MS studies of the formed oligocarbonate m/z = 280.079 for electrolytes containing the conductive salt LiPF\u003csub\u003e6\u003c/sub\u003e or LiFSI. The electrolyte was analyzed after storage at 55 °C and extracted from large format cylindrical LICs after the formation and after the cell was aged for 42 days at 55\u0026nbsp;°C.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/88d7870e8889e77cabc80410.png"},{"id":88814113,"identity":"445b247a-9b14-409c-9372-ad18ffaf9994","added_by":"auto","created_at":"2025-08-11 16:07:02","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1645235,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/bdd15fa2-5707-4d5c-a8f6-c54000500ca5.pdf"},{"id":81189300,"identity":"f2ddb461-d5f6-4014-987f-c82fd9de45c7","added_by":"auto","created_at":"2025-04-23 08:57:04","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":61302,"visible":true,"origin":"","legend":"","description":"","filename":"ElectrolytedegradationSupplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6339226/v1/328ddde2d51a51a1062fde81.docx"}],"financialInterests":"","formattedTitle":"Analysis of electrolyte degradation products in cylindrical automotive lithium-ion cells during thermal aging","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe transportation sector has a significant impact on the European CO\u003csub\u003e2\u003c/sub\u003e emissions, and its electrification is considered as the key factor to become the first climate neutral continent [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Lithium-ion cells (LICs) which are used for energy storage in electric vehicles play an essential role to reach these goals [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Charging and discharging of the LIC leads towards degradation reactions in the electrolyte which are effecting the cell lifetime [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eElectrolytes in commercial LICs consist of a conductive salt dissolved in a mixture of organic carbonates (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and electrolyte additives [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Lithium hexafluorophosphate (LiPF\u003csub\u003e6\u003c/sub\u003e) is the most used conductive salt with high specific conductivity and high dissociation constant [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. A drawback to this conductive salt is the formation of degradation reactions which occur at increased temperatures (60\u0026deg;C). The thermal instability of LiPF\u003csub\u003e6\u003c/sub\u003e leads towards the decomposition of the conductive salt to the highly reactive compound phosphorous pentaflouride (PF\u003csub\u003e5\u003c/sub\u003e) which degrades to numerous further products [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. A second and often used lithium salt is lithium bis(fluorosulfonyl)imide (LiFSI). LiFSI has some advantages compared to LiPF\u003csub\u003e6\u003c/sub\u003e; it is more stable against moisture, has a higher conductivity resulting in a higher cell performance. LiFSI decomposes at higher temperatures which makes the electrolyte more resistant towards increased temperatures [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. A variety of inorganic and organic degradation products have been reported in literature on electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e resulting in a decrease of the cell performance [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Thermal aging of the conductive salt leads towards the formation of hydroflouric acid (HF) alkylflourids and organic phosphates [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Degradation reactions between the conductive salt and the solvent result in a variety of degradation products e.g. organofluorophosphates and oligocarbonates [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eLICs are subject to a variation of electrolyte degradation reactions effecting their cell lifetime and performance [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The electrolyte analysis of automotive cylindrical cells includes some challenges due to the low amount of excess electrolyte. For a quantitative electrolyte analysis, a representative sample is needed to be extracted from the cell materials. Therefore, the tightly wound cell materials are rewound in a specially designed extraction chamber, to increase the contact between the extraction solvent and the electrolyte [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe specific conductivity indicates changes in the electrolyte composition and is a measure for the entire electrolyte, which is afterwards supported with further analysis methods. Many electrolytes with a variety of conductive salts and solvents have been studied in literature regarding their specific conductivity [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Dudley \u003cem\u003eet. al\u003c/em\u003e [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e] investigated a variety of conductive salts and Landesfeind \u003cem\u003eet. al\u003c/em\u003e. [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] studied the electrolytes at different temperatures using a two-electrode system. The analyses performed in literature have been conducted before the electrolyte was filled into a LIC. The effect of the electrolyte aging on the specific conductivity hasn\u0026rsquo;t been studied before. It is known that the specific conductivity is related to the concentrations of the lithium salts and their decomposition products, ion chromatography-mass spectrometry (IC-MS) and high performance liquid chromatography-mass spectrometry (HPLC-MS) are performed to investigate the formed degradation products [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e], [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this study fundamental investigations of how thermal aging changes the electrolyte composition is performed and what effect these degradation reactions have on the specific conductivity of the electrolyte. In addition, the changes of electrolyte components are studied by HPLC-MS to understand the effect of the conductive salt on the aging process. Finally, the degradation products of electrolytes in large format LICs during thermal aging are investigated.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e1. Specific conductivity during electrolyte storing at elevated temperatures\u003c/h2\u003e \u003cp\u003eElectrolytes in lithium-ion cells (LICs) in general consist of a conductive salt as lithium hexafluorophosphate (LiPF\u003csub\u003e6\u003c/sub\u003e) or lithium bis(fluorosulfonyl)imid (LiFSI) dissolved in organic solvents. The amount of conductive salt highly effects the specific conductivity of the electrolyte and therefore the charging and discharging behavior of the cell [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The effect of increased temperatures on the specific conductivity of electrolytes with various conductive salt concentrations were investigated and are displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Therefore, the electrolytes were stored in pouch bags inside the drying oven at 55\u0026deg;C for up to 42 days. For verification of the conductivity results the samples were analyzed three times and the results, including the standard deviation are listed in tables S 1 \u0026ndash; S 10 of the supplementary information.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eElectrolytes with the conductive salt LiPF\u003csub\u003e6\u003c/sub\u003e in EC/EMC (1:1) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e left) showed increasing specific conductivity up to a conductive salt concentration of 1.1 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with a maximum conductivity of 9.1 mS cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 25\u0026deg;C and a decreasing conductivity for higher conductive salt concentrations, as reported in literature [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. At a conductive salt concentration of 1.1 M LiPF\u003csub\u003e6\u003c/sub\u003e the highest number of free ions is reached, resulting in the highest specific conductivity. By increasing the conductive salt concentration further ion pairs are formed, decreasing the ion mobility and therefore the specific conductivity of the electrolyte [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Storing the electrolyte at increased temperatures lead towards changes in the electrolyte composition, indicated by the reduction of the specific conductivity. In the investigated concentration range, it can be observed that low concentrations of conductive salt lead towards less conductivity loss during thermal aging. It can be concluded that the LiPF\u003csub\u003e6\u003c/sub\u003e concentration highly effects the thermal stability of the electrolyte during the aging at increased temperatures.\u003c/p\u003e \u003cp\u003eElectrolytes based on LiFSI in EC/EMC (1:1) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e right) generally show higher conductivities compared to electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. For electrolytes containing LiFSI the maximum specific conductivity of 9.7 mS cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is achieved at a concentration of 1.1 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Electrolytes containing 0.6 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e LiFSI have a low conductivity decrease of less than 0.2 mS cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Comparing this to conductive salt concentrations of 1.7 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e a higher decrease in conductivity of 1.2 mS cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e can be observed.\u003c/p\u003e \u003cp\u003eBy comparing the specific conductivities of the two conductive salts LiPF\u003csub\u003e6\u003c/sub\u003e and LiFSI similarities can be observed. Both conductive salts show a significant specific conductivity loss at high salt concentrations. Due to the generally higher conductivity of LiFSI the conductivity loss is less significant compared to LiPF\u003csub\u003e6\u003c/sub\u003e. The degradation process of LiPF\u003csub\u003e6\u003c/sub\u003e containing electrolytes starts at lower temperatures, compared to LiFSI [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The significant decrease in the specific conductivity indicates that degradation reactions occur within the LiPF\u003csub\u003e6\u003c/sub\u003e containing electrolytes, changing the composition of the electrolyte.\u003c/p\u003e \u003cp\u003eThe selected conductive salt highly effects the conductivity of the electrolyte and therefore the internal resistance of the cell. A maximum of specific conductivity leads towards a low internal resistance of the cell resulting in the optimized charging and discharging behavior for the cell. Correlations between the conductive salt concentration and the degradation process of the electrolyte at elevated temperatures could be observed. Therefore, the changes in the conductive salt concentrations are examined in more detail in the following chapter.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003e2. Effect of thermal aging on conductive salt concentration\u003c/h3\u003e\n\u003cp\u003eIn the next step, the impact of the thermal aging process on the conductive salt concentration is investigated. To investigate changes in the conductive salts during the aging period of 42 days (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) two replicates of each electrolyte were analyzed by means of IC-CD-MS.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe conductive salt quantification of LiPF\u003csub\u003e6\u003c/sub\u003e electrolytes (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e left) shows that the aging process at 55\u0026deg;C over the period of 42 days has no significant impact on the conductive salt concentration of the entire electrolyte. Degradation products such as difluorophosphate (PO\u003csub\u003e2\u003c/sub\u003eF\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e), dimethylphosphate (PO\u003csub\u003e4\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) and methyl fluorophosphate (PO\u003csub\u003e3\u003c/sub\u003eFCH\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) identified in previous studies could be detected in the aged electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. It can be concluded that the concentration range of the degradation products is at least an order of magnitude lower than the amount of conductive salt used in the electrolyte. The specific conductivity measurements (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) clearly show a decrease in conductivity during the aging period, outlining the high impact of the degradation products on the electrolyte performance despite their low concentration.\u003c/p\u003e \u003cp\u003eQuantification of LiFSI containing electrolytes (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e right) displays, similar to LiPF\u003csub\u003e6\u003c/sub\u003e, no significant changes in the conductive salt during the investigated aging period. Furthermore, it can be observed that electrolytes with low and high conductive salt concentrations show similar aging behavior during the 42 days at 55\u0026deg;C.\u003c/p\u003e \u003cp\u003eIt can be concluded that the thermal aging of the electrolyte doesn\u0026rsquo;t significantly change the overall conductive salt concentration of the investigated electrolytes. Despite no visible decrease in the conductive salt concentration many ionic degradation products of LiPF\u003csub\u003e6\u003c/sub\u003e containing electrolytes could be observed in the thermally aged samples, which have been reported in literature [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The analyzed degradation products occur in a different concentration range compared to the conductive salt. Even though only traces of the degradation products could be determined in the aged electrolytes the specific conductivity decreases during the aging process.\u003c/p\u003e\n\u003ch3\u003e3. Effect of conductive salt on the formation of oligocarbonates\u003c/h3\u003e\n\u003cp\u003eIn this chapter the impact of the selected conductive salt for the electrolyte and its effect on the formed degradation products is investigated. The specific conductivity measurements (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) display that degradation reactions occur at increased conductive salt concentrations, indicating that degradation products are formed.\u003c/p\u003e \u003cp\u003eOligocarbonates represent one of the main electrolyte degradation products in commercially used LIC electrolytes and have been reported in previous studies [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. Spotte-Smith \u003cem\u003eet. al\u003c/em\u003e. [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e] and Shi \u003cem\u003eet. al.\u003c/em\u003e [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] proposed reaction mechanisms for linear and cyclic carbonates in lithium-ion cell electrolyte to degrade towards oligocarbonates (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn this reaction a nucleophilic species reacts with a linear carbonate to form an ethoxy anion (1). The ethoxy anion triggers the decomposition of the cyclic carbonates and by further reaction with a linear carbonate the oligocarbonates can be formed (2). The end groups of the linear carbonate determine the end groups of the oligocarbonate [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFor the identification of various oligocarbonates high performance liquid chromatography-mass spectrometry (HPLC-MS) measurements of the electrolyte containing 1.5 M LiPF\u003csub\u003e6\u003c/sub\u003e or 1.5 M LiFSI were performed. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the chemical structures of the investigated oligocarbonates and the correlation to the conductive salt. For most of the identified oligocarbonates no standards were commercially available. Therefore, instead of quantifying the oligocarbonates the identified analytes were compared by the detected area in the HPLC-MS. For validation of the presented results two identical sample preparations of the electrolyte were performed and analyzed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe HPLC-MS measurements show that electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e have high concentrations of the oligocarbonates compared to electrolytes containing LiFSI (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e), indicating that LiPF\u003csub\u003e6\u003c/sub\u003e electrolytes catalyze the reaction of EC and EMC towards the oligocarbonates. The HPLC-MS measurements show that the choice of conductive salt has an effect on the degradation products formed at increased temperatures.\u003c/p\u003e \u003cp\u003eThe formation of oligocarbonates is triggered by a nucleophilic species reacting with the linear carbonates of the electrolyte, as shown in the mechanism in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. Degradation products of electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e have been studied in literature and a frequently reported analyte is the difluorophosphoric anion [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The difluorophosphoric anion is formed during the decomposition of LiPF\u003csub\u003e6\u003c/sub\u003e in contact with water [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The formation of difluorophosphoric anion is analyzed by means of IC-CD-MS in the thermally aged electrolytes with conductive salt concentrations from 0.6 M \u0026ndash; 1.8 M (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt can be observed that traces of the difluorophosphoric anion can be identified in electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e after the thermal aging process. In the thermally aged LiFSI electrolytes the difluorophosphoric anion could not be identified. The IC-CD-MS analysis showed that electrolytes with higher LiPF\u003csub\u003e6\u003c/sub\u003e concentrations show larger amounts of the decomposition product after the aging process compared to lower salt concentrations. The formation of the difluorophosphoric anion and other anionic degradation products in electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e could initiate the reaction mechanism towards the oligocarbonates (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Anionic degradation products after the decomposition of the conductive salt could be identified for LiPF\u003csub\u003e6\u003c/sub\u003e and not in LiFSI electrolytes. The formation of oligocarbonates due to anionic degradation products could explain the increased concentrations of oligocarbonates for electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eThe formation of oligocarbonates could be confirmed by HPLC-MS and these analysis results underline the specific conductivity measurements. A significant difference in the oligocarbonates formation could be determined by variation of the conductive salt. LiPF\u003csub\u003e6\u003c/sub\u003e showed higher concentrations for all investigated oligocarbonates. The formation of the degradation product difluorophosphoric anion was confirmed in electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e. Anionic decomposition products can induce the formation of oligocarbonates, resulting in increased oligocarbonates concentrations for LiPF\u003csub\u003e6\u003c/sub\u003e electrolytes.\u003c/p\u003e\n\u003ch3\u003e4. Effect of conductive salt concentration on the formation of oligocarbonates\u003c/h3\u003e\n\u003cp\u003eThe effect of the selected conductive salt on the formed oligocarbonates at elevated temperatures was shown. Electrolytes with LiPF\u003csub\u003e6\u003c/sub\u003e display an increased amount of oligocarbonates and electrolytes with a higher conductive salt concentration generally have a lower specific conductivity after the aging process at elevated temperatures. In this chapter the effect of the LiPF\u003csub\u003e6\u003c/sub\u003e concentration on the amount of formed oligocarbonates is investigated. Therefore, the electrolytes with a concentration variation of the conductive salt LiPF\u003csub\u003e6\u003c/sub\u003e from 0.6 M to 1.8 M were stored in the drying oven for 42 days and analyzed my means of HPLC-MS (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e displays the formation of the oligocarbonate for several different conductive salt concentration. It can be observed that higher salt concentrations of LiPF\u003csub\u003e6\u003c/sub\u003e show increased amounts of oligocarbonates, underlining the specific conductivity results (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Furthermore, the HPLC-MS results in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e align with the IC-CD-MS results of the previous chapter. For increasing conductive salt concentrations higher amounts of anionic degradation products can be identified after the aging at elevated temperatures. The anionic decomposition products can induce the reaction of the carbonate solvents to oligocarbonates, resulting in higher oligocarbonate concentrations at higher salt concentrations.\u003c/p\u003e \u003cp\u003eThe specific conductivity measurements show a decreasing conductivity and HPLC-MS results confirm an increasing amount of degradation products in the thermally aged electrolytes with high conductive salt concentrations. The HPLC-MS measurements (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e) of variating conductive salt concentrations confirm that the degradation products increase with higher conductive salt concentrations. These findings support the suggested mechanism by Spotter-Smith and Shi which suggests that the reaction is accelerated by high conductive salt concentrations [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e\n\u003ch3\u003e5. Electrolyte degradation analysis of large format cylindrical LICs\u003c/h3\u003e\n\u003cp\u003eThe performance and lifetime in large format LICs are highly important topics in the automotive industry. In the following chapter the electrolytes of automotive cells are analyzed and the effects of formed degradation products after the aging at 55\u0026deg;C are investigated. The cylindrical cells were filled with different electrolytes to investigate the effect of the conductive salt in thermal aging of the cell. Cells were filled with 1 M LiPF\u003csub\u003e6\u003c/sub\u003e in ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (1:1) or 1 M LiFSI in EC/EMC (1:1). The formation protocol was applied, and the cells were stored at 55\u0026deg;C for 42 days at 2.8 V. For authentication of the results the electrolyte was extracted from two identical large format LICs.\u003c/p\u003e \u003cp\u003eA computer tomography (CT) image was conducted after the electrolyte was filled into the cell (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e left) and the excess electrolyte was highlighted in blue by gray level analysis. A second CT image was taken after the formation and the aging process at elevated temperatures was performed (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e right). The CT image shows no excess electrolyte in the cell, indicating that during the formation and aging process of the cell the electrolyte was soaked into the electrode materials. For the electrolyte analysis of the large format automotive LICs an electrolyte extraction from the cell materials is necessary. The cells were extracted using a previously developed method [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Here, the electrodes are extracted using dimethyl carbonate and rewinding them in a specially designed extraction chamber to increase the contact between the extraction solvent and the cell materials.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAfter the electrolyte extraction the conductive salt concentration was analyzed by means of IC-CD-MS. The electrolyte was analyzed before filling it into the cell, after the formation process and after the aging period of 42 days (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). For the electrolytes extracted from the formed and aged cells the recovery rate for the lithium concentration of the previous developed extraction method was included [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The quantification of the conductive salts LiPF\u003csub\u003e6\u003c/sub\u003e and LiFSI over the investigated aging period shows similar results for both salts. It can be observed that a conductive salt reduction occurs after the formation process which is mainly associated with the to the formation of the solid electrolyte interface building a protective layer on the anode material [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. The measurements also show that no conductive salt reduction occurs over the aging period of 42 days at 55\u0026deg;C, confirming the results from Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn a next step the extracted electrolyte samples were analyzed by means of HPLC-MS. The effect of the cell formation and the thermal aging process of the cells on the formation of oligocarbonates in the electrolytes using LiPF\u003csub\u003e6\u003c/sub\u003e or LiFSI as the conductive salt (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e) was investigated.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe HPLC-MS measurements show an increased concentration of oligocarbonates the electrolytes using LiPF\u003csub\u003e6\u003c/sub\u003e conductive salt compared to LiFSI. This confirms the results from the electrolyte storage experiments indicating that the oligocarbonates are formed intensively by electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e. It can be observed that the oligocarbonate concentration of the extracted electrolytes is significantly higher after the formation of the cells for both conductive salts compared to the thermally aged electrolytes. Indicating that the formation of the oligocarbonates can be ignited chemically and electrochemically. Electrolytes containing the conductive salt LiPF\u003csub\u003e6\u003c/sub\u003e show a higher amount of chemically induced reactions towards the selected oligocarbonates compared to electrolytes containing only LiFSI. The amount of electrochemically induced reactions during the formation process of the cell towards the oligocarbonates are similar for both conductive salts.\u003c/p\u003e \u003cp\u003eThe results achieved in the electrolyte storage experiments could be confirmed on large format cylindrical LICs. Furthermore, it could be shown that the degradation from carbonates towards oligocarbonates can be chemically and electrochemically ignited. Understanding and improving electrolyte degradation reactions in LICs towards longer lifetime and higher performance can have a significant impact on the automotive industry.\u003c/p\u003e "},{"header":"Conclusion and outlook","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003cp\u003eIn this research the thermal aging of electrolytes and its effect on changes in the composition were investigated. The impact of the conductive salt variation between LiPF\u003csub\u003e6\u003c/sub\u003e and LiFSI on the formed degradation products were studied. For the first time the effect of thermal aging on LIC electrolytes was investigated by means of a combination of specific conductivity measurements, and characterization of the formation of oligocarbonates and degradation products of lithium salts. It was found that thermal aging of electrolytes with higher conductive salt concentrations showed a significant decrease in specific conductivity. The conductive salt highly effects the formation of degradation products like oligocarbonates. Electrolytes containing LiPF\u003csub\u003e6\u003c/sub\u003e led to significantly higher concentrations of the formed oligocarbonates compared to electrolytes with LiFSI. Furthermore, by increasing the LiPF\u003csub\u003e6\u003c/sub\u003e concentration in the electrolyte the amount of formed oligocarbonates was increasing.\u003c/p\u003e \u003cp\u003eUnderstanding the degradation reactions of LiPF\u003csub\u003e6\u003c/sub\u003e and LiFSI, as representatives of the most used conductive salts, paves the way to the development of new high performing electrolytes. The electrolyte effects the aging process of LICs and by improving the electrolyte performance regarding cell lifetime a significant impact on large format automotive cells can be achieved.\u003c/p\u003e \u003c/div\u003e"},{"header":"Experimental Part","content":"\u003ch3\u003e1. Chemicals\u003c/h3\u003e\n\u003cp\u003eThe conductive salts, lithium hexafluorophosphate (LiPF\u003csub\u003e6\u003c/sub\u003e), lithium bisfluorosulfonylimid (LiFSI) and the solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were obtained in battery grade. The standard solution potassium chloride (1413 \u0026micro;S cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) from Hanna Instruments (V\u0026ouml;hringen, Germany) was used for conductivity measurements. Deionized water was produced by Millipore Milli-Q IQ7000 from Merck (Darmstadt, Germany). Deionized water and LC-MS grade acetonitrile from Merck (Darmstadt, Germany, 99.9%) were used with formic acid (Avantor, Radnor, USA, 99.0%) for HPLC-MS measurements. The chemicals ethylene carbonate (99.0%), ethyl methyl carbonate (98.0%), diethyl carbonate (99.0%) and dimethyl carbonate (99.0%) were obtained from Thermo Scientific (Dreieich, Germany) for HPLC-MS standard solutions. Samples were diluted in anhydrous acetonitrile (Merck, Darmstadt, Germany, 99.0%). The following chemicals were obtained from Merck (Darmstadt, Germany) for IC-CD-MS measurements: Sodium carbonate (99.9%), LC grade acetone (99.0%), sulforic acid (97.0%) and nitric acid (65.0%).\u003c/p\u003e\n\u003ch3\u003e2. Electrolyte mixing and aging at elevated temperatures\u003c/h3\u003e\n\u003cp\u003eThe electrolytes were mixed in an argon filled glovebox (H\u003csub\u003e2\u003c/sub\u003eO\u0026thinsp;\u0026lt;\u0026thinsp;1 ppm; O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1 ppm). The produced concentrations can be identified in the supplementary information table S 11 \u0026ndash; S 12. Ion chromatography-mass spectrometry was performed to verify the conductive salt concentration of the produced electrolytes. Inside the dry room with a dew point lower then \u0026minus;\u0026thinsp;60\u0026deg;C 4 mL of the sample was transferred into pouch bags (10 cm \u0026times; 2 cm) for LICs from Showa Denko with an inner polypropylene foil and vacuum sealed. The electrolytes were stored in a Thermo Scientific HeraTherm oven at 55\u0026deg;C for 7, 14, 28 and 42 days.\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3. Cell assembly\u003c/h2\u003e \u003cp\u003eIn this study large format cylindrical LICs with a diameter of 46 mm and height of 95 mm were produced in the pilot plant at the BMW Group Munich, Germany. The cell is comprised of a nickel manganese cobalt (NMC) cathode material in a 8:1:1 ratio and a silicon-graphite anode material. The electrodes have a specific mass loading, defining the active material amount on the electrodes. The cathode has a specific mass loading of m\u003csub\u003eNMC\u003c/sub\u003e = 195\u0026thinsp;\u0026plusmn;\u0026thinsp;5 g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e and the anode of m\u003csub\u003eSi\u0026minus;Gr\u003c/sub\u003e = 100\u0026thinsp;\u0026plusmn;\u0026thinsp;5 g m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. The principle of the electrolyte filling process for cylindrical automotive cells was recently published [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. The LICs were filled with 42\u0026thinsp;\u0026plusmn;\u0026thinsp;5 g 1 M LiPF\u003csub\u003e6\u003c/sub\u003e or 1 M LiFSI in EC/EMC (1:1).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e4. Cell formation and aging at elevated temperatures\u003c/h2\u003e \u003cp\u003eThe formation was performed in a climate chamber (Li plus GmbH) at 25\u0026deg;C using a Maccor series 4000. Starting with the PreCharge the cylindrical automotive cells were charged with constant current (CC) and 0.1 C (battery capacity) to 3.3 V. After the PreCharge the cells were sealed in the dry room. During the formation the cells were charged to 4.2 V using constant current-constant voltage (CC-CV) with 0.1 C. The cell was hold at 4.2 V with constant voltage for 1 h and discharged with 0.1 C to 2.8 V. The cells were stored at 30% state of charge (2.8 V) in the climate chambers for 42 days at 55\u0026deg;C.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e5. Cell disassembly and electrolyte extraction\u003c/h2\u003e \u003cp\u003eThe automotive cell was disassembled in an argon filled glovebox. The cell can and isolation tapes were removed. The cell material was disconnected from the discs and the current collectors were removed. The electrolyte extraction of the entire cylindrical automotive cell was performed in the glovebox. The principle of the developed electrolyte extraction method was described previously [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. For the extraction the jelly roll was transferred into the designed extraction chamber and completely rewound after the addition of the extraction solvent DMC. A homogeneous mixture of the electrolyte in the cell materials and the added solvent is achieved by an extraction period of 48 h while shaking at 10 osc min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. An aliquot of the electrolyte extract was taken for analysis.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e6. Conductivity measurements\u003c/h2\u003e \u003cp\u003eThe conductivity studies were performed using an Autolab AUTM101.S from Metrohm (Filderstadt, Germany) controlled by NOVA 2.1.6. A TSC microcell 1600 closed GC (rhd instruments, Darmstadt, Germany) with a platinum beaker electrode and glassy carbon electrode was used for the conductivity measurements. The microcell was connected to the temperature controlled microcell HC basic package from rhd instruments (Darmstadt, Germany).\u003c/p\u003e \u003cp\u003eImpedance measurements were performed outside of the glovebox in a frequency range of 200 kHz to 0.1 kHz with an amplitude of 10 mV. For the determination of the cell constant a potassium chloride standard solution with a conductivity of 1413 \u0026micro;S cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e from Hanna Instruments (V\u0026ouml;hringen, Germany) was used at 25\u0026deg;C resulting in a cell constant of 1.27\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Afterwards the microcell was cleaned using a water isopropanol mixture and dried in the vacuum chamber at 60\u0026deg;C. The microcell was transferred into a glovebox and injected with 900 \u0026micro;L of electrolyte sample and sealed airtight. Electrolytes with a different conductive salt concentration were analyzed at 25\u0026deg;C. The specific conductivity is measured three times for each electrolyte sample. The results, including the standard deviation are listed in table S 1 \u0026ndash; S 10 in the supplementary information.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e7. High performance liquid chromatography \u0026ndash; mass spectrometry (HPLC-MS)\u003c/h2\u003e \u003cp\u003eThe samples were separated by means of a Vanquish\u0026trade; HPLC (Thermo Scientific, Dreieich, Germany) consisting of a Vanquish\u0026trade; binary pump, an Vanquish\u0026trade; autosampler, a Vanquish\u0026trade; column compartment with a Vaquish\u0026trade; diode array detector. The HPLC was connected to a Orbitrap Exploris 120\u0026trade; (Thermo Scientific, Dreieich, Germany) using a heated electrospray ionization (H-ESI) at 2.0 kV for the ionizsation. Samples were analyzed in positive and negative mode in a mass range from \u003cem\u003em/z\u003c/em\u003e 40\u0026ndash;1000. Data evaluation was performed with Trace Finder\u0026trade; 5.1 General Quan (Thermo Scientific, Dreieich, Germany) and Compound Discoverer\u0026trade; 3.3 SP2 (Thermo Scientific, Dreieich, Germany).\u003c/p\u003e \u003cp\u003eSeparation of the samples was performed by a pentafluorophenyl column (150 \u0026times; 2.1 mm) with a pore diameter of 2.7 \u0026micro;m from Raptor\u0026trade; (Restek, Bellefonte, USA) at 45\u0026deg;C. A gradient system of formic acid (0.02%) and acetonitrile (2%) in water (solvent A) and formic acid (0.02%) and water (5%) in acetonitrile (solvent B) was used at a flow rate of 0.4 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The gradient system started with solvent A (100%) for 1 min. From 1\u0026ndash;10 min solvent B was increased to 100% and hold for 1 min. Afterwards, solvent B was decreased to 0% and hold at solvent A (100%) for 4 min. Electrolyte samples were diluted 1:50 and extracted electrolyte samples were diluted 1:5 in anhydrous ACN inside the glovebox and sealed airtight. An aliquot of 1 \u0026micro;L was directly injected into the HPLC-MS system and a double determination was performed.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e8. Ion chromatography \u0026ndash; conductivity detection - mass spectrometry (IC-CD-MS)\u003c/h2\u003e \u003cp\u003eIC-CD-MS data was acquired using a Professional IC AnCat MSM-HC MCS by Metrohm (Filderstadt, Germany) connected to a 6130 single quadrupole MS from Agilent Technologies (Santa Clara, USA). The anionic species were analyzed in the mass spectrometer. Negative electrospray ionization mode (ESI) with 3 kV was performed in a mass range of \u003cem\u003em/z\u003c/em\u003e 50\u0026ndash;300.\u003c/p\u003e \u003cp\u003eThe cationic species were separated using a Metrosep C4 100/4.0 column connected to a Metrosep C4 Guard 4.0 column. Separation of the anionic species was achieved on a Metrosep A Supp 5-250/4.0 column coupled to a Metrosep A Supp Guard 4.0 column. Cationic eluation was achieved by 2 mM HNO\u003csub\u003e3\u003c/sub\u003e in water with a flow rate of 0.9 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Anionic eluation was performed using a 10 mM NaHCO\u003csub\u003e3\u003c/sub\u003e and acetone (30%) in water with a flow rate of 0.7 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Electrolyte solutions of LiPF\u003csub\u003e6\u003c/sub\u003e (2 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and LiFSI (2 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) in DMC were prepared and diluted in water. Quantification was performed in the range of 0.1 to 2.0 mol L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e using an external six-point calibration. Samples were diluted 1:1000 in water and an aliquot of 10 \u0026micro;L (cation) and 20 \u0026micro;L (anion) was injected into the IC-CD-MS.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e9. Computer tomography (CT)\u003c/h2\u003e \u003cp\u003eCT scans were conducted on a FF35 CT X-ray system using a FXT 225.48 fine focus directional tube and a 4343CT digital flat image detector with CsI scintillator. A current of 200 \u0026micro;A and a voltage of 220 kV with a performance of 44 W was used to scan the automotive LICs. Data visualization was performed using VGStudio Max 3.5 by Volume Graphics and electrolyte illustration was conducted by gray-level analysis.\u003c/p\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing interest or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors would like to thank Simon Krebs and Cornelius Hupbauer (BMW Group Munich) for conducting the CT measurements and the grey level analysis. 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ATR-FTIR spectroscopy\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"monatshefte-fur-chemie-chemical-monthly","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mccm","sideBox":"Learn more about [Monatshefte für Chemie - Chemical Monthly](https://www.springer.com/journal/706)","snPcode":"706","submissionUrl":"https://www.editorialmanager.com/mccm/","title":"Monatshefte für Chemie - Chemical Monthly","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Extraction, Green Chemistry, High pressure liquid chromatography, Mass spectrometry, Material Science","lastPublishedDoi":"10.21203/rs.3.rs-6339226/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6339226/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe impact of battery electric vehicles (BEVs) using lithium-ion cells (LICs) as energy storage systems is increasing and topics as cell performance and lifetime are becoming more important. The electrolyte composition can be a limiting factor for the lifetime of the cell and highly effects the performance of LICs. In this research the aging process of LIC electrolytes at increased temperatures (55\u0026deg;C) is investigated. First, the electrolyte is analyzed separately from the cell materials and afterwards the electrolyte is extracted from thermally aged LICs to investigate the aging effect on the electrolyte and the cell. By performing specific conductivity measurements of thermally aged electrolytes with different conductive salts and various concentrations, representative information for the entire electrolyte was obtained. High performance liquid chromatography-mass spectrometry measurements revealed that the selected conductive salt has a significant impact on the quantity of formed oligocarbonates. Furthermore, a correlation between the LiPF\u003csub\u003e6\u003c/sub\u003e concentration and the amount of formed oligocarbonates could be identified. Finally, large format cylindrical LICs containing LiPF\u003csub\u003e6\u003c/sub\u003e or LiFSI electrolytes were investigated for their electrolyte degradation products after the thermal aging process. Understanding the thermal aging process of different electrolyte compositions and transferring the knowledge to large format LICs is a key factor for the automotive industry aiming for the development of long-lasting, high-performance cells.\u003c/p\u003e","manuscriptTitle":"Analysis of electrolyte degradation products in cylindrical automotive lithium-ion cells during thermal aging","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-23 08:56:59","doi":"10.21203/rs.3.rs-6339226/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2025-05-08T14:11:51+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-04-01T15:25:15+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-04-01T12:42:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Monatshefte für Chemie - Chemical Monthly","date":"2025-03-30T11:15:36+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"monatshefte-fur-chemie-chemical-monthly","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"mccm","sideBox":"Learn more about [Monatshefte für Chemie - Chemical Monthly](https://www.springer.com/journal/706)","snPcode":"706","submissionUrl":"https://www.editorialmanager.com/mccm/","title":"Monatshefte für Chemie - Chemical Monthly","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"05a581da-b255-4617-9f99-fc75b6c6be87","owner":[],"postedDate":"April 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-08-11T16:00:17+00:00","versionOfRecord":{"articleIdentity":"rs-6339226","link":"https://doi.org/10.1007/s00706-025-03358-w","journal":{"identity":"monatshefte-fur-chemie-chemical-monthly","isVorOnly":false,"title":"Monatshefte für Chemie - Chemical Monthly"},"publishedOn":"2025-08-04 15:57:15","publishedOnDateReadable":"August 4th, 2025"},"versionCreatedAt":"2025-04-23 08:56:59","video":"","vorDoi":"10.1007/s00706-025-03358-w","vorDoiUrl":"https://doi.org/10.1007/s00706-025-03358-w","workflowStages":[]},"version":"v1","identity":"rs-6339226","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6339226","identity":"rs-6339226","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}