Post-Mortem Investigations of Fluorinated Flame Retardants for Lithium Ion Battery Electrolytes by Gas Chromatography with Chemical Ionization

Using flame retardants (FRs) in lithium ion battery (LIB) electrolytes is usually a tradeoff between electrochemical performance and electrolyte flammability. Fluorinated FRs are a promising class of FRs which are currently under investigation. During this work, three FRs originating from triethyl phosphate with varying degree of fluorination were investigated regarding their electrochemical stability on cathode (LiNi0.33Co0.33Mn0.33O2, NCM) and anode (graphite) in half cells. During long-term cycling, changes in performance were observed. Especially on the anode side the FR addition showed a decrease in performance in comparison to the standard electrolyte (DEC/EC 1:1, 1M LiPF6). The electrolytes containing the three FRs were extracted from the cells and analyzed regarding their changes in composition and structural degradation. The decomposition products were investigated by gas chromatography (GC) with electron impact (EI) ionization and mass selective (MS) detection. To obtain more information with regard to the identification of unknown decomposition products further GC‐MS experiments with positive chemical ionization (PCI) and negative chemical ionization (NCI) were performed. Twelve different volatile organic decomposition products were identified. These decomposition products can be subdivided regarding their basic structure. Ether based, carbonate based and phosphate based fluorinated and non-fluorinated decomposition products were identified. Furthermore, possible formation pathways for all groups of decomposition products were postulated taking existing literature into account

Electrolyte Extraction—Sub and Supercritical CO2

This chapter reports on experiments aimed at investigating the capability of pressurized carbon dioxide to extract the electrolyte from commercial available LIBs on a laboratory scale. Two different phase conditions of carbon dioxide (subcritical and supercritical) and two different extraction (static and dynamic) have been considered and analyzed for their strengths and weaknesses. Furthermore, the addition of co-solvents is examined with regard to their contribution to higher recovery rates. After reporting the optimized extraction method, the extracted electrolyte was analyzed by gas and ionic chromatography methods for potential de-composition products and their relative amount

Influence of temperature on the aging behavior of 18650-type lithium ion cells: A comprehensive approach combining electrochemical characterization and post-mortem analysis

The understanding of the aging behavior of lithium ion batteries in automotive and energy storage applications is essential for the acceptance of the technology. Therefore, aging experiments were conducted on commercial 18650-type state-of-the-art cells to determine the influence of the temperature during electrochemical cycling on the aging behavior of the different cell components. The cells, based on Li(Ni0.5Co0.2Mn0.3)O2 (NCM532)/graphite, were aged at 20 °C and 45 °C to different states of health. The electrochemical performance of the investigated cells shows remarkable differences depending on the cycling temperature. At contrast to the expected behavior, the cells cycled at 45 °C show a better electrochemical performance over lifetime than the cells cycled at 20 °C. Comprehensive post-mortem analyses revealed the main aging mechanisms, showing a complex interaction between electrodes and electrolyte. The main aging mechanisms of the cells cycled at 45 °C differ strongly at contrast to cells cycled at 20 °C. A strong correlation between the formed SEI, the electrolyte composition and the electrochemical performance over lifetime was observed

Supercritical carbon dioxide extraction of electrolyte from spent lithium ion batteries and its characterization by gas chromatography with chemical ionization

The aging products of the electrolyte from a commercially available state-of-the-art 18650-type cell were investigated. During long term cycling a huge difference in their performance and lifetime at different temperatures was observed. By interpretation of a strong capacity fading of cells cycled at 20 °C compared to cells cycled at 45 °C a temperature depending aging mechanism was determined. To investigate the influence of the electrolyte on this fading, the electrolyte was extracted by supercritical fluid extraction (SFE) and then analyzed by gas chromatography (GC) with electron impact (EI) ionization and mass selective detection. To obtain more information with regard to the identification of unknown decomposition products further analysis with positive chemical ionization (PCI) and negative chemical ionization (NCI) was performed. 17 different volatile organic aging products were detected and identified. So far, seven of them were not yet known in literature and several formation pathways were postulated taking previously published literature into account

Fast screening method to characterize lithium ion battery electrolytes by means of solid phase microextraction – gas chromatography – mass spectrometry

Several electrolytes of commercially available lithium ion batteries (LIBs) were analyzed by solid phase microextraction – gas chromatography – mass spectrometry (SPME-GC-MS). The uptake and subsequent injection of the conducting salt LiPF6 into the GC system was prevented by using a headspace SPME setup. Thus, a removal step prior to the GC-MS measurements was not necessary and it was possible to analyze the untreated electrolyte without injecting the hazardous LiPF6 into the GC system. Furthermore, all SPME experiments were carried out at room temperature to exclude further thermal alteration of the electrolyte during sampling. In LIB electrolytes, different linear and cyclic carbonate solvents and additives such as succinonitrile (SN) and fluoroethylene carbonate (FEC) could be identified using the SPME-GC-MS setup. Moreover, the aging products dimethyl-2,5-dioxahexane dicarboxylate (DMDOHC) and ethylmethyl-2,5-dioxahexane dicarboxylate (EMDOHC) were identified in the electrolyte of aged 18[thin space (1/6-em)]650-type cells. In the case of the cells of one specific supplier, various additional hydrocarbons were detected via SPME-GC-MS. These compounds could not be obtained when a GC-MS setup with conventional liquid or headspace injection is used. Consecutive experiments were carried out by extracting the electrolyte components directly from the headspace above anode, separator and cathode of an aged 18[thin space (1/6-em)]650-type cell, which confirmed the findings of the prior analysis of pure electrolytes. Within this work it was possible to develop a method for the investigation of LIB electrolytes and their decomposition products with high sensitivity and low GC column bleeding

Impact of cycling at low temperatures on the safety behavior of 18650-type lithium ion cells: Combined study of mechanical and thermal abuse testing accompanied by post-mortem analysis

The impact of cycling at low temperatures on the thermal and mechanical abuse behavior of commercial 18650-type lithium ion cells was compared to fresh cells. Post-mortem analyses revealed a deposition of high surface area lithium (HSAL) metal on the graphite surface accompanied by severe electrolyte decomposition. Heat wait search (HWS) tests in an accelerating rate calorimeter (ARC) were performed to investigate the thermal abuse behavior of aged and fresh cells under quasi-adiabatic conditions, showing a strong shift of the onset temperature for exothermic reactions. HSAL deposition promotes the reduction of the carbonate based electrolyte due to the high reactivity of lithium metal with high surface area, leading to a thermally induced decomposition of the electrolyte to produce volatile gaseous products. Nail penetration tests showed a change in the thermal runaway (TR) behavior affected by the decomposition reaction. This study indicates a greater thermal hazard for LIB cells at higher SOC and experiencing aging at low temperature

Influence of electrolyte additives on the cathode electrolyte interphase (CEI) formation on LiNi1/3Mn1/3Co1/3O2 in half cells with Li metal counter electrode

Traditional solid electrolyte interphase (SEI) forming additives of vinylene carbonate (VC), fluoroethylene carbonate (FEC) and ethylene sulfite (ES) are studied with respect to their impact on the formation and growth of the cathode electrolyte interphase (CEI) layer. T-half cells are assembled and undergo three different electrochemical investigation plans: after formation (0.1C, 5 cycles) and long term cycling (0.1C, 5 constant current cycles + 1C, 100/150 constant current/voltage cycles), scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and gas chromatography-mass spectrometry (GC-MS) are combined to investigate morphology, CEI composition, CEI thickness and aging products for cells with different electrolyte systems. The obtained results reveal a significant influence of these additives on the CEI composition and CEI growth. With the help of SEM, it is found that large areas of electrolyte decomposition products are formed at the aged electrode surfaces (=after cycling), with the exception when 2 vol% of FEC is added into the reference electrolyte. From XPS measurements, CEI thicknesses are calculated. The reference electrolyte with 2 vol% of FEC shows the thinnest layer after long time aging (0.8 ± 0.2 nm). For the addition of 2 vol% of VC, an incremental growth of the CEI thickness occurs from the 100th to 150th cycle (from 1.0 ± 0.1 nm to 2.9 ± 0.4 nm). By correlating the CEI thickness values with the electrochemical performance, it can be observed that for lithium metal based half cells, the existence of a thinner CEI layer corresponds to a better cycling behavior, with 2 vol% of FEC showing the highest discharge capacity of 114.4 ± 0.2 mAh/g after 150 cycles at 1C. GC-MS shows that both VC and FEC help to prevent fast electrolyte aging

Influence of the Fluorination Degree of Organophosphates on Flammability and Electrochemical Performance in Lithium Ion Batteries: Studies on Fluorinated Compounds Deriving from Triethyl Phosphate

Three symmetric substances originating from triethyl phosphate were specifically synthesized with varying degree of fluorination at the side chain. Different concentrations of each phosphate were evaluated as co-solvent with regard to their flammability and the electrochemical cycling performance. With higher degree of fluorination and a higher amount of the phosphate in the electrolyte, the self-extinguishing time (SET), a value to determine and compare the flammability of electrolytes, could be significantly lowered to yield a non-flammable electrolyte mixture. A specifically designed SET device is introduced, which offers more accurate results due to lowered standard deviations by minimizing random and systematic errors. As the application of phosphates as co-solvents results in a trade-off in cycling performance, a thorough determination in regard to the ionic conductivity, the anodic oxidation stability and the compatibility with anode and cathode material was carried out in half- and full-cells. The manuscript strives to establish a deeper understanding of the influence that the utilization of phosphates as co-solvents entail with special focus on the fluorination degree. It could be shown that the partially fluorinated phosphate offers the best cycling results and therefore the lowest trade-off in performance, while a severe improvement in SET could be achieved compared to the reference electrolyte

Phosphorus additives for improving high voltage stability and safety of lithium ion batteries

Three phosphorus containing molecules, tris(2,2,3,3,3-pentafluoropropyl) phosphate (5F-TPrP), tris(1,1,1,3,3,3-hexafluoropropan-2-yl) phosphate (HFiP) and tris(1,1,1,3,3,3-hexafluoropropan-2-yl) phosphite (THFPP), were investigated as high voltage and flame retardant electrolyte additives for lithium ion batteries. The effect of the oxidation state of the phosphorus atom as well as the influence of branched vs. linear fluorinated propyl groups were investigated regarding cycling performance and flammability of the resulting electrolyte. In the case of a high voltage battery application, all three investigated molecules showed an improvement regarding the cycling performance in NCM111/Li half-cells. Post mortem analysis of the NCM111 electrodes via SEM and XPS indicates that the different groups of two phosphates (5F-TPrP vs. HFiP) have an impact on the thickness, morphology and composition of the cathode electrolyte interphase (CEI). If the electrolyte formulation contains the linear side group (5F-TPrP), the thickness of the CEI increases, whereas for the branched group (HFiP) it decreases compared to the CEI formed in 1 M LiPF6 EC:DEC (1:1) used as reference electrolyte. Furthermore, addition of at least 20 wt.% of 5F-TPrP to the reference electrolyte formulation resulted in a non-flammable electrolyte formulation