Louis Hartmann

The beneficial effect of fluoroethylene carbonate (FEC) as a co-solvent and electrolyte additive on the cycle life of lithium-ion batteries, particularly those employing high-voltage cathode active materials, has been demonstrated in numerous studies. To further elucidate the effect of varying FEC content, as well as the impact of formation temperature on the properties of the graphite SEI, we performed formation between 10 °C and 60 °C with cells containing either 2 wt% or 20 wt% FEC in LP-57. The irreversible capacity loss due to SEI formation and the arising intercalation resistance (representing the sum of charge-transfer and SEI resistance), determined by means of electrochemical impedance spectroscopy, increased with higher temperature and FEC content. The impact of both quantities on the full-cell performance was assessed in coin and T-cells, revealing enhanced cycling stability for cells that were formed at elevated temperatures, despite a higher susceptibility to lithium plating accompanied by a decreased rate capability for their T-cell analogues when evaluating the rate performance. Employing on-line electrochemical mass spectrometry (OEMS), we further show that higher temperatures and higher FEC contents give rise to a more pronounced, but also more condensed, gas evolution during the initial cycles.

Extending the cycle-life of state-of-the-art lithium-ion batteries (LiBs) is, amongst other aspects, predicated on optimizing the properties of the solid electrolyte interphase (SEI), such as its stability and resistance.^[1] It is formed at the negative electrode during the first cycles by electrolyte decomposition and acts as a passivating layer between the electrode surface and the electrolyte.^[2,3] Since the SEI formation is accompanied by an irreversible loss of cyclable lithium, its stability during cycling is crucial for improving the charge/discharge efficiency of LiBs. Furthermore, the arising resistance associated with the SEI formation, predominantly depending on the SEI thickness, its chemical composition, and its lithium-ion conductivity, all impact the rate performance of the anode.^[4] Two of the most crucial factors impacting SEI formation are temperature and electrolyte composition (i.e., using electrolyte additives).^[5] Additives are added to the electrolyte to inhibit the predominant reduction of other electrolyte constituents and thereby form an SEI with improved stability during cycling.^[1,6,7] One of the most prominent additives for LiBs is fluoroethylene carbonate (FEC), which is preferentially reduced compared to common carbonate solvents (e.g., ethylene carbonate, ethyl methyl carbonate) due to its higher reduction potential.^[7] In this study, we investigated the influence of the formation temperature between 10 and 60 °C, as well as the concentration of FEC (2 and 20 %_wt in EC:EMC 3:7 wt/wt with 1 M LiPF₆, Gotion_, USA) on the characteristics of a synthetic graphite (SMG-A5, Resonac, Japan) anode, both in SMG/Li half-cells and in Ni-rich NCM/SMG full cells. We quantified the first-cycle irreversible capacity loss (ICL) in SMG/Li half cells with a lithium reference electrode after two 0.1 C formation cycles and compared the FEC containing electrolytes to the baseline- (LP-57) and an LP-572 (1 M LiPF₆ in EC:EMC 3:7 wt/wt + 2 %_wt vinylene carbonate (VC), Gotion_, USA) electrolyte. Employing electrochemical impedance spectroscopy in an SMG/Li half-cell setup with a µ-reference electrode and a free-standing graphite electrode,^[8,9] we could further quantify the intercalation resistance (R _Int, representing the sum of the charge-transfer and the SEI resistance) after an identical formation protocol at 40 % SOC. Both quantities, the ICL and R _Int, increase with increasing formation temperature and higher FEC content. The impact of an increasing R _Int on the rate performance was tested in Ni-rich NCM/SMG full cells with a lithium reference electrode, revealing a decreased rate capability and higher susceptibility for lithium plating for cells that were formed at higher temperatures and contained 20 %_wt FEC. Additionally, the cycling stability of Ni-rich NCM/SMG full cells at 25 °C was assessed in coin cells. Despite an increased ICL and R _Int after formation, cells that were formed at elevated temperatures and contained 20 %_wt FEC showed enhanced cycling stability compared to those that were formed at a lower temperature and contained less FEC. Finally, on-line electrochemical mass spectrometry (OEMS) was employed to quantify the evolved gases during the formation of a Ni-rich NCM/SMG full cell containing either of the FEC-containing electrolytes at different temperatures. In accordance with an increased ICL and R _Int, a higher formation temperature, as well as a higher FEC content, gave rise to a more pronounced gas evolution during formation. References: [1] D. Y. Wang, N. N. Sinha, J. C. Burns, C. P. Aiken, R. Petibon, J. R. Dahn, J. Electrochem. Soc. 2014, 161, A467–A472. [2] E. Peled, J. Electrochem. Soc. 1979, 126, 2047–2051. [3] E. Peled, S. Menkin, J. Electrochem. Soc. 2017, 164, 1703–1719. [4] S. Solchenbach, X. Huang, D. Pritzl, J. Landesfeind, H. A. Gasteiger, J. Electrochem. Soc. 2021, 168, 110503. [5] J. Asenbauer, T. Eisenmann, M. Kuenzel, A. Kazzazi, Z. Chen, D. Bresser, Sustain. Energy Fuels 2020, 4, 5387–5416. [6] T. Taskovic, L. Thompson, A. Eldesoky, M. Lumsden, J. R. Dahn, J. Electrochem. Soc. 2021, 168, 010514. [7] K. U. Schwenke, S. Solchenbach, J. Demeaux, B. L. Lucht, H. A. Gasteiger, J. Electrochem. Soc. 2019, 166, A2035–A2047. [8] S. Solchenbach, D. Pritzl, E. J. Y. Kong, J. Landesfeind, H. A. Gasteiger, J. Electrochem. Soc. 2016, 163, A2265–A2272. [9] R. Morasch, B. Suthar, H. A. Gasteiger, J. Electrochem. Soc. 2020, 167, 100540. Acknowledgements: This work is financially supported by BMW AG. The authors thank BMW AG for their financial support.

The trend for increased nickel content in layered transition metal oxide cathode active materials and increasing charging cut-off voltages aggravates aging of lithium-ion battery cells at high state of charge (SOC). We investigate the calendaric aging behavior of large-format automotive prototype cells and laboratory single-layer pouch cells at high but realistic cell voltages/SOCs and demonstrate that electrolyte oxidation in combination with follow-up reactions can cause a significant loss of the LiPF₆ salt in the electrolyte. For this, we analyze the LiPF₆ concentration in aged cells, the generation of H₂ upon storage, and the cell resistance for different aging conditions. We show that the LiPF₆ loss is a critical aging phenomenon, as it cannot readily be detected by capacity fading measurements at low/medium C-rates or by cell resistance measurements, while it severely reduces rate and fast-charging capability. Under certain circumstances, LiPF₆ loss can even lead to a temporary capacity increase due to conversion of the conducting salt in the electrolyte to cyclable lithium in the active material. Finally, we suggest a possible reaction mechanism and a simple accounting model to keep track of how different side reactions involved in LiPF₆ loss change the cyclable lithium inventory of a lithium-ion cell.

The evolution of gases is often associated with the decomposition of the electrolyte or active materials. Thus, its detection can be powerful for understanding degradation mechanisms in Li-ion batteries (LIBs). Here, we present an evaluation method for gas detection and quantification by on-line electrochemical mass spectrometry (OEMS) when using volatile electrolytes (e.g., electrolytes with linear alkyl carbonates) and a new OEMS cell design for improved leak tightness. With a significant fraction of the gases in the cell head-space being electrolyte vapor, we observe a pressure/time-dependency of the electrolyte background in the mass spectrometer, for which we here developed a correction method. We apply this method for the temperature-dependent gas analysis of a graphite/NCM831205 full-cell with an LP57 (1 M LiPF₆ in EC:EMC 3:7 wt:wt) electrolyte. We conclude that the activation energy of the gas evolution associated with the formation of the solid-electrolyte interphase (SEI) is ∼15–20 kJ mol⁻¹. Furthermore, we identify a significant temperature dependence of the lithium alkoxide triggered trans-esterification of EMC with an activation energy of ∼70 kJ mol⁻¹. Lastly, the temperature-dependent analysis reveals the relation between the evolution of hydrogen related to water and HF impurities during the initial SEI formation and in situ generated protons.

Li- and Mn-rich layered oxides such as Li_1.14(Ni_0.26Co_0.14Mn_0.60)_0.86O₂ (LMR-NCM) are potential next-generation cathode active materials (CAMs) for lithium ion-batteries, promising an increased energy density at lower materials costs compared to state-of-the-art CAMs. However, its commercial viability is still inhibited by its strong gassing, poor cycling stability, and voltage fading, so various approaches such as post-treatments or additives are being investigated. Here, it will be shown that the cycling performance of LMR-NCM//graphite coin-cells is drastically improved when assembled with 300 °C dried glassfiber (GF) separators (“GF-cells”) compared to cells with Celgard (CG) separators dried at 70 °C (“CG-cells”). The origin of this phenomenon is investigated by online electrochemical mass spectrometry (OEMS), TGA-MS, water absorption, and XPS measurements. These reveal that the superior performance of the GF-cells can be ascribed to the bulk water absorption capability of the 300 °C dried glassfiber material as well as its ability to scavenge HF, whereby H₂O and HF are produced by the (electro)chemical oxidation of the electrolyte and the decomposition of the LiPF₆ salt. Similar performance enhancements can be observed for 300 °C dried SiO₂ nanoparticles added to the LMR-NCM cathodes or for an HF/H⁺ scavenging electrolyte additive.

Li-ion batteries (LiBs) are known to evolve gases during their first few formation cycles due to the formation of a solid electrolyte interphase (SEI) at the anode active material particle surface. Simultaneously, the electrolyte can react by other mechanisms leading to the formation of volatile species. ^1,2 At a high state-of-charge, the cathode can also release reactive oxygen which causes the formation of CO and CO₂ from the oxidative decomposition of the electrolyte.^3,4 Consequently, by the analysis of evolved gas species one can study the effect of electrolyte additives, reveal substantial degradation processes within LiBs, and rationalize optimized formation strategies.^4–6 The evolution of gaseous species is typically quantified in-situ by monitoring the cell pressure (e.g., via the so-called Archimedes principle) or by online mass spectrometry.^5,7 Concerning the latter, an on-line electrochemical mass spectrometer (OEMS) was developed for the application in LiB research and proved to be particularly powerful, since it allows a quantitative analysis and the identification of the evolved and consumed gas during the operation of a battery.^4,7 The quantification of OEMS data, however, is challenging when the vapor pressure of the electrolyte is high, which, e.g., is the case for linear carbonates, particularly at elevated temperatures. Mixtures of linear and cyclic carbonates are widely used in LiB applications due to their low viscosity, high salt solubility, and high conductivity. In contrast to the cyclic ethylene carbonate (EC) with a vapor pressure of 0.25 mbar, the vapor pressure for the typically used linear carbonates dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) can vary from 23 to 80 mbar at room temperature.⁸ Their vapor pressure further increases by a factor of ~3 when increasing the temperature from room temperature to 45 °C.⁹ Thus, a significant share of the gas in the head-space of the OEMS cell consists of electrolyte vapor. As they undergo fragmentation in the mass spectrometer, a background signal is superimposed on the relevant mass traces related to hydrogen, ethylene, carbon monoxide, oxygen, and carbon dioxide.¹ Furthermore, the cell pressure inherently decreases due to the continuous leak through the capillary (with a leak rate of ~1 µL/min) that connects the OEMS cell (~10 mL volume) with the mass spectrometer. Because of the liquid reservoir of electrolyte in the OEMS cell and its constant vapor pressure, this results in an enrichment of the gas phase with electrolyte vapor. Analyzing the pressure-dependent flow of gaseous species into the mass spectrometer allowed us to introduce corrections for signal normalization, electrolyte background change, and signal quantification. In the present study, we applied these corrections in the investigation of the influence of LP57 (1 M LiPF₆ in EC:EMC 3:7wt/wt) and LP47 (1 M LiPF₆ in EC:DEC 3:7wt/wt) electrolyte on the gassing characteristics during the formation of full-cells with a graphite anode and a Ni-rich cathode from low (10 °C) to high (45 °C) temperatures. From that, we were able to conclude how the formation temperature influences the gas evolution related to SEI formation as well as the trans-esterification of the electrolyte and the lattice oxygen reactivity towards different electrolytes. A comparison to a model electrolyte (1.5 M LiPF₆ in EC) with a low vapor pressure (< 0.25 mbar) allowed us to assess the detection limits of all the signals depending on the electrolyte’s vapor pressure and to evaluate the validity of our proposed signal corrections. Acknowledgment: The authors thank BMW AG for their financial support. References: B. Strehle, S. Solchenbach, M. Metzger, K. U. Schwenke, and H. A. Gasteiger, J. Electrochem. Soc., 164, A2513–A2526 (2017). T. Taskovic, L. M. Thompson, A. Eldesoky, M. D. Lumsden, and J. R. Dahn, J. Electrochem. Soc., 168, 010514 (2021). L. De Biasi, A. O. Kondrakov, H. Geßwein, T. Brezesinski, P. Hartmann, and J. Janek, J. Phys. Chem. C, 121, 26163–26171 (2017). R. Jung, M. Metzger, F. Maglia, C. Stinner, and H. A. Gasteiger, J. Electrochem. Soc., 164, A1361–A1377 (2017). C. P. Aiken, J. Xia, D. Y. Wang, D. A. Stevens, S. Trussler, and J. R. Dahn, J. Electrochem. Soc., 161, 1548–1554 (2014). D. Schreiner, T. Zünd, F. J. Günter, L. Kraft, B. Stumper, F. Linsenmann, M. Schüßler, R. Wilhelm, A. Jossen, G. Reinhart, and H. A. Gasteiger, J. Electrochem. Soc., 168, 030507 (2021). M. Metzger, C. Marino, J. Sicklinger, D. Haering, and H. A. Gasteiger, J. Electrochem. Soc., 162, A1123–A1134 (2015). V. Pokorný, V. Štejfa, M. Fulem, C. Červinka, and K. Růžička, J. Chem. Eng. Data, 62, 4174–4186 (2017). S. Hess, M. Wohlfahrt-Mehrens, and M. Wachtler, J. Electrochem. Soc., 162, A3084–A3097 (2015).

Batteries using Ni-rich cathode active materials are the most common cells in the automotive sector. They offer a high energy density at competitive prices which makes them attractive in a wide-range of applications.^[1,2] However, a higher Ni-content results in more challenges for battery lifetime in comparison to low-Ni materials. These challenges are aggravated at elevated temperatures and high SOCs, stemming from detrimental side-reaction, such as parasitic heat and oxygen release.^[3–6] In this study, we investigated an aging effect which is relevant in a wide-range of cell formats ranging from small single layer pouch cells to automotive cells. Upon storing cells at a high SOC at elevated temperatures (30 days, 45 °C) a significant drop in rate-performance was visible afterwards. At the same time, no active Li-loss and no significant increase in 10 second DCIR resistance at 50%SOC was seen. We propose a mechanism of LiPF₆-loss during storage as the main reason for the worsened rate-performance. By conducting ion chromatography (IC) and NMR analysis of electrolytes from aged cells, it was found that cells stored at an upper cut-off potential of 4.2 to 4.4 V lose a significant fraction of LiPF₆ after a 30-day storage at 45 °C. In addition to these storage experiments, cells with fresh electrolyte, but lower electrolyte salt concentrations (1 M LiPF₆ to 0.2 M LiPF₆) were built, with the goal of using them as a calibration curve to map the worsened rate performance after storage onto a corresponding electrolyte salt concentration without the need of a destructive analysis of the cells. As seen in Figure 1, the rate performance of freshly built single layer pouch cell is highly dependent on the electrolyte salt concentration. While all cells show good discharge capacities at C/10, at higher discharge rates the lower concentration cells show a worsened performance. 1 M LP572 cells show excellent rate-capabilities up to a rate of 2C and a 0.2 M cell already has a significant drop in discharge capacity at C/2. Using OEMS, electrochemical impedance measurements and theoretical models we further elucidate the mechanism of LiPF₆-loss at elevated temperatures and high SOCs. References: [1] D. Andre, S. J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos, B. Stiaszny, J. Mater. Chem. A 2015, 3, 6709–6732. [2] M. S. E. Houache, C. H. Yim, Z. Karkar, Y. Abu-Lebdeh, Batteries 2022, 8, 70. [3] C. P. Aiken, E. R. Logan, A. Eldesoky, M. Metzger, J. R. Dahn, 2022, 169, 050512. [4] B. Strehle, F. Friedrich, H. A. Gasteiger, J. Electrochem. Soc. 2021, 168, 050512. [5] A. Eldesoky, E. R. Logan, A. J. Louli, W. Song, R. Weber, S. Hy, R. Petibon, J. E. Harlow, S. Azam, E. Zsoldos, J. R. Dahn, J. Electrochem. Soc. 2022, 169, 010501. [6] R. Jung, P. Strobl, F. Maglia, C. Stinner, H. A. Gasteiger, J. Electrochem. Soc. 2018, 165, A2869–A2879. Figure 1: Calibration discharge rate-test for identical single layer pouch cells with varying conductive salt concentration of 1 M (black line and dots), 0.8 M (blue line and dots), 0.6 M (green line and dots), 0.4 M (yellow line and dots), 0.2 M (purple line and dots) LiPF₆. 300 µL LP572 was used, and the charge was done at either C/10 or C/3 (CCCV) while the discharge was done at C/10, C/3, C/2, 1C, 2C, 3C, 4C, 5C. Figure 1

Prussian White is a promising active material for the positive electrode of sodium-ion batteries as it is comprised of Na, Mn, Fe, C, and N and thus offers high sustainability and low cost. However, exposure of Prussian White to moisture results in chemical changes due to the formation of surface contaminants, as well as structural changes due to the absorption of water into the bulk crystal structure. Here we report an analysis of the formation rate of surface contaminants and bulk water absorption by weight tracking, infrared spectroscopy, and X-ray diffraction over extended periods of storage in high relative humidity air for fully sodiated Na_1.8Mn_0.8Fe_0.2[Fe(CN)₆]_0.9 and partially sodiated Na_1.3Mn_0.8Fe_0.2[Fe(CN)₆]_0.9. Fully sodiated Prussian White gains almost 20% in mass due to the formation of interstitial water during 20 h of storage in 100% relative humidity at 25 °C. Surface hydroxides and carbonates are found after storage and a structural change from the rhombohedral to a monoclinic crystal structure is observed. It is found that vacuum drying of Prussian White powder or electrodes at 150 °C can remove the majority of interstitial water and restore the rhombohedral crystal structure, but not remove surface contaminants. Prussian White immersed in water during aqueous electrode processing also shows interstitial water and a monoclinic crystal structure, but no surface contaminants. This suggests that aqueous electrode processing of Prussian White is feasible when effective drying strategies are employed. Indeed, Prussian White electrodes made from H₂O-based slurries with CMC/NaPAA binder vacuum-dried at 150 °C show higher specific capacity and similar capacity retention in full cells as Prussian White electrode made from NMP-based slurries with PVDF binder.

Fluoroethylene-carbonate (FEC) is a common co-solvent for high-voltage cathodes and for silicon-based anodes in lithium-ion batteries. However, FEC has a limited thermal stability when used with LiPF6 as conductive salt, and its decomposition can trigger detrimental side reactions. Here, we will examine the reaction mechanism of FEC with LiPF6, confirming that vinylene-carbonate (VC) and HF are produced at elevated temperatures. By full-cell cycling at 45°C in a T-cell setup with a micro-reference electrode (µ-RE), we can show by electrochemical impedance spectroscopy that these side reactions not only lead to an impedance increase of the anode and the cathode, but also trigger transition metal dissolution. By comparison of FEC and ethylene-carbonate (EC) as cyclic carbonate, we demonstrate that FEC has no advantage at high-voltage operation compared to EC, when employing cathode materials or cathode potentials for which no lattice oxygen is evolved. Finally, we use multi-layer pouch-cells to analyze the gassing of an EC- and FEC-based electrolyte upon extended charge/discharge cycling at 45°C, showing that the latter leads to cell bulging upon extended charge/discharge cycling at 45°C due to the oxidation of the VC formed by the thermal decomposition of FEC above ~4.4-4.5 V vs. Li+/Li

Unwanted self-discharge of LFP/AG and NMC811/AG cells can be caused by in situ generation of a redox shuttle molecule after formation at elevated temperature with common alkyl carbonate electrolyte. This study investigates the redox shuttle generation for several electrolyte additives, e.g., vinylene carbonate and lithium difluorophosphate, by measuring the additive reduction onset potential, first cycle inefficiency and gas evolution during formation at temperatures between 25 and 70 °C. After formation, electrolyte is extracted from pouch cells for visual inspection and quantification of redox shuttle activity in coin cells by cyclic voltammetry. The redox shuttle molecule is identified by GC-MS and NMR as dimethyl terephthalate. It is generated in the absence of an effective SEI-forming additive, according to a proposed formation mechanism that requires residual water in the electrolyte, catalytic quantities of lithium methoxide generated at the negative electrode and, surprisingly, polyethylene terephthalate tape within the cell.