Thermal Stability Of Electrolytes Research Articles

An in situ fabricated multifunctional gel electrolyte for lithium–sulfur batteries

Despite the high-energy lithium–sulfur (Li–S) batteries being a promising secondary batteries system, their commercialization is seriously hindered by the polysulfides shuttle, unstable lithium anode and safety hazards in the traditional liquid electrolytes. Herein, a multifunctional gel polymer electrolyte (ANPD-GPE) is fabricated, which is composed of Nafion-coated Al2O3 nanofibers and in-situ polymerized 1,3-dioxolane (DOL). Lewis-acidic Al2O3 has a strong effect on the Lewis base of bis(trifluoromethanesulphony)imide anions, facilitating the transport of Li+ and resulting in a reduced Li+ concentration gradient. In addition, the replacement of a flammable polyolefin separator by the inorganic Al2O3 robust nanofibers skeleton can effectively promote the structural and thermal stability of electrolytes. The introduction of Nafion effectively inhibits the shuttle effect vis polymerization of DOL and the electrostatic repulsion of its anions. The in-situ polymerized elastic poly-DOL (PDOL) can stabilize the lithium anode because of its accommodation for the fluctuating interface during cycling. Consequently, Li–S cells with ANPD-GPE present improved electrochemical performance in terms of cathode cyclability and lithium anode stability.

(Invited) Some Aspects of Electrode Kinetics and Electrolyte Stability in Vanadium Flow Batteries

The redox flow battery (RFB) has seen an explosion in research activity in recent years due to its attractiveness for large-scale energy storage applications (e.g. grid level storage). RFBs1,2 offer many advantages over conventional batteries including their relatively high reliability and long cycle life, as well as the ability to scale their energy and power outputs independently. The vanadium flow battery (VFB)1,3-6 is currently the most promising of these technologies, with a number of large-scale applications and demonstration projects already underway.6 The power density of VFBs is limited primarily by the kinetics of the redox reactions at the carbon electrodes while the energy density is limited primarily by the concentration of vanadium in the electrolyte. In this talk we will review our work on two aspects of VFBs – kinetics on carbon electrodes and thermal stability of electrolytes – which impact these two limitations respectively.We have investigated3,7,8 the kinetics of the VII-VIII, VIII-VIV and VIV-VV redox couples for a range of different carbon materials using a variety of techniques, and it is clear that the kinetic rates depend strongly both on the type of carbon used and on the preparation of the electrode surface. We will discuss our investigations of the effects of anodic and cathodic pretreatments of carbon on electrode kinetics in the VFB. Using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), we showed for four different types of carbon that electrode treatments at negative potentials enhance the kinetics of VIV-VV and inhibit the kinetics of VII-VIII while electrode treatments at positive potentials inhibit the kinetics of VIV-VV and enhance the kinetics of VII-VIII. These observations may explain conflicting reports in the literature. We examined in detail the potentials required for activation and deactivation of electrodes. The results suggest that interchanging the positive and negative electrodes in a vanadium flow battery (VFB) would reduce the overpotential at the negative electrode and so improve the performance. This is supported by flow-cell experiments.3 Thus, periodic catholyte-anolyte interchange, or equivalent alternatives such as battery overdischarge, show promise of improving the voltage efficiency of VFBs.We have developed9-11 a standard methodology for measuring the thermal stability of vanadium catholytes based on their induction time for precipitation of V2O3. Using this, we have investigated the thermal stability of typical vanadium flow battery (VFB) catholytes at temperatures in the range 30–70°C for VV concentrations of 1.4–2.2 mol dm-3 and sulfate concentrations of 3.6–5.4 mol dm-3. In all cases, V2O5 precipitates after an induction time, which decreases with increasing temperature. Plots of the logarithm of induction time versus the inverse of temperature (equivalent to Arrhenius plots) show excellent linearity and all have similar slopes, yielding a value of 1.791±0.020 eV for the activation energy. The logarithm of induction time also increases linearly with sulfate concentration and decreases linearly with VV concentration. The slopes of these plots give values of concentration coefficients β S and β V5 which quantify the effects of concentration on induction times. Combining these with the Arrhenius slope, we have constructed a model11 to predict the stability of sulfate-based vanadium catholytes. The model can also be used as a basis for accelerated testing12 of electrolyte stability.We have investigated a range of additives and shown that many suggested additives are ineffective. However, the Group V elements phosphorus and arsenic in the +5 oxidation state have a significant stabilizing effect.13 We have also used our methodology for measurement of thermal stability to examine the stability of VFB catholytes under actual operating conditions. Results on the effect of state of charge, and deterioration and recovery of stability will be discussed. REFERENCES M. Skyllas-Kazacos et al., J. Electrochem. Soc., 158, R55 (2011). A. Weber et al., J. Appl. Electrochem., 41, 1137 (2011). M. A. Miller et al., J. Electrochem. Soc., 163, A2095 (2016).D. Oboroceanu et al., J. Electrochem. Soc., 163, A2919 (2016).C. Petchsingh et al., J. Electrochem. Soc., 163, A5068 (2016). Á. Cunha et al., Int. J. Energy Res., 39, 889 (2015).A. Bourke et al., J. Electrochem. Soc., 163, A5097 (2016). A. Bourke et al., J. Electrochem. Soc., 162, A1547 (2015).D. Oboroceanu et al., J. Electrochem. Soc., 163, A2919 (2016).D. Oboroceanu et al., J. Electrochem. Soc., 164, A2101 (2017).D. N. Buckley et al., J. Electrochem. Soc., 165, A3263 (2018).D. N. Buckley et al., J. Electrochem. Soc., 168, 030530 (2021).D. Oboroceanu et al., J. Electrochem. Soc., 166, A2270 (2019).

Thermal Stability of Polyethylene Oxide Electrolytes in Lithium Nickel Manganese Cobalt Oxide Based Composite Cathodes

Thermal runaways induced by parasitic reactions are one of the greatest intrinsic risks for lithium-ion batteries. Therefore, the thermal stability of the electrolyte in contact with electrode materials is of utmost importance for safe battery usage. While solid state electrolytes are said to be safer than liquid ones, appropriate data about their thermal stability is nearly completely missing in literature. To fill this gap, thermogravimetric analysis and differential scanning calorimetry coupled with mass spectrometry was used to analyze the thermal decomposition of composite cathodes in an argon atmosphere. The samples consisted of different polymer electrolytes mixed with lithium nickel manganese cobalt oxide (NMC622). The results show that all examined solid electrolytes are stable up to 300 °C. Above this temperature, decomposition progress depends on the lithium salt. The cathode active material also reacts with the polymer electrolytes at high temperatures. Due to this, the energy output during decomposition increases with regard to the polymer fraction. Such knowledge is fundamental for the practical use of solid polymer electrolytes.

(Invited) Influence of Salt Anion on the Electrochemical and Thermal Stability of Electrolyte

Electrolytes for Lithium ion batteries mainly consist carbonate solvents and lithium salt. It has been well known that the species and component of solvents greatly affect the stability of electrolyte. While the role of salt anion in electrolyte stability has been neglected. In this presentation, we will reveal the influence of salt anion, mainly PF6 -, on the electrochemical stability and thermal stability of electrolyte. Specifically, on the anode surface, PF6 - shows great influence on the reduction stability and passivation capability of the induced solid electrolyte interphase (SEI) film; on the cathode surface, especially at high voltage, PF6 - affects the oxidation stability of solvent. Moreover, co-oxidation of PF6 - would generate HF. On the other hand, PF6 - reduces the thermal stability of the carbonate-based electrolyte, which would be significantly exacerbated when there are transition metal ions dissolved from cathode in the electrolyte. This overlooked effect is mainly due to the high catalytic effect of transition metal ions on the thermal decomposition of PF6 - to produce PF5. Understanding the mechanisms of the effect of salt anion on the stability of the electrolyte would provide a strong theoretical guidance for the design and development of the next generation lithium-ion battery electrolyte.

The Role of TiO2 Particles on the Ion Conduction Mechanism in Polyether Electrolyte Having Cyanoethoxy Sidechain

Although solid-state polymer electrolytes (SPE) have attracted much attention as a safe electrolyte material for lithium-ion batteries (LIB), one major limitation of SPE resides in the low ionic conductivity[1]. Previous study reported the improvement of ionic conductivity by adding inorganic particles into polymer electrolytes[2]. However, a molecular-level understanding of its effect on the Li+ conduction mechanism is still lacking.In this study, we select polyether having cyanoethoxy sidechain as a polymer matrix, and clarify the effect of TiO2 nanoparticles on the Li+ conduction mechanism. Differential scanning calorimetry and thermogravimetric analysis confirmed that glass transition temperature (T g) and the thermal stability of electrolytes were not affected by TiO2. On the other hand, electrolyte with TiO2 showed higher ionic conductivity than the electrolyte without TiO2. Various (electro)chemical and spectroscopic analyses were performed to clarify the key descriptor for the ionic conductivity. Infrared spectroscopy confirmed that the Li+–CN interaction was not affected by the existence of TiO2. However, it also suggested two types of the dissociated anion (free anion and anion interacting with the surface hydroxyl group on TiO2) in the electrolyte with TiO2. Based on these results, observed improvement of Li+ conduction for TiO2-contained electrolytes can be due to the trapping of bulky anion by surface hydroxyl groups on TiO2.In conclusion, we clarify that the anion trapping by the surface hydroxyl group on TiO2 nanoparticles could facilitate Li+ conduction, probably due to the absence of bulky anion, which sterically inhibits Li+ transportation. The surface hydroxyl groups on inorganic fillers thus play a vital role in facilitating Li+ conduction.[1] X. Zhigang et al., J. Matter. Chem. A, 2015, 3, 19218–19253.[2] F. Croce et al., Nature, 1998, 394, 456–458.

Infrared Optical Measurements of Thermal Breakdown in Battery Electrolytes

Electrolyte thermal stability is critical for both battery safety and extending battery life. Battery fires are often the result of thermal breakdown of the liquid electrolyte leading to a flammable mixture of volatile products. Even in the absence of fire, electrolyte decomposition leads to battery swelling, harmful precipitates, reduced ion conductivities, and undesirable battery chemistry. Current methods of determining electrolyte stability using ex-situ techniques, such as thermal gravimetric analysis or differential scanning calorimetry, can over predict thermal stability, do not address materials compatibility within batteries, and cannot identify chemical decomposition mechanisms. Infrared optical detection of thermal stability offers an in-situ approach providing: speciation, identification of chemical bonds involved in decomposition, real-time detection, and chemical kinetics. In this work, the thermal stability of the room temperature ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO4]) is investigated using infrared (IR) spectroscopy. Quantitative IR absorption spectral data are measured for the heated ionic liquid. Spectra have been collected between 25°C and 100°C using a heated optical cell. Multiple samples and cell path lengths were used to determine quantitative values of the absorption coefficient. These results were compared to previous computational models of the ion pair to identify each absorption band with corresponding chemical bonds. The quantitative spectra were then used to measure the rate of thermal decomposition at elevated temperatures revealing the onset of breakdown above 75°C.

Conductivity and thermal stability of proton-conducting electrolytes at confined geometry of polymeric gel

The proton-conducting gel electrolytes based on poly(methyl methacrylate) (PMMA) doped by acid solutions in aprotic solvents were synthesized and discussed in this work. The gel conductivity as a function on concentrations of acid and polymer as well as of molecular mass of PMMA has been analyzed. The thermal stability of electrolytes was estimated and discussed. Extreme dependence of the conductivity on concentration is found to be for the gel (at a concentration of PMMA from 5 to 15 wt.%). The increase of electrical conductivity in the concentration range from 5 to 10 wt.% of PMMA with an increase in viscosity of the system is discussed as an indication of an involvement of the polymer matrix in increasing the mobility of the charge carriers in frame of Grotthuss mechanism.

Proton conducting gel electrolytes based on polymethylmethacrylate doped with sulfuric acid solutions in N,N-dimethylformamide

In the work presented time-stable proton conducting gel electrolytes based on polymethylmethacrylate doped with sulfuric acid solutions in N,N-dimethylformamide were synthesized. The dependences of gels conductivity on the acid concentration and the molecular mass of PMMA at 25–65 °C are studied. The thermal stability of electrolytes was estimated. The FT-IR (ATR) spectroscopy was used for understanding of mechanism of proton transfer.

Thermal stability and flammability of electrolytes for lithium-ion batteries

Safety is the key-feature of large-size lithium-ion batteries and thermal stability of the electrolytes is crucial. We investigated the thermal and flammability properties of mixed electrolytes based on the conventional ethylene carbonate–dimethyl carbonate (1:1wt/wt)–1M LiPF6 and the hydrophobic ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14TFSI). The results of thermogravimetric analyses and flammability tests of mixed electrolytes of different compositions are reported and discussed. An important finding is that though the mixtures with high contents of ionic liquid are more difficult to ignite, they burn for a longer time, once they are ignited.

Proton-conducting gel electrolytes based on poly(methyl methacrylate) doped with phosphoric acid in N,N-dimethylformamide

Time-stable proton-conducting gel electrolytes based on poly(methyl methacrylate) doped by o-phosphoric acid solutions of different concentrations in DMF are prepared. The dependences of the electrical conductivity of the gels on the concentrations of the acid and polymer and the molecular mass of PMMA at 25–65°C are studied. The thermal stability of electrolytes is estimated. It is found that an increase in the content of the polymer in gels changes the thermal behavior of the samples.

Thermal stability of LiPF 6-based electrolyte and effect of contact with various delithiated cathodes of Li-ion batteries

Thermal stability of LiPF 6-based electrolyte (1 M LiPF 6/EC + DMC) was studied by in-situ FTIR spectroscopy and C80 calorimetry, which indicated that the electrolyte underwent furious polymerization and decomposition reactions and sharp heat flow was generated below 225 °C. The thermal stability of the electrolyte in contact with various delithiated cathodes (Li x CoO 2, Li x Ni 0.8Co 0.15Al 0.05O 2, Li x Ni 1/3Co 1/3Mn 1/3O 2, Li x Mn 2O 4, Li x Ni 0.5Mn 0.5O 2, Li x Ni 0.5Mn 1.5O 4 and Li x FePO 4) was also investigated by C80 calorimetry. The results show that the cathode materials except for Li x FePO 4 usually have an enhancement effect on the decomposition of the electrolyte, but Li x FePO 4 exhibits a suppression effect on the reactions of the electrolyte. Li x FePO 4 is found to be with excellent thermal stability. Among the other cathodes, Li x CoO 2, Li x Ni 0.8Co 0.15Al 0.05O 2, Li x Ni 0.5Mn 0.5O 2 and Li x Ni 0.5Mn 1.5O 4 promote the decomposition of electrolyte by releasing oxygen and thus considered not favorable for safety, but Li x Ni 1/3Co 1/3Mn 1/3O 2 with a lesser reaction heat and Li x Mn 2O 4 with even less heat flow in the low temperature range (50–225 °C) are believed as promising cathodes for better safety. By comparing X-ray diffraction (XRD) patterns of these cathode materials at room temperature and those heated to 300 °C in the presence of the electrolyte, we have found that Li x FePO 4 only has experienced tiny structure change, which is greatly different from the other cathode materials.

Effect of vinyl acetate plus vinylene carbonate and vinyl ethylene carbonate plus biphenyl as electrolyte additives on the electrochemical performance of Li-ion batteries

For application to Li-ion batteries, we studied the electrochemical behavior and thermal stability of the following two combinations of binary electrolyte additives in a triphenylphosphate (TPP)-containing ionic electrolyte: vinyl acetate (VA) plus vinylene carbonate (VC), and vinyl ethylene carbonate (VEC) plus biphenyl (BP). Mesocarbon microbeads (MCMB) and LiCoO 2 were used as the anode and cathode materials, respectively. Cyclic voltammetry (CV), differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) were used for the analyses. These results confirmed the capability of the VEC + BP electrolyte additive to improve the cell performance and electrolyte thermal stability in TPP-containing solutions in Li-ion batteries.

Effects of functional electrolyte additives for Li-ion batteries

The electrochemical behaviour and thermal stability of functional electrolyte additives for Li-ion batteries is investigated. The Li-ion cell systems is comprised of an anode of mesocarbon microbeads (MCMB) and a cathode (LiCoO 2) in a solution of 1.1 M LiPF 6 dissolved in ethylene carbonate and ethylmethyl carbonate (EC:EMC; 4:6, v/v). Vinyl acetate (VA) and vinylene carbonate (VC) in an ionic electrolyte containing triphenylphosphate (TPP) are tested as functional electrolyte additives. The main analysis tools used in this study are cyclic voltammetry (CV), differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM). Cells containing VA or VC exhibit excellent irreversible capacity, coulombic efficiency, rate capability and cycleability. These features confirming the effectiveness of VC addition for improving both the cell performance and the thermal stability of electrolytes in TPP-containing solutions for Li-ion batteries.

Additives for Stabilizing LiPF6-Based Electrolytes Against Thermal Decomposition

Thermal stability of lithium-ion battery electrolytes composed of in a mixture of organic carbonates was probed. Generation of via the thermal dissociation of is the primary source of thermal decomposition. It was found that the addition of low concentrations (3-12%) of Lewis basic additives provides a dramatic increase in the thermal stability of the electrolyte. The increased stability is linked to the formation of base: complexes which reversibly sequester , preventing the decomposition of the electrolyte. Lewis bases investigated include pyridine, hexamethoxycyclotriphosphazene, and hexamethylphosphoramide. Our results can serve as a valuable model for stabilizing organic electrolytes against thermal decomposition initiated by Lewis acids.

Thermal Stability of Electrolytes with Mixtures of LiPF6 and LiBF4 Used in Lithium-Ion Cells

Thermal stability studies of electrolytes with mixtures of and were carried out using differential scanning calorimetry. The solvent was a mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in the volume ratio of 3:3:1, respectively. We expected the occurrence of two independent exothermic peaks associated with at the lower temperature and lithium at the higher temperature, due to decomposition reactions resulting in the Lewis acids and Instead, the mixed salt electrolyte exhibited a single exothermic peak. We deduced that the HF produced by the reaction of with solvent was the reason for the existence of one exothermic reaction peak. The HF may react with to give which is very unstable and decomposes easily to HF and at a lower temperature than the decomposition temperature of itself. By comparison, a thermal study of a mixed salt electrolyte including and showed that the exothermic reaction of with solvents is also influenced by HF produced in the reaction of with solvents but that the strength of the influence is small compared with its effect on an electrolyte mixture including © 2004 The Electrochemical Society. All rights reserved.

Thermal Stability of Electrolytes with LixCoO2 Cathode or Lithiated Carbon Anode

AbstractFor Abstract see ChemInform Abstract in Full Text.

Thermal stability of electrolytes with Li xCoO 2 cathode or lithiated carbon anode

The thermal stability of electrolytes with Li x CoO 2 cathode or lithiated carbon anode was reviewed including our recent results. From our experiments, it was found that Li x CoO 2, delithiated by a chemical method using H 2SO 4 showed two exothermic peaks, one beginning at 190 °C and the other beginning at 290 °C. From high-temperature XRD, it was found that the first peak, from 190 °C, was the phase transition from a monoclinic ( R-3 m) to a spinel structure ( Fd3 m). The spinel structure Li x CoO 2 showed a very small cycling capacity. Probably, cation mixing was induced by the heat treatment. The DSC measurements of Li 0.49CoO 2 with 1 M LiPF 6/EC+DMC showed two exothermic peaks. The peak starting at 190 °C probably resulted from the decomposition of solvent due to an active cathode surface, and the peak starting at 230 °C was electrolyte oxidation caused by released oxygen from Li 0.49CoO 2. From DSC profiles of chemically delithiated Li 0.49CoO 2 and 1 M PC electrolytes with various Li salts, it was found that the inhibition effect of the surface reaction starting at 190 °C was large when LiBF 4, LiPF 6, and LiClO 4 were used. The thermal stability of electrochemically lithiated graphite with 1 M LiPF 6/EC+DMC and PVdF-binder has been investigated. DSC revealed a mild heat generation starting from 130 °C with a small peak at 140 °C. The mild heat generation continued until a sharp exothermic peak appeared at 280 °C. The lithiated graphite with the electrolyte without PVdF-binder did not show the small peak at 140 °C. The peak at 140 °C seems to be caused by the reaction (the solid electrolyte interphase (SEI) formation) of the electrolyte and lithiated graphite, which surface is covered by poly(vinylidene fluoride) (PVdF)-binder without formation of SEI at a lower temperature.