Helmar Wiltsche

Thermal-runaway experiments on consumer Li-ion batteries with metal-oxide and olivin-type cathodes

Li-ion batteries play an ever-increasing role in our daily life. Therefore, it is important to understand the potential risks involved with these devices. In this work we demonstrate the thermal runaway characteristics of three types of commercially available Li-ion batteries with the format 18650. The Li-ion batteries were deliberately driven into thermal runaway by overheating under controlled conditions. Cell temperatures up to 850 °C and a gas release of up to 0.27 mol were measured. The main gas components were quantified with gas-chromatography. The safety of Li-ion batteries is determined by their composition, size, energy content, design and quality. This work investigated the influence of different cathode-material chemistry on the safety of commercial graphite-based 18650 cells. The active cathode materials of the three tested cell types were (a) LiFePO4, (b) Li(Ni0.45Mn0.45Co0.10)O2 and (c) a blend of LiCoO2 and Li(Ni0.50Mn0.25Co0.25)O2.

Thermal runaway of commercial 18650 Li-ion batteries with LFP and NCA cathodes – impact of state of charge and overcharge

Thermal runaway characteristics of two types of commercially available 18650 cells, based on LixFePO4 and Lix (Ni0.80Co0.15Al0.05)O2 were investigated in detail. The cells were preconditioned to state of charge (SOC) values in the range of 0% to 143%; this ensured that the working SOC window as well as overcharge conditions were covered in the experiments. Subsequently a series of temperature-ramp tests was performed with the preconditioned cells. Charged cells went into a thermal runaway, when heated above a critical temperature. The following thermal runaway parameters are provided for each experiment with the two cell types: temperature of a first detected exothermic reaction, maximum cell temperature, amount of produced ventgas and the composition of the ventgas. The dependence of those parameters with respect to the SOC is presented and a model of the major reactions during the thermal runaway is made.

Mercury determination in soil by CVG-ICP-MS after volatilization using microwave-induced combustion

Microwave-induced combustion (MIC) was applied for mercury volatilization from soil with subsequent determination by cold vapor generation coupled with inductively coupled plasma mass spectrometry (CVG-ICP-MS). Samples of soil were mixed with microcrystalline cellulose (300 mg), pressed as pellets and combusted in closed quartz vessels pressurized with 20 bar of oxygen. Mercury was volatilized from the sample matrix during combustion and quantitatively absorbed in a suitable solution. The type and concentration of absorbing solution (diluted or concentrated nitric or hydrochloric acids, or even water) as well as the use of a reflux step after combustion were studied. Accuracy was evaluated using soil certified reference materials. Using 0.25 mol L−1 HNO3 as absorbing solution with a reflux step of 5 min the agreement with reference values was better than 95%. The limit of detection was 0.006 μg g−1 Hg (using 300 mg of sample mass) and the residual carbon content for MIC digests was always below 1%. The main advantage of the proposed method is related to the complete separation of the analyte from the sample matrix. Up to eight samples could be simultaneously combusted in only 25 min. Taking into account that only 6 ml of very diluted nitric acid solution (0.25 mol L−1) were used, the proposed MIC method coupled with CVG-ICP-MS can be considered in good agreement with green analytical chemistry recommendations.

Characterization of a multimode sample introduction system (MSIS) for multielement analysis of trace elements in high alloy steels and nickel alloys using axially viewed hydride generation ICP-AES

A commercial multimode sample introduction system (MSIS) was characterized for simultaneous multielement hydride and nonhydride-forming elements (As, Bi, Co, Cr, Fe, Mn, Mo, Ni, Sb, Se, Sn, Ti, V, W) in high alloy steels and nickel alloys by axially viewed ICP-AES. Vapor formed by the NaBH4 reaction and aerosol from a specially designed dual conical spray chamber and pneumatic nebulizer were injected simultaneously into a robust plasma. Spectroscopic effects caused by major and minor elements and nonspectroscopic interferences induced by matrix concomitants were evaluated both for conventional nebulization and for hydride generation. The objective of the study was to establish optimized operating conditions whereby both nonhydride and hydride forming elements could be determined simultaneously. It was observed that the sensitivities of the ‘hydride generating’ elements using a MSIS, were similar to those observed using conventional hydride generation. In comparison to conventional pneumatic nebulization, hydride LODs, were enhanced significantly by factors varying from about 2–20. Limits of detection using the MSIS, for nonhydride elements (Co, Cr, Fe, Mn, Mo, Ni, Ti, V, W) were similar to those reported for the conventional sample introduction system. Several fundamental parameters such as Mg II 280.270/Mg I 285.213 nm ratios, excitation temperature (Tex) and the effect of hydrogen were also determined. The effect of hydrogen, formed during hydride generation, on intensities of nonhydride and hydride forming elements and excitation temperature was evaluated. This is important because hydrogen can modify the properties of the ICP. The effect of variable concentrations of tartaric acid, L-cystein, EDTA, and 1,1,3,3-tetramethy-2-thiourea as retarding agents for reducing deleterious effects of Ni was investigated. The accuracy of the procedure was verified by analyzing several certified reference metallurgical samples. An important conclusion of this multielement study is that the application of the MSIS for simultaneous determination of hydride forming and non-hydride forming elements in complex metallurgical samples by ICP-AES is constrained by matrix derived spectral line interferences.

Matrix effects of carbon and bromine in inductively coupled plasma optical emission spectrometry

In inductively coupled plasma (ICP) based techniques the signal enhancing effect of carbon on some elements like arsenic or selenium is well documented. However, there is a large spread in the reported magnitude of this effect and whether it can be observed for other elements too. In this investigation we studied the effect of larger amounts of carbon on a total of 157 emission lines of 36 elements. A strong instrument dependence of the “carbon enhancement effect” was encountered in inductively coupled plasma optical emission spectrometry (ICP-OES), despite the use of the same sample solutions and the same sample introduction system. Several potential enhancement sources (carbon in the form of methanol, phenylalanine and CO2 as well as bromine) were compared. By tapping the high voltage power supply of the RF generator, current and voltage fed to the power oscillator could be recorded simultaneously with the emission line signal. From these data it was concluded that the carbon-based matrix effect is a combination of five factors: (1) depending on the source of carbon, changes in the sample nebulization; (2) carbon induced charge exchange reactions; (3) plasma impedance changes caused by the introduction of large quantities of carbon into the ICP: depending on the RF generator used, this effect causes power regulation problems and results in higher RF power coupled to the discharge; (4) thermal pinch effect – the ICP discharge shrinks and becomes smaller; (5) the state of matter (gaseous or liquid) of the introduced carbon sources is relevant to the magnitude of the carbon enhancement effect.

Simultaneous determination of As, Bi, Se, Sn and Te in high alloy steels—re-evaluation of hydride generation inductively coupled plasma atomic emission spectrometry

Hydride generation (HG) coupled to a simultaneous multi-element axially viewed inductively coupled plasma atomic emission spectrometer (ICP-CCD-AES) was used to determine trace contaminants (<10 mg kg−1) of As, Bi, Se, Sn, and Te in chemically resistant high alloy steels and Ni alloys. These refractory and chemically resistant samples were quantitatively digested using a mixed acid microwave-assisted procedure. EDTA, tartaric acid and 2-pyridinecarbaldehyde oxime were evaluated as interference-retarding agents. Accuracy was evaluated using spiked synthetic steel solutions and CRM steel and Ni alloys. Limits of quantification (LOQs) calculated for the solid material varied from about 1–10 mg kg−1 and satisfactory data were obtained for As, Bi, Se, Sn and Te. However, the LOQs for Sb were degraded and, as a result, this LOQ could not be determined with satisfactory accuracy in the samples tested.

Visualization, velocimetry, and mass spectrometric analysis of engineered and laser-produced particles passing through inductively coupled plasma sources

Velocities of particles passing through the load coil region of an inductively coupled plasma (ICP) attached to a quadrupole mass spectrometer (MS) were measured by particle image velocimetry (PIV). Particles were produced either by laser ablation (LA) of solid targets or from drying analyte-spiked microdroplets ejected by a piezoelectrically actuated quartz capillary. For instance, velocities determined under conditions typically applied to LA-ICP-MS analyses were found to range between 10 and 20 m s−1, depending on the axial position. Our data, furthermore, evidence significant changes of the gas velocity upon modifications of the ICP operating conditions such as plasma power, gas flow rate, and torch injector diameter if helium is admixed in excess of >50% of the total gas flow passing through the injector. For instance, an increase of the ICP RF power from 800 to 1600 W resulted in particle velocity gradients up to 15 m s−1 kW−1 measured after the third turn of the RF-coil. Temporal changes in velocity, i.e. particle accelerations over the axis of the load coil region were specified to 300–1000 m s−2. In addition, ICP-MS analyses of laser-produced aerosols carried out at constant volumetric flow rates but reduced injector diameters made signal intensities of elements such as Y, Ce, or U drop by up to two orders of magnitude suggesting incomplete particle evaporation as well as notably different aerosol penetration depths. Sensitivities measured in this case turned out to correlate with boiling points of the respective oxides rather than the element-specific ionization potentials commonly observed. The mechanisms controlling gas velocity and sensitivity variations are discussed and consequences on LA-ICP-MS analyses are drawn.

High pressure microwave-assisted flow digestion system using a large volume reactor-feasibility for further analysis by inductively coupled plasma-based techniques

A new high pressure (40 bar) continuous flow system with a large volume reactor (13.5 mL heated volume inside the microwave cavity) has been developed. The microwave-assisted digestion of the samples occurred in a coiled perfluoroalkoxy (PFA) tube reactor. As the mechanical stability of the PFA-tube is insufficient at the used digestion conditions (40 bar, >200 °C), it was placed inside an autoclave constructed from a thick-walled borosilicate tube and pressurized by nitrogen. Nitric acid and mixtures of HNO3 with HCl and/or HF were used for sample digestion and no elevated blank levels caused by contamination with corrosion products from the flow digestion system were encountered. For glucose, glycine and phenylalanine a residual carbon content (RCC) of 2.3 ± 0.5, 37 ± 3 and 77.9 ± 0.7% (mean ± standard deviation, n = 5), respectively, was obtained under optimized digestion conditions (500 W microwave power and 5.0 mL min−1 carrier flow rate). The accuracy of the method was evaluated using certified reference materials (NIST SRM 1577b, SRM 1515). The determined values were in good agreement with the certified ones using inductively coupled plasma optical emission spectrometry (ICP-OES) for analyte quantification. Moreover, a comparison between closed vessel microwave-assisted digestion and high pressure flow digestion was performed using several plant- and animal tissue samples. These materials were less finely ground than CRMs making slurry generation more difficult. Nevertheless, the element concentrations obtained by ICP-OES after flow digestion were in good agreement with those from closed vessel batch digestion.

A new approach for the digestion of diesel oil by microwave-induced combustion and determination of inorganic impurities by ICP-MS

The presence of trace elements in fuels with high vapor pressure, such as diesel oil, can cause several problems, such as the poisoning of automotive catalysts and environmental pollution; thus strict quality control is required. Despite its high digestion efficiency for some organic samples, microwave-induced combustion (MIC) is still not applied to flammable liquids because unpredictable fast reactions can result in the risk of explosions. In this study, the use of quartz wool as a flame-retardant to control the combustion rate of volatile fuels in closed systems for the further determination of trace elements via inductively coupled plasma mass spectrometry (ICP-MS) is proposed. Diesel oil is used as an example of application in this approach, and Cd, Co, Cr, Cu, Mn, Ni, Pb and V are determined in the digests via ICP-MS. The proposed system allows a safe burn of up to 400 mg of diesel oil using a pressurized oxygen atmosphere (20 bar) without a dangerous pressure increase or damage risk to the microwave system. Nitric acid (2, 4, 7 or 14.4 mol L−1) is evaluated as an absorbing solution, and quantitative results for all the analytes are obtained using the HNO3 concentration of 4 mol L−1. Using the proposed method, a very high digestion efficiency is obtained (>99%) and the accuracy is evaluated using a certified reference material, which presents an agreement higher than 97% with the certified values. In addition, the possibility of using diluted acid solutions allows negligible blank values and consequently low limit of quantification values in the range of 0.001 to 0.20 μg g−1 for all the analytes.

A novel approach to high pressure flow digestion

A new high pressure flow digestion system has been developed for sample digestion at a pressure of up to 40 bar and a temperature of about 230 °C. The reaction with acids takes place in a PFA tube and is heated by microwave radiation in a multimode cavity. As the PFA tube cannot withstand the harsh digestion conditions without support, it is placed inside a coiled glass tube pressurized by 40 bar nitrogen thus forming an autoclave. Corrosion of system components by acid fumes and related sample contamination is circumvented by establishing a slow but steady flow of the high pressure nitrogen countercurrent to the sample flow. The presented system does not constrain the selection of the digestion reagent. Acid cocktails of nitric acid with hydrochloric and/or hydrofluoric acid as well as hydrogen peroxide were successfully used for the digestion of various samples. The method accuracy was validated with five certified reference materials (BCR 62, DORM-2, NIST SRM 1515, NIST SRM 1567, NIST SRM 1568) and good agreement between the determined and the certified values was obtained for Al, Ca, Cr, Cu, Fe, Mg, Mn, Ni, Pb, and Zn using inductively coupled plasma optical emission spectrometry (ICP-OES) for analyte quantification. The flow digestion of the CRMs resulted in clear solutions with residual carbon concentrations (RCC) between 11 and 40%. Spike recoveries of Al, As, Ba, Be, Bi, Cd, Co, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Sb, Se, Sr, Ti, V, and Zn were between 94 and 105%. For Hg the spike recovery was 89%. The fully automated high pressure flow digestion system is capable of digesting up to 6 samples per hour.