39
The heating principle is that the gas in the measuring cell is extracted by a pump and pumped through the heating oven. The gas mixture enriched with the water from the sample is then fed back into the measuring cell (Figure 2).
With this circulation, the gas is practically ‘dry titrated’ and no separate, time-consuming gas drying is required. Another great advantage is that there is no evaporation of methanol from the Karl Fischer reagent, which would lead to contamination of the laboratory air.
In our investigations, we found that standard addition using a NIST-traceable liquid standard can be very helpful in verifying the measurement method in your own laboratory.
This standard addition is performed by adding a specifi ed amount of liquid standard (e. g. 1 ml 1000 ppm standard) to a precisely defi ned volume of the electrolyte sample in a sealed vial with a septum cap. Another vial with the same amount of sample but without standard addition is prepared at the same time.
The standard is dosed using a disposable syringe, and a disposable syringe without fi lling is also briefl y pierced through the second vial. Both vials are shaken briefl y. 1 ml of each sample is measured quickly. With this procedure, the difference between the two types can be determined and the correctness of the analysis can be verifi ed based on the recovery of the standard. The moisture in the vials, cannulas and handling steps that occurs during the procedure is also taken into account. The advantage of liquid standard dosing is that the dosed amount can be placed exactly in the range of the sample to be analysed. Because in the coulometric titration, only the absolute titrated amount of water (µg) in the titration cell is important; depending on the sample volume, this can refl ect different concentrations.
Figure 4 shows the high reproducibility when repeating the measurement of highly hygroscopic battery electrolytes. This can be achieved with careful work and a high density of the titration cell.
Figure 2: Gas extraction principle with closed loop.
The powdery, granulated, pasty samples or foils are weighed into glass vials with a tight-fi tting septum cap. For the measurement, a double-needle system pierces the septum and transports the heated water vapor into the measuring cell.
In addition to working with a constant temperature, the sample can also be heated with a temperature ramp. The heating rate is freely programmable in the range from 0.1 to 10°K/min. This computer-controlled temperature gradient heating is a quick way to develop optimal heating methods for different sample types. There are interesting starting points for further interpretation of the measurement results. In this way, the Karl Fischer titration represents an instrument for determining substance-specifi c properties.
In the following, a number of examples will be used to show which peculiarities can occur when determining the water content in battery materials and which methods are suitable.
3. Water content determination in electrolytes
Determining the water content in basically anhydrous electrolytes is one of the most demanding tasks. Electrolytes are measured using direct coulometric titration. The use of an oven technique is not recommended because the conducting salts can be thermally unstable.
Careful sampling is the basis for correct measured values. The highly hygroscopic samples absorb water as soon as they are fi lled into a sample container for transport to the laboratory, especially if the bottles are not fi lled to the brim and then sealed airtight. If possible, the bottle should not be opened to take an aliquot of the sample; a lid with a septum cap is recommended here. Due to the high volatility and high concentration of conducting salts, drawing it up in the cannula quickly leads to crystallisation. The sample should be withdrawn slightly in the cannula.
Sampling in a dry glove box atmosphere can be recommended. SO2
In addition, contact with atmospheric moisture leads to the decomposition of the conducting salt and corrosion of the steel cannulas, particularly in the case of mixtures containing SO2
do not interfere with the titration, mercaptans can be masked with N-ethylmaleimide.
. The use of disposable syringes and cannulas has proven itself. , S2
However, some additives falsify the results, e. g. dimethyl sulfoxide gives low results, sulfonyl and thionyl chloride react with water. In the case of sluggish titrations and constantly increasing drifts, Karl-Fischer reagents without alcohol can be used. These are on offer recently e.g. from Honeywell specifi cally for the battery materials [6].
A typical measurement curve can be seen in Figure 3. - and sulfonates
Figure 4: Reproducibility of water determination in electrolytes with LiALCl4 evaluation.
4. Measurements in solid electrode materials
The powdery, granulated, pasty samples or foils are weighed into glass vials with a tight-fi tting septum cap. For the measurement, a double-needle system pierces the septum and transports the heated water vapor into the measuring cell.
In addition to working with a constant temperature, the sample can also be heated with a temperature ramp. The heating rate is freely programmable in the range from 0.1 to 10 °K/min. This computer-controlled temperature gradient heating is a quick way to develop optimal heating methods for different sample types. There are interesting starting points for further interpretation of the measurement results.
Figure 5 shows an example of temperature-controlled heating of the moisture from carboxymethyl cellulose. The curve can be used to determine the optimal heating temperature (here 150°C) at which the water can be separated from the solid sample very quickly without the sample thermally decomposing.
*SO2 – one software
Figure 3: Titration curve of electrolytes with LiALCl4
*SO2
.
Figure 5: Water release from carboxymethyl cellulose as a function of temperature (temperature program 50-250 °C, 3K/min).
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