Set-up and measurement procedure

aVoltage relaxationUT11 at 90 % SoC

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bVoltage relaxationUT11at 75 % SoC

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cVoltage difference∆UT11−T44at 90 % SoC

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dVoltage difference∆UT11−T44 at 75 % SoC Figure 5.3 –Voltage relaxationUT11a, b and voltage difference∆UT11−T44c, d of cell D after a

5 % discharge step with 2.5 A for different temperatures

5.2 Set-up and measurement procedure

Cells were charged and discharged with a BaSyTec CTS battery cycler. The battery cycler operating software BaSyTest allows additional input values via a programmable software interface, thus the voltage and temperature measurement accuracy was improved by connecting an Agilent 39720A with two 34901A 20 channel armature multiplexer. The Agilent 39720A was used as an 18-bit (5.5 digits) multimeter with a maximum absolute voltage error of 0.0020 % in the used 10 V range. The drift error is specified by the manufacturer as 0.0005 % for the measured value over a time period of 90 days at room temperature.

The temperature dependency of the voltage for the investigated cells was less than 600 µVK⁻¹ for all and less than 100 µVK⁻¹ for most SoC. The conducted temperature protocol considered the need for a large temperature range∆T due to the small

tem-aTop view bFront view

Figure 5.4–Copper block used for ATM with two holes for cylindrical cells with 18.2 mm diameter

perature induced voltage changesdU/dT, the time period, the temperature control unit needed to adjust the temperature accordingly and the necessity to avoid any negative impact of inadequate high or low temperatures on the cell. High temperatures above 40^◦C foster accelerated ageing due to side reactions as e.g. electrolyte decomposition as well as self-discharge of the cell, while low temperatures reduce the lithium-ion diffusion within the cell, delaying the voltage relaxation process.

Two different temperature profiles were used for the thermodynamic measurements, depending on the used set-up. Recurring check-ups were conducted in a Memmert IPP 200 climate chamber where the cell temperature was controlled by forced convection.

The large volume of the climate chamber slowed the heating and cooling process down, increasing the overall measurement time. As an advantage, the larger volume allowed a larger number of parallel measurements. Thus, the characterisation check-ups during the ageing period were carried out in the temperature chamber. To compensate for the limited cooling capability of the climate chamber and to reduce measurement time, a starting temperature of 26^◦C instead of 30^◦C was used, followed by two temperature steps of 4 K each, down to 18^◦C. The cells were fully charged and, based on the previous results, stepwise discharged with a current of 0.08C for 30 min, resulting in 25 discharge steps with 4 % capacity each.

Detailed measurements with a high number of SoCs at BoL and EoL were carried out inside a custom-made copper block with cylindrical holes, illustrated in Fig.5.4. The copper block was coupled to a Julabo F12-MA Refrigerated/Heating Circulator with heating and cooling capacities of 2 kW and 0.16 kW respectively. The cells were connected to the battery cycler with spring loaded Kelvin probes by mounting two front panels on the copper block. Due to the differences in cell dimension between different manufactures, two copper blocks with 18.2 mm and 18.4 mm holes were manufactured to compensate diameter variations. For BoL and EoL testing, the number of measurement points was increased and the cells were step-wise discharged with 0.1C for 6 min, resulting in 100 SoC steps based on the cell’s nominal capacityCn.

For half-cell measurements, coin cells were placed with thermally conductive pads

5.2. SET-UP AND MEASUREMENT PROCEDURE 87 on a liquid cooled plate (aluminium plate, copper tubing) and insulated. The small stack height of the coin cell ensures a uniform temperature in z-direction.

To minimise measurement time without violating the equilibrium conditions, the heat conductivity through the cell needs to be known. Due to the different number of windings, varying coating thickness and the unknown heat transfer rate between the different layers, a simulation of the values might be unreliable. Therefore, the heat transfer rate was determined experimentally by equipping cells of type A and C with thermocouples. All temperature measurements, inside as well as outside, were conducted with T-type thermocouples (class A) with an accuracy of±0.5 K from Labfacility Ltd.

The conductor thickness was 200 µm for surface temperature measurements and 76 µm for in-cell measurements. One was mounted at the surface of the cell, one in the cell core. To achieve this, the thermocouple was introduced through a hole in the bottom after drilling a 1 mm diameter hole into the cell in an argon filled glove-box. To avoid electrical contact and to minimise any chemical contamination of the cell, the exposed metal junction of the thermocouple was insulated using Apiezon^©wax. The feed through hole was sealed with a two-component epoxy glue. The modified cells were fully charged and then discharged to 60 % SoC. Resting for 60 hours allowed the cells to achieve quasi-equilibrium conditions with a negligibly small voltage relaxation. After inserting the cells into the copper block, the heating circulator conducted the desired temperature profile.

Fig. 5.5a shows the temperature of the circulator’s water bath, the surface temperature of cell A as well as the respective core temperature. Starting at 30^◦C, the circulator cooled the system to the pre-set temperature of 25^◦C within approximately 9 min. This is the minimum time achievable, as it is limited by the cooling capability of 0.16 kW of the used circulator. Fig. 5.5b demonstrates the temperature difference between the water bath and the cell surface as well as the difference between cell surface and cell core. Due to the high thermal conductivity of copper, only minor differences between the water bath and the cell surface temperature are measurable. However, the lower thermal conductivity of the electrode layers inside the cells results in a temperature difference of up to 4^◦C. A time delay of 5 min until the core temperature of the cell equalises is observed. Including a tolerance, the necessary homogeneous temperature distribution after a temperature step of∆T=5^◦C is achievable in an overall time of 15 min. Due to the higher heating capability of 2 kW, the temperature of 30^◦C is achieved within 10 min, starting from 20^◦C. Based on these findings, measuring the temperature of the cell surface is sufficient as long as the cell is given enough time to equalise. For the following data processing, the initial 14 min after a temperature change need to be discarded.

The last 5 % of the data of a temperature step are considered for the calculation, as a uniform temperature distribution throughout the cell is assumed. Fig. 5.6 illustrates the procedure where the four resulting data points are used as input for a linear regression approximation.

The linear regression model is calculated by

y= β₀+β₁x_i+e [₇₆] _(5.1)

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aTemperature of the Julabo water bath, cell surface and cell core

Figure 5.5–Applied temperature profile a and measured time delay between water bath and different positions of cell A b

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Figure 5.6–Temperature induced voltage change of cell A at an arbitrary SoC a and calculated regression line using voltage data at equilibrated conditions b