A scalable, spatially resolving isoperibolic battery calorimeter (IBC) with high sensitivity and low noise

Stefan M. Sarge

Physikalisch-Technische Bundesanstalt, Bundesallee 100, 38116 Braunschweig, Germany

With the ever-increasing number of electrochemically active materials used in modern high performance high energy batteries, there is always a desire to study their thermodynamic states during charging/discharging under various conditions (state of charge (SOC), state of health (SOH), temperature, pressure, abuse conditions that eventually lead to malfunction and in the worst case to thermal runaway (TR)). These thermodynamic data are crucial i.a. for the design of the heating/cooling system, e.g. in electric vehicles (EV). In electrochemical systems such as batteries, the potentiometric method is often used in conjunction with electrochemical impedance spectroscopy (EIS) to determine the so-called reversible or entropic heat and the irreversible heat.
Traditionally, however, thermodynamic data is determined by means of calorimetry.
This contribution describes the design of an isoperibolic battery calorimeter, which is particularly suitable for pouch-type cells and enables the determination of heat capacities Cp, reaction or phase transformation enthalpies Delta-H, free enthalpies Delta-G and entropy changes Delta-S as well as Joule heats Q(loss) under various conditions (temperature, SOC, SOH, charge/discharge rate). The design of the calorimeter also allows the heat effects to be assigned to specific areas of the battery, which can be of interest if the battery is later only heated or cooled from one side. A simple change in the temperature control of the calorimeter makes it possible to determine the thermal conductivity in the plane and through the plane of the cell.
The calorimeter essentially consists of five discrete components [1]:
1. two arrays of heat flow sensors between which the pouch cell under investigation is located and which are thermally well connected (bonded) to
2. the isothermal environment, the temperature of which is controlled by one or two laboratory thermostats.
3. two heating foils are glued to the heat flow sensors for calibration, which make it possible to convert the primary output signals of the heat flow sensors (voltage) into the desired quantity (heat flow rate) by applying a known electrical power.
4. a thermally insulating shield protects the calorimeter and its isothermal environment from external temperature fluctuations and draughts.
5. control and data acquisition hardware and software enable the various measurement tasks to be carried out, and evaluation software calculates the desired variables from the measured temperatures, heat flow rates, electrical currents and voltages.
Eight to 140 commercially available Peltier elements utilising the Seebeck effect are used as heat flow sensors and individually scanned. For all components, special emphasis was placed on easy and cheap availability in order to minimise financial loss in the event of an accident. Several calorimeters of this type have been built in our laboratory to analyse cells of different sizes. Their general performance parameters are
• cell geometry: 30 x 30 x 5 mm³ to 120 x 300 x 15 mm³ (approx. 0.5 Ah to 40 Ah)
• temperature range: -10 °C to 60 °C, noise: 0.005 K, measurement uncertainty: 0.1 K
• heat flow range: 10 mW to 5 W, noise: 2 mW to 20 mW, measurement uncertainty: 2 %
The design of these calorimeters combines two capabilities that could previously only be realised using several devices and in independent measurements: The heat flow measured as a function of SOC allows the phase transformations in the graphite anode [2] to be clearly identified and their changes with decreasing SOH of the cell based on the position of the peaks. The magnitude of the signals provides information about any parasitic heat flows that increase with age, caused by side reactions that impair the efficiency [3]. The high spatial resolution of the calorimeter in conjunction with the signal-to-noise ratio, which is approx. 50 times better than that achieved to date [4], then allows this phenomenon to be assigned to a specific area of the cell, which may be subject to particular thermal stress and therefore ageing due to design conditions.

References

[1] Physikalisch-Technische Bundesanstalt, „Calorimeter and measuring system with such a calorimeter“, Utility Patent No. DE 20 2019 101 393 (2019). https://register.dpma.de/DPMAregister/pat/register?AKZ=2020191013931
[2] Dahn, J.R.: “Phase diagram of LixC6”, Phys. Rev. B 44 (1991) 9170-9177. https://doi.org/10.1103/PhysRevB.44.9170
[3] Logan, E.R.; Dahn, J.R.: „Measuring Parasitic Heat Flow in LiFePO4 /Graphite Cells Using Isothermal Microcalorimetry“, J. Electrochem. Soc. 168 (2021) 120526. https://doi.org/10.1149/1945-7111/ac405b
[4] Hu, Yang; Choe, Song-Yul; Garrick, Taylor R.: “Measurement of two-dimensional heat generation rate of pouch type lithium-ion battery using a multifunctional calorimeter”, J. Power Sources 532 (2022) 231350. https://doi.org/10.1016/j.jpowsour.2022.231350