Lithium ion batteries find application in industrial and consumer markets that require back-up or portable power. The cell performance, no memory effect and high power density has powered a wide range of technologies, which otherwise may not have been possible.
Lithium ion batteries, now form the basis of electric vehicles. Lithium ion technology is beneficial in many ways but a key disadvantage is the potential for thermal runaway. Since a number of new applications are emerging and with the ever-increasing demand, it is not very clear how durable or how safe the packs will be. The study of fuel cells and battery materials is done using thermal analysis and calorimetry. Isothermal calorimetry along with charging/discharging in normal and exaggerated situations is highly critical for thermal management research and estimation of long term effects. This paper details the applications of isothermal calorimetry.
The calorimetry testing discussed was conducted using the NETZSCH ARC 254-system as shown in Figure 1, equipped with the battery cycler interface kit and the 18650 VariPhi sensor.
Figure 1. NETZSCH ARC 254-system
The ARC 254 is an accelerating rate calorimeter specifically designed for adiabatic operation. The integration of VariPhiTM sensor ensures that true isothermal calorimetry experiments can also be performed
Differences between isothermal calorimetry and adiabatic calorimetry are listed below:
- Adiabatic calorimeters can study thermal safety as they can test samples even in adverse conditions.
- In adiabatic calorimetry, self-heating by the sample causes the sample temperature to increase which then leads to more heat production and eventually to thermal runaway. This self-heating of a battery pack is abnormal and cannot provide indirectly or directly heat information needed for thermal management.
- Isothermal calorimetry maintains the sample at a pre-defined temperature and measures the heat required to hold the sample at this fixed temperature.
- It is not possible for an adiabatic calorimeter to hold a battery or any system that is not uniform in temperature adiabatically at a specific temperature.
- Adiabatic calorimetry can only measure exotherms precisely and not endotherms.
- Isothermal calorimetry can measure both endotherms and exotherms accurately while keeping the temperature of the sample constant.
Unique Features of NETZSCH ARC Line
The unique features of NETZSCH ARC line are listed below:
- The patented VariPhi technology enables the operation of the ARC in such a way as to obtain isothermal and heat capacity data.
- Isothermal calorimeters are generally not designed to handle the potential risk of thermal runaway and explosion but combining the VariPhi and the ARC provides the benefit of performing the isothermal test in a robust ARC calorimeter.
The VariPhi technology was introduced over five years ago whereas the 18650 3D sensor assembly is very recent and compatible with prevailing VariPhiTM systems. The sensor is shown in Figure 2
Figure 2. VariPhi Sensor
The image of the sensor within the calorimeter is shown in Figure 3
Figure 3. VariPhi Sensor in calorimeter
The features of the sensor assembly are listed below:
- The sensor comprises a silver body.
- The inner diameter of the cylindrical sensor is designed to provide a perfect fit with a standard 18650 cell.
- Excellent measurement data is obtained when the battery is in good thermal contact with the metal surfaces.
- A sensitive thermocouple present at the center of mass detects the temperature of the housing.
- The exterior of the sensor is covered with a heater to ensure that the cylinder is maintained at a constant temperature.
- By carefully measuring the heating energy and temperature accurate isothermal temperature control is possible.
- Connections to the cycler are also provided within the isothermal zone. In certain cases the cycler software directly communicates with the ARC software. In case that cannot be done, an optional interface kit independently reads the current and voltage into the ARC software. This is performed using two high-accuracy 6 1/2 digit agilent multimeters.
- Voltage can be calculated using exclusively designed shunt resistors can easily be interchanged to adapt to differing current ranges.
- The interface kit also has a special tube heater assembly to enable wiring connections in and out of the calorimeter.
- The tube heater can also heat the wires going in and out of the calorimeter to minimize any heat exchange that might otherwise be present
A series of tests were done using a commercially available 18650 cell to gauge the system performance. An example of the results obtained is shown in the graph of heater power and temperature with respect to time in Figure 4.
Figure 4. Cell Heater Power and Cell Temperature versus Time for an Isothermal Battery Cycling Test
The conditions and results obtained from the test are listed below:
- The cell is held constant at 50°C while the cell is being charged and discharged at 500 mA.
- The temperature of the system was maintained at 50°C with only a slight diversion of 0.02°C when the instrument switched between charging and discharging modes.
- Charging or discharging of the battery was done three times and then the system was maintained constant, without cycling, to establish a baseline. This baseline or reference is a function of instrument condition and the self-discharge rate of the cell.
- This reference is used to compute the total heat lost or gained during cycling simply by integrating the peaks.
- Discharging is exothermic and charging is endothermic. The endotherms for any given cycle are smaller than the exotherm. The reason for this is more clearly illustrated in the test sequence summarized in Figure 5.
Figure 5. Constant power cycling of a cell held isothermally at 50°C
- In this test series the current for charging and discharging was increased after every cycle. In the initial few cycles the power is more or less a step pattern. As the current increases, the power curve gets distorted.
- This is because the cycler is trying to charge the battery beyond its capacity. This can be associated with the Li-ion “pressure” increasing due to mass transfer limitations at the interfaces.
- The sample power is the inverse of the heater power. The heater power is plotted with current as a function of time in Figure 6. The heater power is directly proportional to the current.
Figure 6. Cell Heater Power and Cell Current versus Time for an Isothermal Battery Cycling Test
- As the current decreases, the endothermic reaction rate also decreases which results in a decrease in the power needed to keep the battery at isothermal temperatures.
- As shown in Figure 7, the increase in voltage by the battery cycler as adding charge to the cell becomes very tough. Cell heat is not correlated as directly as it is with the current and this is especially clear during the discharge cycle where voltage drops but the cell heat remains relatively constant.
Figure 7. Cell Heat Flow and Cell Voltage versus Time for an Isothermal Battery Cycling Test
Battery cycling inside a calorimeter can provide understanding into the underlying phenomena taking place in the cell. Since this unique isothermal system is built into a robust calorimeter it is quite possible to perform isothermal cycling tests at temperatures close to or even above the specified operating temperature to get a better understanding of how the cell will behave in a range of conditions. For large cells this could result in power output which outpaces the cooling capability of the calorimeter. In such cases thermal runaway and explosion may occur so testing must always be done at a level that ensures safety. These types of calorimeters are best suited for looking at thermal runaway of cells within packs for the design of safety features and to minimize the risks of internal shorting from leading to thermal runaway. The exclusive combination of adiabatic and isothermal testing in a single unit provides the user with the necessary flexibility and safety to run a comprehensive testing program.
This information has been sourced, reviewed and adapted from materials provided by NETZSCH-Gerätebau GmbH.
For more information on this source, please visit NETZSCH-Gerätebau GmbH.