Isothermal Calorimetry can aid in the design and performance of 18650 cells. Using the VariPhi 3D sensor in the ARC® 254, true isothermal tests were conducted on a Li-ion 18650 cell. Heat is measured directly using this technique, there is no need to assume or calculate a heat capacity. The blue “baseline” shown in the diagram represent the amount of heat to hold the cell isothermally at 50C. Endothermic behavior, highest during the start of the charge cycle can be see as heater power above the baseline, exothermic behavior, as seen in the discharge cycles is below the baseline. Isothermal testing gives you the heat-output as a function of time, temperature, and cycling conditions. This data can be used to calculate thermal loads and battery efficiency.
Scanning data of a commercial coin cell LiR2032 from room temperature to 300°C at 1°C/min, showing multiple reactions during coin cell disintegration. It is evident that the exothermic SEI decomposition is followed by the melting of the separator at approximately 130°C (sharp endothermic event). This endotherm occurs nearly simultaneously with the reaction between electrolyte and lithium. The decomposition of electrolyte and reaction between lithium and binder happen in higher temperature.
- Type of sample: Coin Cell LiR2032
- Isothermal mode with temperature set at 40°C
- Charging/Discharging cycle
Constant current Constant voltage(CC-CV) – 40 mA from 4.2 V to 3.0 V
The data shows that charging is characterized by a short endotherm followed by a small exothermic reaction while discharging is characterized by a long exothermic reaction. Cycles are reliably repeatable.
The same data was used to calculate the cell efficiency during charging and discharging. Also note that the isothermal temperature varies only slightly through the discharge and charge cycle.
A series of tests were conducted on a single GM pouch (15Ah) cell to measure the heat output at different operating temperatures. A standard CC/CV* protocol from 2.5V to 4.15V at 1C**until the current dropped down to 750mA was used for all tests. A single example of a charge and discharge can be shown in Figure 1 which was conducted at 20°C.
When the battery is cycled at 40°C, the heat generated by the cell and the Joule heating represents 2.37% of the total energy supplied by the battery; at 0°C, it is more than 10.84% of the total energy. The IBC 284 is sensitive to measure low level differences of heat rate generation as well as peak heating which varies widely depending on whether the battery is tested at 0°C, 20°C or 40°C.
The second graph highlights the temperature dependent behavior of the battery. In terms of efficiency, the difference between 0°C and 40°C is lower than what we measure during a discharge; the heat generated is just 417.129mW higher, which increases the efficiency from 94.69% at 0°C to 98.72% at 40°C (Figure 2).
* Constant Current/Constant Voltage
** Charging capacity measured in Amp hours
When engineering a new lithium ion cell, different battery components must be analyzed together to determine the chemical compatibility and likelihood for thermal decomposition. Proper measurement of reaction rates and thermodynamics is improved by running larger samples sizes in cases such as these where there are multiple phases reacting.
The following graphs depicts the reactions between different electrolytes and a custom anode material. These adiabatic tests, available from the ARC® 244/254, or the MMC 274 Nexus®, show the variety of different reactions that can occur. The onset of the reaction is an important criteria but so is the relative sizes of the reaction rates in determining which combinations will lead to a safer battery.