Batteries based on lithium ion technology provide several benefits in portable power applications, thanks to their superior performance aspects over other technologies. However, the possibility for thermal runaway to take place both under normal situations and in situations of mishandling is one of the key disadvantages of such energy storage devices and raises concern over their safety.
Contact of the highly reactive cell components with oxygen, short circuits, and overcharging are the damage mechanisms that have the potential to discharge the whole amount of energy stored in the device within a few minutes or even seconds. This may result in spontaneous increase in temperature. Moreover, combustion of the battery components may occur when they come into contact with oxygen due to mechanical damage. Hence, care must be taken to ensure the safety of such devices. This article discusses the application of adiabatic calorimetry to perform controlled penetration of a lithium ion cell (18650) with a nail inside the device.
Penetration Test Procedure
A special holder was designed to penetrate the 18650 cell that allowed a metal perforator to pierce the cell utilizing a compressed air control system. This results in an electrical short circuit because of the penetration of the metal perforator into the battery cylinder laterally, i.e., at right angle to the lithium ion cell’s layer structure. Subsequent to the penetration, the cell is opened, which makes the battery components that are sensitive to air to react with oxygen.
The holder for piercing the batteries was designed to perform this nail test inside the NETZSCH ARC 254 calorimeter. The objective is to determine the increase in pressure and temperature during penetration. However, some tests were also performed in a hood outside the calorimeter for demonstration purposes. The vertical, cylindrical penetration sample holder along with the horizontally fixated 18650 cell is depicted in Figure 1 (upper figure). A metal perforator placed in the center of the sample holder pierces the battery at the press of a button, thus causing abrupt destruction of the cell, as shown in Figure 1 (lower figure).
The Adiabatic Calorimeter ARC 254
Highly energetic materials are typically analyzed either as individual substances or as substance mixtures in adiabatic calorimeters. Hence, adiabatic calorimeters or accelerating rate calorimeters, as shown in Figure 2, can record key parameters, including the adiabatic pressure and temperature increments, the reaction temperature (onset), and the corresponding rate at which these take place. The adibatic temperature increase can be used to determine the reaction enthalpy as well as the time duration which passes until the optimum temperature increase rate, i.e., the time-to-maximum rate is achieved.
Minor adaptions to the NETZSCH ARC 254 enable it to analyze entire battery cells with respect to their safety features. The penetration sample holder is an additional version now available, enabling penetration tests to be performed inside the calorimeter.
Penetration Test in the NETZSCH ARC 254 Calorimeter
The 18650 cell is placed into the penetration holder, followed by the fixation of a thermocouple onto the outer sheathing of the battery. The pressure-tight calorimeter chamber is then attached to a pressure cell, enabling the detection of pressure, temperature, pressure increase rate and temperature increase rate before, during and after penetration.
The set of measurement results for a charged 18650 cell at room temperature is illustrated in Figure 3. The cell was maintained at a consistent temperature inside the calorimeter over a period of 18 minutes. The metal perforator pierced the battery cell for a period of 10 seconds and was then pulled out from the battery. The electrical short circuit increased the temperature during the reaction to a maximum value of 614 °C at a heating rate of approximately 1700 K/min. The detected pressure increase was relatively minimal because the size of the calorimeter chamber is comparatively larger than the battery itself.
Figure 3. Results of a penetration test on a commercially available lithium ion cell in the NETZSCH ARC 254
Analysis of the impact of mechanical damage to lithium ion batteries is gaining interest, like the development of new cells and cell components. Mechanical damage, the influence of temperature, differing charge levels, and the reaction of cell components with oxygen during destruction of a cell are important factors that influence the assessment of measurement results. The combination of the NETZSCH ARC254 calorimeter and the penetration sample holder enables the analysis of the impact of the penetration of whole lithium ion batteries. The measurement results in the form of pressure and temperature increments enable the categorization of individual cells and the comparison of cells made from different components.
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.