Accelerating Rate Calorimetry and Complementary Techniques to Characterize Battery Safety Hazards

Rechargeable batteries, specifically lithium-ion chemistry, have allowed functioning of an all-electric society encompassing all aspects of daily life such as transportation, communication, and entertainment. For these energy storage applications, charge capacity equates to range or runtime. Maximizing these parameters leads to aggressively high energy lithium-ion cells. Unfortunately, as electrical energy increases within lithium-ion cells, so does detrimental energy release when a failure occurs¹. A number of regulatory agencies, professional societies, and independent laboratories have developed standards to better characterize the safety of rechargeable batteries. One method used to quantify the thermal intensity of a battery safety event is accelerated rate calorimetry (ARC)^2,3. This type of calorimetry is performed near-adiabatically to capture explicit heat generation from a material or battery cell at the onset of an exothermic reaction, then through thermal runaway and combustion type reaction processes. The ARC instrument provides an opportunity to characterize the worst-case heat, pressure, and gas generation from an exothermic material reaction in a safe and controlled laboratory environment.

The ARC instrument was first developed in the 1970s to simulate exothermic runaway reactions from hazardous and reactive chemicals at safe scales and evaluate the hazards of reactive chemicals to devise safety procedures for handling, usage, storage, and transportation⁴. In the early 1980s, ARC was first used for the purpose of studying thermal runaway reactions in lithium cells. The ARC operates through “adaptive adiabatic control”, which means the calorimeter temperature tries to match the cell temperature while a reaction is occurring. There is also no heat exchange between the sample being tested and the surrounding environment. In doing so, as the cell self-heats and its temperature rises, heat transfer between the cell and its surroundings is minimized. A schematic of the ARC chamber with heating elements and locations for lithium-ion cell testing is shown in Figure 1.

The ARC instrument is available in several sizes to accommodate a wide range of battery materials, cell components, cells, batteries, and battery modules, as shown in Table 1. The ARC also offers a range of thermal analysis testing protocols, including the most prevalent for lithium-ion battery safety characterization known as heat-wait-seek (HWS). ARC measurements can be performed in an “open” or “closed” testing configuration. The main difference between these two testing configurations is the ability to perform pressure and gas sampling measurements in the closed system. The open configuration lends itself to visual observation through use of a high temperature camera or borescope^4,5. The use of a small spherical pressure vessel or “bomb” has been utilized in the ARC to measure reaction heat release from battery electrode materials⁶. Typically, heat release is governed by the lithium concentration in the materials and intensifies in the presence of organic electrolyte solvents and lithium salts^7,8. At the cellular level, an extended volume ARC is required to safely retain the heat, pressure, and gas release from the thermal runaway process. Additionally, features can be incorporated into the ARC instrument to induce battery failures via nail penetration, electrochemical overcharge, or external short circuit.

Sandia National Laboratory has historically been a leader in ARC characterization of batteries in support of the U.S. Departments of Energy and Transportation. Sandia has published many reports highlighting its importance in generating critical safety data, which has influenced federal policy and safety standards^9,10. In the report, they provide optimal test parameters, data collection, and reporting criteria⁹. Most of the recommended practices are adopted in this article to characterize the thermal hazard of a single cylindrical lithium-ion cell under thermal runaway utilizing the HWS protocol. Specifically, the ARC can provide objective quantitative evidence of factors affecting the safety of lithium-ion batteries and battery materials (i.e., maximum temperature, heating rate as function of time/temperature, vent gas as a function of time/temperature, and chemical analysis of hazardous substances from vented gas and smoke) during a battery failure.

The most commonly used ARC testing protocol for battery safety testing is HWS. The HWS protocol offers accurate detection of exothermic reactions occurring within lithium-ion cells and is more accurate than a simple ramped heating mode. This is the standard method for battery thermal runaway characterization. The chamber is heated to an initial start temperature, then a wait time is applied that depends upon the sample mass and heat transfer properties. After this step, the calorimeter seeks for an exotherm greater than the set sensitivity (e.g., 0.02 °C/min). If no exotherm is observed in the allotted time period, the chamber again heats by a defined temperature step (e.g., 5 °C), and the process is repeated. Figure 2 shows the process flowchart for HWS (Figure 2A) and experimental data illustrating the various stages of HWS through the first several iterations (Figure 2B).

Complete definitions of each of the testing steps in the HWS protocol are as follows. Heat mode is the power given to chamber heaters to elevate chamber and device under test (DUT) temperature. Wait mode occurs when thermal equilibrium is established between the calorimeter and bomb or test article. Seek mode occurs when calculations of change in temperature are determined, and the time relates to the change in sensitivity, typically 0.02 °C/min. Cool mode is initiated at the end of a test, when a maximum temperature or pressure has been achieved. The traditional cooling mechanism involves flowing an inert gas such as nitrogen into the chamber. Alternatively, liquid nitrogen may be introduced into the chamber to expedite cooling. Exotherm mode refers to an increase in temperature observed after a seek step is termed exotherm. This describes an environment in which self-heating of the test article is greater than the selected sensitivity, typically 0.02 °C/min. Exotherm mode continues until the rate of self-heating falls below the desired sensitivity, at which point another heat mode is triggered, and the heat-wait-seek sequence continues until a maximum temperature or pressure limit is reached.