Accelerating Rate Calorimetry and Complementary Techniques to Characterize Battery Safety Hazards

Summary

A method to characterize the potential failure hazards of lithium batteries is achieved with accelerating rate calorimetry. Heat and pressure release, visual observation of the failure event, and the capture of evolved gases are collected in this experiment to identify the worst credible threats of batteries taken to failure.

Abstract

The hazards associated with lithium-based battery chemistries are well-documented due to their catastrophic nature. Risk is typically qualitatively assessed through an engineering risk matrix. Within the matrix, potentially hazardous events are categorized and ranked in terms of severity and probability to provide situational awareness to decision makers and stakeholders. The stochastic nature of battery failures, particularly the lithium-ion chemistry, makes the probability axis of a matrix difficult to properly assess. Fortunately, characterization tools exist, such as accelerated rate calorimetry (ARC), that characterize degrees of battery failure severity. ARC has been used extensively to characterize reactive chemicals but can provide a new application to induce battery failures under safe, controlled experimental conditions and quantify critical safety parameters. Due to the robust nature of the extended volume calorimeter, cells may be safely taken to failure due to a variety of abuses: thermal (simple heating of cell), electrochemical (overcharge), electrical (external short circuit), or physical (crush or nail penetration). This article describes the procedures to prepare and instrument a commercial lithium-ion battery cell for failure in an ARC to collect valuable safety data: onset of thermal runaway, endotherm associated with polymer separator melting, pressure release during thermal runaway, gaseous collection for analytical characterization, maximum temperature of complete reaction, and visual observation of decomposition processes using a high temperature borescope (venting and cell can breach). A thermal “heat-wait-seek” method is used to induce cell failure, in which the battery is heated incrementally to a set point, then the instrument identifies heat generation from the battery. As heat generates a temperature rise in the battery, the calorimeter temperature follows this temperature rise, maintaining an adiabatic condition. Therefore, the cell does not exchange heat with the external environment, so all heat generation from the battery under failure is captured.

Introduction

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.

Protocol

1. Calibration of calorimeter NOTE: It is important to calibrate the calorimeter to accommodate any changes in heat transfer conditions to/from the same cell (e.g., connecting large diameter electrical cables to the cell) or replacement of the main measurement thermocouple. The instrument should be recalibrated after a period of 2–3 months, as thermocouple responses can change with prolonged use. Use a small spherical vessel or “bomb” for calibration of the calori…

Representative Results

Representative data from the HWS experiment of a fully charged 18650 commercial lithium-ion battery cell is provided in Figure 4A,B. The figure shows cell temperature as a function of time during a “closed” ARC testing set-up. Basic thermal features (Tonset, Tmax, and ΔT) are highlighted in the figure. The location of Tonset is the beginning of the exothermic step, which continues until Tmax is reached. Cell voltag…

Discussion

The HWS testing procedure accomplished with the ARC instrument is critical to determining the worst credible safety threat posed by a lithium-ion battery. The measurements of self-heat onset temperature and maximum temperature during thermal runaway provide the necessary objective data to accurately assess the safety of lithium-ion cells. Through the use of ARC-based experiments, battery safety metrics can be measured in a controlled and reproducible manner.

One limitation of the ARC instrumen…

Materials

borescope	Optronics		Rigid, high temperature borescope
Energy Lab Potentiostat	Princeton Applied Research / Ametek		potentiostat capable of collecting open circuit voltage, galvanostic/potentiostatic battery cycling and electrochemical impedance spectroscopy
Extended Volume Accelerating Rate Calorimeter	Thermal Hazard Technologies		Mid-sized system, sample range: components to batteries. Working volume: 0.57 m3
high temperature tape	non specific
lithium-ion battery cell	various		rechargeable mixed metal oxide versus graphite lithium-ion cell in 18650 form factor
mat heater	Omega		form factor and size dependent upon battery cell for heat capacity measurements
spherical bomb	Thermal Hazard Technologies		small volume bomb for calibration of ARC