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....

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.

1. Use a small spherical vessel or “bomb” for calibration of the calori.......

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

Name	Company	Catalog Number	Comments
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