The safety performance of lithium-ion (Li-ion) cells and batteries is critical to the successful implementation of this battery chemistry. The thermal abuse tolerance of the cells is a complex function of the interactions of the individual cell components at elevated temperatures. Complex reactions that generate heat, exothermic byproducts, and gas are the two main problems in Li-ion cells that lead to low abuse tolerance. Abuse conditions can result in uncontrollable chemical reactions that lead to thermal runaway. Cells should be designed so that, when they are exposed to extreme conditions (high temperature, short circuit, and overcharge), the battery cell fails gracefully. The NASA Glenn Research Center is using a variety of thermal safety analysis techniques to identify the nature and source of thermal instability in Li-ion batteries under various operational and abusive conditions. One type of study is called accelerating rate calorimetry.
An accelerating rate calorimeter (ARC, ref. 1) is an adiabatic calorimeter. Samples are loaded into titanium or hastelloy bombs suspended from the top of the calorimeter. A thermocouple is attached to the sample bomb to measure variation in temperature. A controller is programmed to increase the calorimeter’s temperature via a predetermined profile. The sample temperature increases because of convection and conduction. If the sample undergoes chemical reactions that generate heat, the sample temperature will rise. If the self-heating rate is greater than the threshold level, then the ARC proceeds into the exotherm mode and the self-heating is followed until the rate falls below the detection limit or until the end-point temperature is reached. The adiabatic self-heating rate of the sample can be measured as a function of time and temperature.
Glenn performs experiments on commercial Li-ion cells and the key electrochemical components using the ARC (see the photographs). Experiments evaluating exothermic behaviors of electrolyte materials and anode and cathode materials at different states-of-charge are conducted.
The blast enclosure and control rack of the ARC.
Sample bomb suspended from the top of the ARC. A thermocouple is attached to the bomb.
An initial test to determine when self-heating would be detected in the ARC was performed using a 2-g electrolyte sample. The electrolyte sample, consisting of 1-M lithium hexafluorophosphate (LiPF6) salt in 1:1 ethylene carbonate (EC):dimethyl carbonate (DMC) solvent, was contained in the sample bomb. A starting temperature of 40 °C, heating temperature of 5 °C/min, waiting time of 15 min, and slope sensitivity of 0.020 °C/min were used. The top and bottom graphs show temperature and pressure data plotted against time, and self-heating data plotted against the temperature profile for the electrolyte sample, respectively. The exothermic onset temperature for the electrolyte was 180 °C. After the initial electrolyte reactions, the self-heating rate increased in an almost linear mode before reaching a maximum at 265 °C. The second exotherm was detected at around 332 °C, with a pressure increase in both cases.
Temperature and pressure data versus time for 1-M LiPF6 in 1:1 EC:DMC electrolyte.
Self-heating rate and pressure versus temperature for 1-M LiPF6 in 1:1 EC:DMC electrolyte.
In order to improve the safety of Li-ion cells, it is necessary to understand the thermal stability and heat generation from the decomposition and exothermic reactions of these cells. There is a need to study the role of the components on the initialization temperature of thermal runaway and relative heat generation. The use of calorimetric methods, such as the ARC, can provide this information and, therefore, will help alleviate thermal runaway and will result in safer Li-ion cell designs.
Find out more about the research of Glenn’s Electrochemistry Branch: http://www.grc.nasa.gov/WWW/Electrochemistry/
Glenn contacts:
Doris L. Britton, 216-433-5246, Doris.L.Britton@nasa.gov
Thomas B. Miller, 216-433-6300, Thomas.B.Miller@nasa.gov
Michelle A. Manzo, 216-433-5261, Michelle.A.Manzo@nasa.gov
Authors:
Doris L. Britton and Thomas B. Miller
Headquarters program office:
Exploration Systems Mission Directorate
Programs/projects:
Exploration Space-Rated Lithium-Ion Battery Task, Extravehicular Activities
Last updated: December 17, 2007
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216-433-8258
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