Battery Power - November 2011 - (Page 16)

Feature Safety and Preventing Thermal Run Away in Large Li-Ion Batteries David White, Senior Member Technical Staff Leon Adams, Sales/Marketing Manager SouthWest Electronic Energy Group Due to time and safety precautions built into vehicles, industry and consumers have grown comfortable with the safety risk associated with gasoline engines, even though gasoline presents a high energy safety risk. However, electric vehicles powered by Li-Ion battery packs are newer and not so well understood. This article will discuss the safety advancements that should be built into Li-Ion battery packs. It highlights battery management systems (BMS) to operate safe, efficient and reliable packs that prevent thermal runaway to make Li-Ion inherently safe for vehicles, bringing it into mainstream safe operation similar to gasoline powered vehicles. Different Li-Ion Cell Chemistry and Construction Can Reduce Thermal Run Away What is Li-Ion Thermal Run Away? Li-Ion thermal run away is a complex combination of chemical reactions and/or shorts inside the cell that are initiated by excessive heat from inside or outside the cell. It is not a classic electrical fire. The chemical reactions generate additional heat which causes a positive feedback cycle to intensify the heat until there are no reactive agents left within the cell. What does Thermal Run Away Look Like? Thermal run away can look like smoldering or a fire with a flame. Figure 1 illustrates a moderate size battery pack after a single 2.2 Ah 18650 Li-Cobalt Oxide cell experienced thermal run away. Notice that the thermal run away reaction was vigorous enough to cause an eruption in the battery pack case. However, the battery pack construction, which utilized thermal heat spreading , prevented a chain reaction of other cells into thermal run away. Chemists and cell manufacturers have developed modifications to Li-Ion cell chemistries of the anode, cathode, electrolyte, separator and case to reduce the incidence and intensity of thermal run away. However, these changes come with some trade-offs such as lower cell capacity and/or higher cost. For example, regarding the anode, exchanging Lithium-titanate for the carbon anode removes the passivation breakdown reactivity of the electrolyte with the anode, minimizing energy release, and reducing internally generated heat that facilitates thermal run away. The trade-off is cell capacity reduced to half or less of the standard Li-Cobalt oxide chemistry. Regarding the cathode, Lithium-iron phosphate offers significantly less reactivity compared to Lithium Cobalt oxide, thus it does not as easily decompose and release oxygen to source the internal burn of electrolyte if thermal run away approaches. However, it also causes a reduction to about half of the cell capacity of Li-Cobalt oxide chemistry. Perhaps more promising is Nickel, Manganese and Cobalt oxide chemistries. Developed to reduce the expensive Cobalt content of the cathode, they reduce the reactivity of the cathode and limit thermal impact yet maintain the cell energy capacity equal to Li-Cobalt oxide, a seemingly preferred trade-off. In addition, changes to electrolyte, separator material, and case construction are all evolving to maximize safety and temperature range while attempting to maintain or increase cell Watthour capacity. Li-Ion Battery System Design Can Reduce or Eliminate Thermal Run Away There is no perfect cell chemistry with respect to performance and thermal run away trade-offs. Further, there is no one cell design that is functionally best for any or all power applications. Key to a safe and efficient Li-Ion battery is to conduct cell selection and battery system design with ample forethought of the target application in order to facilitate safety, performance and cost efficiency. Selecting and Obtaining the Right Cell First, select a safe cell chemistry and case construction that meets all customer and system design functional requirements. Perform in house or third party cell tests to verify that the cell meets battery system design requirements and assure that specs do not change without pre-approval. Qualify cell manufacturer(s) by confirming that a documented and verified quality system emphasizing continuous improvement is in place. Screen all incoming cells to verify continuing conformance to expected quality standards. Figure 1. Spontaneous Thermal Run Away of a Single 18650 Cell in a Moderate Size Battery Pack 16 Battery Power • November/December www.BatteryPowerOnline.com http://www.BatteryPowerOnline.com

Table of Contents for the Digital Edition of Battery Power - November 2011

Battery Power - November/December 2011
Contents
Vishay Intertechnology Introduces Powdered-Iron- Based Rx Coil for Wireless Charging
GenSet Eliminator, Designed to Reduce Fuel Consumption and Promote Safety.
The Challenges of Manufacturing Lithium-Ion Batteries for the Electric Vehicle Industry
Considerations for Customized Battery Packs
Efficient Power Protection Means Less Cost, Not Less Reliability
Safety and Preventing Thermal Run Away in Large Li-Ion Batteries
ZigBee Resource Guide
Batteries
Charging & Testing
ICs and Semiconductors
Components
Power Supplies
Industry News
Research & Development
Marketplace
Calendar of Events

Battery Power - November 2011