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Johns Hopkins Sensor Detects Overheating in Li-ion Batteries

Stories of exploding lithium-ion batteries could become much rarer if a team of scientists at Johns Hopkins University succeed in licensing new technology they've created. Researchers at the Baltimore university have developed a sensor that can warn of impending failure in the batteries, which power millions of notebooks, mobile phones, and electric vehicles.

The scientists in the institution's Applied Physics Laboratory in Laurel, MD have figured out a way to detect unsafe thermal conditions at the critical instant when they occur and before the battery cell vents or sets itself and the battery on fire. The ultimate use of the sensor is to handle battery management and performance.

This isn't a minor issue. Overheated Li-ion batteries were at the center of multiple laptop recalls in 2006. This type of failure is typically the result of "thermal runaway," a condition that occurs once a cell reaches a critical temperature.

More recently, GM put out word to owners of its new electric car, the Volt, to bring in their vehicles for servicing after the batteries caught fire during crash tests. (In that case, thermal runaway wasn't the cause; test crashes damaged the plastic casing in which the batteries were maintained, which led to coolant being leaked; those leaks caused electrical shorts, which in turn sparked the fires.)

"An abnormally high internal cell temperature is a nearly universal manifestation of something going awry with the cell," said Rengaswamy Srinivasan, one of the chemists at Johns Hopkins who invented the new sensor technology. "These changes can occur within seconds, leading to a potentially catastrophic event if corrective measures are not taken immediately. When things start to go wrong inside the cell, time is not on your side."

The team found that a very small alternating current, when applied to a Li-ion battery at specific frequencies, changes in direct proportion to the temperature of a critical interface between components of the battery.

In turn, explained Srinivasan, that current can be used to measure the temperature of the protective layers between those components. "These layers are where the conditions that lead to thermal runaway and catastrophic cell failure begin. This discovery enables us to detect potentially unsafe thermal conditions before surface-mounted temperature sensors, which are the current state of the art, are able to register that any change has taken place."

The sensor created by the team uses a simple electrical connection at the positive and negative terminals of the cell. It operates with power from the battery being monitored. A single sensor can monitor multiple cells in a battery pack.

The team said that battery makers can integrate the new technology into their Li-ion products to increase their safety and performance.

The lab has applied for patents on the new sensor and is looking for licensing opportunities.

"At the heart of lithium battery safety is not only the development of safer battery chemistries but also the availability of accurate and reliable technologies that measure the actual battery cell temperature," noted Michael Hickman, who's managing commercialization of the sensor. "This technology provides the most accurate and immediate method available for measuring the true temperature of a lithium-ion cell; and, it is the only method for measuring a cell's temperature where it counts: inside the cell where temperature changes originate."

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