We demonstrate, in three different rechargeable lithium-ion cells, the existence of an intrinsic relationship between a cell’s internal temperature and a readily measurable electrical parameter, namely the phase shift between an applied sinusoidal current and the resulting voltage. The temperature range examined spanned from −20 to 66 °C. The optimum single frequency for the phase measurement is in the 40–100 Hz range, allowing for a measurement time of much less than a second; the phase shift in this range depends predominantly on temperature, and is almost completely independent of the state-of-charge. Literature reports suggest that the observed dependence of the phase shift on temperature arises from the ionic conduction of the so-called solid-electrolyte-interphase layer between the graphite anode and the electrolyte.

A meter measuring the phase shift across this interphase is analogous to a thermometer reporting the temperature, thereby providing feedback for rapid corrections of any operating conditions that might lead to the catastrophic destruction of the cell. This level of monitoring and control is distinctly different from the present safety-enabling mechanisms: typically positive thermal coefficient ceramics/plastics, or “shutdown” separators based on polyethylene that act to often permanently shut down current flow through the cell.

—Srinivasan et al. (2011a)

The sensor operates through a simple electrical connection at the positive and negative terminals of the cell and can operate using power from the battery it is monitoring. With multiplexing circuitry a single sensor can monitor multiple cells in a battery pack.

We discovered that we can measure the temperature of the protective layers between the electrodes and the electrolyte of the battery during normal operation. 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.

Ultimately, the new sensor enables battery management systems to more closely manage battery performance and, more importantly, to detect unsafe thermal conditions at the critical moment when they occur, and before the cell vents or sets itself and the battery on fire. By integrating this technology into their products, manufacturers of batteries, battery management systems, and battery solution providers can increase both the safety and performance of their products.

—Rengaswamy Srinivasan

APL has applied for US and international patents for the sensor and is pursuing licensing opportunities.

Resources

• Srinivasan, R. (2012) Monitoring Dynamic Thermal Behavior of the Carbon Anode in a Lithium-ion Cell Using a Four-probe Technique. Journal of Power Sources, Vol. 198: 351-358 doi: 10.1016/j.jpowsour.2011.09.077

• Srinivasan, R., Carkhuff, B.G., Butler, M.E. and Baisden, A.C. (2011a) Instantaneous Measurement of the Internal Temperature in Lithium-ion Rechargeable Cells. Electrochimica Acta, Vol. 56: 6198-6204 doi: 10.1016/j.electacta.2011.03.136

• Srinivasan, R., Carkhuff, B.G., Butler, M.E. and Baisden, A.C. (2011b) An External Sensor for Instantaneous Measurement of the Internal Temperature in Lithium-ion Rechargeable Cells. Proceedings of the SPIE conference on Defense, Security and Sensing, 25–29 April 2011. Paper No. 8035-13.