Battery safety is key to the acceptance and penetration of electric vehicles into the marketplace. When battery internal shorts occur, they tend to surface without warning, and usually after the cell has been in use for several months. While some failures simply result in the cell getting very hot, in extreme cases, cells go into thermal runaway, igniting the device in which they are installed. The most publicized failures involved burning laptop and cellphone batteries, and resulted in millions of recalls.

The internal short placed within a pouch cell. (Dirk Long, NREL)

This type of field failure is caused by a latent flaw that results in a short circuit between electrodes during use. As electric car manufacturers turn to Li-ion batteries for higher energy storage capability, solving these safety issues becomes significantly more urgent. Due to the dormant nature of this flaw, battery manufacturers have found it difficult to precisely identify and study.

A device was developed that introduces a latent flaw into a battery that may be activated to produce an internal short circuit (ISC). A method was produced for placing a thermal switch into the battery that can be activated by temperature. Exemplary embodiments may be utilized with spirally wound and stacked flat-plate cells containing any of the common lithium-ion, lithium sulfur, or lithium air electrochemical components. The thermal switch placed in the battery would only activate once a predetermined temperature is achieved. Therefore, the battery can be used and cycled as long as the thermal switch temperature does not exceed the switch’s activation temperature. Once the thermal switch is activated, the cathode and anode of the battery would be electrically connected, and an internal short would result.

The switch consists of a piece of non-conductive material — typically a piece of separator material already used within the battery — whose length and width can be as small as 0.010" or as long as several inches. An electrically conductive material is placed on the piece of separator material. A partial list of how an electrically conductive material can be deposited are through chemical vapor deposition, plasma-enhanced chemical vapor deposition, physical vapor deposition, thermal evaporator, electron beam evaporator, sputtering, and plating. A sheet of electrically conductive material can also be cut to the desired shape and placed, melted, or glued onto the non-conductive material.

The electrically conductive material would have a melting point between 10 and 150 °C, and have a thickness of less than 500 microns. The thermal switch would consist of two pieces of electrically conductive material that are separated by a small gap — between 0.0005" and 0.25".

Exemplary positioning of the switch includes but is not limited to: 1) between the cathode and anode, 2) between the cathode and the negative electrode, 3) between the anode and the positive electrode, and 4) between the positive and negative electrodes.

Producing a true internal short, the device is small compared to other shorting tools being developed by industry, and does not rely on mechanically deforming the battery to activate the short, as do most of the other test methodologies. With the internal short in place, the battery can be used and cycled within normal operating conditions without activating the internal short device. This allows the battery to be aged prior to activation.

For more information, contact Matthew Keyser at This email address is being protected from spambots. You need JavaScript enabled to view it..


Tech Briefs Magazine

This article first appeared in the July, 2017 issue of Tech Briefs Magazine.

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