Lithium-ion (Li-ion) battery designs may soon become safer if manufacturers adopt a device that tests the effects of a latent internal short circuit and related escalating temperatures.

Li-ion batteries power everything from cellphones to laptops to electric vehicles (EVs) because of their low weight and high storage capacity. However, the batteries can be more prone to overheating than less-efficient rechargeable batteries because of an electrolyte that can be thermally unstable at high temperatures.

The Internal Short Circuit (ISC) device, a tiny, implantable diagnostic tool designed and patented by the National Renewable Energy Laboratory (NREL) to emulate latent defects that lead to overheating, allows manufacturers to study battery responses and determine design solutions.

NREL engineer Matthew Keyser holds a sheet of copper discs, which emulates latent defects that can cause thermal runaway in lithium-ion batteries. Image credit: Ellen Jaskol, NREL. While naturally occurring internal short circuits are caused by a multitude of factors, they are usually triggered by a minor internal flaw, such as a foreign particle introduced to the battery cell during the manufacturing process. Knowing how to predict the behavior of a battery cell containing such a flaw may help designers prevent the circumstances that lead to thermal runaway.

If an internal short does occur, containing thermal runaway to a single cell can limit the damage. The NREL device can help battery manufacturers determine how to best minimize the spread of thermal runaway through design measures, such as placing barriers between cells, ensuring that vents are directed away from other cells and taking special precautions with electrical connections between cells.

The device is the first tool capable of replicating a naturally occurring internal short without tampering with the cell exterior. As such, it represents an advance from current methods used to induce battery short circuits, such as nail penetration, rod penetration, crushing the battery, applying voltage or increasing the battery's temperature.

"When you put a nail through a battery, your control over what actually causes the short is minimal," says Matthew Keyser, NREL senior engineer and one of the inventors of the device. "As the nail is driven deeper into the cell, it impacts different components and compromises the structural integrity of the cell."

In contrast, the NREL device acts as a thermal switch contained completely within the cell, delivering consistent and controllable reactions. The device can be placed in any location within a cell and produces all four types of shorts—electrode to electrode, electrode to cathode, electrode to anode and cathode to anode—each of which leads to different responses, from benign to severe.

The device is comprised of a small copper and aluminum disc, a copper puck, polyethylene or polypropylene separator and a layer of wax the diameter of one strand of hair. After implanting the device in a cell, an internal short circuit is induced by exposing the cell to higher temperatures and melting the wax layer, which is then wicked away by the separator, cathode and anode, leaving the remaining metal components to come into contact and induce an internal short. Sensors record the cell's reactions.

Research was required to identify the best, but sometimes unconventional, components for the device, such as the best wax. Wax has a melting point that ranges from 30 to 150 degrees Celsius. However, researchers soon discovered that paraffin wax—the material used in candles—was too brittle and would crack when wound into the roll of a battery cell.

They settled on microcrystalline wax, which is more pliant and is used in a wide range of non-technical applications, including cosmetics and hairspray. A mixture of microcrystalline with paraffin wax created the optimal level of tackiness, malleability and firmness to produce a consistent internal short circuit when melted.

After testing Li-ion batteries at different cycle stages for the past five years, Keyser and his team are entering into conversations with battery manufacturers to produce the device on a larger scale.