While naturally occurring internal short circuits are caused by a multitude of factors, these events are usually triggered by a minor internal flaw, such as a small foreign particle that was introduced to the battery cell during the manufacturing process.

Knowing how to predict the behavior of a battery cell through the use of NREL’s ISC device may also prevent circumstances that lead to thermal runaway. If an internal short does occur, containing thermal runaway to a single cell can limit the damage. The 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 ISC concept represents an evolutionary step forward from the 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. Built into the battery being tested, the new NREL tool is the first capable of replicating a naturally occurring internal short without tampering with the cell exterior.

When you put a nail through a battery, your control over what actually causes the short is minimal. As the nail is driven deeper into the cell, it impacts different components and compromises the structural integrity of the cell.

—Matthew Keyser

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

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

In-depth research was required to identify the best but sometimes unconventional components for the device, such as the right wax. Wax has a melting point ranging from 30 to 150 ˚C, but researchers soon discovered that paraffin wax, the same material used in candles, was too brittle and would crack when wound into the jelly roll of a battery cell.

They finally settled on microcrystalline wax, which is much 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 perfect level of tackiness, malleability, and firmness to produce a consistent internal short circuit when it 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 about producing the device on a larger scale. NREL continues to work with NASA on further improving spacesuit battery safety with modeling, and more automotive battery manufacturers are embracing the benefits delivered by the ISC device.

As NASA and our partners run additional tests with the ISC device, and use that data to validate our models, that builds our confidence that we’re simulating correctly and can apply our models to other systems, like automotive systems.

—NREL Senior Engineer Kandler Smith

In addition to the ISC device, NREL researchers are evaluating battery safety issues at multiple scales using a variety of models and simulation tools. At the particle scale, investigations of surface modifications help prevent electrolyte decomposition and subsequent gas generation. Pack-level modeling explores the propagation of stress build-up during abuse scenarios.

The trials with NASA have more than proven the benefits of the ISC device in terms of consistent and reproducible results. Working with other longstanding partners, battery makers, and industry groups like the US Advanced Battery Consortium (USABC) helps make batteries safer, and ultimately improve the performance of EDVs [electric-drive vehicles].

—Matthew Keyser, NREL senior engineer and one of the inventors of the device