This is the third in a series of posts from the IEEE Spectrum article, “How to Build a Safer, More Energy-Dense Lithium-ion Battery,” authored by Ashok Lahiri, Nirav Shah, and Cam Dales of Enovix. The article describes and illustrates how thermal runaway can occur in a conventional Li-ion battery.
The polymer separator is an inactive material and has to be physically longer and wider than the electrodes to make sure that the electrode edges do not touch each other. One way to increase the energy density is by slimming down the separator. If it gets too thin, however, the battery becomes prone to shorting out.
Another problem is the presence of microscopic metal particles—introduced unavoidably during assembly—which can accumulate on an electrically active spot, creating a major short circuit that shunts enough current between the electrodes to sharply raise the temperature. That heat, in turn, may affect neighboring areas, setting off what’s known as thermal runaway, which can produce an explosion and fire. It’s practically impossible to eliminate metal particles, because they are generated by the cutting, rolling and winding machinery during the production and assembly processes.
Additional problems can occur during charging, when lithium ions flow from the lithium metal oxide cathode to the graphite anode (the standard anode material in virtually every Li-ion battery used in mobile devices). Normally, the lithium ions fit into the gaps in the crystal lattice structure of the graphite—a process known as intercalation. But a high charge current, a local lack of active anode material, or a low ambient temperature can cause lithium ions to instead plate on the surface of the anode. Lithium metal may then accumulate as threadlike structures known as dendrites, which grow as the cell is charged and discharged, eventually puncturing the separator and creating a short, which can lead to thermal runaway. Finally, conventional Li-ion batteries can become unstable if they get too warm, which also can lead to thermal runaway.
Thermal Runaway: The construction of a conventional Li-ion cell, adapted from magnetic-audio-recording tape production techniques, makes it susceptible to thermal runaway, which can result in catastrophic damage from explosions or fires.
The article also describes how our 3D cell architecture improves battery safety.
The other great advantage of our design is improved safety. How do we achieve that? For one thing, we use a better separator.
In a conventional Li-ion cell, the separator is typically made of a plastic or polymer material because it must be flexible enough to roll up. As a result, conventional separators are more likely to fail under high temperatures. Our flat design can accommodate a ceramic separator, which is far more tolerant of heat.
Also, our silicon anode’s ample capacity to absorb lithium without swelling makes it much less susceptible to lithium plating, even with a high charge current. Should an electrical short occur anyway, our use of many distributed electrodes—as opposed to long sheets—will limit the current that can flow between any individual anode/cathode pair, which greatly reduces the risk of thermal runaway.
Our cathode design is safer, too. Typically, when cathode material hits a critical temperature (as can happen near a short), it spontaneously breaks down, releasing oxygen that can fuel a fire. This breakdown can proceed from cathode particle to cathode particle as the next particle hits the critical temperature, fueling a thermal runaway. Our architecture breaks the cathode up into hundreds or thousands of tiny segments separated by silicon, which conducts heat nearly as well as aluminum, making it hard for a runaway reaction to get started. By contrast, a conventional wound battery’s cathode is one long sheet, allowing runaway reactions to quickly spread through the device.
All these features, taken together, essentially eliminate the danger of explosion and fire.
The last excerpt in this series will describe how our we use photolithography and wafer processing techniques borrowed from the solar cell industry to produce our 3D silicon lithium-ion cell.