Lithium-metal batteries hold almost twice the energy of their widely used lithium-ion counterparts and they’re lighter. That combination offers the prospect of an electric vehicle that would be lighter and go much farther on a single charge. But lithium-metal batteries in the laboratory have been plagued by premature death, lasting only a fraction of the time of today’s lithium-ion batteries.
Researchers have created a lithium-metal battery that lasts for 600 cycles, meaning it can be fully charged and discharged 600 times before it dies. While the lithium-ion batteries used in electric vehicles today hold less energy, they last longer — typically at least 1,000 cycles. But vehicles won’t go as far on one charge as they would with an effective lithium-metal battery.
Instead of using anodes with more lithium, the team found a way to increase the battery’s lifetime by using incredibly thin strips of lithium just 20 microns wide, far thinner than the width of a human hair. The lithium-metal battery has an energy density of 350 watt-hours per kilogram (Wh/kg) and after 600 cycles, the battery retained 76 percent of its initial capacity. The battery is a pouch cell, which more closely mirrors real-world conditions than does a coin cell — a less realistic type of device used in many battery research projects.
The scientists found that thicker strips contribute directly to battery failure. That’s due to complex reactions around a film on the anode known as the solid electrolyte interphase (SEI). The SEI is the byproduct of side reactions between lithium and the electrolyte. It acts as an important gatekeeper that allows certain molecules to go from the anode to the electrolyte and back again while keeping other molecules at bay.
An SEI working effectively allows certain lithium ions to pass through but limits unwanted chemical reactions that reduce battery performance and accelerate cell failure. A primary goal for researchers has been to reduce unwanted side reactions between the electrolyte and the lithium metal — to encourage vital chemical reactions while restraining unwanted ones.
The team found that thinner lithium strips are adept at creating a good SEI, while the thicker strips have a higher chance of contributing to harmful SEI; the researchers use the terms “wet SEI” and “dry SEI.” The wet version retains contact between the liquid electrolyte and the anode, making important electrochemical reactions possible.
But in the dry version, the liquid electrolyte doesn’t reach all of the lithium. Simply, because the lithium strips are thicker, the electrolyte needs to flow into deeper pockets of the lithium and as it does so, it leaves other portions of the lithium dry. This stops important reactions from occurring, effectively smothering necessary electrochemical reactions, and contributes directly to the early death of the battery.