Lithium, the third element on the periodic table and lightest of the metals, is a chemical celebrity. It’s got a lot of uses, from making soap to treating depression, but nowhere is lithium more useful than in batteries. The last decade saw a boom in battery production that shifted global lithium use drastically. In 2011, 27 percent of all lithium was used for batteries; in 2020, it was 65 percent. Whatever device you’re reading this on right now is almost certainly powered by a Lithium-ion battery.

Li-ion batteries are the standard in consumer electronics and electric vehicles (EVs), but they do not contain lithium metal. Rather, the cathode (positive electrode) of Li-ion batteries is made of a lithium compound. This compound releases lithium ions, which travel through a liquid electrolyte to the anode (negative electrode) and back again, crossing a thin separator in between. The anode in most Li-ion batteries is graphite, the very same substance you sharpen on your pencil.

Lithium-ion vs. Lithium Metal Batteries

Unfortunately, Li-ion batteries are not the best we can do. Not by a long shot. Our batteries would be much better if we replaced the graphite anode with—here comes that battery superstar again—lithium metal.

So-called lithium metal batteries outshine Li-ion batteries in two key ways. For one, graphite has a specific capacity of 372 mAh/g, which refers to how many lithium ions the anode can store. Lithium metal has ten times that specific capacity, 3860 mAh/g. Two, lithium is about 26 percent as dense as graphite, meaning lithium metal batteries would be a lot lighter than Li-ion.

So why don’t we use lithium metal batteries already?

“When it comes to lithium metal batteries, cycling is always very poor,” explained Jie Xiao of the U.S. Department of Energy’s Pacific Northwest National Laboratory, one of the authors of a new study on lithium metal batteries: Balancing interfacial reactions to achieve long cycle life in high-energy lithium metal batteries.

It doesn’t do any good to make a high energy battery if it fails after just a few cycles of charging and discharging. It’d be like building a Ferrari that breaks down after 100 miles. To be practical in real world applications, batteries need a relatively long and stable lifetime, and current lithium metal batteries live fast and die young. Therefore, extending cycle life is one of the main goals of lithium metal battery researchers like Xiao.

How Much Lithium is Too Much For an Anode?

Xiao and her colleagues focused on one simple question in their research: how much lithium should you put in an anode?

At first glance, the solution seems obvious: the more lithium, the better, since more lithium gives you more capacity and energy. But the researchers found that more isn’t always better. Their study tested different thicknesses of lithium metal anodes in a prototype pouch cell with an energy density of 350 Wh/kg.

In a pouch cell—which, Xiao points out, is more realistic than the coin cells typically used in research settings—there are multiple layers of cathodes and anodes stacked together. For the 350 Wh/kg pouch cells used in the research, the thickness of these anodes was varied by the researchers. They tested lithium thicknesses of 100 microns, 50 microns, 20 microns, and 0 microns (anode-free, in a sense, though lithium ions are deposited during cycling to form a natural lithium anode).

The researchers found that the thinner the lithium metal anodes, the longer the lifetime of the battery, with the peak performance occurring in the 20 micron pouch cells. Why? It boils down to something called the solid electrolyte interface, or SEI.

“The liquid electrolyte inside the battery continuously reacts with lithium metal, forming a passivation film. We call it a solid electrolyte interface,” Xiao explained.

In order for the anode reaction to proceed, that SEI must be in direct contact with the liquid electrolyte. If the SEI is “dry”, it will act as an insulator and increase the cell’s impedance, hastening cell failure. There’s a limited amount of liquid electrolyte in a given cell, so the smaller the surface area of the SEI, the more of it that’s in contact with the electrolyte.

During the course of cycling, lithium metal anodes form channels that penetrate below the surface. These holes form their own SEI layer, contributing to the total surface area of SEI that must be covered by liquid electrolyte. That’s why thinner lithium anodes end up being better for cell performance: the thinner the anode, the shallower the channels, and the more SEI that’s wetted by the liquid electrolyte.

This gif illustrates the concept:

“Dry SEI will not contribute to the electrode chemical reaction in the battery, but it adds the highly resistive green layers, which increase the cell impedance. So the battery on the right will actually fail faster than the one on the left,” Xiao described.

The Optimal Thickness of Lithium Anodes

So does this mean that thinner lithium anodes are always the answer? Not necessarily. Xiao and her colleagues didn’t test any thicknesses below 20 microns (other than the anode-free case, which performed the worst of all the pouch cells), simply because no suppliers currently produce strips of lithium thinner than this. Testing 10 microns, for example, may produce even better results, or it may prove too thin. Further research is required, and Xiao is eager to test thinner anodes as soon as a supply is available.

Ultimately, the researchers concluded that the thickness of lithium metal anodes is dependent on the cell itself. The electrolyte, cathode, cell size, and other architectural details all interact to determine a cell’s performance. The optimal thickness of the lithium anode must therefore align with those other factors.

“We believe there’s always an optimized lithium metal thickness for each different cell energy and the dimension of the pouch cell,” Xiao summarized. “The 20 microns does not apply to all different lithium metal batteries. It really depends on how we design the batteries.”

The Battery500 Initiative

The new research is part of a larger project called Battery500, a consortium led by the Pacific Northwest National Laboratory that’s working to bring lithium metal batteries to the mainstream, particularly for EV applications. The name comes from the consortium’s goal of producing a battery pack with a specific energy of 500 Wh/kg, twice that of current EV batteries.

The Battery500 consortium has a long road ahead of it. Cycling stability is one of many problems that must be solved to bring commercial lithium metal batteries to the EV industry. Another is the problem of dendrites, which are pointy protrusions that form on lithium metal anodes over the course of several charge and discharge cycles. Dendrites can puncture the separator in the electrolyte and short the cell, causing it to fail and potentially combust.

“We are still focusing on the fundamental challenges to extend the cycling stability as much as possible, and then we can start to understand the thermal stability of lithium metal later,” Xiao said.

The work must proceed one step at a time. Since every part of a battery interacts with every other, researchers must test each individual parameter—e.g., anode thickness—in isolation. Xiao illustrated the complexity with a familiar analogy.

“A battery is like a human being,” she said. “You have a liquid electrolyte which is the blood. You have the head, you have the shoulders. For the human being to work, we have to make sure every component is working perfectly by itself, and also coordinating with each other. It’s the same thing for batteries. For a lithium metal battery to cycle as stable as possible, we need to make sure each component is working perfectly and compatible with the rest.”

Ultimately, it’s still early days for lithium metal batteries, but the Battery500 consortium has every confidence in that celebrated element.

“Lithium metal batteries are considered as the future battery technology. They’re quite promising, but there’s still a lot of work to do and challenges to address,” Xiao concluded.