Better living through electricity

Energy storage has come a long way since Italian physicist Alessandro Volta stacked pairs of copper and zinc discs separated by brine-soaked cloth, creating the first battery as we know it.

In Volta’s achievement, the zinc discs ended up with extra electrons, making them electrically negative. The copper discs, on the other hand, ended up missing electrons, which made them electrically positive. When the two were connected, electrons moving from the zinc back to the copper created an electrical current.

Battery basics have stayed much the same since Volta created that first battery in 1800. Batteries have two sides — or electrodes — separated by an electrolyte. The side with extra electrons (the zinc discs in Volta’s battery) is known as the anode, while the side missing electrons (the copper discs) is the cathode. The electrolyte (in Volta’s case, the brine) allows charged ions to travel from one side to the other while barring electrons from doing the same.

The details, however, are very different. Instead of copper and zinc, modern batteries rely on lithium, the lightest metallic element and the third-lightest overall. And instead of the weak current created by Volta’s pile of discs, modern batteries can hold enough energy to power a vehicle for 300 miles or more.

The future is electric

Electricity powers our lives, and ORNL researchers are deeply involved in every aspect of the electricity ecosystem, including improved batteries and battery charging (see “Going wireless for better vehicle charging”). ORNL research also runs the gamut from basic research on new materials (see “Ensuring the supply of critical materials”) to applied research focused on the deployment of new and improved technologies (see “New center houses ORNL electricity research”).

Batteries for electric vehicles, or EVs, are central to transportation research because they have the potential to wean us away from gasoline and diesel fuel, which are responsible for the emission of more than 2 billion tons of carbon dioxide annually in the United States alone.

To realize their potential, however, batteries will have to get better. They will have to charge faster, last longer on a charge and be both safer and cheaper — a key goal, given that the cost of a battery makes up roughly 30 percent of an EV’s price tag.

Pursuing solid electrolytes

One important focus of battery research is the electrolyte, the ion-conducting medium between anode and cathode. Traditionally, the electrolyte has been a liquid, which limits both the safety and stability of a battery.

Much of the problem is that liquid electrolytes are flammable. When things go wrong — say a battery is damaged or overheats — it can burst into flames.

“The reason your phone might catch fire is there’s a flammable solution in your battery,” said ORNL materials scientist Andrew Westover. “If you set a match to it, it’ll burn like gasoline.”

ORNL researchers are addressing this shortcoming by developing solid electrolytes, which do not burst into flame.

The lab has a long history working with solid electrolytes. In 1991, for example, ORNL researchers developed a solid electrolyte called Lipon, which enabled thin-film batteries for applications such as “hazard cards,” which alerted the wearer to the presence of hazardous gases in the environment.

Making modern batteries with a solid electrolyte, however — ones able to power EVs for 300 miles or more — is a mammoth undertaking. One challenge is to get an adequate connection between the electrolyte and the anode and cathode.

“There are scientific bottlenecks,” explained Jagjit Nanda, leader of ORNL’s Energy Storage Group. “The interfacial transport between a solid–solid interface is potentially very disruptive, unlike a liquid–solid interface, because the liquid can penetrate into the pores of the electrodes.”

Westover said the Energy Storage Group is working on several approaches to a solid electrolyte, using sulfides, oxides, combinations of ceramic and polymer, and even Lipon.

“There’s probably about eight or nine criteria that really need to be met,” he said, “but if you really boil it down, ultimately, it’s, ‘Can I pass a significant amount of lithium back and forth a thousand times?’”

Rethinking the anode

Traditional lithium-ion batteries use an anode made of graphite, which stores the lithium ions. One area of research focuses on the development of batteries that don’t depend on graphite. Instead, researchers at ORNL are studying lithium metal batteries, which forgo the graphite and use lithium metal as the anode.

This approach means that the resulting battery will be lighter and more powerful.

“In the case of a traditional battery, the anode needs a graphite host — the lithium has to go into something,” Westover said. “And if that something weighs extra, it’s the weight of that lithium you put there plus the weight of that host.

“What you're doing with a lithium metal battery is essentially saying, ‘OK, we’re just going to take this host and throw it out. Basically, you’re removing a lot of dead weight and reducing the volume.”

The resulting battery takes a new approach, Nanda said.

“The lithium is stored in the cathode, which is a lithium reservoir. Then you pull the lithium from the cathode fast, and it goes through the solid electrolyte. When you charge, the ions are splayed on a current collector. And when you discharge, the battery does the work, and the lithium is restored to the cathode efficiently.”

If this approach is successful, the energy density of the resulting battery will nearly double, from 250 to up to 500 watt-hours per kilogram, according to Westover.

“Essentially, that means if you had the same weight of battery, it would last twice as long without needing to charge,” he said.

The challenges to this approach include longevity — typically, these batteries can’t go through nearly as many charging cycles — and the creation of lithium formations called dendrites.

“When you plate lithium metal without a host to contain it,” Westover said, “oftentimes what happens is you form something called lithium dendrites — long lithium needles that will pierce the electrolyte and cause a short between your anode and your cathode.”

This is especially bad news if you have a liquid electrolyte, which is subject to fire.

How these issues will be resolved remains to be determined, Westover said.

“It's pretty easy to say we use a lithium metal, but there's actually a lot of questions,” he said. “Do we need a little bit of lithium there to get us started, like a seed layer? How much do impurities affect performance? And then, really importantly, how does the interface with the lithium metal shape and form?”

Cobalt-free cathodes

Another focus of ORNL research is development of a battery cathode that doesn’t use cobalt, a metal that is both expensive and prone to supply issues.

Cobalt — in the form of lithium cobalt oxide — has been critical to the success of lithium-ion batteries since they were developed in the 1990s, according to Ilias Belharouak, head of the lab’s Electrification Section. In particular, he said, cobalt is critical to the stability and performance of the battery.

Unfortunately, we’re not going to have enough cobalt to go around, and supply bottlenecks may be especially troublesome for North America, which has relatively little cobalt.

“Cobalt is not sustainable,” Belharouak said. “Why? Because we don't have a lot of it in the U.S. If you take the U.S. and Canada, they only produce about 4 percent of the worldwide supply.”

For years, companies have worked to reduce the amount of cobalt in lithium-ion batteries. Tesla is using a substitute, called NCA, that is 80 percent nickel, 15 percent cobalt and 5 percent aluminum. According to Belharouak, that’s the least cobalt we have in a commercial car battery so far, but it’s not good enough.

“Even with that 15 percent, the cobalt needs if we were to scale these technologies to, let's say, millions of EVs, will be massive.”

In response, Belharouak’s group has developed a cobalt-free cathode using a material known as NFA, which is a mixture of nickel, iron and aluminum. The new material promises to deliver the same stability and all the benefits of cobalt — including fast charging, cost effectiveness and long lifetimes — while cutting costs. In addition, the NFA cathodes will be able to use the same manufacturing infrastructure as cobalt-based cathodes.

These benefits — especially strong performance — are critical for any cathode material that can replace cobalt, Belharouak said. In essence, the new materials must be as good as the old ones.

“The question is, are we going to get the same performance as we do with today’s lithium-ion batteries? You cannot have people, instead of utilizing their phone for a whole day, having to recharge many times a day. People enjoy certain standards, and you have to at least meet those standards.”

An electric future

This ORNL research supports a vehicle industry that is moving rapidly toward EVs and away from the internal combustion engine. The United Kingdom, for instance, plans to phase out new gasoline- and diesel-powered vehicles by 2030, and General Motors has announced plans to go all-electric by 2035.

“I think the battle for electric vehicles has pretty much been won,” Westover said. “It's just a matter of time until it dominates the market.”