Solid State Battery Tech For EV Cars: Challenges Lie Ahead

The phrase "what's under the hood" is rapidly reaching obsolescence. For cars powered by internal combustion, the engine is often the most important factor in the car's overall performance. But today, with EVs coming to the fore, that question is moot—and not just because what's usually under their hoods are a variety of non-traditional components and maybe a little storage.

For today's consumers contemplating the purchase of an EV, the single most important component is the battery. All mass-market EVs today rely on lithium-ion battery packs to power the electric motors that make them go. (Although the chemistry is sometimes different, lithium remains the key element.) Those batteries are large and heavy, often taking up the entire floor of the car; some even gobble up portions of the trunk and the transmission tunnel, too.

Today's batteries are also sensitive to temperature extremes, still charge slowly relative to gas fill-ups, degrade over time, and can turn into terrifyingly intense infernos. Although lithium-ion battery design has gotten far better in recent years (making individual cells far less explodey), their fundamental structure means fire will always be a risk.

But there's a new type of battery in development that could revolutionize EV performance, resulting in packs that offer more energy at reduced weight and with less risk of explosion. They're called solid-state batteries, and although they hold a ton of potential, there are still a number of hurdles to overcome before they come to market.

To understand what makes solid-state batteries different, we'll have to get a little technical. So buckle up, but we'll try to keep this light.

Nearly all batteries, including traditional lithium-ion batteries and solid-state batteries (which also use lithium ion as their core chemistry), share the same basic architecture. On one side there's the cathode, which serves as the positive terminal when discharging. On the other side lives the anode, which is the negative terminal. What happens internally varies depending on the battery type, but in general a reaction causes electrons to flow through from one side to the other, creating a circuit and powering your phone or watch or car or whatever.

The lithium-ion batteries in your smartphone or EV are designed to be as small as possible, so the anode and cathode are sandwiched right next to each other, sometimes even wound around each other in the case of cylindrical batteries. Letting these two components touch would cause an electrical short, which is very bad news. So to keep things separate, a membrane called a separator is sandwiched in between. It's usually a thin plastic film.

But in order for the battery to work, ions need to flow from the anode to the cathode, or vice versa, depending on whether the battery is providing power or being recharged. How do you get them to flow through a plastic separator? You use a liquid electrolyte solution and make sure the separator is porous enough to allow it to flow.

In a solid-state battery, it's conceptually the same basic structure: anode and cathode with a separator in between. However, in this case the separator is the electrolyte, a solid, often ceramic material that allows the flow of ions directly.

It sounds like a minor change, but it has huge implications—both positive and negative. Before we look into the challenges, let's run through some solid-state advantages.

The energy density of a battery is how much actual electricity it can output for a given weight or volume. This is key because a battery with higher density means less weight, which could actually increase the range of an EV even if the battery's electrical output stays the same.

"As you move toward solid-state batteries, the reason that they're so useful, and the reason you get this over-performance benefit from them is that they allow the use of higher-energy-density anodes," said Rory McNulty, co-author of Benchmark Mineral Intelligence's Solid-State and Lithium Metal Batteries Report. McNulty says this increase could mean batteries that are three times more energy-dense than today's lithium-ion cells.

To put that into perspective, on average, an 80-kilowatt-hour battery pack in an EV today weighs about 1,000 pounds. At three times the density, an 80-kWh solid-state pack would weigh just 333 pounds. Less weight in an EV means more range.

"Solid state batteries, generally speaking, depending on how thin you can get the electrolyte, should be able to charge much faster than [today's liquid-electrolyte] lithium-ion batteries. Without the safety concerns," McNulty said.

Specific projections for solid-state battery packs are all over the place, but many solid-state startups estimate a full charge in approximately 10 to 15 minutes. A full charge in today's typical lithium-ion batteries easily takes an hour or more at a fast charger.

Under normal conditions, an EV equipped with lithium-ion batteries is perfectly safe. However, if a battery starts to get too hot due to damage or improper charging, it can start a chain reaction. Those liquid electrolytes that fill the batteries? Well, they're very flammable.

"When a battery goes into what's called thermal runaway, you get reactions with that liquid," McNulty said. "Those chain reactions generate a lot of heat, which then speeds up the reactions further and causes a fire."

With solid-state batteries, there are no liquid electrolytes, so even when you're charging at incredible rates the risk of fire stays low.

Construction of a lithium-ion battery can be a lengthy process. After the cell is constructed, there's a filling and conditioning phase where the liquid electrolyte is applied. "You gently, gently charge and discharge the battery, allowing the electrodes to form their protective coating, almost like a preparation for the battery to enter its normal life," McNulty said. "Now, with a solid-state separator, you don't need those steps, so you remove up to three weeks of processing time from your manufacturing line."

In an age of rapid manufacturing and just-in-time supply logistics, taking three weeks out of the overall manufacturing process for a car would be huge.

All of that sounds great, and it's no wonder there are dozens of startups working on bringing solid-state batteries to market, many with big funding from major OEMs and optimistic projections of product launches by 2025. That may be optimistic, though. Let's look at some of the roadblocks.

Although the internal constituents of batteries vary based on construction, lithium is a key factor in most. Globally, lithium prices have tripled in the past year alone, and that's despite global lithium production tripling in just the past five years. There is, quite simply, a global shortage of the stuff.

The problem is that solid-state batteries could actually use even more lithium than today's lithium-ion packs. Remember those higher-density anodes mentioned above? They'll likely be made of pure lithium metal. "Now, lithium metal can increase the specific energy of your battery by up to three times but it comes as pure lithium, which means the lithium intensity is also increased," McNulty said, noting this will exacerbate the lithium shortage.

"It's going to be between five and 10 times the amount of lithium for the same battery," said Dr. Jordan Lindsay, research and innovation manager at Minviro, a U.K. consulting firm that quantifies the environmental impacts of raw material production. "So, if you can nail recycling for that, cool. But if not, we're already projecting difficulties with supply chains for normal lithium-ion, so I have no idea how we resource solid-state."

According to Lindsay, as of now there are no effective ways to recycle solid-state packs.

"One of the issues with solid-state is that we're going to have to get better at recycling lithium. Currently, with lithium-ion batteries, you can recycle nickel, cobalt, manganese pretty well—aluminum and copper from the cell components pretty well," Lindsay said. "But, graphite and lithium are the issue. They're the sticking point from wholesale, closed-loop battery recycling. They're still working out the efficiencies of it."

Recycling could help address the supply chain issues, while easing environmental concerns, but Lindsay worries it may not come soon enough to address the situation: "There is this huge criticality on materials, and I don't think it's been addressed as seriously as it could be."

He continued: "I think there is this sort of messy scramble to try and get ahead and the industrial side of recycling for batteries because it's really important in terms of LCA [life cycle assessment], not necessarily an environmental impact, because the recycling processes are quite energy intensive by themselves, but in terms of reducing strain on supply chains. It's essential. We will not be able to make all these batteries without recycling."

Quick refresher: In solid-state batteries, the anode and cathode are separated by a solid electrolyte. That means smaller, denser batteries and higher-density, pure lithium for one of the electrodes.

So far, so good, right? Well, researchers have been spotting a problem, one that also plagues the lithium-ion batteries that power today's EVs, especially when they are repeatedly charged at high-power, fast-charge stations. As these batteries age, the shape of the lithium electrode has been changing, growing in weird, organic ways. The lithium is forming what are called dendrites, branching structures of metal that literally grow into the solid electrolyte.

Eventually, those dendrites grow long enough to reach through to the other side of the electrolyte, shorting out the battery pack. Again, that's bad news. Recent MIT research has determined the dendrites form due to internal stresses within the battery construction. By applying other physical stresses, those researchers found they can inhibit the dendrite growth. However, as these results are fresh out of the lab, it could be years before a solution can be applied to mass-manufacture.

Perhaps the biggest drawback of all is cost. Solid-state batteries not only require higher densities of rare metals, but their construction technique is wholly different from that of today's lithium-ion cells. That means new factories, new procedures, and new benefits of scale manufacturing that are still being invented.

There is, however, potential for these batteries to be even cheaper—eventually. "The first commercialization of a solid-state battery will not be cost-competitive with [today's] lithium-ion batteries; it will come at a cost premium," McNulty said. "But those benefits in safety and drive range and that kind of thing would likely make up for that. It's over time, over the first five to 10 years of commercialization, that it will begin to become cost-competitive as the technology improves."

Some manufacturers are reportedly producing solid-state batteries in cars within the next few years, but it's clear this will be on an extremely limited scale to start.

"So if we're talking about mass production, I would say 2030 is an optimistic suggestion for when the first solid-state batteries will begin to be purchasable on scale by consumers," McNulty said. "And those first vehicles really will be testbed systems that will be really expensive. They'll be high-performance, but I think the idea of those vehicles would be to get used to the technology, and I don't think there'll be wanting to build significant quantities of those until they're sort of mastered what the technology is like in the natural EV application."

McNulty says 2032 to 2035 is a more realistic estimate for when we might see solid-state-battery EVs in mass production. That gives battery developers about a decade to figure out the recycling and supply chain issues. But Minviro's Lindsay is optimistic that we might also be more frugal with our car battery construction by then: "It sounds sort of silly and simple, but just halving the battery size would reduce the strain loads," he said. "I think the conversation has to be about halving battery size, making batteries that function how people need."

Getting over your range anxiety, then, might be the final key to making solid-state batteries work.