People are slowly but surely embracing electric vehicles (EVs), but the pace of that transition still needs to accelerate for the world to hit its net-zero emissions target in 2050. Despite the exponential improvements of EVs, many drivers are still reluctant to leave behind the convenience of their petrol-powered cars. Along with cost, concerns over a lack of charging stations and battery life were cited as the main barriers for US consumers buying an EV in an Ipsos Mori survey last year. For car manufacturers, much of this comes down to the persistent restrictions on range and longevity of the incumbent lithium-ion (Li-ion) batteries under EVs’ bonnets.

However, a team of scientists at Harvard University believe they have taken an important step toward solving these quandaries. Researchers at the School of Engineering and Applied Sciences (SEAS) have developed a new “solid-state” battery that can charge in the time it takes to fill up a petrol tank, and endure 3–6 times more charge cycles than the typical EV battery.

Solid-state batteries have long been considered the holy grail for a widespread transition to electrified transportation, and the race to commercialise them has sped up in recent years. The likes of Toyota and Volkswagen are developing their own versions, which they hope to get into vehicles by the end of the decade. With the boost of this latest innovation from Harvard, are solid-state batteries finally ready to live up to their hype?

The benefits of solid over liquid electrolytes

Today, Li-ion batteries rule the roost; they are used in everything from mobile phones and laptops to EVs and energy storage systems. Researchers and manufacturers have driven down the price of Li-ion batteries by 90% over the past decade and believe they can make them cheaper still. They also believe they can make an even better lithium battery.

These batteries use a liquid electrolyte to move ions between a cathode and anode when discharging and charging. However, the liquid is flammable and prevents the addition of materials that extend the life of the battery. Researchers believe one solution would be to use solid instead of liquid electrolytes.

These solid-state batteries promise a wide variety of advantages over their liquid-based counterparts. Above all, they offer a higher energy density; meaning they can store more energy per unit volume or weight, leading to either a longer battery life or smaller, lighter battery packs. They also promise a longer cycle life; withstanding more charge-discharge cycles without degrading, thereby increasing the lifespan of the battery. The use of a solid electrolyte also enables much faster charging without the risk of battery damage due to more efficient ion transport.

Solid-state batteries can operate across a wider temperature range than liquid-based batteries, allowing for better use in extreme weather. They are generally considered safer because a solid electrolyte reduces the risk of short circuits and overheating, which can lead to fires or explosions in liquid-based batteries. Finally, the solid electrolyte can be made from a wider range of cheaper and more environmentally friendly materials.

Overall, solid-state batteries have the potential to revolutionise the battery industry by offering improved performance, safety and longevity compared with traditional lithium-ion batteries. “Because of their high energy density, solid-state batteries will be most appropriate for EVs rather than [stationary] energy storage systems, and can really be a key contributor to the electrification of heavy transport,” says Teo Lombardo, an energy modeller for transport at the International Energy Agency (IEA).

“A leap forward” in solid-state battery design

The SEAS researchers developed a postage stamp-sized battery using a “pouch cell” design, rather than the typical “coin cell” variant. The battery retained 80% capacity after 6,000 charging cycles and performed well at low temperatures. It outperformed other solid-state batteries as the researchers found a way to make it with a lithium-metal anode, which has ten times the capacity of the typical graphite anode.

The new multi-layer, multi-material design was able to overcome the pervasive problem of “dendrites” – root-like structures that grow from the surface of the anode into the electrolyte. These can pierce the barrier separating the opposing cathode, leading the battery to short-circuit and, sometimes, catch fire.

The battery’s extended lifespan – equivalent to around 30 years – could markedly reduce the cost of EVs, while the ability to charge the battery in a matter of minutes gives it an exceptional power density that could lend itself to other applications.

“We were able to charge the battery in 5–10 minutes for 6,000 cycles; it usually takes EV batteries several hours to charge and they have between 1,000 and 2,000 cycles," says Xin Li, associate professor of materials science at SEAS, and the lead researcher on the project. "Our research also shows you could use other materials as an anode, such as silver, magnesium or silicon. It is definitely a leap forward towards the scaling of mass production for solid-state batteries.”

“From the lab to the real world”

Not everyone is convinced, however. “The current challenge of solid-state batteries is implementation and scale-up, rather than getting something even better at the cell level,” says Lombardo.

From an engineering perspective, a challenge that the industry has yet to overcome is manufacturing a solid-state battery pack that is able to endure extremely high pressure while also being able to “breathe” – expand and contract. “The solution to this problem could negate the energy-density gains of solid-state batteries, so that is really a question the industry needs to answer in the coming years through the scale-up process,” says Lombardo.

From the safety perspective, another problem that solid-state manufacturers need to overcome is that even if a solid-state battery does not catch fire when it short-circuits, other materials in the engine might. “Again, this is an engineering challenge that needs to be tested and verified on the industrial level,” says Lombardo.

Finally, there is the considerable hurdle of building a supply chain for solid-state batteries. According to Lombardo, battery supply chains require high-quality materials in very high volumes, because a battery fails to work with even a miniscule amount of contaminants. “That takes a long time to build,” he says. “That is also because the wider battery field is exponentially growing, so solid-state isn’t entering into a fixed market but rather one where every technology – including the traditional lithium-ion battery – is improving crazily fast and you need to gain some space in it.”

For Lombardo, the success of solid-state batteries will not come through new academic breakthroughs – “important though this study is”, he caveats – but rather how industry will solve the remaining engineering challenges and develop the associated supply chain.

“Solid-state batteries have loads of potential, but how the industry solves these [engineering] challenges will determine whether they take over the EV battery market or whether they remain a niche application for very long-distance cars and trucks,” he says.

According to recent research from Focus, an AI analysis platform that predicts technological breakthroughs based on global patent data, solid-state battery technology is improving at a rate of 31% year-on-year. Although impressive, that is currently not a sufficient pace to disrupt the incumbents – with Li-ion batteries improving at a similar 30.5%.

The IEA has forecast that solid-state batteries will play an important role in the net-zero transition, particularly to decarbonise heavy transport through applications such as electric trucks. “But it is important that we don’t over or under-hype the industry,” says Lombardo. If solid-state batteries do succeed in fulfilling their potential, it will be sometime in the 2030s, he predicts. “Right now, they really need to be moved from the lab to the real world.”

For his part, Li believes it will be around 2030 before solid-state goes mainstream. “Before then, there are still many technical barriers to overcome,” he says. “The [recent] breakthroughs don’t necessarily bring that 2030 date forward, they make that date possible.”