Imagine a smartphone that charges faster, lasts longer and is more eco-friendly – all at a lower cost. Aluminium-ion batteries (AIBs) could make this dream a reality, and scientists are working to unlock their potential as a more abundant, affordable and sustainable alternative to the lithium-ion batteries currently used in mobile devices, electric cars and large-scale energy storage. As part of this effort, Dmitrii A Rakov and colleagues at the University of Queensland, Australia recently overcame a technical hurdle with an AIB component called the solid-electrolyte interphase. Their insights could help AIBs match, or even surpass, the performance of their lithium-ion counterparts.

Like lithium-ion batteries, AIBs contain an anode, a cathode and an electrolyte. This electrolyte carries aluminium ions, which flow between the positively-charged anode and the negatively-charged cathode. During discharge, these ions move from the anode to the cathode, generating energy. Charging the battery reverses the process, with ions returning to the anode to store energy.

The promise and the problem

Sounds simple, right? But when it comes to making AIBs work effectively, this process is far from straightforward.

Aluminium is a promising anode material – it is lightweight and stores a lot of energy for its size, giving it a high energy density. The problem is that AIBs are prone to instabilities as they cycle between charging and discharging. During this cycling, aluminium can deposit unevenly on the anode, forming tree-like structures called dendrites that cause short circuits, leading to battery failure or even safety risks.

Researchers have been tackling these issues for years, trying to figure out how to get aluminium to deposit more evenly and stop dendrites from forming. An emerging focus of this work is something called the solid-electrolyte interphase (SEI).  This thin layer of organic and inorganic components forms on the anode as the battery charges, and like the protective seal on a jar of jam, it keeps everything inside fresh and functioning well.

In AIBs, though, the SEI sometimes forms unevenly or breaks, like a seal on a jar that doesn’t close properly. When that happens, the aluminium inside can misbehave, leading to performance issues. To complicate things further, the type of “jam” in the jar – different electrolytes, like chloroaluminate ionic liquids – affects how well this seal forms. Some electrolytes help create a better seal, while others make it harder to keep the aluminium deposits stable.

Cracking the code of aluminium deposition

In their study, which is published in ACS Nano, the Queensland scientists, together with colleagues at the University of Southern Queensland and Oak Ridge National Laboratory in the US, focused on how the aluminium anode interacts with the liquid electrolyte.  They found that the formation of the SEI layer is highly dependent on the current running through the battery and the type of counter electrode (the “partner” to the aluminium anode). Some currents and conditions allow the battery to work well for more cycles. But under other conditions, aluminium can build up in uneven, dendritic structures that ultimately cause the battery to fail.

To understand how this happens, the researchers investigated how different electrolytes and cycling conditions affect the SEI layer. They discovered that in some cases, when the SEI isn’t forming evenly, aluminium oxide (Al₂O₃) – which is normally a protective layer – can actually aggravate the problem by causing the aluminium to deposit unevenly. They also found that low currents can deplete some materials in the electrolyte, leading to parasitic reactions that further reduce the battery’s efficiency.

To solve these issues, the scientists recommend exploring different aluminium-alloy chemistries. They also suggest that specific conditioning protocols could smooth out the SEI layer and improve the cycling performance. One example of such a conditioning protocol is pre-cycling, which is a process where the battery is charged and discharged in a controlled way before regular use to condition it for better long-term performance.

“Our research demonstrates that, like in lithium-ion batteries, aluminium-ion batteries also need pre-cycling to maximize their lifetime,” Rakov tells Physics World. “This is important knowledge for aluminium-ion battery developers, who are rapidly emerging as start-ups around the world.”

By understanding the unique pre-cycling needs of aluminium-ion batteries, developers can work to design batteries that last longer and perform more reliably, bringing them closer to real-world applications.

How far are we from having an aluminium-ion battery in our mobile phones?

As for when those applications might become a reality, Rakov highlights that AIBs are still in the early stages of development, and many studies test them under conditions that aren’t realistic for everyday use. Often, these tests use very small amounts of active materials and extra electrolyte, which can make the batteries seem more durable than they might be in real life.

In this study, Rakov and colleagues focused on understanding how aluminium-ion batteries might degrade when handling higher energy loads and stronger currents, similar to what they would face in practical use. “We found that different types of positive electrode materials lead to different types of battery failure, but by using special pre-cycling steps, we were able to reduce these issues,” Rakov says.

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Which battery technologies are you focusing on at Argonne?

We work on everything. We work on lead-acid batteries, a technology that’s 100 years old, because the research community is saying, “If only we could solve this problem with cycle life in lead-acid batteries, we could use them for energy storage to add resilience to the electrical grid.” That’s an attractive prospect because lead-acid batteries are extremely cheap, and you can recycle them easily.

We work a lot on lithium-ion batteries, which is what you find in your electric car and your cell phone. The big challenge there is that lithium-ion batteries use nickel and cobalt, and while you can get nickel from a few places, most of the cobalt comes from the Democratic Republic of Congo, where there are safety and environmental concerns about exactly how that cobalt is being mined, and who is doing the mining. Then there’s lithium itself. The supply chain for lithium is concentrated in China, and we saw during COVID the problems that can cause. You have one disruption somewhere and the whole supply chain collapses.

We’re also looking at technologies beyond lithium-ion batteries. If you want to start using batteries for aviation, you need batteries with a long range, and for that you have to increase energy density. So we work on things like solid-state batteries.

Finally, we are working on what I would consider really “out there” technologies, where it might be 20 years before we see them used. Examples might be lithium-oxygen or lithium-sulphur batteries, but there’s also a move to go beyond lithium because of the supply chain issues I mentioned. One alternative might be to switch to sodium-based batteries. There’s a big supply of soda ash in the US, which is the raw material for sodium, and sodium batteries would allow us to eliminate cobalt while using very little nickel. If we can do that, the US can be completely reliant on its own domestic minerals and materials for batteries.

What are the challenges associated with these different technologies?

Frankly, every chemistry has its challenges, but I can give you an example.

If you look at the periodic table, the most electronegative element is lithium, while the most electropositive is fluorine. So you might think the ultimate battery would be lithium-fluorine. But in practice, nobody should be using fluorine – it’s super dangerous. The next best option is lithium-oxygen, which is nice because you can get oxygen from the air, although you have to purify it first. The energy density of a lithium-oxygen battery is comparable to that of gasoline, and that is why people have been trying to make solid-state lithium-metal batteries since before I was born.

The problem is that when you charge a battery with a lithium metal anode, the electrolyte deposits on the lithium metal, and unfortunately it doesn’t create a thin, planar layer. Instead, it forms these needle-like structures called dendrites that short to the battery’s separator. Battery shorting is never a good thing.

Now, if you put a mechanically hard material next to the lithium metal, you can stop the dendrites from growing through. It’s like putting in a concrete wall next to the roots of a tree to stop the roots growing into the other side. But if you have a crack in your concrete wall, the roots will find a way – they will actually crack the concrete – and exactly the same thing happens with dendrites.

So the question becomes, “Can we make a defect-free electrolyte that will stop the dendrites?” Companies have taken a shot at this, and on the small scale, things look great: if you’re making one or two devices, you can have incredible control. But in a large-format manufacturing setup where you’re trying to make hundreds of devices per second, even a single defect can come back to bite you. Going from the lab scale to the manufacturing scale is such a challenge.

What are the major goals in battery research right now?

It depends on the application. For electric cars, we still have to get the cost down, and my sense is that we’ll ultimately need batteries that charge in five minutes because that’s how long it takes to refuel a gasoline-powered car. I worry about safety, too, and of course there’s the supply-chain issue I mentioned.

But if you forget about supply chains for a second, I think if we can get fast charging with incredibly safe batteries while reducing the cost by a factor of two, we are golden. We’ll be able to do all sorts of things.

For aviation, it’s a different story. We think the targets are anywhere from increasing energy density by a factor of two for the air taxi market, all the way to a factor of six if you want an electric 737 that can fly from Chicago to Washington, DC with 75 passengers. That’s kind of hard. It may be impossible. You can go for a hybrid design, in which case you will not need as much energy density, but you need a lot of power density because even when you’re landing, you still have to defy gravity. That means you need power even when the vehicle is in its lowest state of charge.

The political landscape in the US is shifting as the Biden administration, which has been very focused on clean energy, makes way for a second presidential term for Donald Trump, who is not interested in reducing carbon emissions. How do you see that impacting battery research?

If you look at this question historically, ReCell, which is Argonne’s R&D centre for battery recycling, got established during the first Trump administration. Around the same time, we got the Federal Consortium for Advanced Batteries, which brought together the Department of Energy, the Department of Defense, the intelligence community, the State Department and the Department of Commerce. The reason all those groups were interested in batteries is that there’s a growing feeling that we need to have energy independence in the US when it comes to supply chains for batteries. It’s an important technology, there’s lots of innovations, and we need to find a way to move them to market.

So that came about during the Trump administration, and then the Biden administration doubled down on it. What that tells me is that batteries are largely bipartisan, and I think that’s at least partly because you can have different motivations for buying them. Many of my neighbours aren’t particularly thinking about carbon emissions when they buy an electric vehicle (EV). They just want to go from zero to 60 in three seconds. They love the experience. Similarly, people love to be off-grid, because they feel like they’re controlling their own stuff. I suspect that because of this, there will continue to be largely bipartisan support for EVs. I remain hopeful that that’s what will happen.

• Venkat Srinivasan will appear alongside William Mustain and Martin Freer at a Physics World Live panel discussion on battery technologies on 21 November 2024. Sign up here.

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In this episode of the Physics World Weekly podcast I explore routes to more sustainable solar energy. My guests are four researchers at the UK’s University of Oxford who have co-authored the “Roadmap on established and emerging photovoltaics for sustainable energy conversion”.

They are the chemist Robert Hoye; the physicists Nakita Noel and Pascal Kaienburg; and the materials scientist Sebastian Bonilla. We define what sustainability means in the context of photovoltaics and we look at the challenges and opportunities for making sustainable solar cells using silicon, perovskites, organic semiconductors and other materials.

The post How to boost the sustainability of solar cells appeared first on Physics World.