How can battery-powered aircraft get off the ground?

Host: Benjamin Thompson

Welcome back to the Nature Podcast. This week, getting battery-powered planes off the ground.

Host: Nick Petrić Howe

And the latest from the Nature Briefing. I’m Nick Petrić Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson.

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Interviewer: Benjamin Thompson

Over a hundred years ago, a 52 metre-long airship called La France completed an 8 kilometre round trip through the skies near Paris. The voyage was one of the very first powered flights and, in this case, that power came from a battery. There are a handful of small battery-powered planes operating right now, but jet fuel is what‘s powering the vast majority of planes that fly above our heads every day. One of the problems is that batteries are heavy, although not quite as heavy as the one from La France, which weighed several hundred kilograms. And modern batteries also take up a lot of room – not ideal if you want things like passengers and cargo. This week in Nature, there’s a Perspective article written by a group of researchers that looks at what needs to happen in order for batteries to power planes in the future. To find out more, I spoke to one of the authors, Venkat Viswanathan from Carnegie Mellon University in the US. I started by asking him why there’s so much interest in reviving battery-powered flight and what the state of the field is at the moment.

Interviewee: Venkat Viswanathan

I think the big change that has happened is batteries have been getting better and better and impressively, since 1991 when Sony first introduced the lithium-ion battery, is slowly you’ve been moving upmarket. First, it was about the small handheld devices. Second, it was sort of the laptops. And then the same laptop-type batteries then started to power cars, and then I guess at the end of 2020 we’re starting to see larger vehicles like pickup trucks and so on. And we’re successively packing in more and more energy, purely by electric power and by batteries. We still have a long way to go to continue to electrify all planes, but slowly we’ll start again in the same way that we did with cars. We’ll do the same thing in aviation where we’ll do small planes, then bigger planes, and then the very large planes hopefully someday.

Interviewer: Benjamin Thompson

One of the issues you highlight that’s preventing current batteries being used to power planes is weight, and comparably aviation fuel gives a lot more power for the same amount of weight. What’s the disparity now between lithium-ion batteries and current aviation fuel?

Interviewee: Venkat Viswanathan

If you compare just on a fuel basis, it’s about 30-40 times more. But the electric drivetrain is much more efficient than the comparable gas turbine, and so overall it’s roughly 10 times more energy per weight, and so that’s the disparity. That’s the gap we’re trying to address. And I think what will happen is some of that gap might be addressed by reducing the amount of power needed, which is what we’ve seen in cars as well, where some of the electric cars are more aerodynamic than comparable internal combustion engine vehicles. But then a large portion of the gap has to be addressed by improving the battery technology.

Interviewer: Benjamin Thompson

And it seems like future batteries have got a lot of challenges to overcome. For example, in your article you say that an aeroplane on the tarmac can be very, very hot, but yet at high altitude can be very, very cold. You need a huge amount of power to just get an aeroplane off the ground in the first instance. They’re just a couple of things there, but what does an ideal battery need to be able to do? What does a future battery look like?

Interviewee: Venkat Viswanathan

Yeah, so a future battery has to be much lighter, so that’s the first and foremost. So, it has to be able to pack a lot more energy. Second, they also need to have sort of a wide safe temperature range of operation. Batteries today are quite finicky, right? They only operate in a narrow thermal window well. And it has to have exceptional safety, so it has to have the ability to do many thousands of cycles and not have failure, which leads to battery fires. So, I think those are some of the key elements that are needed.

Interviewer: Benjamin Thompson

And where are we then? Where do we need to be?

Interviewee: Venkat Viswanathan

The best batteries today are somewhere between 180-220 watt-hours per kilogram. Watt-hour is a unit of energy. Now, if you want to do small commuter aircraft with about sort of 20 seats or so, then you need another factor of 5 from where we are. And then much larger aircrafts need maybe another factor of 2 from that. So overall, about 10 times more energy dense from where we are today. It’s a big ask, so we have to basically do more than what we have done over the last 150 years in the transition from lead-acid batteries to today’s lithium-ion batteries. We need something like that, so a leap like that, to be able to power the kinds of aircrafts we’ve been discussing.

Interviewer: Benjamin Thompson

So, it seems then, Venkat, that today’s current lithium-ion batteries aren’t necessarily suitable for this endeavour. And in your Perspective, you and your co-authors have maybe had a little bit of a look at some battery technologies that are, it has to be said, pretty new, kind of almost on the drawing board, but that might be suitable to provide this very specific kind of power-to-weight ratio that will be required. What are some of the things that are being looked at?

Interviewee: Venkat Viswanathan

The key idea is the following. What causes the weight inside a lithium-ion battery? Very simply, in a lithium-ion battery, on one side we have graphite, which is carbon, which is one of the host materials, and then on the other side we have some kind of a transition metal oxide, be it cobalt, nickel, iron, that stores and hosts the lithium, and they’re heavy. And the core concept that we advocate in this perspective is we need to lose the transition metal.

Interviewer: Benjamin Thompson

And one technology in particular that you mention a few times are CFx batteries. How do they differ and what advantages do they have?

Interviewee: Venkat Viswanathan

The key difference with CFx is that we now do away with all of the transition metal, and simply the carbon and the fluorine act as the host for lithium. And you can get lithium CFx batteries today. So, you can get them today and they deliver unprecedented performance, except that you can only use them once. So, I think that’s the challenge that we have ahead, is to turn it from being primary, which is one use, to secondary which is they should last thousands of times. I think the aircraft economics really hinges on having many, many thousands of cycles. But I’m very optimistic because we already have existing proof that we can actually do these things, right? So, we can make these devices, so all that has to happen – albeit it’s hard – but the only thing that has to happen is you have to make it rechargeable.

Interviewer: Benjamin Thompson

Let’s say, hypothetically, that one does make the grade and is tested and is safe and is ready to go, what sort of a difference would a battery-powered plane make to things like emissions?

Interviewee: Venkat Viswanathan

We have to sort of fast forward to when this will be ready. What we are doing is we are taking the emissions that would come directly from burning jet fuel to two categories of emissions. One is the emissions associated with making the battery, and the second is the source of electricity that is used to charge the battery pack, which comes from the grid. And I think, globally, the grid is turning greener and greener and greener, so the answer to that question is it depends on the source of electricity. I think it’s difficult to give you sort of a single number because that depends on what kind of aircraft you have, how large the aircraft is and so on. In the long term, with countries committed to this net zero emissions goal and the grid becoming primarily renewable power, the emissions associated with flying can come down dramatically.

Interviewer: Benjamin Thompson

And of course, it was over 100 years ago that the first battery-powered flight was demonstrated. Obviously it’s difficult to say, but conservatively, maybe, when you think we might be able to expect commercially viable electric-powered aeroplanes to be taking off around the world every day?

Interviewee: Venkat Viswanathan

So, if you look at the arc of technological progress, right, new material innovations take on the average of about two decades. I think three things are in favour to be able to shrink that time. One is our ability to watch at what happens inside a battery, what happens to the atoms, how they rearrange as they are undergoing these processes that are discharging and charging. Second, we have been able to now build robotic setups that can rapidly search and accelerate and find what kind of chemicals will give us the kind of performance we need. And third is machine learning, which allows the data to be analysed and you find new insights in ways that are unprecedented. All these three things considered, I think we can shrink that time. So, looking ahead, I think I’m very optimistic that if we start today and make a massive investment in aviation-specific batteries, over a short half-decade, we will be able to commercialise new battery innovation and start to unlock different categories of electric aviation.

Interviewer: Benjamin Thompson

That was Venkat Viswanathan from Carnegie Mellon University. Head over to the show notes where you can find a link to his perspective article.

Host: Nick Petrić Howe

Coming up, how sharing saliva could help babies identify close relationships. Right now, though, it’s time for the Research Highlights with Shamini Bundell.

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Shamini Bundell

When it comes to their trunks, elephants have some nerve. In fact, the nerve circuitry in the trunk of a female African elephant can weigh around 1.6 kilograms. That’s thanks in part to one of the largest known nerve bundles in a living organism. Elephants constantly use their trunks to touch each other or their surroundings. To investigate the nerves that carry sensory information from an elephant’s elongated nose to its brain, researchers examined the heads of eight Asian and African zoo elephants that had either died of natural causes or been put down because of health problems. They found that the elephants’ sensory-nerve fibres can be more than 2 metres long, and that the nerve that carries tactile signals from the trunk to the brain is more than three times as thick as the nerve responsible for relaying visual information. The elephant’s trigeminal ganglia, the bundle of nerve-cell bodies that sits just below the brain, is one of the largest structures used for carrying sensory information known in any animal. The findings suggest that elephants have an extraordinary sense of touch. Feel your way over to Current Biology to pack your trunk with that research.

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Shamini Bundell

Physicists have recorded images of an object using light that’s never actually touched it. Researchers shot a laser through a crystal that splits some of the beam’s photons into pairs. Each pair contained two photons at different wavelengths – for example, one in the infrared and one in the visible spectrum. The infrared photons were routed to the object to be imaged where they interacted with it. Meanwhile, the visible photons travelled a different path that didn’t approach it. The two beams were then directed back to the crystal, where information was transferred from the infrared to the visible light. The visible light then carried that information to a detector that recorded an image, even though those particles had never been near the object. The process, called quantum holography, could lead to new methods of imaging samples that are transparent to visible light. That research is available in the visible spectrum in Science Advances.

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Host: Benjamin Thompson

Finally on the show, it’s time for the Briefing Chat, where we discuss a couple of articles that have been highlighted in the Nature Briefing. Nick, what have you got for us this week?

Host: Nick Petrić Howe

Well, Ben, I’ve been reading in Stat about an investigation of mental health apps and how many of them actually work.

Host: Benjamin Thompson

And these, Nick, I guess are apps on people’s phones, and there’s a bunch of them right, which can help with maybe breathing exercises and mindfulness, that sort of thing.

Host: Nick Petrić Howe

Yeah, that sort of thing. There’s actually a huge range of them – everything from counselling all the way to meditation and that sort of thing. So, this was by some researchers at the University of Wisconsin-Madison, and they were making their own meditation app. And when they were making it, they wanted to check that it was actually going to work very well. And so, what they did is they went back to the literature to see what has worked and how they’ve been assessed to think of the best way to assess their own app. And what they ended up doing was grouping together quite a few meta analyses that have already been done into an even bigger meta review to try and assess how many of these apps actually work and what levels of effects you could see.

Host: Benjamin Thompson

And tell me about the data that they pulled together then, Nick. What are we talking about?

Host: Nick Petrić Howe

Well, they grouped together 14 meta analyses and that included 145 trials with 50,000 patients, so a huge amount of data, and with all of this massive amount of data, they failed to find convincing evidence in support of any mobile phone based intervention on any outcome. So, it didn’t seem like any of these apps were having at least a convincing level of effect.

Host: Benjamin Thompson

And what sort of outcomes were they looking at, Nick, and what did people report that they found from using these apps?

Host: Nick Petrić Howe

So, the outcomes they were looking for were what the apps actually set out to do. So, for example, it could be something like making people feel more mindful, making them feel more calm or something like that. And they wanted to see if there were effects there. And what they found was that there were effects but they just didn’t meet the very high bar that they’d set, which was this sort of level of convincing evidence because, in a lot of cases, they wouldn’t compare to other kinds of treatments, so they wouldn’t compare the app to, say, talking to a counsellor or something like that. And also, there was an element of publication bias that they identified as well, so this is people only publishing results that were favourable. They weren’t showing all the negative results as well. So, it’s hard to say that the evidence is convincing when you only have the favourable results.

Host: Benjamin Thompson

So, that’s not to say then that these apps are inherently without their use, it’s just maybe the ways they’ve been tested haven’t proved to a particular level that they are themselves useful.

Host: Nick Petrić Howe

Exactly. And the authors themselves say that this is actually quite promising because this is a very high bar that they’ve set, this sort of convincing evidence. But if you sort of look at a slightly lower bar, which they call highly suggestive evidence, there are quite a few that actually meet that criteria. It’s just that the effects diminish as you are more and more rigorous in how you assess them. And while the effects may be small, the advantage of these apps is that they’re widely accessible. Not everyone will be able to go and talk to a counsellor or have many therapy sessions or whatever it is, but they may be able to access an app, and also it’s widely scalable as well. You can get apps to hundreds of thousands of people very, very quickly. So, in the future, they think that more tests will be done, more rigorous tests, and better ways to assess these apps will become apparent and we will get an idea of which interventions are actually going to be very, very helpful for people.

Host: Benjamin Thompson

Well, nice one, Nick. That’s an interesting story indeed, and I’m looking forward to seeing those next rounds of analyses when they come out. I’ve also got a story that was reported in Stat, but this one was from a paper in Science, and it’s a story that’s maybe helping to make sense of how young children and infants identify some of the social relationships in their worlds, and it revolves around saliva.

Host: Nick Petrić Howe

Oh, okay. I was not expecting that to be the next word coming from you. I guess how young are we talking and how does saliva factor into discerning relationships?

Host: Benjamin Thompson

Well, in this work, they’ve looked at sort of very, very young infants and toddlers and slightly older children. But in terms of saliva, I think it’s an interesting one. So, kind of the central point of this work is that saliva helps children to distinguish what are called ‘thick’ relationships. Now, these are relationships between people who have a close bond. So, swapping saliva, things like kissing or maybe sharing eating utensils, those sorts of things identifies these thick relationships. And if you’re a parent, you’ll know that there’s a lot of that that happens between a child and a parent. But what’s interesting about this work is that much of it looks at how small children use saliva sharing to define relationships between other people that they could see.

Host: Nick Petrić Howe

Right, and I’m really curious now how on Earth they actually work out what the baby is seeing and how they define relationships. What were the experiments involved in this?

Host: Benjamin Thompson

There’s a bunch of experiments in this work, and I’ll maybe give you a little flavour of a couple of them. Let’s start with the slightly older children, so 5-7-year-olds. Now, they looked at cartoons and were more likely to predict that characters that shared utensils or that licked the same food item were likely to be from within the same family. Whereas characters in these cartoons that shared toys could be in a family or they could be in a friendship group, so kind of a differentiation there. But when it comes to the younger children, so toddlers and infants, the researchers used some puppets to try and work out what was going on, and this is kind of neat. So, in one of the experiments, they got two actors to sit down and between them they had like a glove puppet. Now, one of these actors kind of ate a piece of orange and shared that with the puppet, so saliva-sharing behaviour there. And the other actor played with a ball and shared that with the puppet in the middle, so non-saliva sharing. And then what happened is this puppet showed some signs of distress, and these very young children, these infants and these toddlers, were more likely to look first and longer at the orange-sharing actor than at the actor with the ball, suggesting that they thought that this was a close relationship, a thick relationship, and that this actor was perhaps more likely to provide some care or some comfort to this puppet.

Host: Nick Petrić Howe

Wow, okay. That’s a really interesting study design. And what do they think are the sort of implications of this? What could this tell us about relationships and about how infants view them?

Host: Benjamin Thompson

I mean, from what I understand, it really does give an insight into how children figure out the world around them. Now, I would say that in this study it was quite a diverse group of children that were involved in the research, but they were all from the US, and culturally and around the world there are different kind of societal norms and of course there are other behaviours as well that can be used to identify thick relationships. But this is maybe one of them, and in this paper they say that it might be helpful, for example, for young children to identify kin, maybe parents or siblings, versus non-kin, so maybe a nursery teacher or a nanny or so forth. So, two groups who give them care but obviously very, very different social relationships.

Host: Nick Petrić Howe

Wow, that is absolutely fascinating research, and I always love experiments that involve puppets, so thanks for bringing that to the Briefing chat, Ben. And listeners, if you’re interested in more stories like these then why not check out the Nature Briefing. We’ll put a link of where to sign up along with the articles we discussed in the show notes.

Host: Benjamin Thompson

And that’s all for the podcast this week. As always, if you want to get in touch, we’re on Twitter – we’re @NaturePodcast. Or you can send us an email – we’re podcast@nature.com. I’m Benjamin Thompson.

Host: Nick Petrić Howe

And I’m Nick Petrić Howe. Thanks for listening.