Lithium-ion batteries power the world, but with lithium running low, we desperately need a viable alternative. Here’s why common salt may be our best bet
THEY are the widgets that quietly power our lives: lithium-ion batteries. Our phones, laptops and increasingly our cars rely on them. They already seem ubiquitous, yet the real battery revolution is still coming. Just take electric vehicles: in 2019, the number of electric cars on the world’s roads was just over 7 million, but that is expected to shoot up to some 200 million by 2030. And then consider our hopes of running the future on green electricity from wind turbines and solar panels. That will also depend on huge batteries that can store electricity for when it is needed, smoothing out peaks and troughs in demand.
Firms around the world are ploughing billions into battery factories to meet the demand. But that is going to require a lot of lithium. So much, in fact, that it isn’t obvious if we can mine enough of it to keep up, at least not without ravaging the environment even more. There may come a point where lithium becomes too scarce or expensive to be the key ingredient in this revolution.
What if we could make batteries using something so common that you almost certainly have it in your kitchen? Researchers have for years been working on batteries based not on lithium, but its close chemical cousin sodium, one half of sodium chloride or common table salt. It hasn’t been easy. You might even say it has been a grind. But at last we might have a way out of this lithium bottleneck. Might the batteries of the future be made from salt?
To understand how a condiment can ride to our rescue, it first helps to know the basics of how batteries work. Think of them as a circuit that is part electrical, part chemical. Things kick off at the battery’s anode, made of a material that can release electrons and, in the case of a lithium-based battery, lithium. Switch on whatever device is connected to the battery, and it sucks electrons from the battery to power itself. Meanwhile, positively charged lithium ions, having lost their electrons, diffuse from the anode, through a liquid or gel electrolyte, and insert themselves into pores in the cathode.
Eventually, the anode runs out of electrons, at which point you will see that low-battery icon flashing balefully on your screen. But the delightful thing about a rechargeable battery is that the process works both ways. Provide it with an external source of electricity by connecting it to the mains and everything happens in reverse: the lithium ions flow back to the anode and recombine with new electrons, ready for the process to start afresh. A typical lithium-ion-based smartphone battery can be recharged an impressive 500 times or so before there is a notable loss of performance.
The lithium-ion battery’s path to superiority began in 1980, when chemist John Goodenough, now at the University of Texas at Austin, developed a prototype that was more powerful than any other at the time. Together with Akira Yoshino and Stanley Whittingham, he won a Nobel prize for the work in 2019.
Lithium batteries soon came to dominate the market and they have remained peerless for reasons of inescapable chemistry. In the periodic table, lithium appears at the top of the group 1 metals, a set of elements whose atoms tend to bear a charge of +1. Lithium is the smallest and lightest of the bunch and so has the highest charge density, meaning that a lithium battery can pack in more ions and so hold more power than a battery of the same weight made from another group 1 metal. It is easy to see why that is attractive for smartphone users and electric car makers.
But lithium batteries come with serious environmental drawbacks. While lithium isn’t the rarest of metals, sizeable production happens in two places: mines in Australia and salt flats in the “lithium triangle” around the borders of Chile, Bolivia and Argentina. In South America, lithium brines are sequentially dissolved and allowed to evaporate to remove impurities. This requires about 1.9 million litres of water per tonne of lithium, a prodigious amount that leaves local farms and communities parched. With lithium found in so few countries, there is also a risk of geopolitical ructions between producers and big consumers, such as China, if – as is predicted – the supply becomes more scarce.
There are efforts to get around these difficulties (see “Fresh lithium“). But our best batteries have another grave problem: cobalt. Goodenough’s design, still in use today, uses a cathode made of lithium cobalt oxide. Cobalt is rare stuff indeed. Around two-thirds of mined cobalt comes from one country, the Democratic Republic of the Congo. Much of the metal is dug up by miners, including children, who often work without safety equipment in awful conditions and earn $3 a day or less. Another type of lithium battery uses a cathode made of manganese and nickel, which are both also rare.
“Purifying a tonne of lithium brine can require 1.9 million litres of water”
Sodium’s potential as a replacement for lithium is suggested by a glance at the periodic table. It sits in the square below lithium, also in group 1, but weightier. While having almost the same chemistry as lithium, it has none of the environmental baggage or geographical limitations. “Sodium is so democratic,” says battery researcher Maria Helena Braga at the University of Porto in Portugal. The US Geological Survey doesn’t even attempt to put a number on the size of Earth’s salt reserves, simply saying: “World continental resources of salt are vast.”
Sodium isn’t an automatic solution though. Largely, that is because it is quite a bit heavier, with a relative mass of 23 to lithium’s 7. This is reflected in the standard potentials of the two metals, an indication of the maximum amount of work that a battery made from them can do. Lithium, at -3.03 volts, has the best value of any metal, with sodium trailing behind at -2.71 volts. “Sodium is heavier, it has a lower voltage,” says Nuria Tapia-Ruiz, a battery researcher at Lancaster University, UK. “To make it comparable to a lithium-ion battery, we need much more material, and so we are going to make heavier batteries.” This is why sodium batteries tend to conjure up images of electric vehicles with all the dynamism of a milk float.
Big, but not bad
But a bulky battery isn’t always bad. “If you want to store energy from solar panels or a wind farm, what you want is a very big battery. You don’t necessarily worry about energy density or how heavy it is,” says Robert Armstrong at the University of St Andrews, UK. Spurred by this thought, research into sodium cells – and all sorts of other battery designs – has been going on for ages (see “Bizarre batteries“).
It isn’t possible to simply use sodium ions in existing lithium batteries. Instead, each of the three battery components must be redesigned. But in doing so, we have learned that sodium batteries have benefits that go beyond the environment.
First, the cathode, which in lithium-ion batteries requires metals such as cobalt. The good news is that we have already learned to make sodium battery cathodes from layers of more sustainable metal oxides, such as magnesium, iron and copper. “We’re always trying to avoid cobalt and nickel,” says Tapia-Ruiz. These cathodes have made it into working batteries, including those made by HiNaBattery Technology in China.
Second is the anode. This is made of graphite in lithium-ion batteries, but the pores of this material are too small for sodium. The best alternative found so far is an engineered form of charcoal, which has bigger pores. Not enough is yet known about how much charcoal expands and contracts as sodium ions move in and out – too much of this and the battery will lose performance and possibly short circuit. Tapia-Ruiz says she and others are trying different alloys and forms of carbon to find the best option.
Third is the most challenging component, the electrolyte. The trouble is that in metal-ion batteries of all kinds the electrolyte can react with the anode and cathode, forming a layer on them that depletes performance. This happens in lithium-ion batteries, but it isn’t a problem because the layer remains stable after the first charging cycle. In sodium battery prototypes, however, the solid layer tends to build up. Getting a working sodium battery, then, involves redesigning each of the three components and getting them to work together seamlessly.
In June 2020, Yuehe Lin at Washington State University and his team did just that, reporting a prototype sodium-ion batterythat had a capacity similar to some lithium-ion batteries and that could be recharged more than 1000 times while maintaining 80 per cent of its performance. The crucial ingredient was a highly concentrated electrolyte that didn’t lose performance even if some of it reacted with the electrodes.
Prototypes like this aren’t, of course, finished products that can be slotted into a camera or other device. Getting to that point requires plating the electrodes onto metals so they connect neatly to electronic circuits, among other things. Happily, this stage of development yields more good news for sodium.
In a lithium-ion battery, the cathode is plated on to aluminium. But that same metal can’t be used at the anode because lithium ions can form an alloy with it, and so copper is used instead. Unfortunately, having different metals at each end means the battery always has an electric potential, even when not in use. As a result, lithium-ion batteries can short circuit, overheat and catch fire when disconnected. This is especially a risk when lots of batteries are being shipped around together. Sodium batteries can use aluminium at both cathode and anode, which eliminates this problem at a stroke.
On your bike
Sodium-ion batteries might be heavier than lithium ones, but with advantages at almost every other turn, that is starting to look like a worthy compromise. That’s certainly the attitude of Faradion, a company based in Sheffield, UK. It produces a 1-kilogram sodium-ion battery that it says has a similar performance to a lithium cell. In 2015, the firm demonstrated an electric bike powered by its product. “You can certainly see them competing with lithium-ion,” says Armstrong.
Perhaps the most original approach to sodium batteries comes from a firm called Natron Energy. The company’s founder Colin Wessells developed an electrode material based on the pigment Prussian blue. This iron-based molecule has pores that are much bigger than a sodium ion and so it can let them in and out with almost no resistance, giving it a long life. “There’s basically no wear out mechanism,” says Jack Pouchet at Natron. “We have shown 37,000 cycles with no end in sight.” The company is selling its wares mostly to data centres, servers that support the internet. These need extra battery power during periods of peak energy demand and as an insurance against mains power outages. For applications like this, a heavy battery isn’t a problem.
“Sodium has the same chemistry as lithium, but none of the environmental baggage”
More conventional sodium battery technology is set to improve quickly, according to forecasts from Stefano Passerini at the Karlsruhe Institute of Technology in Germany and his colleagues. The researchers totted up the materials needed to make lithium-ion and sodium-ion batteries with a capacity of 11.5 kilowatt hours, about a third of what is required in a small electric car. Then they repeated the exercise considering advanced prototype batteries and expected future developments. The results suggest that we can shave 32 kilograms off a lithium battery and 42 kilograms off a sodium battery of this capacity. The price of sodium batteries is set to come down quickly too. They will be competitive with lithium batteries by about 2025, estimates Passerini.
We shouldn’t necessarily expect sodium batteries to directly replace lithium ones. Instead, it might make sense to use sodium cells in certain applications and so, hopefully, take pressure off our lithium reserves. What is important is that we can store electricity from renewable sources without wrecking the planet in the process. If they are to aid in that goal, batteries will, one way or another, need a total recharge.
The vast majority of the world’s lithium comes from just two places: Australia and South America. In both cases, its extraction is damaging to the environment. But there are plans afoot to get lithium from other areas using gentler methods.
Some of the action is happening in Cornwall, an area of the UK best known for its beautiful beaches. A company called Cornish Lithium has discovered that beneath the peninsula’s granite bedrock are pools of lithium-rich hot brines.
The firm wants to get at it in a relatively environmentally friendly way, drilling 1-kilometre-deep boreholes and pumping the brine to the surface. The liquid will then be fed though a column of beads that lithium ions cling to, with the remaining water then washed back underground.
If it works, it could provide a much needed raw material for battery makers in the UK. “The world is hurtling down this lithium route,” says Cornish Lithium’s CEO Jeremy Wrathall. “Either we have to find a way of mining lithium in an environmentally benign way or we go to another technology.”
There isn’t going to be a single type of battery that will address all the world’s energy storage needs, which is why people have been dreaming up all sorts of variations
Lithium and sodium are both good battery ingredients (see main story). However, their ions can only carry an electrical charge of +1. Why not use an ion that can carry a greater charge – like magnesium, with its +2 charge? Several research teams are working on just this. It is early days, but magnesium could one day be the basis of batteries more powerful and safe than those made with lithium or sodium.
A major selling point of sodium batteries is that they can be made from a plentiful resource, salt. And what better place to find salt than in seawater? This is why Stefano Passerini’s team at the Karlsruhe Institute of Technology in Germany has developed a prototype battery based on seawater, with the sodium that is naturally dissolved in it carrying the charge. Passerini says he already has keen interest from investors in South Korea.
Maria Helena Braga at the University of Porto in Portugal has been working on an unusual battery with John Goodenough, the Nobel prizewinning inventor of the lithium-ion battery. The key component is the electrolyte, which is made of glass spiked with sodium ions, which can travel through it. Every material needed is easy to source. “It’s the most eco-friendly cell you can find,” says Braga. The battery apparently has extraordinary properties: Braga says it can outperform lithium-based batteries; the one in her office has been powering an LED for five years. Others are having trouble replicating the device. Still, with backing from the likes of Goodenough, this is one battery to watch.
Think of fuel cells as batteries that you charge by adding fuel rather than plugging them into the mains. John Andrews at RMIT University in Melbourne, Australia, has developed one that splits protons from water, which are then stored inside the battery. To release this power, oxygen from air is fed through the machine, which combines with the protons to produce water and electricity. “It’s a very neat principle,” says Andrews. “The challenge is to make it work in a practical device.”
Otherwise known as flow batteries, these work on a similar principle to regular batteries, but all the components are dissolved in liquids. Chemist Lee Cronin at the University of Glasgow, UK, and his team have developed one such battery based on an enormous tungsten-containing molecule. The advantage is that a charged-up liquid battery could be pumped into a car quickly, much as petrol is today. The main barrier at the moment is that all that electrical charge makes the liquid electrolyte sticky and therefore difficult to pump.