Batteries - a guide to the future

Batteries that store renewable energy are essential for the Paris climate agreement to work. Viviane Richter describes the rechargeable batteries that could make it happen.

A new all-solid lithium-sulfur battery

Turning solar and wind electricity into a 24/7 power source as reliable as coal. Eliminating the "range anxiety" that stops people switching from petrol to electric cars. Stopping the irritation of flat smartphones or laptops.

Those are just a few of the advantages that affordable long-lived rechargeable batteries, capable of delivering a sustained high-powered output over weeks instead of days could offer.

According to the experts, we have the technology. What we don't have is the magic mix of affordability, lightness and power delivery in a single battery. Instead, rechargeable batteries are diversifying, spawning a range of storage tools, each best suited to a particular niche. Here we meet the five frontrunners.

Reigning champion - the lithium ion battery

Potential use: Almost everything - devices, electric cars, household renewable energy storage.

Advantages: Proven, mature technology.

Disadvantages: Most gains in energy storage have already been made. Some safety concerns.

Lithium ion battery are the reigning champions when it comes to energy density - the amount of electric charge stored by weight.

And the technology is well proven. Even sexy. Witness the sleek Tesla Powerwall, a large lithium ion battery pack that, connected to solar panels, could enable a household to disconnect from the grid. Meanwhile, Tesla CEO Elon Musk says the company's Gigafactory in Nevada will produce enough lithium ion batteries to fit half million electric cars a year at a third of current costs.

You can think of the lithium ion battery as a tiny water tower. When you charge your phone, electrons are pumped up into a storage tank called the "anode", where they're captured by lithium ions. Turning on your phone is like turning on the tap: electrons come flooding out of the lithium, and through your phone's circuitry - before being collected in a receiver tank called the "cathode".

At the same time, the lithium ions cross the battery directly to reach the cathode. Here, electrons and lithium ion recombine - but as a spent force. Charging the battery re-energises the pair by pumping them back up to the anode.

But there are safety concerns. Solvents in lithium ion batteries, such as diethyl carbonate, are inflammable and sometimes things go wrong. For example, in 2013 lithium batteries caused a fire on a Boeing's 787 Dreamliner at Boston's Logan International Airport, grounding the fleet.

Such mishaps could be avoided if a new technology developed by Chunsheng Wang and colleagues at the University of Maryland in Baltimore pans out - a non-flammable salt solvent that can carry lithium ions, creating a battery with the same power as a conventional lithium ion battery. By fine-tuning the chemistry the team thinks it can push the battery's performance further. He published his development in Science in November

Still, the lithium ion battery won't be the champion forever. When it comes to energy density, lithium ion batteries may be the best we've got, but the technology's already been pushed "almost as far as it can go", says Adam Best, a materials engineer at Commonwealth Scientific and Industrial Research Organisation in Melbourne.

Second-in-command - the lithium sulfur battery

Potential use: Devices, electrical cars, household renewable energy storage.

Advantages: Potentially 5x more energy dense than lithium ion

Disadvantages: shorter lifespan than lithium ion batteries.

Lithium sulfur is the next battery we'll see rolled out commercially, says Cameron Shearer, materials engineer at Flinders University in Adelaide. Otis Energy, a UK-based company, has announced that next year they will sell this type of battery for electric cars and solar energy storage. But they could soon be powering smartphones too.

Whereas conventional lithium ion batteries use a slab of graphite to capture lithium ions at the anode, this battery uses a lightweight slither of lithium itself. And at the cathode, the battery uses sulfur to soak up the spent lithium and electrons - again, a lighter option than the mixture of metals in a conventional lithium ion battery cathode.

Their lighter construction and favourable electrochemistry means that, at least on paper, a lithium sulfur battery can hold five times more electrons than lithium ion batteries, weight for weight - and at lower cost (sulfur is cheap).

But the lithium sulfur battery is not long-lived - the lithium and sulfur tend to react together and clog pores in the cathode, and the sulfur begins to decompose. One solution could be to wrap the battery in a thin protective polymer coat - this helps hold the sulfur together, according to work from chemists at the Toyota Research Institute of North America in Michigan published in Energy and Environmental Science in November.

Shearer is a strong supporter of lithium sulfur batteries, predicting their reliability and lifespan will match lithium ion batteries within 10 years.

Lives fast, dies young - the lithium air battery

Potential use: Devices, electric cars.

Advantages: Very light, and on paper 10x more energy dense than lithium ion.

Disadvantages: Very short lifespan (so far).

Rather than using sulfur to soak up lithium at its cathode, this battery uses oxygen from the air. When the battery is charged, the oxygen is exhaled again. As a result, these batteries are exceptionally light - on paper they could store 10 times more energy than a lithium ion battery of the same weight. That's your smartphone powered for more than a week on one charge - or an electric car with range longer than a petrol car with a full tank.

"If you can make lithium-air work, you've pretty much got the most energy dense device available to man," says Best.

Sadly, so far the lithium air battery's lifespan is even worse than lithium sulfur's. And for the same old reasons - clogged cathodes, in this case because the lithium reacts with other molecules in the air.

"It's losing around 10 to 20 % [capacity] each time it's cycled," says Best. In October in Science, researchers at the University of Cambridge revealed a better type of lithium air battery. It contains a different anode that happily tolerates lithium hydroxide particles that form when lithium reacts with moisture in the air. But reaction with nitrogen still clogs the cathode.

We're unlikely to see lithium air move from lab bench to the market any time soon. "It's 10 to 20 years away, if that," says Best.

Slow, chubby and cheap - the sodium ion battery

Potential use: devices, electric car, household renewable energy storage

Advantages: Lower cost than lithium ion.

Disadvantages: Lower performance than lithium ion.

Lithium has so far quenched our thirst for portable electronics, but demand for lithium is rising - an electric car battery requires about four kilograms. And lithium isn't cheap, which will continue to push up the price of batteries as lithium becomes scarcer.

Enter the sodium ion battery. It works like a lithium ion battery but uses cheaper, more abundant sodium as its electron source. The drawback is that it is not as energy dense - sodium atoms are bigger and heavier than lithium.

For now, that takes it out the running for mobile applications such as electronic devices and electric cars, but not for solar energy storage, where cost is critical and size and weight less of an issue. Faradion, a UK-based company, and Aquion Energy, a US company, are already selling sodium ion batteries for these purposes.

And if lithium prices do spike? That could provide the incentive for mass manufacture of sodium ion batteries - they could be ready for use in phones and laptops, in perhaps as little as five years, according to some estimates.

The alien - the flow battery

Potential use: Storage of renewable energy.

Advantages: Cheap and reliable.

Disadvantages: Low energy density. Must be stationary because of the heavy liquids and pumps.

The flow battery promises to be the heavy-duty storage workhorse.

Flow batteries don't use an anode or cathode - they store energy by shuttling electrons between two large tanks of liquid. In place of lithium, most flow batteries use vanadium. Where lithium ions need to latch on to an anode or cathode to store electrons, vanadium ions are stable in solution.

Flow batteries have a long lifetime as there is no solid anode or cathode to degrade. What's more, it's easy to increase capacity - simply feed in more solution.

Their drawback is low energy density. The ions need a lot of liquid to be stable and this liquid needs to be pumped around to keep the ions evenly distributed. That makes the battery heavy and big - the smallest are about the size of a bar fridge. You wouldn't use one in an electric car, let alone a phone.

But at least one company, Australian firm Redflow, already sells flow batteries - for renewable energy storage at remote mine sites.

Home use is also not out of the question. "The Tesla Powerwall is made to show off," says Cameron Shearer of Flinders University in Adelaide. "You'd have a flow battery hidden in your wall."

How we're designing new batteries has changed. "We're moving from a sort of cottage industry to a much more sophisticated atomic design of materials," says Anthony Vassallo, battery chemist at the University of Sydney. High-resolution microscopy techniques are enabling scientists to custom-design anodes, cathodes and other battery parts from the atoms up. Within five to 10 years, he estimates, nano-engineered battery components will be mass produced on 3D printers.

Perhaps these could be the enabling technologies that springboard advanced batteries such as lithium air out of the lab and into real world renewable energy storage.

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