Explainer: how do batteries work?

Batteries! They’re not the sole solution to the climate and energy crisis, but they’re certainly going to play a large role. Along with pumped hydro, they’re likely to provide much of the energy storage needed to firm renewables in the next decade. And they’re constantly getting cheaper and more effective.

It seems there’s a new type of battery getting announced nearly every week – in the past 12 months, Cosmos has covered quantum batteries, paper batteries, cellulose batteries and graphene batteries .

So it’s a good time to go back to basics: how do batteries work, and what role do they play in the grid?

How do batteries work?

The short answer is that batteries perform a chemical reaction which releases energy. The energy is released in the form of electricity, and connecting a battery to a circuit allows the reaction to happen.

To understand the slightly longer answer, it’s worth looking at the structure of atoms. Atoms have a nucleus, made up from positively charged protons and chargeless neutrons, surrounded by a cloud of negatively charged electrons.

Lithium atom drawing: four blue balls and three red balls bunched together in the centre, with three grey balls orbiting around the outside
A drawing of a lithium atom: protons (red) and neutrons (blue) in the nucleus, electrons (grey) whizzing around the outside. Credit: Richie Bendall – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=102688272

The electrons can move easily from one atom to another. If you can move the electrons in a consistent flow, you can create heat, light, and a variety of other useful things – with electricity.

A battery has one substance, or reagent, which supplies electrons (called an anode), and another reagent which accepts them (called a cathode). The electrons are sent between reagents via a circuit in the device you’re trying to power, letting electricity flow. 

For this reaction to work, something positively charged also has to move from the anode to the cathode to balance the negatively charged electrons. Atoms with fewer electrons than protons – called positive ions, or cations – do this job.

Diagram of beaker with liquid and two added rectangles: an anode and a cathode, connected by a wire labelled with electrons and 'current supply'. In the beaker, dotted arrows represent anions and cations moving between anode and cathode
A simple electrochemical cell. Credit: Emanuele Oddo – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=80156641

The positive ions don’t move through the circuit, but through a separate bridge connecting the anode and the cathode. This is the electrolyte – it can be a liquid solution, or a solid. 

This whole process is referred to as an electrochemical cell. Batteries can just use one cell, or they can have several electrochemical cells connected to each other. 

Most commercial batteries actually rely on chemical reactions that are more complicated than simply shunting electrons and one type of positive ion around (lithium-ion included). There can be several different types of metals, salts and other reactants involved.

But batteries all, ultimately, need a cathode, an anode, and an electrolyte.

What’s the difference between a non-rechargeable and rechargeable battery? 

What it’s made of. In a non-rechargeable battery, the reaction which creates electrons only works one way: once the substance in the cathode has accepted the electrons, it can’t be changed back into its former, electron-less self. 

In a rechargeable battery, it is possible to reverse this reaction. An external power source can remove electrons from the cathode and add them to the anode, leaving them fresh and ready to react again. 

The substances which make up the anode, cathode, and electrolyte will determine whether or not the reaction is reversible.

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Why do rechargeable batteries run down? 

Because there’s a bunch of different substances reacting with each other in a battery to make those electrons flow, there are a few opportunities for things to go wrong. 

Chemical reactions rarely make exactly the products you want – there are usually small amounts of by-products.

“When that chemical reaction is occurring, sometimes it doesn’t perfectly reverse when you recharge it. That can lead to things like dendrite growth – which are kind of tiny crystal growths,” says Professor Lachlan Blackhall, head of the Battery Storage and Grid Integration Program at the Australian National University.

“Over time that effectively reduces the ability for that chemical reaction to continue to happen in a reversible way.”

Researchers and manufacturers are getting better at finding battery chemistries with cleanly reversible reactions, that barely run down over time.

Tesla’s battery at the Hornsdale Power Reserve in South Australia has a warranty of 15 years, for instance, but operator Neoen is confident that the batteries will still retain the majority of their capacity at this time and will be capable of operating beyond it depending on market conditions and other factors.

Line of large white batteries
Tesla Powerpack batteries in September 2017, ahead of their installation at Hornsdale. Credit: Mark Brake / Stringer

What’s the difference between a phone battery and a grid-scale battery?

“In many cases, they’re actually identical chemistry,” says Blackhall.

“A grid scale battery, it just has hundreds or thousands of little lithium-ion cells, but packed and managed within a battery module.”

Big batteries aren’t singular electrochemical cells. They’re actually series of smaller units, lined up in one place.

That said, it’s not a guarantee that your phone is identical to a grid battery. The requirements for the battery all inform what it might be made of. Weight is an absolutely critical feature in a car battery, for instance, but it’s less relevant for a static home battery. There, you might be able to use cheaper but heavier materials.

Which raises another question:

Why do batteries need lithium?

Are lithium-ion batteries going to be the gold standard forever? Could we make them with a cheaper substance?

The reason lithium is thrown around as the “queen of batteries” is its basic chemistry. As the lightest metal, with only three protons, lithium is essentially guaranteed to be the most energy-dense way of making a battery. (For more on the importance and prevalence of lithium, watch our Cosmos Briefing.)

So, where weight or size must be minimised (read: in anything that has to move), it’s unlikely that lithium batteries will be overtaken.

But grid-scale batteries, and home batteries, can be much larger and heavier than car or phone batteries. Sodium is an economically viable alternative to lithium in this range, for example.

Flow batteries are starting to be adopted as big batteries too. Flow batteries operate with a dramatically different structure to traditional batteries – they may never run down, and they can work very efficiently without lithium. Vanadium is a particularly popular candidate here.

At the other end of the scale, small electronics don’t necessarily need powerful batteries. Here, we can get even more creative with chemistry – for instance, using biodegradable materials like paper and carbon-based polymers.

How do they work in the grid?

Pumped hydro is a cheaper way to store energy than batteries. But batteries have a feature which gives them a key advantage in the energy transition: they can send power to the grid almost instantly.

“The chemical reactions can occur sufficiently quickly that you can draw current from the cell very quickly,” says Blackhall.

“Your output can be almost instantaneous.”

Even the fastest forms of fossil fuel generators can’t match this – such as gas power plants used for peaking energy.

“Gas peakers, they can take 15 minutes to go from no power output to power output, just because you’ve got to spin the turbine up,” says Blackhall.

Currently, the electricity grid is designed to accommodate this sort of timing. But as we transition our energy systems further, we may be able to get more flexible.

“Because you can control batteries so quickly, we may even end up with a new operating paradigm for our electricity system once we have completely shut down all synchronous generators,” says Blackhall.

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