How do batteries work?

Battery! They are not the only solution to the climate and energy crisis, but they will certainly play an important role. Along with pumped hydro, they are likely to provide much of the energy storage needed to firm up renewables over the next decade. And they are constantly becoming cheaper and more efficient.

It seems that a new type of battery is announced almost every week – in the past 12 months, Cosmos covered quantum batteries, paper batteries, cellulose batteries and graphene batteries.

So now is a good time to get 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 that releases energy. The energy is released in the form of electricity and connecting a battery to a circuit allows the reaction to occur.

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

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

Electrons can move easily from one atom to another. If you can move electrons in a steady stream, you can create heat, light, and a variety of other useful things – with electricity.

A battery contains a substance, or reactant, that provides electrons (called anode), and another reagent that accepts them (called a cathode). Electrons are sent between reactants through a circuit in the device you are trying to power, allowing electricity to pass.

For this reaction to work, something positively charged must also 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 a beaker with liquid and two rectangles added: an anode and a cathode, connected by a wire labeled with electrons and
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 do not 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 called electrochemical cell. Batteries can use only one cell or 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 some type of positive ion around (lithium-ion included). There can be several different types of metals, salts, and other reagents involved.

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

What is the difference between a non-rechargeable battery and a rechargeable battery?

What it’s made of. In a non-rechargeable battery, the reaction that creates electrons only works in one direction: once the substance in the cathode has accepted the electrons, it can no longer become itself, without electrons.

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

Why do rechargeable batteries run out?

Because there are a bunch of different substances reacting with each other in a battery to get those electrons flowing, there are a few occasions for things to go wrong.

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

“When that chemical reaction happens, sometimes it doesn’t reverse perfectly when you recharge it. This can lead to things like the growth of dendrites – which are a 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, this effectively reduces the ability of this chemical reaction to continue reversibly occurring.”

Researchers and manufacturers are getting better at finding battery chemistries with cleanly reversible reactions, which barely deplete over time.

Tesla’s battery at the Hornsdale Power Reserve in South Australia has a 15 year warrantyfor example, but operator Neoen is confident that the batteries will still retain the majority of their capacity at this time and will be able to operate beyond that depending on market conditions and other factors.

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

What is the difference between a phone battery and a network-wide battery?

“In many cases, it’s actually identical chemistry,” says Blackhall.

“A grid-scale battery, it’s just hundreds or thousands of little lithium-ion cells, but packaged and managed in a battery module.”

Large batteries are not singular electrochemical cells. They are actually series of smaller units, lined up in one place.

That said, your phone is not guaranteed to be the same as a grid battery. The battery requirements all state what it could be made of. Weight is an absolutely essential characteristic in a car battery, for example, but it is less relevant for a static household battery. There you may be able to use cheaper but heavier materials.

Which raises another question:

Why do batteries need lithium?

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

The reason lithium is considered 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 to make a battery. (For more on the importance and prevalence of lithium, watch our Cosmos Briefing.)

So where weight or size need to be minimized (read: in anything that needs to move), lithium batteries are unlikely to be outdone.

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

Flow batteries are begins to be adopted like big batteries too. Flow batteries work with a radically different structure to traditional batteries – they can never discharge 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 be even more creative with chemistry – for example, using biodegradable materials like paper and carbon-based polymers.

How do they work in the grid?

Pumped hydroelectricity is a cheaper way to store energy than batteries. But batteries have one feature that gives them a key advantage in the energy transition: they can send electricity to the grid almost instantaneously.

“The chemical reactions can happen fast enough that you can draw current from the cell very quickly,” Blackhall says.

“Your output can be almost instantaneous.”

Even the fastest forms of fossil fuel generators cannot match this, such as the gas-fired power plants used for energy peaks.

“Gas spikes can take 15 minutes to go from no power to power output, just because you have to spin the turbine,” says Blackhall.

Currently, the electricity network is designed to adapt to this type of schedule. But as we continue to transition our energy systems, we may be able to become more flexible.

“Because you can control the batteries so quickly, we might even find ourselves with a new operating paradigm for our electrical system once we completely shut down all synchronous generators,” Blackhall says.