Batteries and lithium
Batteries are getting a lot of attention these days. It wasn’t that long ago that they were just something that were not included in your kid’s latest toy.
Now batteries look like they will help transform our world. Small batteries allow our mobile devices to operate well away from the nearest power point. Larger batteries are helping our electricity grids cope the the variable electricity production from the sun and the wind. Batteries will replace fuel tanks in our cars, enabling vehicles to run on electricity generated from any source. I wrote about the effect electric vehicles will have in this article.
Batteries are already helping us reduce our dependence on fossil fuels and open up more of the energy world to using electricity as its energy carrier.
Lithium is emerging as a key material for the 21st century. The main reason lithium will be important will be its use in lithium-ion batteries.
So what are batteries, exactly? Batteries have been with us for a long time. Over 200 years ago, Alessandro Volta, an Italian chemist and physicist, developed one of the earliest batteries. His “voltaic pile” (modest, wasn’t he?) was built using alternating plates of copper and zinc in a stack. Each plate was separated by cloth or cardboard soaked in brine (salty water). By connecting a wire to each end of the stack, electricity would start flowing.
For the next 70 years, the entire development of electricity — as a scientific field of study and an industry — was dependent on batteries like this. It was only in the 1870s that the first electrical generator was developed, allowing electricity to be produced energy sources other than batteries.
Now I’d like you to bear with me here — I’d like to give an explanation of what electricity is and what is going on inside a battery. Feel free to skip ahead if you remember your high school physics and chemistry.
What is electricity and how is chemistry involved?
Electricity is the movement of electrons — the tiny charged particles that surround the central nucleus of atoms (the clump of protons and neutrons in the middle).
To help you understand electricity a bit more, we need to go into a bit of chemistry. Here’s a diagram of a simple atom — in this case a lithium atom.
The atom’s protons in its nucleus are positively charged. Lithium always has 3 protons. The number of protons determines what element something is. If it had 1 proton it would be hydrogen, 2 protons would be helium, and so on. You can see a list of elements here showing the number of protons for each element.
Lithium has 4 neutrons in the picture. Neutrons, as their name suggests, have a neutral charge, meaning they are neither positively or negatively charged. They’re not relevant to this article so don’t worry about them for now.
The electrons are negatively charged particles. Most of the time, electrons remain close to their atom’s nucleus. This attraction is due to the electromagnetic force — a fundamental force of nature — which pulls objects with opposite charges closer together and repels objects of the same charge. The positive charge of the nucleus (from its protons) and the negative charge of the electrons keep the electrons moving around close to the nucleus.
If the number of protons and electrons in an atom is the same (like in the diagram above — 3 protons and 3 electrons) then we say its charge is balanced (i.e. it’s zero) — the 3 positive charges of the protons are balanced by the 3 negative charges of the electrons. This makes the atom’s overall charge +3–3 = 0 — making an overall charge of zero. We say this atom is neutrally charged.
It’s possible for an atom to gain or lose electrons. If this happens, the atom becomes negatively charged (if it gains electrons) or positively charge (if it loses electrons).
If a lithium atom loses an extra electron (to have 2 in total) then its total charge would be +3 from its protons and -2 from its electrons, for an overall charge of +1. This would make the lithium atom positively charged.
When an atom is charged (positive or negative) we call it an ion. When positive and negative ions meet, they are also attracted to each other by the electromagnetic force, just as smaller protons and electrons are attracted to each other.
A good example of ions coming together is table salt — sodium chloride — consisting of 1 positively charged sodium ion and 1 negatively charged chlorine ion.
When together, chemists call this NaCl (Na for sodium, Cl for chlorine). When in crystalline form, table salt is bound together very strongly. The electromagnetic force holds sodium and chlorine ions together very tightly into a dense solid.
If we put salt in water, the water molecules help the sodium and chlorine ions to separate and float around in the water. The ions retain their individual charges. We call the sodium ion (with a +1 charge) Na+. And we call the chlorine ion (with a -1 charge) Cl-. Once dissolved, the salt water is full of ions floating around all over the place.
Because the ions are free to move around in the water, and because the overall number of Na+ and Cl- ions is the same, this salty water remains neutrally charged. However, the presence of these charged particles now allows for easy movement of charges through the water. We will get to this later when we talk about a battery’s electrolyte.
The electrons matter because it is the movement of trillions of electrons that make up what we call electricity. This movement is called the current, measured in amperes (amps for short). The current is analogous to a current in a river, denoting a flow of water — though here it is talking about a flow of electrons.
When trillions of electrons start moving we can use their energy to do useful things. How do they get moving? Well, in certain materials — especially metals like copper, aluminium, or steel — we have lots of atoms that are packed close together. The atoms are electrically conductive — they allow electrons to move easy from each atom to its neighbours. The overall charge of all these atoms does not change — the electrons simply flow between atoms when provided with some external force.
What causes electrons to start moving? It is possible to give energy to electrons. This energy is provided in the form of voltage (also called electrical potential, or just potential).
Voltage is a measure of how much electrical energy each unit of charge has. Energy is measured in joules and charge is measured in coulombs. A coulomb is the total (negative) charge of 6.24 x 1018 electrons — that’s 6.24 million million million electrons. We can also measure positive charges of protons or positive ions in coulombs, but usually we’re taking about electricity (negative electrons).
Electricity tends to flow from places with high voltage to places with low voltage. This is similar to how fluids flow from areas of high pressure to areas of low pressure, or how objects to to fall from high places to low places.
By connecting an electrically-conductive object like a metal wire to a source of voltage, we can make electricity flow. This actually requires the wire to be connected to a voltage difference — for example, to the positive and negative terminals of a battery. The battery provides a voltage difference — one end of the wire has a high voltage and the other has a low voltage. This differences causes electrons to start to flow. This flow is what we call electricity.
The electricity flowing through the wire carries energy (voltage) and we can harness this energy to create light (such as in a light bulb), produce heat (like in an electric tea kettle), induce motion (using an electric motor) or perform calculations (as in a computer’s CPU).
How does a battery work?
Now we have that all that basic science stuff out of the way, we’re ready to look at how batteries work.
All batteries are variations of the same basic idea. They have four main components:
- An anode. This is the place where electrons are produced. It’s the source of high voltage.
- A cathode. This is the place where electrons are absorbed. It’s the source of low voltage.
- An electrolyte. This is a solid or liquid material between the anode and the cathode. It allows for ions, either positive or negative, to flow between the anode and cathode. However, it won’t allow individual electrons to flow through it.
- A circuit. This is the electrical wiring that you connect to the battery to do stuff. If there is no circuit attached to the battery’s anode and cathode, no electricity will flow.
Here’s where I need to describe some chemistry. Many atoms or compounds (combinations of elements) have a tendency to produce a certain amount of voltage when they either gain or lose electrons.
When the battery’s anode and cathode are not connected by a circuit, nothing happens. The cathode and anode each have voltage. However, the electrons are unable to move through the electrolyte, so they just stay put.
When a battery is connected to a circuit, electrons are free to flow out of the anode, through the circuit, and into the cathode. This makes the anode negatively charged (because it is a source of negative electrons). It simultaneously makes the cathode positively charged.
Why does this flow of electrons happen? It’s because of the chemistry happening at the cathode and the anode.
At the anode, a chemical reaction occurs that releases electrons — these electrons are the ones that flow out the terminal and through the circuit. The exact reaction depends on what the anode and electrolyte are made of.
I will give you a very simple example, using a variation of Volta’s battery. Volta used a brine (salt water) electrolyte and anodes made of zinc and copper. However, a better approach is to replace the brine with an acid mixed in water. Acidic electrolytes contain lots of hydrogen ions (H+) each with a positive charge of +1. That’s all an acid is — something containing lots of hydrogen ions.
Any acid will do — it could be a food acid as found in fruit juice (citric acid) or a powerful acid like sulphuric acid. The H+ ions are very important, as we’ll see shortly.
So let’s look at a simple zinc/copper battery with an acid-based electrolyte.
When the circuit is connected, here’s what happens.
At the zinc anode, the zinc atoms each lose 2 electrons (written as e-, where the minus represents a charge of minus 1 for each electron). This produces a zinc ion with a charge of +2 (written as Zn2+). These zinc ions are now free to move into the salt water electrolyte. Essentially the zinc metal starts to dissolve into the electrolyte. Simultaneously, the 2 electrons that have been produced move through the zinc metal anode and out of the battery through the circuit.
The chemical reaction for the anode is written as:
Now our electrolyte has gained an extra ion with a charge of 2+. This makes our electrolyte positively charged.
Over at the cathode, electrons are arriving from the circuit, having been sent from the zinc anode via wires and through devices to do something useful. The copper cathode in this case does not undergo a chemical reaction — it is simply an electrical conductor to let electrons get to the electrolyte.
Our positively-charged electrolyte (containing H+ ions from the acid and some Zn2+ ions from our anode) is attracted to the negatively charged electrons in the copper.
The hydrogen ions combine with the electrons to form hydrogen gas — a pair of hydrogen atoms joined together (written as H2). This makes hydrogen gas form and bubble up from the copper cathode.
The chemical reaction for the cathode is written as:
(Above, two hydrogen ions and two electrons need to come together to make each hydrogen gas molecule containing two hydrogen atoms).
How do these reactions cause voltage?
You’ll notice that these reactions cause electricity to flow from the anode to the cathode. Why does this flow happen? Earlier I said that electricity always flows from a place of high voltage to a place of low voltage. This means that the anode (the source of electrons) must be at a higher voltage than the cathode (the receiver of electrons). If this were not true, electricity would not flow.
The two reactions I showed above (with zinc and hydrogen) are known as half reactions. Each describes half of an overall reaction where electrons are gained and lost. In the case above, the full reaction is zinc metal dissolving in acid to form hydrogen gas.
Each half reaction produces a certain amount of voltage. Under perfect conditions, the above zinc reaction produces a voltage of 0.76 volts. The hydrogen reaction produces a voltage of zero volts. This means that the voltage difference between the anode and the cathode is 0.76 volts.
So this means the zinc anode has a higher voltage than the hydrogen-making copper cathode. Remember that electricity wants to flow from a high voltage to a low voltage? Well, we can use this voltage to push electrons through a circuit from the anode to the cathode.
0.76 volts isn’t much. For comparison, a standard AA battery produces 1.5 volts, and that’s not enough to even feel with your fingers. But it is enough to make a little electricity flow. If we want more voltage, we can connect multiple anode/cathode pairs together to allow the voltages to add together. With enough pairs we can get theoretically get whatever voltage we want, though in practice there are limits.
So to sum all that up — half reactions at the battery’s anode and cathode set up a voltage difference which we can use to produce electricity.
Problems with the zinc / copper / acid battery
The zinc/copper battery is useful because it helps explain the basics of what a battery is doing. But in practice it’s not a very good battery. Here are some of its problems:
- It’s dangerous. The cathode is producing hydrogen gas. Hydrogen is flammable. If this is not adequately vented it can build up, increasing the chances of a fire or even an explosion.
- The anode is dissolving, which means eventually it will disappear into the electrolyte.
- Hydrogen ions are being lost from the acidic electrolyte, being replaced by zinc ions dissolving from the anode. This eventually results in the electrolyte running out of hydrogen ions. This slows down the number of electrons that can be taken into the cathode, and causes the voltage of the cathode to become closer to the anode’s. The effect is that the voltage produced by the battery reduces, producing less electricity.
- Zinc and copper are heavy, making this battery a poor choice when considering how to store a certain amount of electricity in a given volume or mass.
In the two centuries since Volta, we have learned to make much better batteries. So now let’s look at the best available today.
What about lithium-ion batteries?
Now we can look at what lithium-ion batteries are and why they are so useful.
Lithium-ion batteries, as their name suggests, use chemistry involving lithium ions (lithium with one electron taken away, with a charge of 1+). These Li+ ions are very useful in batteries for a number of reasons:
- Half reactions on anodes incorporating lithium have high voltages.
- Lithium is the lightest metal available, and therefore gives the potential for a lighter battery than if heavier metals are used.
- They enable a lot of electrical energy to be stored in a very small volume. This means they have a high energy density, making them ideal for mobile devices or electric vehicles, where space is at a premium.
- The chemical reactions in them are reversible meaning we can reverse the flow of electricity and make the reactions go backwards. This makes the rechargeable, unlike the Zinc/Copper/acid battery we looked at earlier.
There are many types of Li-ion batteries, but we can look at a common example here — those using graphite anodes and cobalt oxide cathodes. They are made up of:
Anode: A graphite (carbon) material intercalated with lithium atoms. Intercalated means that the lithium atoms are inserted into gaps between the carbon atoms. This forms our negative electrode, or source of electrons.
Cathode: One common type uses a cobalt oxide material intercalated with lithium atoms. This is our positive electrode, or receiver of electrons. There are many other types of cathode used, but this will do for this article. It can also be intercalated with lithium atoms.
Electrolyte: A lithium salt in an organic (hydrocarbon) solvent. The salt provides lithium Li+ ions that can move around. The organic solvent is present to prevent lithium from catching fire — lithium is highly flammable when it comes into contact with moisture from the air. This is why Li-ion batteries are sealed and covered in warnings not to open them.
The reactions are very interesting. During use of the battery, here’s what is happening for a common Li-ion battery type with a graphite anode and cobalt oxide cathode (note there are many other types):
Anode reaction:
Carbon (chemical symbol C) in graphite form starts off intercalated with lithium atoms. When the circuit is connected, lithium atoms give up an electron to the circuit and leaves a Li+ ion (which is positively charged because it just lost a negative electron). The reaction is written as:
This reaction produces lithium ions (Li+) wich leave the anode and enter the electrolyte. The electrons (e-) go via the circuit as electricity.
Cathode reaction:
A Cobalt oxide cathode is made of cobalt and oxygen atoms (with 1 cobalt atom to every 2 oxygen atoms, giving the formula CoO2).
When an electron is received by the cathode, it meets up with a lithium ion from the electrolyte. The lithium ion moves from the electrolyte and is inserted (intercalated) into the cathode.
The reaction is written as:
Electrolyte:
The electrolyte serves as a storage of lithium ions. Notice in the anode and cathode reactions that the anode sends lithium ions into the electrolyte while the cathode removes them from the electrolyte. Hence there is no overall change the quantity of lithium ions in the electrolyte.
Benefits of Li-ion batteries
A typical lithium-ion battery can produce 3.2 volts, enough to operate a mobile device like a computer or a phone. By combining these batteries we can produce much higher voltages, such as those needed to drive a vehicle’s electric motors.
Li-ion battery designs are being improved all the time. By varying the exact electrolyte materials (including the solvents used) and the cathode construction and physical layout, it is possible to get ever larger amounts of electricity into small spaces.
Recharging
I mentioned earlier that lithium-ion batteries can be recharged. We do this by applying an external source of voltage (from a power point, usually) at a higher voltage than the battery to reverse the electricity flow in the circuit.
The reactions at each electrode then go into reverse. The anode becomes intercalated again with lithium atoms. The cathode gives up more and more lithium atoms to the electrolyte.
Charging is complete when either the anode is full or the cathode is empty of lithium atoms. This is the reverse of when the battery is drained, which is when the anode is empty of lithium or the cathode is full.
One key feature we want in batteries is the ability to be recharged quickly. This is one of the key challenges for battery designers. During charging, heat is produced. Given the small size of Li-ion batteries, it can be a challenge to dissipate this heat. If you charge too quickly, the temperature of the battery can rise to unacceptable levels. The electronics incorporated into every battery ensure that charging is done at a rate that does not cause overheating.
Another problem with Li-ion batteries is overcharging. If you continue to apply a charging voltage to the battery after all the anode spaces for lithium have been filled, other chemical reactions can start to take place. These can start to generate gases inside the battery, including carbon dioxide and hydrogen.
Remember above that I said Li-ion batteries are sealed? This means generating gases is a problem. If gases build up too much, the pressure can cause the battery to burst like a balloon, spilling its flammable contents and causing a fire and injury.
For this reason, Li-ion batteries and chargers are equipped with overcharging protection. This ensures that charging is stopped before a dangerous overcharging situation occurs.
How much lithium is in a lithium-ion battery?
A lithium-ion battery contains many materials other than lithium. These materials are used to make the organic electrolyte, the anode and cathode, the electronics that control charging and discharging, and an outer case to keep the battery contents sealed from the moisture in the air.
For larger battery packs, which are comprised of many smaller batteries connected electrically, there will be additional packaging and copper as well.
The Argonne National Laboratory in the US estimates that lithium only makes up less than 3% of the mass of a typical lithium ion battery. This is because the weight of other components (made from denser materials) tend to dominate, and also because lithium itself is such a light material.
For this reason, it’s important to recognise that while lithium is a vital chemical for li-ion batteries, it is just a tiny part of the whole battery’s materials. It is also a small part of the battery cost, at least for now.
In future, however, the rapidly growing electric vehicle industry will require many more lithium-ion batteries than are made today. Given the rapid growth of the electric vehicle industry, with each vehicle requiring large lithium-ion batteries, this will require a much larger supply of lithium that humanity has needed in the past. Where will we get that lithium? Can we recycle it? What are the environmental impacts of the lithium industry?
David T. Kearns PhD is an energy, process and sustainability consultant with a background in process/chemical engineering. David is the owner and principal consultant of Sustainable Services, offering technical, environmental and training services across a range of energy and resource industries. David is a lecturer in Sustainable Processing at Monash University and an occasional lecturer at the University of Melbourne.
