Lithium-ion batteries explained in 4 levels of difficulty
I made a video on the same topic. It goes through most of the technical stuff I explain in this article.
It’s dark and cold in Montreal. I’m running late coming home from school. My fingers feel like icicles as I tap on my screen trying to wake up my phone. My phone is dead. My battery has let me down once again. And the result is I’m going to be in big trouble with my parents.
I recognize my battery problem is a small problem, which could be solved by carrying my portable charger (which I always forget). But imagine this happening with your car, or with your access to renewable electricity.
Lithium-ion batteries are one of the most important technologies being worked on today. They are a critical factor in the electrification of our planet. From electric cars to renewable energy storage lithium-ion batteries are being used to make better and greener tech.
That’s why a few months ago I decided I wanted to learn more about the electrochemical processes behind these batteries that we are more and more reliant on. To help me with my learning I broke it down into different levels of complexity.
This article is a very technical look at what happens inside lithium-ion batteries.
Menu for this article:
Level 1: High-level explanation of the core components (cathode, anode, electrolyte)
Level 2: Description of the materials used and more details on the chemical reactions
Level 3: Electrolyte solvent choices and Cathode structure implications
Level 4: The metrics, measurements and voltage profiles
Level 1: High-level explanation of the core components (cathode, anode, electrolyte)
Lithium is an atom that has 3 electrons orbiting around it. Two of its electrons are in the inner layer and 1 electron is in the outer layer. The lithium atom wants to get rid of this one electron in the outer layer (called the valence electron). This is because it wants to fuse with another atom to get 8 valence electrons that give it stability.
Every battery is composed of a positive terminal and a negative terminal. The positive terminal is called the cathode and the negative terminal is called the anode.
During the discharge, the lithium atoms start in the anode (when your phone is at a hundred percent). These lithium atoms are wedged in between the layered planes of graphite, this is called intercalation.
As I said before, lithium wants to get rid of its one valence electron, so when there is a pathway from the negative terminal to the positive terminal (in the form of a circuit), the negative valence electrons are attracted to the positively charged cathode. The movement of these electrons gives your phone the power it needs to operate.
Since lithium is now missing an electron it becomes an ion. It leaves the anode and tries to rejoin its electron at the cathode, to balance the positive charges and the negative charges.
The thing is, it needs a safe journey from the anode to the cathode. This is where the electrolyte comes in. The electrolyte in a lithium-ion battery is usually some sort of solute dissolved in liquid solvents.
When a lithium-ion get’s to the cathode it gets intercalated in the cathode structure. The inverse happens when the battery is charged.
Redox reactions
The basis of all batteries is two very simple chemical reactions.
Reduction + Oxidation = Redox Reactions
Reduction is the process of gaining electrons. Oxidation is the process of losing electrons. As a battery charges and discharges, the cathode and the anode lose and gain electrons resulting in Redox reactions. This is very simple but will become critical to understand when we go deeper into the topic of batteries.
Takeaways:
- Lithium is a light metal that wants to get rid of the electron in its outer layer
- The anode is the negative terminal, the lithium atoms start here during the discharge
- The cathode is the positive terminal, the lithium atoms start here during the charge
- When there is a pathway from the negative terminal to the positive terminal in the form of a circuit, the valence electrons leave because they are attracted by the positive terminal. This creates a current that powers electric cars, phones etc…
- The lithium atoms become ions and move through an electrolyte.
- Reduction (gaining electrons) + Oxidation (losing electrons) = Redox reactions
Level 2: Description of the materials used and more details on the chemical reactions
As we saw previously the anode, the cathode and the electrolyte are the foundation of a lithium-ion battery. But there are many considerations involved in the selection of the materials in the battery. There are also a few other important components namely the current collector for the anode and the cathode, the SEI (solid electrolyte interface) and the separator. It is also relevant to consider how these materials impact the battery’s chemical reactions. So what are all these parts composed of?
Anode materials
The anode that is in your phone and most lithium-ion batteries is made up of a crystallized lithium metal graphite structure. Graphite is pure carbon, so for every six carbon atoms, there is one lithium atom intercalated in the graphite structure. The molecular representation of this is LiC6.
Current Collector for the Anode
Next to the anode, there is a current collector. A current collector is exactly what it sounds like, the part of the battery that collects the current (collecting the electrons that are being ripped away from their lithium atoms). The current collector next to the anode is composed of copper because it does not react with lithium.
Cathode materials
A cathode is usually a lithium metal oxide. Meaning a combination between lithium, oxygen and some kind of metal. The oxygen and metal bond together to form layers of octahedral metal oxide structures separated by the sheets of lithium atoms that are intercalated.
For example, the battery in your phone has a cathode made out of lithium cobalt oxide (LiCoO2).
Current Collector for the Cathode
The current collector next to the cathode is usually composed of aluminum.
Electrolyte components
An electrolyte is mainly composed of three things. Lithium salts, organic solvents and an additive. Lithium salt, usually lithium hexafluorophosphate (LiPF6), is dissolved in organic solvents, meaning solvents that are carbon-based (solvents with carbon atoms in them). Examples of organic solvents used in an electrolyte are ethylene carbonate, dimethyl carbonate, diethyl carbonate etc…
The lithium salts are important because there are already lithium ions in the electrolyte solution which lets the lithium ions being released by the anode/cathode pass through more efficiently.
Solid Electrolyte Interface (SEI)
The additive is another organic molecule that helps with the longevity of a battery. The additive reacts with the surface of the anode and cathode to create Solid Electrolyte Interfaces (SEI). This is basically a protective film over the anode and cathode that extends battery life to thousands of cycles, helping with the longevity of the battery.
Separator
In the middle of the electrolyte is a porous polymer separator. The point of the separator is for the anode and the cathode to not touch as this could short out the battery
Takeaways:
- The anode is composed of a crystallized lithium metal graphite structure, 6 carbon atoms per every lithium atom (LiC6)
- The current collector for the anode is copper
- The cathode is composed of lithium + some sort of metal + oxygen = Li metal oxide. Example: LiCoO2.
- The current collector for the cathode is aluminum
- Electrolyte, lithium salts dissolved in carbon-based solvents as well as a carbon-based additive
- The additive forms an SEI which is a protective film over the anode and cathode structures. This extends battery life
- There is a porous separator in the electrolyte that let’s lithium ions pass through but prevents the anode and the cathode from touching each other
Level 3: Electrolyte solvent choices and Cathode structure implications
In Levels 1 and 2 we described all the different parts of a battery. Cathode, Anode, Electrolyte, porous separator, current collectors, and the SEI. As well as we described the materials that go into these parts. In Level 3 we’re going to go deeper into solvents that compose electrolytes and different types of cathode structures.
Electrolyte solvents
As we said previously lithium hexafluorophosphate is dissolved in organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate. But instead of memorizing all of those carbonate names, you can separate these carbonates into two groups: cyclic carbonates and chain carbonates.
Ionic conductivity defines a battery’s ability for ions to move through its electrode structures; a higher conductivity allows for more ions to pass through at a given time, improving the discharge rate. A higher discharge rate means more energy can be pumped out at a given time.
In order to have high ionic conductivity, the organic solvents in the electrolyte should have high solubility in order to separate the lithium salt into ions, and low viscosity for smooth movement of lithium ions coming from the anode and cathode.
Chain carbonates offer low viscosity. Meaning chain carbonates’ molecular makeup results in very little friction. This lets the lithium ions pass through easily. As well as cyclic carbonates offer high solubility meaning they can cut through the ionic bonds in the lithium hexafluorophosphate.
Cathode Structures
There are 3 main crystal structures for cathode materials, each with specific advantages and disadvantages.
- Layered structures
- Spinel structures
- Polyanion structures (also referred to as Olivine structures)
Layered Structures are exactly what they sound like, a structure that has layers one on top of the other like a big lithium atom sandwich. There are several types of layered cathode materials, but one of the most common is lithium cobalt oxide (LCO) or (LiCoO2).
Layered structures are 2 dimensional, meaning that there are two pathways that the lithium-ion can travel through. They offer the highest practical capacity currently up to 180 Amp hours per kg.
Spinel cathode structures are usually made from lithium manganese oxide, which has a spinel crystal structure. Spinel structures are 3 dimensional, meaning that there are three pathways that the lithium-ion can travel through.
Spinel cathodes are made by synthesizing lithium manganese oxide using a solid-state reaction process. This reaction happens when reactants are mixed together and then heated to a high temperature in an oven or furnace, which creates a new compound.
These cathodes have a high energy density and good stability, meaning they can store a lot of energy and won’t break down at the drop of a hat.
Polyanion/olivine cathode structures are a type of lithium-ion battery cathode that is made from materials with an olivine-type crystal structure, such as lithium iron phosphate (LFP) or lithium cobalt phosphate (LCP).
Polyanion/olivine cathodes are known for their good stability and high resistance to capacity fading, which is the gradual loss of capacity that can occur over time in some lithium-ion batteries. But they generally have a lower capacity and a lower voltage output compared to other cathode materials, which can limit their use in certain applications.
Polyanion/olivine structures are 1 dimensional, meaning that there is 1 pathway that the lithium ions can travel through.
Now you can see that battery cell engineers have to make a lot of decisions and that there are implications for the performance of the battery due to the properties of each material.
Takeaways:
- The electrolyte solvents have cyclic carbonates and chain carbonates in their solution
- Cyclic carbonates offer high solubility
- Chain carbonates offer low viscosity
- There are three different types of cathode structures, olivine structures, spinel structures and layered structures
- Olivine structures are 1 dimensional, meaning there is one entry point where the lithium-ions can get intercalated
- Layered structures are 2 dimensional
- Spinel structures are 3 dimensional
Level 4: The metrics, measurements and voltage profiles
Before we head into the magical land of battery graphs. Let’s take a step back and define some important metrics and measurement terms for batteries:
What are Volts? Volts are the pressure in an electrical current that pushes the electrons in a circuit to create a current.
What are Coulombs? A Coulomb is basically a unit containing a huge number of electrons. One coulomb of charge is equivalent to 6,250,000,000,000,000,000 electrons or 6,24 x 10¹⁸electrons.
What are Amps? An ampere or amp is a measure of current. This means it measures how many electrons are flowing through a circuit. One amp is 1 coulomb per second. An amp is also 1000 milliamps.
What is a Watt? One watt is equivalent to one amp under the pressure of one volt.
These terms can also be explained using a simple water flow analogy. Volts are the pressure put on the water to move through the pipe. Coulombs are how much water you have. Current is the rate at which the water is flowing through the pipe. Watts are the amount of water coming out of the pipe.
So now that we understand the units that we measure batteries and electrical circuits, we can start to compare the performance of different batteries.
A common way to compare batteries is using a graphical voltage profile. So what do we learn from voltage profiles?
Voltage Profiles are graphs that allow scientists and researchers to understand the changes happening in a battery. There exist different types of voltage profiles such as charge voltage profiles and discharge voltage profiles.
In the Discharge Voltage Profile above the x-axis shows how the capacity is changing, meaning how much energy the lithium-ion battery is generating. And on the y-axis, there is the voltage measurement which shows the pressure on the electrons generated by the battery.
If you look at the graph above you can see that over time the more electricity that is discharged (mAh) the less pressure (volts) is maintained. This phenomenon is also even more dramatic at different temperatures.
There is also a degradation of the voltage as the battery discharges. You can imagine this like an untied balloon full of air. When you let go of the balloon the airflow coming out of the balloon at high pressure will cause the balloon to fly across the room, but as the air pressure is reduced the airflow slows down and the balloon eventually comes to stop. The same can be observed of a battery that has a high voltage (pressure) because it is fully charged, but as it is discharged the voltage falls.
Voltage profiles are one of the primary ways that scientists evaluate the effectiveness of their batteries in order to optimize their material choices.
Takeaways:
- Volts are the pressure in an electrical current
- A coulomb is 6,24 x 10¹⁸ electrons
- An amp is a measure of current
- One watt is one amp under the pressure of one volt
- Voltage profiles allow scientists to understand batteries better by seeing the changes taking place during the charge and discharge
Conclusion
Lithium-ion batteries are one of those technologies that are revolutionizing the energy industry. Continued innovation in this space is critical to the electrification of our planet and the elimination of our reliance on fossil fuel-based energy systems.
Understanding the chemistry behind these batteries lets us delve deeper into the research and development that is necessary for the innovation and optimization of these critical products.
Flashback 2 months ago, batteries were a mysterious technology to me. But after months of research, I now have way more appreciation for my phone dying during a cold winter night.
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