Lithium-ion batteries are found in almost every portable electronic device. They're in smartphones, laptops, and even in our cars. In fact, batteries are one of the keys to realizing a 100% renewable energy future.
In 2018, there were over five million electric cars on the road, which includes both hybrid vehicles and fully battery powered electric cars. And their popularity only continues to grow. Since batteries are powering more and more of our lives, why don't we explore how exactly batteries work, and what makes lithium-ion batteries so special?
There are a bunch of batteries out there, made of different materials, in different shapes, and with different charge capabilities. But on the most basic level, batteries are composed of electrochemical cells. And the materials that make up an electrochemical cell can create the positive and negative sides you see on the either end of that battery.
Inside a single electrochemical cell, there are a few main parts that help the cell create electricity. Two electrodes, which are the materials that make the battery ends positive or negative. The negative side is called the anode, and the positive side is called the cathode. The next part is called the electrolyte, which sits between the anode and the cathode. And this is important, because it's what enables charged ions to flow between the two electrodes. The electrolyte can be liquid or solid, or any material that helps the chemical reaction flow smoothly. And finally, there's a semipermeable layer that keeps everything separate so that we can control the reaction.
Now, if we wanted to power, say, a flashlight, you would add an external circuit that connects the anode to the light bulb and the flashlight to the cathode. When we add a charge to this circuit, we initiate a chemical reaction between the anode and the electrolyte. This releases electrons and leaves leftover ions at the anode. These released electrons will travel through our circuit as electricity, ending up in the cathode. At the same time, the electrolyte will help the ions they left behind at the anode flow through the semipermeable barrier and meet the electrons at the cathode.
This whole process is called a reduction-oxidation reaction, also commonly referred to as a redox reaction. Oxidation is where a material loses electrons, and reduction is when it accept electrons.
When we talk about battery performance, we have to consider both energy and power density. And a good example to highlight the difference between energy and power density, is comparing a mug to a large jug with one of those narrow bottlenecks. If your water represents energy, and you fill both vessels with water, you see that the jug has a greater overall energy storage. It can simply hold more water or energy. But if we were to pour that water out, well, then it's clear that the water or that stored energy comes out of the mug at a much faster rate, demonstrating that the mug has a higher power output.
Energy density is defined as how much energy is within a given mass. So if something has a high energy density, it means it can store a lot of energy in a small amount of mass.
Power density, on the other hand, is defined as, you guessed it, how much power is within a given mass. So when something has a high power density, it can output large amounts of energy in a short amount of time.
So if you have a device with high energy density and low power density, it means that the device can store a lot of energy and doesn't use it up quickly. A good example of this is your very own phone. It actually has a small battery, but can run for a long time.
Now, you may notice your phone doesn't really generate that much power. I mean, it probably has enough power to have all of your apps open while streaming cool science videos, but then you'd probably have to recharge it pretty soon afterwards.
You'll find that most phones today use lithium-ion batteries, and materials are important for chemical reactions in battery cells. So, in the case of lithium-ion cells, both the anode and the cathode are made of materials that can enhance their ability to absorb lithium ions. This means the ions are held inside the structure of the material, and they can't get loose.
In most cases, the anode is made of graphite, which has this structure of carbon atoms. This structure allows the graphite anode to store positive lithium ions, while the cathode, typically made of lithium cobalt oxide, has a structure that also is conducive to storing lithium ions. These enhanced materials are key for a couple of different reasons. It means that the cell can store more energy while remaining small, and that's energy density. And this also means the battery is rechargeable.
When we want to use a lithium-ion battery, it works similarly to our other batteries. As the cell gets used, those electrons are freed from the anode, and they shuffle through an external circuit to the cathode. While the electrons move through the circuit as electricity, the lithium ions left behind travel through the electrolyte to the cathode. And there, they get absorbed and stay put until the device that uses the battery is plugged in and begins the charging cycle. Then they all do the whole process again, but backward.
Also, depending on how much energy density you need, the cathode can be created with different metal oxides for different applications. For example, lithium cobalt oxide is what's used in our phones, while something like a Tesla vehicle uses lithium nickel cobalt aluminum oxide. So you were probably aware that electric cars use lithium-ion batteries, but maybe you didn't know that Tesla cars used such a different kind of lithium-ion battery.
Electric vehicles have been developing for decades now, but they only sort of hit a tipping point recently. I've been driving one for a couple of years, and I just wouldn't go back. And part of that is due to their very innovative battery technology.
Electric vehicles look pretty different under the hood from internal combustion engine cars. I mean, there's not much to see here, it's just a storage space. The batteries that this car uses are individual cells, which are packaged together into modules, and modules joined together to form the battery pack, which actually sits down here along the bottom of the vehicle. And it's really heavy, so it gives the car a low center of gravity. Now, those batteries are pretty impressive. Lithium-ion. In the Model S version of this car, there are 7,000 of them stuck together, and that can give these cars a range over 595 kilometers.
The next most efficient electric cars on the market only have a range of about 415 kilometers. And this huge gap demonstrates that Tesla is leading the charge, at least for now, in terms of energy density and even power density, in a way that makes those electric cars relatively affordable for a mass market.
But lithium-ion batteries do have their downsides. They're not super powerful, they're expensive, the materials they're made from are unsustainable, and their electrolyte can be flammable, making the product potentially hazardous. So clearly there are improvements to be made. But what's it gonna take to make an even better battery?