Tom explores the history, function, and importance of Lithium Ion Batteries.
Featuring Tom Merritt.
MP3
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Episode transcript:
I’ve had my device for a couple years and the batteries are already going…
Should I have charged it differently…?
And is this thing going to explode on me…?
Are you confused?
Don’t be.
Let’s help you Know a Little more about Lithium ion batteries.
Look around your house and you have at least one lithium-ion battery powering something, like a phone or a laptop. Likely you have dozens. Pretty much anything rechargeable from watches to cars use Lithium Ion batteries these days.
The idea of using the metal lithium in batteries was explored by G.N. Lewis in 1912. M. Stanley Whittingham discovered in the early 1970s how to store lithium ions. And rechargeable lithium batteries were demonstrated in 1979 with a prototype developed in 1985. The first commercial lithium-ion battery was introduced by Sony in 1991. In 2019 the Nobel Prize in Chemistry was awarded to John Goodenough, Stanley Whittingham and Akira Yoshino “for the development of lithium ion batteries”.
But how do they actually work?
Those of you who already understand batteries, bear with me, we’ll get to the lithium-ion part quickly.
Any battery relies on a chemical reaction to create a stream of positively-charged ions and negatively-charged electrons. Ions move through the battery itself and electrons move out into anything attached to the battery to provide electrical energy. In non-rechargeable batteries this reaction only occurs in one direction and when you’ve exhausted the process it’s done and the battery is “dead.”
A rechargeable battery uses different chemical reactions that can work in both directions. So “recharging” is really just reversing the chemical reaction so that you can do it again later in the other direction and produce a new stream of electrons.
Another side note here. Battery, is a term for a collection of things all together. Hence a battery of machine guns. What we’re calling a battery here is a collection of power cells. Each cell has its own chemical reaction and sends its electrons out to mix with those from all the other cells to make the electrical output of the battery. If you ever opened up a battery– something you should not do because of the risk of fire and explosion– but if you did, you might find what look like lots of little batteries. Those are the cells. In fact if you’ve ever heard the phrase single-cell battery, that means it’s not really a battery, just one cell. A AA battery is just one cell.
Each cell also has a positive and negative end. You’ve seen that with AA batteries for sure. One end has a plus sign the other a negative sign. The positive end is where the anode sits. The anode in a lithium ion battery is usually made from lithium-cobalt oxide or lithium iron phosphate. Something with lithium. We’ll see why in a minute. The cathode is the negative end and is usually made from some kind of carbon, usually graphite. Between the anode and the cathode is an electrolyte which allows ions to move back and forth and prevents electrons from coming back in. It can be a lot of different chemicals and still work about the same. It’s often made of ether.
All of these materials are lighter than materials used in other kinds of rechargeable batteries which is one of the big advantages of lithium-ion. You can pack in more power without packing in as much weight.
For example. A lead-acid battery, like the one used in cars, can store about 25 watt-hours of energy in one kilogram.
A Nickel-MetalHydride battery can store 60-70 watt hours, or maybe up to 100 watt hours in one kilogram.
But a Lithium Ion battery can store 150 watt-hours per kilogram
OK so let’s charge this lithium ion cell up!
When you charge a lithium ion battery the anode, which is made of some kind of lithium, generates lithium ions. They split off and move through the electrolyte solution to the graphite cathode and stay there. This pushes electrons out from the anode through the circuit where they end up at the cathode and combine with the lithium ions to make lithium. The lithium gets kind of sandwiched between layers of graphite. That stores up potential energy. OK. We’re all charged up with loads of lithium stored in the graphite end of the cell called the cathode.
Now we turn on the device and start drawing power. Lithium ions get drawn out of the graphite through the electrolyte to the anode end and start generating electrons. A micro-perforated separator exists between the two ends that only the ions can move through to the anode, not the electrons. The electrons can’t move back through the electrolyte so they head out through the cathode– the negative end– through the connected circuit and provide electricity to the device. Lithium ions generate about 3.7 volts per cell. A typical AA cell generates 1.5 volts.
Once all the ions have moved out of the graphite cathode into the anode, the electrons stop flowing and your cell stops providing power.
If you turn off your device before that happens, it stops drawing electrons which slows down the flow of them to a near standstill. That process never entirely stops though, which is why a battery will slowly lose power even if you never use it. Lithium Ion batteries lose about 5 percent of charge per month when not used.
Basically it’s one big chemical reaction that sends lithium ions back and forth and electrons out and around a circuit. The direction determines whether you’re charging or discharging.
And that chemical reaction needs minding. A lithium-ion battery needs electronics to make sure it stops charging when it reaches capacity and stops discharging when its out because if you completely discharge a lithium ion battery, it’s ruined. So what’s in those electronics in your battery? Most lithium-ion battery packs include a temperature sensor, a voltage regulator, a voltage tap monitoring the capacity so you can estimate how much charge is left, and a charge state monitor that manages the charging process to keep it as fast as is safe. Finally there’s a connector to the device that not only sends out power but information on the battery.
This is important in part because if you keep charging a lithium-ion cell past its limit it heats up and can cause a “thermal runaway” which will eventually cause it to catch fire or explode. Similarly if the battery gets too hot for any reason, like being out in hot weather, it needs to be shut down or again, thermal runaway. A Current Interrupt Device or CID in lithium ion batteries stops current from running when the voltage reaches its maximum, or the battery gets too hot or internal pressure rises too high.
One way to reduce the fire risk is to use flame-resistant materials. One alternative being explored is using electrically conductive glass instead of the electrolyte. Another is Lithium polymer batteries, or LiPo. These are lithium ion batteries that use a polymer in place of the electrolyte. If you see these on the market, which you will, the polymer is a plastic gel that delivers higher specific energy which is useful when you want to keep the device light. LiPo batteries are common in radio-controlled and mobile devices. Hyundai uses it in its electric cars batteries. Gel-based LiPo cells still have risks due to swelling. Fully solid polymers are still being researched.
Lithium-ion batteries have no memory effect. You may have run across Nickel-metal-hydride Nickel-Cadmium batteries or Nicad batteries that are also rechargeable. If you recharge a nicad before fully discharging, the battery may only charge a partial amount because of the way the chemistry works. It seems to remember where you started recharging it and acts like it only has that much capacity. Lithium ions don’t have this “memory” effect, so you can recharge them when they’re at 20% 50% or 80% without worrying about temporarily reducing the capacity. In fact lithium ion batteries stay healthy with partial discharge where you recharge them before they get near empty.
And lithium ion batteries age. An average expected life no matter how you use it is about two to three years.
So there you go. Moving some lithium ions around makes your phone go. And your laptop and your ring doorbell and so many more things.
Until we get a new breakthrough in battery tech lithium ion batteries are the best energy storage we have for most consumer electronics.
In other words, I hope you know a little more about lithium ion batteries.
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