Lithium-Ion Batteries: How They Work, Last and Stay Safe

9 min read

THE BATTERY IN YOUR POCKET

Why Lithium-Ion Batteries Power Almost Everything

Laptop, smartphone and cordless drill on a wooden workbench. Illustrative image

Lithium ion batteries are rechargeable batteries that store electricity by moving tiny lithium particles back and forth between two sides. Scientists say they have the highest "energy density" you can easily buy, which just means the most power packed into the least weight.

The first one went on sale in 1991. Today the same idea runs smartphones, laptops, drones, electric vehicles and almost every modern home solar battery.

So what is going on inside?

What's Inside

Copper foil, aluminium foil and white separator film beside a sealed battery pouch. Illustrative image

Every lithium-ion cell has four parts. The two sides are called "electrodes", a name for the ends where electricity goes in and out.

The anode (the negative electrode). Usually graphite, the same carbon that is in your pencil. It works like a parking garage, with spots between the carbon layers.

The cathode (the positive electrode). A compound containing lithium, such as lithium iron phosphate. The cathode materials decide most of the cell's personality.

The electrolyte. A liquid the tiny particles swim through. In most cells the electrolyte can burn.

The separator. A thin film with tiny holes that keeps the electrodes apart. If they touched, the cell would short out.

How It Charges And Discharges

Smartphone resting on a wireless charging pad on a wooden table. Illustrative image

The charger pushes lithium out of the positive side, through the electrolyte, and into the graphite garage.

When you use it, the lithium slides back, and the electrons take the long way round through your device. That current is the electricity.

That's the whole trick. The chemical parts don't get used up, so it works again and again. One full charge and discharge is called a cycle.

How many cycles? A good LFP home battery is rated for about 2,500 cycles if you use most of it each day, or 5,000 if you only use half. A lead-acid battery usually manages 500 to 1,000.

The Lithium Family

Cylindrical cells, a silver pouch cell and an enclosed battery module on a bench. Illustrative image

"Lithium-ion" isn't one thing, it's a family. Change the cathode and you change the whole battery, from safety to how long it lasts. Those round cells are often 18650s, named after their size: 18 mm wide and 65 mm long.

NMC (Nickel Manganese Cobalt)

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The high energy density choice, used in many electric cars and e-bikes. However, it handles heat and abuse worse than LFP.

NCA (Nickel Cobalt Aluminum)

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Even more per kilo, and Tesla has used it in its cars. However, it is rated for relatively few cycles and gets touchy when hot.

LCO (Lithium Cobalt Oxide)

Tablet and closed laptop on a wooden desk. Illustrative image

The classic for mobile devices like phones and tablets. Lots of storage, but a short life and the worst temper of the family when overheated.

LFP (Lithium Iron Phosphate)

Enclosed home battery and wall-mounted electrical unit connected by a covered cable duct. Illustrative image

The off-grid favorite. It is a bit bigger and heavier for the same storage, but it offers three big wins:

Much safer. The hardest of the family to push into a fire.
Lasts longer. Thousands of cycles.
No cobalt, a metal that is expensive and often mined in hard conditions.

If you're buying a home solar battery, it's probably LFP.

LTO (Lithium Titanate)

Closed industrial storage cabinet in a bright service room. Illustrative image

Charges fast, handles a wide temperature range and lasts 3,000 to 7,000 cycles. The catch is low capacity and a high price, so it mostly sits in backup power systems, some electric vehicles and solar street lights.

LMO (Lithium Manganese Oxide)

Cordless drill with attached battery resting beside timber offcuts. Illustrative image

It can offer a big burst of power and is safer than LCO, so power tools and medical devices use it. It doesn't last as long as LFP.

What They're Good For

Timber cabin with rooftop solar panels in a meadow. Illustrative image

The primary rule: lithium wins wherever weight matters. That covers most applications, from phones to drones to electric vehicles.

For off-grid setups, rechargeable lithium-ion is now the usual pick. It costs more up front, but it lasts more cycles and weighs much less. If you want something that cannot burn at all, read about saltwater batteries. For how a full home setup fits together, see how home batteries work.

Why They Catch Fire (Sometimes)

Enclosed battery pack on a metal tray with an inspection camera on a tripod. Illustrative image

The primary danger is "thermal runaway". A cell that is damaged, overcharged or too hot can slip into it. Heat breaks down the inside, that makes more heat, and the cell can catch fire.

Once it starts, it is extremely hard to stop. Firefighters need a huge amount of water to cool it, so get out and call the fire brigade.

That is why every good pack has a battery management system (BMS), a small electronic brain that watches every cell's voltage and temperature and cuts the circuit if anything looks wrong.

→ How a BMS keeps a battery safe

What Are The Disadvantages Of Lithium-Ion Batteries?

Portable power station stored indoors beside a window overlooking snow. Illustrative image

Cost. Still more per kWh than lead-acid up front.

Cold. Cold slows the chemical reaction, so performance drops. Most cells require a temperature above freezing (0°C, 32°F) to charge, because a thin metal layer can coat the anode and damage the cell for good. That's why some products come with a heater.

Mining. Lithium, nickel and cobalt all have to be dug up. LFP skips the cobalt.

Recycling. Old cells go to a recycling point, never in the trash.

How To Make One Last

Two battery packs stored separately on a dry shelf. Illustrative image

Don't store it empty or full. Putting it away for months? Leave it about half full.

Don't sit at 100% all the time. A full battery ages faster, so many phones and cars offer a setting that stops early.

Keep it cool. Heat directly speeds up wear, so never leave a phone in a hot car.

Use the right charger. Stick to the charger that came with it, or one listed by a recognised testing lab, so the safety circuits are there. Good chargers provide the right voltage and stop on time.

What's Next

Sealed battery pouch on a white tray beside a laboratory microscope. Illustrative image

Research continues on two big ideas that could improve performance. Solid-state batteries swap the burnable liquid for a solid, which would cut the fire risk.

Silicon anodes could hold up to about ten times more lithium than graphite. However, silicon swells as it fills up and can crack. Labs continue to research ways around that, because it could make phones run much longer.

The Short Version

Two parking lots joined by a liquid: one way fills it, the other way runs your stuff. Keep it cool and half full when stored, and it will last for years.

→ How to build your own home battery

→ Battery size calculator

Sources

Hard Words

Term	What it means
Lithium-ion	The most common rechargeable battery available today.
Electrode	One of the two ends of a cell: the anode or the cathode.
Anode	The negative side, usually graphite.
Cathode	The positive side, a metal compound that holds the lithium.
Electrolyte	The liquid the lithium swims through.
Separator	A thin film that keeps the electrodes apart.
NMC	A high-energy type used in many cars.
LFP	Lithium Iron Phosphate, the safer type in most home batteries.
Battery management system (BMS)	The electronic brain that watches every cell.
Thermal runaway	Heat that makes more heat until a cell catches fire.
Cycle	One full charge and discharge.
18650	A round cell, about 18 mm wide and 65 mm long.

Common Questions

Are phone batteries lithium-ion?

Yes. Almost every smartphone, tablet and laptop runs on lithium ion batteries, often LCO because it packs a lot into a thin slab.

What's the difference between NMC and LFP?

NMC holds more for its weight but is less stable when hot. LFP is heavier but safer and lasts more cycles, so it became the standard for home solar batteries.

Why do lithium-ion batteries catch fire?

Damage, overcharging or heat can start thermal runaway, where heat makes more heat until the liquid inside burns.

How long do 18650 batteries last?

It depends on the chemistry, because 18650 is only a size. Battery University lists about 500 to 1,000 cycles for LCO, 1,000 to 2,000 for NMC and LFP.

Are all 18650 batteries the same size?

Nearly. The name means about 18 mm by 65 mm, but makers vary a little, so check the details on the data sheet before you buy.

Why is cold weather bad for lithium-ion batteries?

Cold slows the chemical reaction, so you get less out, and most cells must not be charged below freezing.