Energy stored as chemistry
A battery does not store electricity the way a bucket stores water. It stores energy in chemical form, in materials that are ready to react. When a circuit is connected, the reaction proceeds and electrons are pushed through the circuit, which is the electric current that powers a device. In a rechargeable battery, the reaction can be driven backward by applying electricity from a charger, which restores the original materials.
A single battery cell has the same few parts, whatever its chemistry:
- Anode (negative electrode). Where the reaction releases electrons during discharge.
- Cathode (positive electrode). Where electrons are taken up during discharge.
- Electrolyte. A liquid or gel that carries charged atoms called ions between the electrodes.
- Separator. A thin porous barrier that keeps the electrodes from touching while letting ions pass.
The key feature is that electrons and ions take different routes. Ions move through the electrolyte inside the cell. Electrons cannot pass through the electrolyte, so they travel through the outside circuit, and that is the path that delivers power.
What happens in a lithium-ion cell
Lithium-ion cells are the common type in phones, laptops and electric cars. In the typical design, the anode is made mostly of graphite, a form of carbon. The cathode is a lithium-containing metal compound, and various compounds are used. Lithium is held in both electrodes as ions, tucked between layers of the material, a process called intercalation. Nothing is dissolved or plated in the ideal case; the lithium simply shifts between the layers like guests moving between two hotels.
Discharging. Lithium ions leave the graphite and travel through the electrolyte to the cathode. Electrons are released at the anode and flow through the outside circuit to the cathode, powering your device along the way.
Charging. A charger forces the process backward. Lithium ions are pushed out of the cathode, travel back through the electrolyte, and settle into the graphite. Electrons are driven the same way around the outside circuit.
The amount of energy stored depends on how many lithium ions the electrodes can hold and on the voltage difference between the two electrodes. That is why a bigger battery stores more, and why different electrode materials give different performance.
Capacity and what the numbers mean
Capacity is often given in milliamp-hours (mAh) for small devices or watt-hours (Wh) or kilowatt-hours (kWh) for larger ones. Watt-hours measure energy directly: a 10 watt device would, in an idealized case, run for 1 hour on a 10 Wh battery. Milliamp-hours measure charge, and need the voltage to compare. Real runtime is shorter than such simple arithmetic suggests because of conversion losses, temperature and changing demand. The ideas of power and stored energy belong to the wider topic of forms of energy.
Why batteries age
Every rechargeable battery gradually holds less energy and may deliver power less readily. Several processes contribute, and which dominates depends on the design and use:
- Surface layer growth. During early charging, part of the electrolyte reacts at the graphite surface and forms a thin protective film. It is needed for stable operation, but it keeps growing slowly over time, using up some lithium and increasing resistance.
- Electrode wear. The electrode materials swell and shrink slightly each time lithium enters and leaves. Over many cycles this can cause cracking and lost contact, so some material no longer takes part.
- Lithium plating. Under some conditions, such as charging very fast or in the cold, lithium can deposit as metal on the anode surface instead of entering it. This can permanently reduce capacity and, in severe cases, create safety problems.
- Heat. High temperatures speed up unwanted side reactions, which is a main reason batteries age faster when hot.
Aging happens even with a battery unused on a shelf, known as calendar aging, and also from use, known as cycle aging. A common way to count use is a full cycle, equal to the use of the battery’s full capacity, though not necessarily in one go. Two half discharges add up to roughly one cycle.
Habits that often help
Manufacturers’ guidance differs by product, and the device’s manual is the best source. In general, the following are often recommended:
- Avoid leaving devices in hot places, such as a parked car in summer.
- Avoid regularly draining a battery fully to zero or keeping it at 100 percent charge for long periods. Many devices now include settings that limit the maximum charge.
- Use the charger designed for the device.
- Do not charge a damaged or swollen battery. Stop using it, and ask the manufacturer or a qualified repair service what to do.
These habits tend to slow wear rather than stop it. Batteries age regardless, and the practical effect varies by model.
Safety and the battery management system
Lithium-ion cells contain a flammable electrolyte, and a damaged or poorly made cell can overheat in a process called thermal runaway. Battery packs therefore include electronics that monitor voltage, current and temperature, and cut off charging or discharging if something goes wrong. Separators, vents and other design features add further protection. Serious failures are rare compared with the huge number of cells in use, but they do happen, which is why damaged batteries should be handled carefully.
End of life
Because the materials are valuable and can be harmful if released, used batteries should not go in ordinary trash. Many places have drop-off points at stores or collection sites, and local rules vary, so household recycling programs are the place to look. Batteries from devices such as wearable devices and earbuds are small but follow the same rule.
Other rechargeable types
Lithium-ion is not the only kind. Nickel-metal-hydride cells are used in some rechargeable AA batteries and in older hybrid cars, and lead-acid batteries still start many gasoline cars. Each type has its own chemistry and trade-offs in cost, weight, lifespan and safety. Research on other approaches continues, but claims about future batteries should be treated cautiously until they are widely available.
Key points
In a lithium-ion cell, lithium ions shuttle between a graphite anode and a metal-oxide cathode through an electrolyte, while electrons travel through the outside circuit to power a device. Charging runs the process in reverse. Side reactions, mechanical stress and heat gradually reduce capacity, which is why all rechargeable batteries eventually wear out.