The big idea
A rechargeable battery stores energy in chemical form and releases it as electricity on demand. When you plug it in, electricity drives a chemical change that “loads” the battery. When you use it, the reverse change happens and the energy comes back out as a flow of electric current.
In lithium-ion batteries, the key action is the movement of lithium ions back and forth between two materials. This is why these batteries are sometimes described as “rocking chair” batteries: ions rock from one side to the other and back again.
The main parts of a cell
A single cell has four essential components.
- The anode, also called the negative electrode during discharge. In most lithium-ion cells it is made largely of graphite, a form of carbon, which has a layered structure that can hold lithium atoms between its layers.
- The cathode, the positive electrode, made of a lithium-containing metal compound. The exact material varies, and it is a major factor in the battery’s cost, energy density, lifespan and safety.
- The electrolyte, a liquid or gel containing lithium salts, which lets lithium ions travel between the electrodes.
- The separator, a thin porous membrane that keeps the electrodes from touching, which would cause a short circuit, while still allowing ions through.
Metal foils, typically copper on one side and aluminum on the other, collect the current and carry it to the outside circuit.
What happens during discharge
When a device is connected and the battery supplies power:
- Lithium atoms in the anode give up an electron and become lithium ions.
- The ions travel through the electrolyte, crossing the separator, toward the cathode.
- The electrons cannot pass through the electrolyte, so they flow through the external circuit instead. That flow is the electric current that powers the device.
- At the cathode, the ions and electrons recombine and the lithium becomes part of the cathode material.
The battery is “empty” in practical terms when most of the lithium has moved to the cathode side, or when the voltage drops to a lower limit.
What happens during charging
Charging runs the process in reverse. A charger pushes electrons back toward the anode using an external power source. Lithium ions leave the cathode, move through the electrolyte, and settle between the layers of the graphite. The energy supplied by the charger is stored in this arrangement of lithium in the anode, which is less stable than the arrangement in the cathode and therefore holds energy, in the way a compressed spring does.
Why use lithium
Lithium is the lightest metal and gives up its electron readily. These qualities allow lithium-ion cells to store a comparatively large amount of energy for their weight and volume, which is called energy density. That is the main reason the technology became dominant in phones, laptops and electric vehicles, where weight matters. Lithium-ion cells can also be recharged many times, and they lose relatively little charge when idle compared with some older chemistries.
Different chemistries
“Lithium-ion” is a family rather than a single recipe. The main variations come from the cathode material. Some chemistries emphasize high energy density, which suits devices and vehicles where space is limited. Others emphasize longer life and tolerance to heat or stress, which can suit stationary storage. The choice involves trade-offs among cost, performance, durability, safety and the materials used, and the preferred balance changes with the application. Questions about sourcing those materials are discussed in the article on battery recycling and materials.
The battery management system
Lithium-ion cells need to be operated within certain limits of voltage, current and temperature. A battery management system, or BMS, is the electronics that monitors cells and keeps them within safe ranges. It can balance charge between cells, limit charge and discharge rates, detect faults and disconnect the battery if something goes wrong.
This is important because overcharging, deep over-discharging, physical damage or overheating can cause damage and, in rare cases, a dangerous event called thermal runaway, in which a cell heats itself in a rapid chain reaction. Well-designed products are built to make this unlikely, but it is a good reason to use approved chargers and to avoid modifying or puncturing batteries.
Why batteries wear out
The process described above is not perfectly reversible. Each cycle causes small side reactions, so capacity declines slowly over time. This gradual change is the subject of the article on battery degradation, and it is why a battery lasts for years rather than forever.
Reading the numbers
The voltage of a cell is set by its chemistry, while the amount of energy it can hold is usually quoted in watt-hours or kilowatt-hours. The way these figures relate to each other is covered in the explanation of battery capacity, and the general unit definitions are in the article on energy units. As a quick example, a battery rated at 3.7 volts and 2 amp-hours holds roughly 3.7 times 2, or 7.4 watt-hours.
Where lithium-ion fits
Lithium-ion is the current workhorse for portable electronics, electric vehicles and many stationary systems. It is not the only option, and research continues on other chemistries, including those that use more abundant materials. For storing energy over very long periods or at very large scale, other methods such as pumped hydro may be a better fit.
Because the energy inside a large battery can be hazardous, working on packs or installations requires training and authorization. The basic principles above are for understanding, not a guide to opening or repairing a battery.