When your phone dies mid-day, you may ask: how do batteries work? In short, a battery converts chemical energy into electricity, sending electrons through a wire while ions move through an electrolyte.
Batteries are one of the building blocks behind the wider range of types of technology you use every day.
| Item | Value |
|---|---|
| Energy conversion in a battery | Chemical energy into electrical energy |
| Lithium-ion energy per unit mass | Three to four times more than other technology |
| Lithium-ion cycle life | At least 500 discharge/charge cycles at 100% |
| Voltage of a single lithium-ion cell | A few volts |
| Thermal runaway trigger | Electrolyte overheating above 100°C |
These figures come from Planète Énergies, updated March 2024. They can change as the technology develops, so check the source before you rely on them.
What is inside a battery?
A battery is a device that converts chemical energy into electrical energy. Rechargeable versions, also called accumulators, can accept and store electric energy and release it when needed, as Planète Énergies explains. Single-use batteries simply supply electricity to a circuit until they run out of charge.
In its simplest form, a battery has two electrodes immersed in an electrolyte, joined outside the electrolyte by a conductive wire. Inside a modern lithium-ion cell, the Ionworks guide describes these layers:
- Positive electrode (cathode): a porous layer typically made of lithium-metal oxide particles, such as lithium cobalt oxide or lithium iron phosphate.
- Negative electrode (anode): similar in structure, but typically made of graphite.
- Separator: a thin, porous membrane that keeps the electrodes apart to prevent short circuits while letting ions pass through.
- Electrolyte: a liquid or gel that fills the pores and helps lithium ions move; it usually consists of a lithium salt dissolved in an organic solvent.
- Current collectors: thin metal foils, typically aluminum for the positive electrode and copper for the negative one.
Together, these parts form a cell, the basic electrochemical unit that stores and delivers energy. In cylindrical or prismatic batteries the layers are rolled up; in pouch batteries they are stacked.
How a battery releases and stores energy
The key idea is that electrons and ions travel by different routes. Electrons go around the outside through the wire, where they can do useful work. Ions go through the electrolyte inside the cell, which keeps the electric balance.
During discharge, the process runs like this:
- The negative electrode (anode) releases electrons.
- The electrons travel along the wire, powering a motor or electronic device.
- The positive electrode (cathode) absorbs the electrons.
- Positive ions move through the electrolyte between the two electrodes to balance the exchange.
Charging reverses the sequence. With an external electricity source connected, lithium ions leave the cathode and are stored in the anode, which returns the cell to its high-energy state, as the Ionworks guide puts it. This reversible movement of ions and electrons is what makes lithium-ion batteries rechargeable.
Real cells also show side effects, such as gas formation, electrode swelling and growth of a surface layer called the solid-electrolyte interphase. These matter for how batteries age, but they do not change the basic picture.
Why lithium-ion cells are so common
Scientists became keen on lithium because it is very lightweight, being the third element in the periodic table. Its atoms can readily release one of their three electrons, creating positively charged Li+ ions, according to Planète Énergies.
Early designs used lithium metal for the negative electrode, but repeated recharging altered the metal. Cathodes are now often made from cobalt oxide and a small proportion of lithium, paired with a graphite anode. The name “lithium-ion” comes from the lithium salts in the electrolyte, which contain a great many lithium ions.
The payoff is practical. Per Planète Énergies’ March 2024 update, these batteries can store three to four times more energy per unit mass than batteries using other technology. They recharge quickly and manage at least 500 discharge/charge cycles at 100%.
A single cell gives only a few volts, so makers group cells together in varying numbers depending on whether the job is a phone or a car. If you want to see where small battery-powered gadgets fit into everyday life, read our explainer on what is the internet of things.
Other rechargeable chemistries
Lithium-ion is not the only option. Rechargeable batteries use combinations of materials that can easily and durably exchange electrons and positive ions.
Vehicles with internal combustion engines most often use lead-acid batteries. They have a negative electrode of lead, a positive electrode of lead oxide, and an electrolyte of sulfuric acid and water. Other materials used in batteries include nickel, cadmium, sodium and sulfur, per Planète Énergies.
Reading battery specs: volts, power and energy
Battery labels use several units that answer different questions. In the words of the EVKX battery basics page (in Portuguese), voltage in volts is the electric potential difference between two points. Power in kilowatts is how fast energy is delivered or received, while energy in kilowatt-hours is the amount delivered, received or stored.
For example, the same page notes that a battery supplying a constant 20 kW for half an hour delivers 10 kWh at its terminals. The energy reaching an electric car’s wheels is lower, because the power electronics and motor also have losses.
Your charged phone spends its stored energy on many things, including its wireless radio. To follow that part of the story, read how does wifi work.
Safety, limits and what comes next
Lithium-ion batteries have a known weakness: thermal runaway. Planète Énergies describes the risk of a battery suddenly catching fire and releasing toxic gases when the electrolyte overheats to above 100°C. In 2013, a battery in a Boeing 787 aircraft caught fire after landing, and thousands of phones and tablets have been recalled in recent years (source).
Investigations have shown that overheating is most often caused by a short circuit from incorrect assembly or impact. As a result, makers follow rigorous processes and fit an electronic battery management system (BMS), which turns the power off if it detects an anomaly. Researchers are also exploring solid electrolytes made of ultrathin polymer films.
Demand is also reshaping industry. As of Planète Énergies’ March 2024 update, the decision to ban the sale of internal combustion engines after 2035 means Europe must catch up in battery production, and recycling is part of those plans.
Frequently asked questions
Does a battery store electricity?
Show answer
Not exactly. A battery stores chemical energy and converts it into electrical energy when a circuit is closed, according to Planète Énergies.
Why can a rechargeable battery be used again and again?
Show answer
Its reactions are reversible. Applying an external voltage moves lithium ions back to the negative electrode and restores the high-energy state, as the Ionworks guide explains.
What is thermal runaway?
Show answer
It is the overheating of the electrolyte to above 100°C, which can lead to fire and toxic gases. Planète Énergies notes that a battery management system helps prevent this by cutting power when it detects an anomaly.
What is the difference between power and energy?
Show answer
Power (kW) is how quickly energy is delivered, and energy (kWh) is the total amount. The EVKX battery basics page gives 20 kW for half an hour as 10 kWh.
