by Chris Woodford. Last updated: July 1, 2014.
Power to go—that's the promise batteries deliver. They give us all the convenience of electricity in a handy, portable form. The only trouble is, most batteries run flat very quickly and, unless you use a specialized charger, you then have to throw them away. It's hard on your pocket and bad for the environment as well: worldwide, we throw away billions of disposable batteries every single year. Rechargeable batteries help to solve this problem and the best kind use a technology called lithium ion. Your cellphone, laptop computer, and MP3 player probably all use lithium-ion batteries. They've been in widespread use since about 1991, but the basic chemistry was first discovered by American chemist Gilbert Lewis (1875–1946) way back in 1912. Let's take a closer look at how they work!
Photo: A lithium-ion battery, such as this one from a laptop, is made from a number of power-producing units called cells. Each cell produces about 3–4 volts, so a lithium ion battery that produces 10–16 volts typically needs three to four cells. This battery is rated as 10.8 volts and has three cells inside.
The trouble with ordinary batteries
If you've read our main article on batteries, you'll know a battery is essentially a chemical experiment happening in a small metal canister. Connect the two ends of a battery to something like a flashlight and chemical reactions begin: chemicals inside the battery slowly but systematically break apart and join themselves together to make other chemicals, producing a stream of positively charged particles called ions and negatively charged electrons. The ions move through the battery; the electrons go through the circuit to which the battery's connected, providing electrical energy that drives the flashlight. The only trouble is, this chemical reaction can happen only once and in only one direction: that's why ordinary batteries usually can't be recharged.
Photo: Ordinary batteries, such as this zinc carbon one, cannot be recharged because the chemical reactions that generate the power are not reversible.
Rechargeable batteries = reversible reactions
Different chemicals are used in rechargeable batteries and they split apart through entirely different reactions. The big difference is that the chemical reactions in a rechargeable battery are reversible: when the battery is discharging the reactions go one way and the battery gives out power; when the battery is charging, the reactions go in the opposite direction and the battery absorbs power. These chemical reactions can happen hundreds of times in both directions, so a rechargeable battery will typically give you anything from two or three to as much as 10 years of useful life (depending on how often you use it and how well you look after it).
How lithium-ion batteries work
Photo: Lithium-ion (Li-ion) batteries are less environmentally damaging than batteries containing heavy metals such as cadmium and mercury, but recycling them is still far preferable to incinerating them or sending them to landfill.
Like any other battery, a rechargeable lithium-ion battery is made of one or more power-generating compartments called cells. Each cell has essentially three components: a positive electrode (connected to the battery's positive or + terminal), a negative electrode (connected to the negative or − terminal), and a chemical called an electrolyte in between them. The positive electrode is typically made from a chemical compound called lithium-cobalt oxide (LiCoO2) or, in newer batteries, from lithium iron phosphate (LiFePO4). The negative electrode is generally made from carbon (graphite) and the electrolyte varies from one type of battery to another—but isn't too important in understanding the basic idea of how the battery works.
All lithium-ion batteries work in broadly the same way. When the battery is charging up, the lithium-based positive electrode gives up some of its lithium ions, which move through the electrolyte to the negative electrode and remain there. The battery takes in and stores energy during this process. When the battery is discharging, the lithium ions move back across the electrolyte to the positive electrode, producing the energy that powers the battery. In both cases, electrons flow in the opposite direction to the ions around the outer circuit. Electrons do not flow through the electrolyte: it's effectively an insulating barrier, so far as electrons are concerned.
The movement of ions (through the electrolyte) and electrons (around the external circuit, in the opposite direction) are interconnected processes, and if either stops so does the other. If ions stop moving through the electrolyte because the battery completely discharges, electrons can't move through the outer circuit either—so you lose your power. Similarly, if you switch off whatever the battery is powering, the flow of electrons stops and so does the flow of ions. The battery essentially stops discharging at a high rate (but it does keep on discharging, at a very slow rate, even with the appliance disconnected).
Unlike simpler batteries, lithium-ion ones have built in electronic controllers that regulate how they charge and discharge. They prevent the overcharging and overheating that can cause lithium-ion batteries to explode in some circumstances.