LITHIUM ION BATTERIES HOW DO THEY WORK

LITHIUM ION BATTERIES HOW DO THEY WORK

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, butrecycling 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), anegative 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.

How a lithium-ion battery charges and discharges

As their name suggests, lithium-ion batteries are all about the movement of lithium ions: the ions move one way when the battery charges (when it's absorbing power); they move the opposite way when the battery discharges (when it's supplying power):

1. During charging, lithium ions flow from the positive electrode (red) to the negative electrode (blue) through the electrolyte (gray). Electrons also flow from the positive electrode to the negative electrode, but take the longer path around the outer circuit. The electrons and ions combine at the negative electrode and deposit lithium there.
2. When no more ions will flow, the battery is fully charged and ready to use.
3. During discharging, the ions flow back through the electrolyte from the negative electrode to the positive electrode. Electrons flow from the negative electrode to the positive electrode through the outer circuit, powering your laptop. When the ions and electrons combine at the positive electrode, lithium is deposited there.
4. When all the ions have moved back, the battery is fully discharged and needs charging up again.

Advantages of lithium-ion batteries

Generally, lithium ion batteries are more reliable than older technologies such as nickel-cadmium (NiCd, pronounced "nicad") and don't suffer from a problem known as the "memory effect" (where nicad batteries appear to become harder to charge unless they're discharged fully first). Since lithium-ion batteries don't contain cadmium (a toxic, heavy metal), they are also (in theory, at least) better for the environment—although dumping any batteries (full of metals, plastics, and other assorted chemicals) into landfills is never a good thing. Compared to heavy-duty rechargeable batteries (such as the lead-acid ones used to start cars), lithium-ion batteries are relatively light for the amount of energy they store.

 
Photo: Lightweight lithium-ion batteries are used in a number of cutting-edge electric cars, including the pioneering Tesla Roadster. It takes roughly 3.5 hours to charge its 6831 lithium-ion cells, which together weigh a whopping one half a tonne (1100 lb). Fully charged, they give the car a range of over 350km (220 miles). Left: You can see the yellow power lead charging the batteries. Right: The batteries are in the large compartment you can see directly above the back wheel. Left photo: Tesla Inside; right photo Shiny New Tesla. Both by courtesy of Steve Jurvetson, published on Flickr in 2007 under a Creative Commons licence.

Who invented lithium-ion batteries?

Handy, helpful lithium-ion power packs were pioneered at Oxford University in the 1970s by chemist John Good enough and his colleagues Phil Wiseman, Koichi Mizushima, and Phil Jones. Their research was published in 1980 and turned into a commercial technology by Sony, who produced the first lithium ion batteries in the early 1990s. Since then, they've become commonplace: around 5 billion are manufactured every year (according to a Bloomberg news report from 2013), most of them in China.