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Lithium Ion Batteries!
Lithium Ion (also known as LiI or Li-Ion) batteries are one of the three major types of rechargeable batteries (the other two being Nickel Cadmium (NiCD) and Nickel Metal Hydride (NiMH).
Of these three battery types, the Li-Ion ones are the king of the hill, cream of the crop, top of the line, master of the... okay, okay.. But they are best for many uses.
So far, the standard size batteries have not seen the development into Li-ion, for several reasons; For one thing, low voltage Li-Ion batteries are difficult to make, so making batteries that could directly replace the AA or other batteries would be difficult - this marked is currently ruled by the NiMH batteries.
Advantages of Li-Ion
The main advantage of Li-Ion is that they pack massive amounts of power. If you have a cell phone, chances are that they use Li-Ion batteries. The same goes for all camcorders, digital cameras, laptops, rechargeable MP3 players, and other devices that need a lot of power.
The advantages of Li-Ion batteries are their power per volume (220 % more than NiCD, and 160 % more than NiMH) - relatively small batteries can hold lots of battery power. In the process of using and re-charging the batteries, very little of the Lithium is "spent", giving the batteries tremendous life spans. Case: Mobile phone batteries. Many of us never, ever, turn off our mobiles. The battery is constantly working - either powering the phone or being recharged - but even after several years of abuse, the batteries are still usable.
Another advantage of Li-Ion is that they have extremely long shelf-life. The batteries self-discharge with less than 5% per month (versus 10% for NiMH and up to 20% for NiCD), meaning that your topped-up batteries stay charged for longer.
Li-Ion batteries are made from Lithium-Cobalt Dioxide. Lithium is non-toxic (when oxydised, at least) and completely recyclable:, which means that they are nicer on the environment too: When the batteries are used up, you hand them back to your dealer. They pass them on to a factory who crack the batteries open and take out their contents. The batteries are then dropped in salt water for a few days. This de-activates the batteries completely, after which the lithium carbonate can be extracted from the resulting lithium salts. This Lithium carbonate is sold back to the battery manufacturers, who can make shiny new batteries!
Disadvantages of Li-Ion
If you ignore the price difference between Li-Ion and NiMH (which I personally think is allowable, considering the difference in performance), Li-Ion only has one major problem; Lithium reacts with other elements extremely easily, so the batteries have to be sealed very well. On some battery packs, however, this sealing is not good enough, and if you drop the battery pack, the sealing might crack slightly, which can be enough to seriously impair the usability of the battery. Not a problem, if you make sure to handle the batteries with care, and don't toss them about.
More about Li-Ion
The first LiI cells were used late in the 1960s, but the technology has gradually evolved the past thirty years, and it was not until the entertainment boom of the 1990s came, that the LiI batteries were put to heavy use.
Li-Ion batteries are built up from one or more Li-Ion cells. (ah - a battery of power cells...). All cells use lithium as the anode metal (negative pole), but there are several variations on the cathode side of the cells. There are also many different ways to construct the batteries, to specifically make them work at very low temperatures (some varieties can work without significant power loss at -50°C), very high temperatures (up to 210°C), for high drain appliances, for endurance performance on low drain, for better performance in environments with strong shocks, vibrations or loud sound (i.e. vibration) etc.
Because of its versatility, the Li-Ion cells can be constructed specifically for its particular use, which again increases the battery life significantly.
Graphite and Coke
The two main types of Li-Ion batteries (barring the possible specialty designs discussed above) are coke and graphite. These two are designations of the material used in the anode. The "coke" one uses a carbon anode, while the graphite one - surprisingly enough - uses graphite.
When the Li-Ion technology was new, Coke core anodes were first used, but they are used less and less nowadays; The graphite versions keep their power at a high voltage right until the batteries are completely discharged*, a higher discharge voltage, and lower temperature when in use.
*) this is true for most rechargeables. If you listen to a Walkman on alkaline batteries, your music will go fainter and the cassette will be turning slower over time. If you use rechargeables, the power just suddenly drops and is gone. This is an advantage in high-drain appliances such as digital cameras (they cannot handle running on half power), but the disadvantage is that you get very little warning before you lose all battery power. This means you have to carry spare batteries with you at all times. Sony have a nice solution to this problem with their Info-Lithium batteries, which keeps track of how much power has been used, and can fairly exactly determine how much battery power is left.