Lithium is the third element on the periodic table, after hydrogen and helium. It is the lightest metal, and you probably use it every day. The batteries in your phones and laptops and most rechargeable batteries you use are lithium ion batteries.
Lithium is used in batteries because it has the highest electrochemical potential of any element. It is so high that it will split water into hydrogen and oxygen (and violently!). This means there is a lot of energy available to exploit. This is also why laptop batteries can sometimes explode; the batteries are sealed very tightly, but if the seal is broken, air and water vapor will come in contact with the lithium and this is unsafe. 10-15 years ago, there weren’t as many lithium batteries in use, but now they are everywhere. Science has made great strides in improving the configurations of the batteries to give more energy, such as increasing the surface area of the lithium portion. Each time you cycle your battery, the lithium undergoes an electrochemical reaction on the draining and again on the charging of the battery. This is also why batteries become shorter lived over time; the high surface areas of new batteries aren’t thermodynamically favorable, and the lithium will become lower surface area with time. Less available surface area means less available energy.
Lithium ion (Li+) has another, very different use. It is used as a mood stabilizer. It is particularly useful at combatting mania. The linked wikipedia page contains its fascinating medical history. It was first used in the 1870’s as a mood stabilizer. Eventually LiCl was marketed as an alternative to table salt (NaCl), to avoid high blood pressure, and its mood properties were forgotten. Early versions of 7 Up contained lithium. Excessive lithium use was found to be deadly, and it was banned as an additive in the 1940s. Then in Australia, it was again discovered to have mood-stabilizing properties. Its therapeutic dose is quite close to its toxic dose, which is maybe why it took a while to gain approval in the US. Studies suggest that water supplies containing lithium may promote longevity and reduce the occurrence of suicide.
Lithium salts also have another really nifty use: cleansing the air in spaceships and submarines. Not only does human breathing consume oxygen; it also produces carbon dioxide, which is toxic when present in high amounts. Several lithium salts can remove carbon dioxide from the air. One even adds oxygen to the air when it removes carbon dioxide.
Elemental lithium is highly reactive, and is a member of the alkali metal group (all of whom react very impressively with water). Below is a video of lithium reacting with water. It bursts into bright red flame:
Another video shows more lithium action:
The people who made the second video have a great youtube channel with videos about all the elements done in a university laboratory environment. Most of them have good footage of reactions as well. I just spent an hour watching their videos, they are very entertaining for people with little knowledge, or a lot. If you have a little time to kill, the videos of sodium and potassium are also good, flammable fun.