Lithium batteries are not allowed on planes as they can cause severe problems if they were to catch fire or explode. There was a case in 2010 where a man tried to check in his bag, and the lithium battery inside it started leaking which then caught fire and caused panic amongst fellow passengers. There isn't just 1 type of lithium battery, they vary greatly, and the more powerful ones can become unstable if damaged, something which is common when checking in luggage. When these batteries get too hot and overheat, they either start to vent or explode, and it usually leads to a fire or chemical burns. If you have ever seen an item on fire, you will know that it is very little you can do to put it out, which poses the most significant danger on an aircraft. The other problem is that when a battery starts to emit smoke or even start a fire in a hold, it is very difficult to detect until it's too late, and often the smoke from a battery fire will be mistaken for another item on fire. This is why it is so vital that passengers cannot bring any lithium battery onto an aircraft.
There are some types of lithium batteries that are allowed on planes, and these are the ones that have been designed specifically for use in a plane. These batteries have been tested and found safe and will not cause a fire or explosion. Airlines often sell these batteries and can usually be found in the duty-free section at an airport. They are usually a little more expensive than a normal battery, but they have been specially designed to meet the safety standards required for air travel. Again, just like with every other type of battery, you should never try to charge one on board an aircraft. There are specific power sockets that have been designed for this purpose and can be found in the seatback in front of you. Using any other type of socket could lead to a fire or explosion. If you are traveling with a laptop, it is always best to bring the charger and plug it into the plane's power socket. This will not only save you from having to buy a new battery when you reach your destination, but it will also help to ensure that your device is fully charged in case of an emergency.
So, if you are traveling with any lithium battery, either in your hand luggage or checked-in bag, please leave it at home. The risks aren't worth it. Instead, buy a battery specifically designed for air travel or use the airline's batteries which can be found in the duty free section. And remember, never try to charge a battery onboard an aircraft.
Another thing to remember is that even if you make it to your destination without any problems caused by a lithium battery, this doesn't mean that the battery is now safe. Lithium batteries are known to have issues once they have been used for a while, so just because yours safely reached its destination does not mean it will be fine on the return journey. The only way to ensure safety is by making sure you don't bring any lithium batteries with you in the first place.