What type of battery does an iPhone 8 take?

The Genius Bar or an authorized Apple service shop will install the correct replacement battery, so you don't need to be concerned about the size of the battery.

You can find a list of authorized shops in your area using this link: Find Locations

It might be a good idea to call the shop in advance to make sure that they have the battery in stock, so your phone won't sit in the shop for 2-3 days waiting for the part to arrive.

About $75 (U.S.)

In case you were thinking about trying to install a 3rd party battery on your own, or have a shop install a non-genuine battery, Battery Health settings will not display correctly on your phone unless a genuine battery is installed. Only authorized Apple shops will have access to a genuine Apple battery.

An iPhone 8 takes a Li-ion battery. You can check your model no. from Settings>General>About. Make your question a bit clearer. Would you like to replace your battery?

You can also prolong the durability of your current iPhone without opening it by using a Powerbank from a reputed brand.