Why are alkalis used in batteries?

In ordinary alkaline batteries it is potassium hydroxide, sometimes called potash or potassium lye.

Other battery chemistries may contain a different alkali, a salt, or an acid.

For example a battery used in most modern US nuclear weapons is called a thermal battery and uses a salt as the electrolyte. When the weapon is fired a heat source rapidly heats the battery, melting the salt and activating the battery powering up the weapon's electronics. Such batteries have an almost infinite shelf life.