Why are alkalis used in batteries?

Answer 1

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.

Answer 2

Batteries based on alkaline electrolytes are safer than the older batteries based on acid electrolytes, mainly because if the battery leaks the alkali will not corrode the metals in the device the battery was used in, while the older acid would corrode and severely damage the metals in the device the battery was used in.

However not all battery types can replace an acid electrolyte with an alkaline electrolyte (e.g. car batteries) and still function.