Instead of generating intense heat in a confined combustion chamber with a hole at the back to allow the hot gas to escape, an ion engine accelerates individual ions and ejects them in a specific direction.

At the moment, ion engines are very limited in their thrust, and there are some fundamental phsical reasons for this.

maybe future developments will lead to other options.

I never found a use for my conceptual invention of the crystalline explosive - that would release stored energy only in a single direction. Maybe rocket thrust could be just such an application - you wouldn't need motors and pumps and tanks. Just a stick of this (imaginary) stuff. Like Niven's "Stage Trees"...