Atoms of lithium (li) and cesium (cs) both lose electrons and become positive ions when they from ionic bonds. which type of atom requires more energy to become an ion? why?

Lithium.

Both Lithium and Cesium are group 1 elements with a single electron in their outermost valence shell. And since opposite charges attract, both elements have their electrons attracted to the nucleus. But the outermost shell for cesium is further away from the nucleus and since the force of attraction follows the inverse square law, the force holding the outermost electron in Cesium is less than the force holding the outermost electron in lithium.