First worn by Lex Luthor, the Kryptonite ring was fashioned out of a sample obtained from Metallo's Krytonite power core. In an attempt to keep Superman at arm's length Lex Luthor wore it at all times. However, continued exposure to the ring's ambient radiation caused him to develop cancer. Despite his best efforts, and the amputation of his hand, the cancer spread eventually killing him.
However, shortly after his death, Luthor's brain was placed in a perfect clone of his original body having no cancer or amputation.
Eventually Superman entrusted the ring to Batman to use against him should he ever loose control. Batman keeps the ring in a special compartment of his Utility Belt.
In the Batman: Hush story line Batman uses the Kryptonite ring against Superman.
When an employee of Lex's discovered that Superman and Clark Kent were the same individual, she told Lex. Lex was outraged over the thought that Superman, with all his power, would pose as a mere human. He had her fired and removed.
She took the ring and planned on confronting Clark herself. Later she was mugged and the ring ended up in Gotham City, where Batman got hold of it.
At the end, Superman decided to entrust the ring to Batman, should something ever happen and he went out of control. Batman later used the ring against Earth 2's Superman but later learned since it was not the Kryptonite from that Superman's world it doesn't affect him.
In the possible future, Tim Drake as Batman used this ring when Superboy tried to stop Batman from hurting Tim Drake as Robin. It is also revealed that Tim inherited the ring after Bruce Wayne's death.