Kryptonite Ring

Batman keeps a kryptonite ring inside a lead-lined compartment in his utility belt so that if Superman ever went rogue he would have a chance of controlling him. Lex Luthor once wore a krytonite ring to protect himself from Superman. Prolonged exposure to the ring caused Luthor to lose his hand.
 
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.