The Sully’s Key is the key that you will use to use the Sully’s Steam Terminal with your Steam Battery.
How to get Sully’s Key
Pass by Sully’s shop and go to the blue building in the middle. This is the living quarters. Jump up the platforms on the left side and then run past the door. You’ll see a robot crab here. Jump up and land on top of him to make him bounce and flip over. When he does, he’ll drop Sully’s key. Pick up the Sully’s key.