One simple exercise that you might do is the static hold, just load the up the bar that you deadlift with a weight that you consider heavy, deadlift the bar and hold it in the top position for as long as you can. Do 3-5 sets of these once a week. Experiment with the weight, go heavy when doing static holds and do it with weight that you can hold for only 10 to 15 seconds.
You might experiment a bit with the grip. Grip the bar so that one arm holds the bar with pronated grip and the other arm has supinated grip. I think it's called alternate grip.
Here's a good article with exercises for the grip
Bodybuilding.com - David Robson - Get To Grips With Hand Strength: Develop Your Grip. .
Also, if you are still struggling with the grip, you might try using wrist straps.