"As I increase the weight on my deadlifts, my grip starts slipping. What can I do? Do I need to use gloves?"
Gripping the bar becomes an issue for just about everyone as the weight increases when you use a conventional grip (as pictured).
Your fingers can easily pry apart with a conventional grip. Gloves and straps are options, but they come with trade-offs. Gloves can help with moisture, but they actually make the the grip thicker, and therefore harder to grip.
Straps, do give you an advantage in your grip but can be cumbersome using all the time. They also do put more stress on the wrist.
Olympic weightlifters often use a hook grip (pictured below). This grip, with chalk, provides a very strong grip. Two fingers reinforce the thumb, keeping the fingers hooked around the thumb. It usually takes a while to get used to this grip. Many lifters who try this grip for the first time complain about irritation on the inside of the thumb. This discomfort diminishes over time as caluses develop.
A great solution is the modified hook grip (below). This grip also reinforces your fingers, but the thumb is on top of the first finger or first two fingers. This position does not cause the same thumb discomfort as with the hook grip and therefore can immediately be adopted.
The modified hook grip can be used for other pulling exercises, like rows. The orientation of the fingers increases the strength and endurance of your grip. Of course, chalk keeps your hands dry and increases the effectiveness of both the hook grip and modified hook grip.