This is a tinned copper ring terminal. It is very oversized and works great on 1/0 and 2/0 awg size cables. Because these are oversized and thick, it is recommended to use a hydraulic crimper. (The Temco brand is the best one I have found)

Bare copper is fairly resistant to corrosion, but when used in humid or wet environments, it can quickly become damaged. By coating bare copper with a metal alloy, like tin, the copper's natural properties are strengthened giving it a longer life.
 
This 5/16" ring fits my grounding bolts perfectly.

Sold individually.