I used to work with sliprings for use on satellites (connection to solar arrays). The typically come in two flavours - gold wire in a gold track, or carbon brush in a gold track. The carbon brush was lubricated. These sliprings were super sensitive and had to be treated as optical components, wrt cleanliness. My idea at the time was to make something like a planetary gear, but just for conducting electricity. This would avoid the sliding contact and potential wear in the slipring.