Blind rivet nuts for ebike battery on frame: aluminium or steel?
Personally?  For a heavy battery mounted on raised/cantilevered supports, I'd suggest looking at a welded support and forgo rivets completely.
If you do use rivets, I'd match the metal to the frame to eliminate galvanic corrosion issues.   Four rivets might not be enough - depending on how your mounts touch the frame, you might want &quot;foot-like&quot; lugs, with one either side and at least one in the foreward or aft direction.
Or look at mounting rivnuts in the frame instead, and then use decent bolts into those to hold your battery frame.
Batteries are surprisingly heavy - I once made a 3 point bracket for inside a frame's main triangle.  It used the bottle cage mounts, along with metal straps to the top and seat tubes.  It was barely capable of holding the batteries even with a triangle of stability.