Out of interest would an M10 aluminium rivnut be Ok to earth battery cable?
The contact area between the Rivnut and metalwork it's fitted to is only small, so while it would work, you need to be mindful as to how much current you put through it and if a high current (ie, cranking), do so only for very short periods, allowing some time in between for the heat to dissipate. Also, being Aluminium, if fitted to steel, there will in time be some corrosion here and it will end up a poor connection.
I feel a Bolt, Nut, Flat Washers and a 3 mm plate about 20 - 25 mm square would be a better way.