In the present study, the thickness of the reduction zone has been set to r e -= 1 nm from the anode surface, which means that reduction reactions are restricted to that area of the simulation cell. Based on work by Lin et al. that estimated the insulating thickness of a layer of Li 2 CO 3 as 3 nm (at which electron tunnelling probability is essentially zero ), our value averages the distance at which appreciable electron tunnelling can occur.