Abstract A LiClO4 SEI film grown on copper was examined with time-of-flight secondary ion mass spectrometry. The SEI porosity profile and Li+ transport processes within the SEI were studied with isotopically labeled 6LiBF4 electrolyte. An ~ 5 nm porous region, into which electrolytes can easily diffuse, was observed at the electrolyte/SEI interface. Below the porous region, a densely packed layer of Li2O and/or Li2CO3 prevents electrolyte diffusion, but Li+ transports through this region via ion exchange.