context: Via this NMR study we identify and quantify some of the key SEI parameters – namely the lithium ion transport and the rate of healing – that are important in controlling the nature of lithium metal deposition. Future studies with a much wider range of additives and electrolytes are in progress to use this methodology to help design an optimal SEI layer on lithium metal that achieves uniform plating and stripping at commercially relevant current densities (>0.5 mA cm−2) with high coulombic efficiencies.