Lithium metal has received a renewed interest as a promising anode material for next-generation, high-energy batteries owing to its high specific capacity (3860 mAh g-1) and low reduction potential (-3.04 V vs. the standard hydrogen electrode). However, lithium metal batteries suffer from low capacity retention, short cycle life and safety problems associated with microstructural and dendritic growth of lithium. In this work, nuclear magnetic resonance (NMR) spectroscopy is used to understand the effect of the solid electrolyte interphase (SEI) on lithium metal deposition. In situ NMR is used to quantify the lithium microstructures formed during plating and allows the current efficiency and porosity of the structures to be estimated. The effect of the fluoroethylene carbonate (FEC) additive is explored along with a range of plating conditions. NMR measurements show that the isotope exchange between a 6Li-enriched lithium metal and a natural abundance electrolyte depends significantly on the electrolyte and the corresponding SEI. A numerical model is developed to describe the processes during isotope exchange and is discussed in the context of the standard model of electrochemical kinetics. The model is used to extract both an exchange current at the open circuit voltage and the SEI formation current as a function of time. In situ NMR methods are then developed to study ‘anode-free’ lithium metal batteries where the lithium is plated directly onto a bare copper current collector from a LiFePO4 cathode. The low cycling stability of lithium metal batteries becomes clear when there is no excess of lithium in the cell. The ‘dead lithium’ and SEI formation can be quantified by NMR and their relative rates of formation are here compared in carbonate and ether-electrolytes. Importantly, the NMR experiments reveal that the dissolution of lithium metal during the periods when the battery is not in use, i.e., when no current is flowing, demonstrating that dissolution of lithium remains a critical issue for lithium metal batteries. Strategies to mitigate lithium corrosion are explored; the work demonstrating that both polymer coatings and the modification of the copper surface chemistry stabilising lithium metal. Overall, this work demonstrates that the NMR approach offers unique insight into the dynamic processes occurring on lithium metal both during electrochemical measurements and at the open circuit voltage.