Noninvasive In Situ NMR Study of “Dead Lithium” Formation and Lithium Corrosion in Full-Cell Lithium Metal Batteries

Capacity retention in lithium metal batteries needs to be improved if they are to be commercially viable, the low cycling stability and Li corrosion during storage of lithium metal batteries being even more problematic when there is no excess lithium in the cell. Herein, we develop in situ NMR metrology to study "anode-free" lithium metal batteries where lithium is plated directly onto a bare copper current collector from a LiFePO₄ cathode. The methodology allows inactive or "dead lithium" formation during plating and stripping of lithium in a full-cell lithium metal battery to be tracked: dead lithium and SEI formation can be quantified by NMR and their relative rates of formation are here compared in carbonate and ether-electrolytes. Little-to-no dead Li was observed when FEC is used as an additive. The bulk magnetic susceptibility effects arising from the paramagnetic lithium metal were used to distinguish between different surface coverages of lithium deposits. The amount of lithium metal was monitored during rest periods, and lithium metal dissolution (corrosion) was observed in all electrolytes, even 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. The high rate of corrosion is attributed to SEI formation on both lithium metal and copper (and Cu⁺, Cu²⁺ reduction). Strategies to mitigate the corrosion are explored, the work demonstrating that both polymer coatings and the modification of the copper surface chemistry help to stabilize the lithium metal surface.