Li-metal is a promising, high-capacity anode that can be incorporated into Li-ion, Li-S, and Li-air battery systems to meet the need for energy-dense batteries. However, lithium is lost both to forming a solid electrolyte interphase (SEI) and electrically disconnected metallic lithium deposits (‘dead’ Li). These losses limit cycle life and ultimately, commercialization. While protection methods, such as coatings and electrolyte additives, have yielded improved cycling efficiencies, identifying how losses are suppressed will allow for mechanistic understanding and improved design. To elucidate of the origins of capacity loss in lithium metal anodes, we have developed a method of monitoring lithium metal in the anode via operando X-ray diffraction (XRD). Throughout cycling, we can understand how much lithium is active, how much ‘dead’ lithium develops, and how the lithium metal corrodes. Our results show the lithium metal efficiency, measured through XRD, is higher than the electrochemical Coulombic efficiency measurements. We also see that the contribution of ‘dead’ lithium to the overall Coulombic efficiency varies much more across electrolytes, cell compression, and cycling conditions than the SEI does. This fundamental knowledge will inform design of protection methods and improve anode performance. In addition to Li loss, the morphology development of Li metal is it is plated has an impact on cyclability. To investigate the dynamic processes occurring during lithium plating, we have performed operando grazing incidence small angle X-ray scattering (GISAXS). GISAXS shows in real time that for certain additives the growth of lithium particles to a fixed, highly monodisperse diameter, followed by columnar growth, which is reversible at low current rates and capacities. Effects of applied current density on SEI formation and lithium plating were studied independently as well as the relationship between the two phenomena.

Monitoring capacity losses through operando X-ray diffraction of lithium metal anodes

2019 Fall National Meeting

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