Modeling of Dead Lithium Formation During Dissolution

Kurzfassung

Lithium metal anode batteries are promising candidates for next generation energy storage systems as they have a high theoretical energy density. In order to make them commercially viable, the cyclabilty has to be improved. It is limited by irreversible loss of active lithium due to the formation of a solid electrolyte interface (SEI) and the formation of unreactive "dead" metallic lithium. The latter is the dominating source of capacity loss in certain battery systems. Experiments showing that during dissolution of lithium whiskers the tip cannot be dissolved hint that mechanical instabilities occur. We developed a generalized phase-field model of the dissolution process in order to gain insights of the underlying mechanisms of "dead" lithium formation and capacity fade. Our phase-field model describes the dissolution of a single lithium whisker in a liquid electrolyte based on lithium surface tension and the interaction between lithium and the SEI, utilizing the framework of non-equilibrium thermodynamics. We study the shape evolution of the lithium whisker under galvanostatic condition and under which circumstances instabilities occur. Our model is able to predict the nucleation of a Rayleigh instability induced by interactions with the SEI. Thus, the lithium whisker does not dissolve completely and an electrically disconnected metallic lithium droplet is formed. This is in agreement with optical microscope observations of lithium dendrite dissolution.