Perspective on the mechanism of mass transport-induced (tip-growing) Li dendrite formation by comparing conventional liquid organic solvent with solid polymer-based electrolytes

Abstract

A major challenge of Li metal electrodes is the growth of high surface area lithium during Li deposition with a variety of possible shapes and growing mechanisms. They are reactive and lead to active lithium losses, electrolyte depletion and safety concerns due to a potential risk of short-circuits and thermal runaway. This work focuses on the mechanism of tip-growing Li dendrite as a particular high surface area lithium morphology. Its formation mechanism is well-known and is triggered during concentration polarization, i.e. during mass (Li+) transport limitations, which has been thoroughly investigated in literature with liquid electrolytes. This work aims to give a stimulating perspective on this formation mechanism by considering solid polymer electrolytes. The in-here shown absence of the characteristic “voltage noise” immediately after complete concentration polarization, being an indicator for tip-growing dendritic growth, rules out the occurrence of the particular tip-growing morphology for solid polymer electrolytes under the specific electrochemical conditions. The generally poorer kinetics of solid polymer electrolytes compared to liquid electrolytes imply lower limiting currents, i.e. lower currents to realize complete concen­tration polarization. Hence, this longer-lasting Li-deposition times in solid polymer electro­lytes are assumed to prevent tip-growing mechanism via timely enabling solid electrolyte interphase formation on fresh Li deposits, while, as stated in previous literature, in liquid electrolytes, Li dendrite tip-growth process is faster than solid electrolyte interphase forma­tion kinetics. It can be reasonably concluded that tip-growing Li dendrites are in general practically unlikely for both, (i) the lower conducting electrolytes like solid polymer electro­lytes due to enabling solid electrolyte interphase formation and (ii) good-conducting electro­lytes like liquids due to an impractically high current required for concentration polarization.