Interactions between Lithium Growths and Nanoporous Ceramic Separators

To enable lithium (Li) metal anodes with high areal capacity that can match advanced cathodes, we investigate the growth mechanisms and the tendency of the deposited metal to penetrate nanoporous ceramic separators across a range of practical current densities. Our results from realistic sandwich cells and special transparent junction cells suggest the existence of three growth modes of lithium, due to the competing reactions of lithium deposition and the solid electrolyte interphase formation. A critical current density (6 mA cm−2), ∼30% of the system-specific limiting current density, is identified as a practical safety boundary for battery design and operation, under which root-growing lithium whiskers are the dominant structure of electrodeposition and can be blocked by the nanoporous ceramic separator. Our operando experiments reveal that metal penetration of the separator does not lead to zero voltage immediately, but rather to sudden, small voltage drops, which should not be treated as benign soft shorts. [Display omitted] •Three current-dependent growth modes during Li electrodeposition are identified•Nanoporous ceramic separators can block Li growths up to a critical current density•Internal shorts at under-limiting currents are due to reaction-limited surface growth•Sudden voltage drops are signs of the metal penetration through the separator Li-ion batteries are energy-dense power sources of cell phones, laptops, and electric vehicles. The basic unit inside is a three-layer stack, i.e., anode, separator, and cathode, fully wetted by organic liquid electrolyte. Removing the ion-insertion anode materials could significantly increase the energy density of the battery. During recharge, Li ions that used to be accommodated by the anode materials will be reduced to form a Li metal anode, without dead weight and volume. The process, however, is notoriously unstable and always forms finger-like structures that can penetrate the separator to short-circuit the battery, through mechanisms more complex than the simple term “dendrite” can reveal. Depending on the current, one may generate tip-growing dendrites, root-growing whiskers, or surface-growing clusters. This study presents the accurate understanding of each growth mode, which is critical for controlling the hazardous instabilities across the entire range of working conditions. Depending on the operating current density, lithium electrodeposition exhibits three distinct growth modes: root-growing whi