Experimental and Modeling Studies of the Hysteresis Behavior and Dendrite Suppression Efficacy of an Electrolyte Additive in Zinc Electrodeposition

During Zn-air battery charging, the electrodeposited Zn metal is known to evolve undesirable dendritic morphology. Here, we report on a novel electrolyte additive benzyltrimethylammonium hydroxide (BTMAH), which effectively suppresses Zn dendrites. In cyclic voltammetry studies of Zn electrodeposition, BTMAH presents a unique hysteresis effect suggesting that the Zn surface is rendered either polarized or depolarized depending on the relative rates of BTMAH transport, adsorption and its deactivation at the Zn surface, as well as the potential scan direction. Mass spectroscopy indicates that BTMAH deactivation is likely not due to its inclusion into the Zn deposit. A mathematical model, which incorporates a dimensionless parameter xi representative of the relative rates of BTMAH transport and surface deactivation, is presented to explain conditions under which BTMAH can suppresses accelerated growth at micro-scale dendrite tips. At high BTMAH concentrations and small dendrite tip radii, BTMAH transport rates exceed surface deactivation rates (xi is large) thereby favoring surface saturation by BTMAH and thus suppressed Zn growth. Experimental data supports model predictions of the concentration-dependent efficacy of BTMAH in suppressing dendrites. BTMAH opens the possibility for 'localized' suppression of dendrites during charging of energy-efficient Zn-metal batteries. (C) The Author(s) 2019. Published by ECS.