Electrochemical Cycling Behaviour and Shape Changes of Zn Electrodes in Mildly Acidic Aqueous Electrolytes Containing Quaternary Ammonium Salts

Abstract

Secondary Zn–based batteries are a valid alternative to Li for stationary storage, but commercial devices are not yet available, chiefly owing to anode shape‐change and passivation issues. Mildly acidic aqueous solutions are actively studied, since they seem to limit unstable growth of Zn, with respect to the alkaline ones, customary for primary batteries. Additives can further improve the performance of mildly acidic electrolytes. In this work we focus on the impact of a series of quaternary ammonium salts (TBAB, CTAB, DMDTDAB, BDMPAC, BPPEI, PDADMAC), selected to represent a comprehensive range of molecular functionalities. Electrochemical measurements (cyclic voltammetry, chronopotentiometry and galvanostatic‐cycling in split‐cells), combined with 2D and 3D imaging techniques (SEM, stereomicroscopy and in situ tomography) were adopted for the assessment Zn behaviour. This multi‐technique approach pinpointed TBAB as the single most effective additive for low‐current density operation, while at high current densities the additive‐free electrolyte allows better cycling performance, coherently with similar results for alkaline electrolytes.

Understanding the Role of Additives on The Electrochemistry and Performance of Zn Energy Storage Devices

“…[26,27] Likewise, several studies have demonstrated the beneficial effect of some QAC, with TBA showing the best results so far. [28,29] The mechanism through which these cationic substances affect the electrochemical process is clear and related to the adsorption on the surface of the electrode, creating a barrier that controls the reaction rate. An exhaustive study of QAC with different chemical structure was conducted by Bozzini et al, [29] showing that inhibition of the cathodic reaction is the main factor leading to the improvement in cycling stability.…”

“…[28,29] The mechanism through which these cationic substances affect the electrochemical process is clear and related to the adsorption on the surface of the electrode, creating a barrier that controls the reaction rate. An exhaustive study of QAC with different chemical structure was conducted by Bozzini et al, [29] showing that inhibition of the cathodic reaction is the main factor leading to the improvement in cycling stability. The synergetic effect between a complexing agent as DMSO together with an adsorbing additive, allows cycling for almost 250 h in comparison with the 120 h achieved in the absence of additives.…”

“…Al [28] achieved great cycling stability by combination of QAC and a 3D copper current collector, thus the improved performance cannot be attributed solely to the additive but to a synergy between both factors. B. Bozzini et al [29] reported relatively poor cyclability (below 100 h) at low current densities and areal capacities (1 mA cm 2 and 0.5 mAh cm 2 ) after addition of several QACs to a 2 M ZnSO 4 electrolyte. These authors also provided a detailed explanation, based on the Zn deposition morphology, of the positive effect that higher current densities can have on the cyclability of Zn//Zn symmetric cells previously observed by Glatz et al .…”

“…On the other hand, quaternary ammonium compounds (QAC) have been known to have a positive effect on the morphology of Zn deposits for decades and are now being considered in the formulation of electrolytes for Zn based devices. [28][29][30] These cationic surfactants adsorb on the surface of Zn forming a barrier that controls the reaction rate, [31][32][33] mitigating common issues of Zn anode such as dendritic growth and H 2 evolution. Even though there are several reports on the use of either DMSO or QAC as additives in neutral electrolytes, the combination of both types of organic substances has not been considered yet, which could bring about further improvements in Zn-devices cycle life.…”

“…Both effects hinder dendrite formation thus increasing the cycling stability of Zn anodes. On the other hand, quaternary ammonium compounds (QAC) have been known to have a positive effect on the morphology of Zn deposits for decades and are now being considered in the formulation of electrolytes for Zn based devices [28–30] . These cationic surfactants adsorb on the surface of Zn forming a barrier that controls the reaction rate, [31–33] mitigating common issues of Zn anode such as dendritic growth and H 2 evolution.…”