Homogeneous Coating with an Anion-Exchange Ionomer Improves the Cycling Stability of Secondary Batteries with Zinc Anodes

Abstract

Limited cycling stability of secondary cells with zinc anodes arises mainly from the high solubility of oxidized zinc species in the alkaline electrolyte resulting in electrode shape change and loss of active material during repeated discharge and charge. We propose and successfully employ a homogeneous coating with an anion-exchange ionomer (AEI) on model electrodes with electron-conductive host structures to confine the oxidized zinc species. Ideally, the confinement of oxidized zinc species reduces the shape change of the electrode and keeps the active material as close as possible at its place of origin. In this work, the confinement concept for the oxidized zinc species is elucidated by means of electrochemical studies and X-ray photoelectron spectroscopy: as intended, an interlayer of zinc oxide forms between the AEI and the surface of the zinc electrode. This interlayer implies that the hydroxide ions are able to pass and react as intended, whereas the migration of oxidized zinc species into the bulk electrolyte is hindered. The coating with an AEI yields a higher amount of restored zinc during electrodeposition in comparison to an uncoated zinc electrode-applying an AEI coating increases the achievable cycle number by up to six times. We investigate the morphology of the cycled electrodes and derive thereby the needs for further material classes that might be employed in the confinement concept. This approach demonstrates the benefit of ion-selective coatings, allowing for the permeation of hydroxide ions but not of oxidized zinc species, a concept which improves rechargeable batteries with zinc anodes, such as zinc-oxygen batteries.

Zinc–air batteries: are they ready for prime time?

“…22 To tackle with the corrosion and dendrite problems, composition or surface modications of Zn anodes are necessary. Studies show that Zn electrodes alloyed with certain metals (e.g., Pb, Cd, Bi, Sn and In) 13,56,57 or modied with additives (e.g., silicates, surfactants and polymers) [58][59][60][61] can suppress H 2 evolution to different extents. The incorporation of alloying metals or additives may increase the conductivity, improve the current distribution and promote the formation of compact, thin Zn deposits.…”

Enhancing the Cycle Life of a Zinc–Air Battery by Means of Electrolyte Additives and Zinc Surface Protection

“…J. Zhu et al [24] tried different ionomer films that were coated on zinc electrodes, demonstrating, in general for all of them, an effective reduction of dissolved zinc discharge products into the electrolyte. In the same way, D. Stock et al [28] demonstrated an effective increment of the cycle life for Zn model electrodes coated with a homogeneous layer of an anion-exchange ionomer. They explained such an increment by a greater amount of restored zinc during electrodeposition thanks to a confinement of oxidized zinc species near the electrode surface.…”

“…Hence, it is expected that this Nafion ® layer spread on zinc particle surfaces would act as a barrier to reduce the diffusion of discharge products (i.e., zincate ions) to the bulk electrolyte. This would keep zincate ions in the proximity of the electrode surface, minimizing the dissolution of zinc (closely related to shape change and dendrite formation), and favoring zinc deposition during battery charging processes [28].…”

“…For the same electrolyte, the ionomer coating (curve b. in Figure 2) induces a slight negative shift (13 mV) of the corrosion potential (third column in Table 2). In this respect, the local ionomer coating may provoke an accumulation of zincate ions at the vicinity of the coated zinc surface [28], thus promoting the negative shift of the potential.…”

“…For the same electrolyte, the ionomer coating (curve b. in Figure 2) induces a slight negative shift (13 mV) of the corrosion potential (third column in Table 2). In this respect, the local ionomer coating may provoke an accumulation of zincate ions at the vicinity of the coated zinc surface [28], thus promoting the negative shift of the potential. As shown in Figure 2 and summarized in Table 2, the blank (ICZP-0 in 6.6 M KOH) has an E corr of −1.427 V Hg/HgO (1.329 V vs. SHE) in good agreement with the literature [19,47].…”