Basal-Plane Orientation of Zn Electrodeposits Induced by Loss of Free Water in Concentrated Aqueous Solutions

Abstract

Concentrated aqueous solutions attract considerable attention because water electrolysis can be suppressed due to a decrease in the amount of free water. The present study focuses on electrodeposition behaviors of metallic zinc (Zn) using concentrated aqueous solutions containing bis(trifluoromethylsulfonyl)amide (Tf2N–) anions. An increase in Tf2N– concentration significantly enhances water-anion interactions, giving characteristic infrared spectra for the breakdown of the hydrogen-bonding networks of water clusters, i.e. loss of free water. For the Tf2N– system Zn electrodeposits with the preferred orientation of hcp basal plane was observed, while, for the SO4 2– system with the presence of the hydrogen-bonding networks, preferred orientation of basal plane was not observed. The preferred orientation of basal plane is not attributed to the adsorption of Tf2N– anions on the electrode, proved by the use of mixed Zn(Tf2N)2-ZnSO4 concentrated solutions. The loss of free water in the concentrated Zn(Tf2N)2 solutions will suppress hydrogen adsorption at the cathode to promote surface diffusion of intermediate Zn+ adions and growth of Zn crystals. Consequently, the promotions and the easier growth of Zn basal planes with the lowest interfacial free energy will enhance the horizontal growth of Zn basal planes.

Selective Zn/Na Ions Insertion into FePO4 Positive Electrode Tuned by Counter Anions in Aqueous Zn‐Based Rechargeable Batteries

“…No significant blueshift was observed in the Raman bands located at 745 cm −1 attributable to CF 3 bending vibration coupled with the S−N stretching mode of TFSA anions even with increasing NaTFSA concentration, meaning that the TFSA anions are not ligands for the Zn ions as well as Na ions, but rather are primarily coordinated by water molecules (Figure 1c). This is substantiated by the existence of the Raman bands between 380 to 425 cm −1 ascribed to [Zn(H 2 O) 6 ] 2+ [33] . In addition, the hydration are also supported by the results shown by Zhang et al .…”

“…This is substantiated by the existence of the Raman bands between 380 to 425 cm À 1 ascribed to [Zn(H 2 O) 6 ] 2 + . [33] In addition, the hydration are also supported by the results shown by Zhang et al; they have found experimentally and computationally using X-ray total scattering, X-ray adsorption spectroscopy, and molecular dynamics simulation that Zn ions undergo six coordination by water molecules through oxygen atoms rather than TFSA anions in the water-in-salt electrolyte composed of 1 m Zn(TFSA) 2 and 20 m LiTFSA. [35] On the other hand, in the FT-IR spectra of the electrolytes with Cl-based salt, the increase in peak intensity attributable to the hydration of Zn ions was minor difference with increasing NaCl concentration from 1 to 6 m (Figure 1b).…”

“…Diffusions of Zn adatoms or adions on the negatively charged Zn surface (active sites such as kink) during the cathodic polarization are assumed to be inhibited by the adsorbed hydrogen atoms and positively charged hydrated Na ions. [33] This should be the cause of the small grains observed at high current densities. On the other hand, in the electrolytes with ZnCl 2 , the mossy form is dominant below 1 mA cm À 2 .…”

“…In the FT‐IR spectra of the electrolyte with TFSA‐based salt, the O−H stretching modes of water molecules in the range from 2900 to 3700 cm −1 shifted toward a higher wavenumber side (Figure 1b). The peak at around 3600 cm −1 is the strong hydration of Zn ions with high charge density, [33] and broad peak between 3250 and 3400 cm −1 is the result of the solvation of water molecules to Na ions. TFSA anions interact with water through the oxygen atoms of sulfonyl group of the anions and resulting decreased free water molecules are also responsible for the peak shift to the high wavenumber side (Figure S1, Supporting Information) [34] .…”

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

“…Problems related to deposits morphologies are more pronounced in alkaline environment, where non-compact structures have a higher tendency to grow. This is one key reason for the increasing interest in alterative water-based electrolytes, among which mildly acidic (in battery chemistries such as zinc-ion battaries (ZIB), [24,25] rechargeable zinc-air batteries [26][27][28][29] and hybrid batteries with a Zn anode and a Li-ion intercalation cathode [16,17] ) and highly-concentrated [30,31] electrolytes: the focus of this work will be on the former approach. It is worth noting that the formation of mossy zinc occurs at lower c. d. s than compact deposits, so that the c. d. working range during charge which allows to have a high cyclability, has not only an upper limit due to dendritic growth but also a lower one, since Zn anodes might have problems also working at low c. d. s: it is worth stressing that the literature on Zn anodes did not address this issue, to date.…”