How does increasing zinc salt concentration suppress side reactions? Optimize aqueous zinc-ion cells with water-in-salt electrolytes

Increasing zinc salt concentration suppresses side reactions by reducing the amount and reactivity of water near the zinc electrode. In dilute aqueous electrolytes, Zn²⁺ is typically surrounded by a water-rich solvation sheath, often represented as [Zn(H₂O)₆]²⁺. Concentrated or “water-in-salt” electrolytes introduce more anions into this sheath, lower water activity, and reduce the water available for hydrogen evolution, corrosion, and active-material dissolution.

Core takeaway: High salt concentration changes both the chemistry of Zn²⁺ solvation and the availability of free water. This makes water-driven parasitic reactions less favorable, although the concentration must be optimized because excessive salt can increase viscosity, reduce conductivity, and create new interfacial problems.

Why Dilute Electrolytes Promote Side Reactions

Zn²⁺ Carries a Water-Rich Solvation Sheath

In a dilute aqueous electrolyte, Zn²⁺ strongly coordinates with surrounding water molecules. Its primary solvation environment is commonly approximated as [Zn(H₂O)₆]²⁺.

These coordinated water molecules accompany Zn²⁺ toward the electrode interface. Before zinc can deposit or intercalate, part of this solvation sheath must be removed, a process known as desolvation.

Coordinated Water Can Participate in Parasitic Chemistry

The water surrounding Zn²⁺ is not chemically irrelevant. At the zinc anode, water can be reduced to hydrogen through the hydrogen evolution reaction, consuming charge without storing it reversibly.

Water can also contribute to zinc corrosion, local pH changes, and the formation of unwanted surface products. These processes lower Coulombic efficiency and make cycling less predictable.

Water Can Destabilize Active Materials

Water-driven reactions are not limited to the zinc electrode. They can promote dissolution or structural degradation of active materials at the positive electrode.

Dissolved species may migrate through the electrolyte and participate in additional parasitic reactions, causing capacity loss and poorer long-term reversibility.

How Higher Salt Concentration Changes the Electrolyte

Anions Enter the Zn²⁺ Solvation Structure

As zinc salt concentration increases, anions become more abundant relative to water. Instead of being surrounded almost entirely by water, Zn²⁺ can form solvent structures containing both water molecules and coordinating anions.

In “water-in-salt” formulations, the salt concentration is high enough that the electrolyte contains very little free water relative to the dissolved ions. The exact solvation structure depends on the salt, anion, solvent, and concentration, so anions should not be assumed to replace every coordinating water molecule.

Free-Water Activity Decreases

The important change is not simply that there is less total water. Concentrated salts reduce water activity, meaning water is less available to participate in electrochemical and chemical reactions.

This raises the practical difficulty of reducing water at the zinc surface and suppresses water-dependent processes such as hydrogen evolution and corrosion.

Desolvation and Interfacial Reactions Are Altered

A Zn²⁺ ion with a modified, anion-containing solvation sheath encounters a different desolvation environment at the electrode interface. The reduced number of tightly coordinated water molecules can lessen water-associated desolvation penalties and limit the direct delivery of reactive water to the zinc surface.

The result is often more reversible zinc plating and stripping, but the outcome depends on the specific electrolyte and electrode interface.

Which Side Reactions Are Suppressed

Hydrogen Evolution Is Reduced

At the zinc anode, hydrogen evolution competes directly with zinc deposition. It consumes electrons, generates gas, and can alter the local pH near the electrode.

Lower water activity and a more favorable interfacial solvation environment reduce the extent of this reaction. This helps increase the fraction of charge used for reversible zinc deposition.

Zinc Corrosion Becomes Less Severe

Water and dissolved ionic species can sustain chemical corrosion even when the battery is not actively cycling. Concentrated electrolytes restrict the water available for these reactions and can change the composition of the zinc-electrolyte interface.

This does not eliminate corrosion, but it can reduce its rate and improve zinc utilization.

Active-Material Dissolution Is Mitigated

A less water-reactive electrolyte can reduce dissolution of susceptible electrode materials. It also limits some coupled reactions involving dissolved species, water, and changes in local acidity or alkalinity.

Lower dissolution helps preserve electrode composition and reduces cross-talk between the positive electrode and zinc anode.

Dendrite Formation Can Be Inhibited

Uneven zinc deposition is influenced by current distribution, surface chemistry, mass transport, and interfacial reactions. Suppressing hydrogen evolution and corrosion removes sources of local pH and surface nonuniformity that can intensify uneven growth.

Concentrated electrolytes may therefore help produce more uniform zinc deposition. They are not, by themselves, a guaranteed dendrite-prevention strategy.

Why Coulombic Efficiency Improves

More Charge Goes to Zinc Deposition

Coulombic efficiency measures how much of the charge used during plating can be recovered during stripping. Hydrogen evolution, corrosion, and electrically isolated zinc all reduce this value.

When concentrated electrolytes suppress these parasitic pathways, a greater proportion of the applied charge contributes to reversible zinc cycling.

Near-100% Efficiency Requires More Than Concentration

High salt concentration can move experimental cells toward Coulombic efficiencies near 100%, particularly when the electrolyte is well matched to the electrode materials and operating conditions.

However, efficiency also depends on current density, areal capacity, zinc excess, separator properties, electrode surface condition, and measurement protocol. Electrolyte concentration should be treated as one part of the cell design rather than a standalone solution.

Understanding the Trade-offs

Excess Salt Can Increase Viscosity

Very concentrated electrolytes are generally more viscous. This slows ion transport and can make electrolyte wetting, separator infiltration, and reproducible cell assembly more difficult.

Higher viscosity can also increase concentration polarization during high-rate cycling.

Ionic Conductivity May Decline

Adding salt initially often improves the number of charge carriers, but conductivity does not increase indefinitely. At sufficiently high concentrations, ion pairing, correlated ion motion, and increased viscosity can reduce effective ionic transport.

The concentration that best suppresses side reactions may therefore not provide the best high-rate performance.

Anions Can Create New Risks

Anions such as TFSI⁻ or Cl⁻ can participate in interfacial reactions and may alter corrosion, passivation, or electrode stability. Their benefits depend on the electrode chemistry and potential window.

A water-in-salt electrolyte must therefore be evaluated for compatibility with both electrodes, the current collectors, and the separator.

Concentration Must Be Optimized Experimentally

The useful operating range is a balance between low water activity, acceptable conductivity, manageable viscosity, and stable electrode interfaces.

Researchers should compare concentration-dependent hydrogen evolution, zinc plating and stripping efficiency, impedance, dissolution, and rate capability rather than selecting the highest possible salt concentration.

How to Apply This to Your Project

The most useful design principle is to optimize water activity and Zn²⁺ solvation together, then verify that transport and materials compatibility remain acceptable.

• If your primary focus is suppressing hydrogen evolution: Use a concentrated or water-in-salt electrolyte that reduces free-water activity at the zinc anode, then verify gas evolution and zinc corrosion under the intended current density.
• If your primary focus is improving zinc plating and stripping efficiency: Select a concentration that produces an anion-influenced Zn²⁺ solvation structure without causing excessive viscosity or transport limitations.
• If your primary focus is preventing active-material dissolution: Evaluate concentrated electrolytes alongside positive-electrode stability, because reduced water reactivity can help but cannot compensate for an intrinsically incompatible salt or anion.
• If your primary focus is high-rate operation: Avoid assuming that the most concentrated formulation is best; measure ionic conductivity, impedance, polarization, and rate capability across a concentration series.
• If your primary focus is dendrite control: Treat concentration as one control variable and combine it with appropriate current density, electrode surface preparation, separator selection, and cell balancing.

By controlling both Zn²⁺ solvation and water activity, concentrated electrolytes can make aqueous zinc-ion cells substantially more reversible and experimentally reliable.

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