Increasing salt concentration generally improves zinc-anode reversibility in aqueous zinc-ion batteries (AZIBs) by restructuring the Zn²⁺ solvation sheath and reducing the amount of reactive, free water. This suppresses hydrogen evolution, corrosion, active-material dissolution, and irregular zinc deposition, which can raise Coulombic efficiency toward 100% and extend cycling stability. However, excessive concentration increases viscosity and can reduce ionic conductivity, rate capability, and overall cell power performance.
The optimum electrolyte concentration balances interfacial stability against ion transport: enough salt should be present to limit water-driven side reactions and dendrite growth, but not so much that viscosity, conductivity losses, and polarization offset those benefits.
How Concentration Changes the Zinc–Electrolyte Interface
Anions Enter the Zn²⁺ Solvation Sheath
In a dilute aqueous electrolyte, Zn²⁺ is strongly coordinated by water, commonly represented as a hydrated species such as ((\mathrm{Zn(H_2O)_6})^{2+}). Increasing the salt concentration introduces more anions—such as TFSI⁻ or Cl⁻—into the primary solvation environment.
This reduces the number of water molecules directly associated with Zn²⁺ and changes the desolvation and deposition process at the zinc surface.
Less Reactive Water Reduces Hydrogen Evolution
Free and coordinated water can participate in parasitic reactions at the zinc anode, particularly the hydrogen evolution reaction (HER). These reactions consume charge without reversibly storing zinc and can increase local pH, corrosion, gas formation, and interfacial instability.
A high-concentration or “water-in-salt” formulation reduces water activity, thereby suppressing these water-driven reactions during zinc plating and stripping.
Interfacial Chemistry Becomes More Stable
Lower water activity can also reduce zinc corrosion and the formation or dissolution of unstable by-products. This helps preserve the active zinc inventory and produces a more favorable interface for repeated deposition and removal.
The result is typically a smaller gap between the charge used for zinc plating and the charge recovered during stripping.
How Anode Reversibility Improves
Coulombic Efficiency Increases
Anode reversibility is commonly assessed through Coulombic efficiency, which compares the charge recovered during zinc stripping with the charge applied during zinc plating. Suppressing HER, corrosion, and electrically isolated zinc reduces irreversible charge loss.
With an appropriately optimized concentrated electrolyte, zinc plating and stripping can approach near-100% Coulombic efficiency. The exact result remains dependent on salt identity, concentration, electrode surface, current density, separator, pH, and cell configuration.
Dendrite Formation Is Reduced
Zinc dendrites develop when deposition becomes spatially nonuniform. Local electric-field intensification and uneven ion transport can cause certain regions to grow faster than others, eventually creating porous or needle-like structures.
Concentrated electrolytes can improve the stability of the zinc–electrolyte interface and help maintain a more uniform surface morphology over extended cycling. This lowers the likelihood of separator penetration, internal short circuits, and progressive loss of active zinc.
Active Material Loss Is Mitigated
Water-driven corrosion and dissolution can continuously remove zinc from the usable electrode. Concentration-driven changes in solvation and interfacial chemistry reduce these losses, helping retain the anode’s capacity over repeated cycles.
This improvement is especially important when comparing cells over hundreds of cycles rather than judging performance from only the first few charge–discharge cycles.
How Whole-Cell Performance Changes
Cycle Life Usually Improves
When parasitic reactions and dendritic growth are suppressed, the cell can maintain more stable capacity and voltage behavior during cycling. The improvement is not simply a result of higher salt content; it comes from preserving the zinc interface and reducing irreversible reactions.
A concentrated electrolyte can therefore provide a more reliable platform for studying cathode behavior because fewer apparent capacity losses originate from an unstable anode.
Rate Performance Can Decline at Excessive Concentration
Increasing concentration also increases electrolyte viscosity and can reduce ionic conductivity. Zinc-ion transport becomes less efficient, particularly at high current densities, which may increase polarization and reduce apparent capacity or power capability.
Thus, an electrolyte that produces excellent low-rate cycling may perform poorly under fast-charging or high-current conditions if its transport properties are inadequate.
Voltage Efficiency and Polarization Must Be Monitored
A concentrated electrolyte may improve Coulombic efficiency while simultaneously increasing overpotential if ion mobility and desolvation become less favorable. Researchers should therefore evaluate not only capacity retention, but also charge–discharge voltage hysteresis, impedance, and rate capability.
These measurements distinguish genuine interfacial improvements from performance changes caused by increased transport resistance.
What Researchers Should Control During Testing
Keep Electrolyte Chemistry Comparable
Salt concentration should not be treated as an isolated variable when changing the electrolyte. Salt identity, anion chemistry, solvent composition, pH, water activity, and additive content can all alter zinc deposition and corrosion.
For example, TFSI⁻- and Cl⁻-containing systems may produce different solvation structures and interfacial reactions even at similar nominal concentrations.
Control Zinc Surface and Cell Construction
Zinc surface roughness, electrode thickness, separator compression, electrolyte-to-capacity ratio, and sealing conditions can strongly affect dendrite formation and parasitic reactions. Nonuniform electrode density or inconsistent compression can obscure the true effect of electrolyte concentration.
Reproducible slurry processing, coating, pressing, and cell assembly are therefore essential when comparing formulations.
Measure Both Half-Cell and Full-Cell Behavior
Zinc plating/stripping tests are useful for isolating anode reversibility. Full-cell tests are necessary to determine whether the electrolyte actually improves practical capacity retention, energy efficiency, and long-term cycling.
A formulation can perform well in a zinc symmetric cell yet provide limited full-cell benefit if it adversely affects the cathode, separator, or overall ionic transport.
Understanding the Trade-offs
High Concentration Is Not Automatically Better
The beneficial effect of concentration has a practical optimum. Beyond that point, rising viscosity and falling conductivity can increase polarization, limit zinc-ion transport, and reduce usable capacity at higher rates.
The best concentration is therefore the one that minimizes irreversible interfacial reactions without imposing excessive transport penalties.
“Near 100%” Efficiency Requires Careful Interpretation
A high Coulombic efficiency indicates improved charge reversibility, but it does not by itself prove that the electrolyte is suitable for a complete battery. Capacity retention, energy efficiency, impedance growth, dendrite morphology, gas evolution, and post-cycling electrode condition must also be considered.
Testing conditions should be reported clearly because current density, areal capacity, zinc excess, and electrolyte volume can materially influence the result.
Additives and pH Can Confound Concentration Effects
Organic or inorganic additives such as SDS, CTAB, and thiourea can modify zinc crystal growth and corrosion behavior. Activated-carbon-containing electrodes may also improve interfacial reversibility, making it difficult to attribute an observed improvement solely to salt concentration.
Because zinc electrochemistry is pH-sensitive, changes in acidity or alkalinity must also be controlled. Acidic-to-neutral and alkaline environments can produce different dissolution, hydroxide, oxide, or zincate chemistry.
Alternative Electrolytes Solve Different Problems
Ionic-liquid electrolytes offer low volatility, non-flammability, high thermal stability, and a wider electrochemical stability window than aqueous systems. Their anion selection can also influence zinc deposit morphology.
However, ionic liquids are not simply a direct replacement for concentrated aqueous electrolytes: they introduce different transport, cost, viscosity, and materials-compatibility considerations.
How to Apply This to Your Research
The most defensible workflow is to screen concentration systematically while measuring both interfacial reversibility and transport performance.
- If your primary focus is zinc-anode reversibility: Increase concentration until HER, corrosion, dissolution, and dendritic deposition are substantially suppressed, then verify the improvement using plating/stripping Coulombic efficiency and post-cycling morphology.
- If your primary focus is long cycle life: Prioritize the concentration that maintains a stable zinc surface and minimizes active-material loss over extended cycling, rather than selecting the formulation with the best initial capacity.
- If your primary focus is high-rate performance: Reject concentrations that cause excessive viscosity, ionic-conductivity loss, or polarization, even if they provide excellent low-rate Coulombic efficiency.
- If your primary focus is mechanistic battery research: Control salt identity, pH, additives, electrode preparation, separator compression, and electrolyte loading so that concentration remains the principal experimental variable.
- If your primary focus is practical full-cell performance: Confirm that the concentrated electrolyte benefits the complete cell, including cathode compatibility, energy efficiency, rate capability, and capacity retention.
The right electrolyte concentration is not the highest achievable value, but the point at which reduced water activity and stabilized zinc deposition outweigh the resulting transport penalties.
Summary Table:
| Aspect | Impact of Increasing Salt Concentration | Key Considerations |
|---|---|---|
| Zn²⁺ Solvation | More anions enter solvation sheath, fewer water molecules coordinated | Reduces free water, alters deposition |
| Interfacial Side Reactions | Suppresses hydrogen evolution, corrosion, and dissolution | Increases Coulombic efficiency |
| Anode Reversibility | Improves plating/stripping efficiency, reduces dendrites | Approach near-100% CE with optimal concentration |
| Cycle Life | Generally improves due to stable interface | Maintains capacity over long cycling |
| Rate Capability | Declines at excessive concentration due to viscosity/conductivity loss | Trade-off between stability and power |
| Optimum | Balances interfacial stability vs. ion transport | Not highest concentration, but point of best performance |
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