What are the primary degradation mechanisms affecting zinc anodes during secondary zinc-air cell testing, and how do they influence battery durability? Key factors include dendrite growth and passivation.

The primary degradation mechanisms are zinc shape change and dendrite growth, corrosion with hydrogen evolution, and passivation by insulating ZnO. During discharge, zinc forms soluble zincate species; during charging, non-uniform redeposition changes the electrode morphology and can create separator-piercing dendrites. At the same time, parasitic corrosion consumes water and generates hydrogen, while dense Type II ZnO films block electrochemical access to active zinc.

Battery durability is governed by how these mechanisms reduce reversible zinc utilization, increase impedance and self-discharge, and ultimately cause capacity failure or internal short circuits. They are interrelated: uneven current distribution and electrolyte concentration gradients can accelerate both dendrite formation and passivation.

How Zinc Anodes Degrade During Cycling

1. Zinc dissolution and electrode shape change

During discharge in an alkaline electrolyte, zinc dissolves and forms soluble zincate ions, commonly represented as [Zn(OH)₄]²⁻. Zinc is therefore removed from some regions of the anode and transported through the electrolyte before charging.

Ideally, charging would redeposit zinc uniformly across the original electrode surface. In practice, zincate concentration gradients and non-uniform current distribution cause some areas to dissolve or redeposit more rapidly than others.

This produces electrode shape change. Portions of the anode can become depleted while other regions accumulate excess zinc, reducing the uniformity of the active surface and limiting reversible zinc utilization.

2. Dendrite and needle-like growth

Localized zinc redeposition can produce mossy, spongy, or needle-like structures known as dendrites. High local current density and zincate concentration gradients make this uneven growth more likely.

Dendrites reduce mechanical and electrical stability in the cell. If they penetrate the separator, they can create an internal short circuit, causing abrupt failure rather than gradual capacity loss.

Even before a short circuit occurs, irregular deposits can isolate portions of zinc from the electrolyte or current collector. This decreases usable capacity and can increase cell impedance over repeated cycles.

3. Corrosion and hydrogen evolution

Zinc is susceptible to parasitic corrosion in aqueous alkaline electrolytes. A key associated side reaction is the hydrogen evolution reaction (HER), in which water is consumed and hydrogen gas is produced.

Hydrogen evolution has several durability consequences:

• Electrolyte depletion: Water consumption changes the electrolyte balance.
• Self-discharge: Zinc is consumed without delivering useful external energy.
• Pressure buildup: Accumulated hydrogen increases internal cell pressure.
• Lower Coulombic efficiency: Some charging current supports side reactions instead of reversible zinc deposition.

Corrosion also promotes inactive or poorly conducting surface products. As a result, the anode may lose active material even when the cell has not delivered its theoretical capacity.

4. Passivation by Type II ZnO

Zinc discharge can lead to the formation of zinc oxide, or ZnO, on the anode surface. A porous and reactive film may still permit electrochemical access, but a dense Type II ZnO layer is substantially more damaging.

This compact film obstructs ion transport and electrically isolates underlying zinc. The result is anode passivation, in which active material remains physically present but becomes difficult or impossible to use.

Passivation increases polarization and cell impedance. It also causes premature capacity loss because the measured capacity becomes limited by access to zinc rather than by the total amount of zinc in the electrode.

How These Mechanisms Reduce Battery Durability

Loss of reversible capacity

Shape change, corrosion, and passivation all reduce the fraction of zinc that can be reversibly cycled. Some zinc is physically displaced, some is consumed by side reactions, and some becomes trapped beneath resistive ZnO.

This causes progressive capacity fade and lowers the practical energy delivered per cycle.

Declining charge efficiency

Hydrogen evolution and corrosion divert current away from zinc deposition. Non-uniform deposition further reduces the fraction of the charging reaction that produces a stable, reversible anode.

The resulting low Coulombic efficiency compounds over many cycles: even a modest loss per cycle can produce substantial capacity loss during extended testing.

Increasing impedance and polarization

Passivation layers and irregular zinc deposits make charge and discharge increasingly difficult. The cell requires greater polarization to sustain the same current, reducing energy efficiency and usable power.

Increasing impedance can also intensify local current-density variations, reinforcing the conditions that caused non-uniform deposition in the first place.

Risk of abrupt failure

Capacity fade is not the only durability concern. Dendrite penetration can cause a sudden internal short circuit, while hydrogen accumulation can create mechanical and safety concerns if gas management is inadequate.

A cell may therefore appear to perform acceptably in early cycling but still possess a significant risk of abrupt failure later.

Why the Mechanisms Must Be Evaluated Together

The failure modes are coupled

Zinc dissolution creates the concentration gradients that influence redeposition. Uneven redeposition creates rough surfaces and localized current densities, which encourage further dendrite growth and non-uniform dissolution.

Corrosion and HER alter the local electrolyte environment, while ZnO formation increases interfacial resistance. These changes can further redistribute current and accelerate morphological instability.

Testing should separate capacity loss from short-circuit risk

A simple cycle-life measurement may show declining capacity but not identify whether the primary cause is passivation, corrosion, or loss of zinc morphology.

Durability evaluation should therefore consider cycling behavior alongside indicators such as Coulombic efficiency, impedance growth, gas generation, electrode morphology, and evidence of separator damage.

Cell fabrication affects the observed degradation

Anode porosity, compaction, particle distribution, separator selection, and surface coatings influence local current distribution and ion transport. Inconsistent electrode preparation can introduce apparent degradation that is actually caused by variations in cell construction.

Controlled pressing, coating, and assembly are therefore important when comparing anode modifications or electrolyte additives.

Understanding the Trade-offs

Greater surface area is not automatically better

Increasing anode surface area can reduce local current density and promote more uniform reactions. However, highly porous structures may complicate electrolyte management, mechanical integrity, and reproducible fabrication.

The useful design target is not maximum surface area alone, but uniform, accessible, and mechanically stable electrochemical area.

Protective layers can impede reaction access

Coatings and passivation-control strategies may suppress corrosion and improve deposition uniformity. If the coating is too resistive, too thick, or poorly adhered, it can also hinder ion and electron transport.

Protective treatments must therefore be assessed for both corrosion resistance and reversible zinc utilization.

Dense compaction can improve contact but restrict transport

Pressing an anode can improve particle-to-particle contact and structural consistency. Excessive compaction, however, may reduce electrolyte penetration and promote concentration gradients that worsen passivation or uneven deposition.

Electrode density and porosity must be optimized together rather than treated as independent variables.

Additives and separators solve specific, not universal, problems

Electrolyte additives may influence zincate transport, corrosion, or hydrogen evolution, while specialized separators can reduce dendrite penetration. Neither necessarily eliminates every degradation pathway.

A modification that improves dendrite resistance may still leave corrosion or ZnO passivation unresolved, so long-term testing should evaluate all three primary mechanisms.

Making the Right Choice for Your Goal

The appropriate durability strategy depends on which failure mode limits the cell under the intended operating conditions.

• If your primary focus is maximum cycle life: Prioritize uniform zinc deposition, controlled electrode porosity, effective separator design, and suppression of both dendrite growth and Type II ZnO formation.
• If your primary focus is high charge efficiency: Focus on reducing zinc corrosion and HER through electrolyte and surface engineering, then verify that charging current is converted predominantly into reversible zinc deposition.
• If your primary focus is preventing catastrophic failure: Emphasize dendrite suppression, separator integrity, and morphology control, while monitoring for short circuits and abnormal impedance changes.
• If your primary focus is reliable materials comparison: Use consistent electrode processing and cell assembly, and correlate cycling data with Coulombic efficiency, impedance, gas generation, and post-test anode morphology.

Understanding which mechanism dominates allows researchers to select interventions that improve not only initial performance, but also the durability that determines whether a secondary zinc-air cell is practically viable.

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