What mechanisms cause internal resistance to rise during cell discharge? Optimize cathode design for stable performance

In zinc-based primary cells, internal resistance rises mainly because discharge progressively obstructs ionic and electronic transport inside the cathode. Insoluble reaction products—diammine-zinc chloride in Leclanché cells and zinc oxychloride in zinc chloride cells—occupy pore volume, while the manganese dioxide active mass becomes less electrically conductive. The result is increasing impedance, polarization, and voltage loss as depth of discharge increases.

Internal-resistance growth is therefore a transport problem inside an evolving porous electrode. Cathode R&D must preserve connected ionic and electronic pathways throughout discharge, not merely maximize initial capacity or initial conductivity.

Why Internal Resistance Increases During Discharge

Insoluble products block the cathode pore network

The zinc–manganese dioxide reaction generates solid products that precipitate within the porous cathode matrix. As these crystals accumulate, they reduce the open pore volume available for electrolyte movement and restrict access to remaining active material.

This is similar to gradually filling a sponge with solid particles: the structure may initially absorb electrolyte easily, but transport becomes increasingly difficult as the pathways narrow or close.

Manganese dioxide becomes less conductive

The manganese dioxide active mass does not retain constant electronic conductivity throughout discharge. Its conductivity decreases as the electrochemical reaction proceeds, increasing the resistance of the solid phase and making electron transport through the cathode more difficult.

This effect adds to the physical pore blockage. The cathode must therefore support both ionic transport through the electrolyte-filled pores and electronic transport through the active-material/conductive-additive network.

Reaction products increase polarization

As ionic and electronic pathways deteriorate, a larger voltage loss is required to sustain the same discharge current. The measured internal resistance consequently rises, particularly at greater depth of discharge.

The increase is not caused by a single uniform layer of resistance. It reflects the combined evolution of the porous structure, reaction-product distribution, active-material conductivity, and electrode utilization.

What This Means for Cathode Material R&D

Optimize porosity for the full discharge profile

Cathode porosity must provide enough interconnected volume for electrolyte transport while leaving sufficient solid contact for electronic conduction and mechanical integrity.

Excessively dense material can produce high initial resistance because electrolyte penetration is restricted. Excessively porous material can reduce volumetric capacity, weaken particle-to-particle contact, and provide less active material per unit volume.

The relevant target is therefore stable transport under product accumulation, not simply maximum initial porosity.

Control particle density and packing

Particle size, packing behavior, and compaction determine how pores are distributed through the cathode. A structure with accessible, interconnected pores is more resilient than one containing isolated pores or narrow transport bottlenecks.

Researchers should evaluate how compaction changes both the initial resistance and the rate at which resistance increases during discharge.

Tune conductive additive ratios

Conductive additives create an electronic network through the manganese dioxide active mass. Increasing their fraction can reduce electronic resistance and help compensate for the declining conductivity of the active material.

However, conductive additives occupy volume that could otherwise contain active material. They can also alter cathode density and porosity, so the correct ratio is a formulation trade-off rather than a simple “more is better” decision.

Design for reaction-product accommodation

Because insoluble products occupy pore volume, the cathode should contain sufficient transport margin to tolerate their formation. The distribution of pore sizes and the location of conductive contacts influence how quickly local blockage becomes a cell-level resistance increase.

A cathode that performs well initially but has no capacity to accommodate reaction products may show a sharp resistance rise at moderate or high depth of discharge.

How Processing Changes Cathode Performance

Precision pressing controls the starting structure

Pressing determines cathode density, porosity, particle contact, and the continuity of conductive pathways. Small changes in compaction can therefore affect both initial impedance and its evolution during discharge.

Laboratory presses are useful because they allow researchers to produce controlled cathode densities and compare resistance growth under repeatable conditions.

Compaction must balance strength and transport

Higher compaction can improve particle contact and mechanical stability, but it can also narrow pores and impede electrolyte movement. Lower compaction may improve electrolyte access while increasing structural weakness or reducing electronic contact.

The optimum processing window is the point at which the cathode maintains adequate ionic and electronic transport without sacrificing practical energy density.

Formulation and processing cannot be optimized separately

The same conductive-additive ratio can behave differently at different compaction levels. Likewise, a target density may require a different particle-size distribution or binder and additive strategy to preserve connected porosity.

Cathode development should therefore treat material formulation, powder packing, and pressing conditions as one coupled design problem.

How to Measure the Relevant Changes

Track resistance against depth of discharge

Initial internal resistance alone is insufficient. Researchers should measure resistance at multiple states of discharge to determine whether a cathode maintains transport pathways as reaction products accumulate.

The key performance metric is the slope and shape of resistance growth, not just the starting value.

Separate loaded-voltage loss from open-circuit behavior

Monitoring loaded voltage, open-circuit voltage, and internal resistance under controlled conditions helps distinguish electrochemical polarization from simple state-of-charge effects.

Temperature must also be controlled because electrolyte conductivity and reaction kinetics are temperature dependent. Otherwise, thermal variation can be mistaken for cathode degradation.

Examine the cathode after discharge

Post-discharge analysis can connect electrical behavior to physical changes, including crystal deposition, pore blockage, density variation, and loss of conductive connectivity.

This correlation is essential: resistance data identifies when performance deteriorates, while structural analysis helps explain why.

Broader Design Lessons from Other Cell Chemistries

Solid-electrolyte cells show the cost of a thickening resistive layer

In lithium–iodine systems, the lithium iodide electrolyte layer grows during discharge. Because ion transport through this solid layer is intrinsically resistive, increasing thickness directly raises internal impedance.

The cathode-processing lesson is analogous: minimize unnecessary transport distance and control layer thickness through precise deposition or formation. This principle is especially relevant when designing cathode/electrolyte interfaces.

Sodium-metal chloride cells show the impact of reaction-front position

In high-temperature sodium-metal chloride cells, the reaction front moves deeper into the positive electrode during discharge. This lengthens the ionic conduction path through the molten-salt region and increases ohmic resistance.

The corresponding processing lesson is to control positive-electrode thickness, uniformity, and contact with the solid electrolyte. Thin, consistent electrode matrices can reduce the distance that ions must travel as discharge proceeds.

Silver oxide cells demonstrate that resistance can also decrease

Not every chemistry experiences rising resistance. In silver oxide cells, poorly conductive silver oxide is converted into highly conductive metallic silver during discharge, so cathode impedance can decline.

This contrast is important for R&D: the direction of resistance change depends on whether discharge products block transport and reduce conductivity or instead create a more conductive active structure.

Understanding the Trade-offs

Lower initial resistance may not mean better discharge performance

A highly conductive cathode can show excellent initial impedance yet experience rapid resistance growth if its pores become blocked quickly. Initial performance and resistance stability are related but distinct design objectives.

Testing only fresh cells can therefore favor formulations that do not perform well at high depth of discharge.

More conductive additive reduces active-material density

Conductive additives improve the electronic network, but excessive loading lowers the fraction of electrochemically active manganese dioxide and may change the cathode’s packing structure.

The correct design balances electronic conductivity, ionic access, volumetric capacity, and resistance growth.

Higher density can improve contact but restrict electrolyte movement

Compaction improves solid-solid contact and can reduce some electronic losses. Beyond an optimum, however, it constricts the pore network and accelerates transport limitations as reaction products accumulate.

Processing targets should be based on discharge testing rather than density alone.

Cathode optimization cannot eliminate all cell resistance

The cathode is a major contributor in these systems, but total internal resistance also includes contributions from the electrolyte, separator or barrier layers, current collectors, interfaces, and contacts.

A cathode formulation that appears optimal in isolation may not be optimal in the complete cell.

How to Apply This to Cathode Development

The most useful development program combines controlled processing with resistance measurements across the intended discharge range.

• If your primary focus is low initial impedance: Increase electronic connectivity through conductive-additive and compaction optimization, while verifying that porosity remains sufficient for electrolyte access.
• If your primary focus is high-depth-of-discharge performance: Design pore volume and connectivity to accommodate insoluble reaction products without closing critical transport pathways.
• If your primary focus is volumetric energy density: Increase cathode density cautiously and quantify the resulting penalty in electrolyte transport and resistance growth.
• If your primary focus is process scale-up: Define a reproducible pressing window for density, porosity, and contact quality, then confirm it with depth-of-discharge impedance testing.
• If your primary focus is mechanism discovery: Correlate resistance-versus-discharge data with post-discharge analysis of product deposition, pore blockage, and conductive-network degradation.

The central R&D objective is a cathode whose ionic and electronic pathways remain functional as its chemistry and microstructure evolve during discharge.

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