Why does the lower discharge plateau around 2.1 V exhibit sluggish reaction kinetics during lithium-sulfur cell testing? Understand the Li2S Passivation Effect

The lower plateau near 2.1 V is sluggish because the reaction ends by forming solid Li₂S, an electronically insulating and electrolyte-insoluble product. As Li₂S precipitates on conductive cathode surfaces, it blocks active interfaces, disrupts electronic contact, and increases charge-transfer resistance. This makes the final sulfur-conversion step particularly difficult at practical discharge rates.

The 2.1 V plateau is not slow simply because lithium ions must travel farther. Its limiting feature is the precipitation and accumulation of insulating Li₂S, which progressively passivates the cathode–electrolyte interface and reduces conversion efficiency.

What Happens Across the Two Discharge Plateaus

The upper plateau is dominated by soluble intermediates

At approximately 2.3–2.4 V versus Li/Li⁺, elemental sulfur is reduced to soluble higher-order lithium polysulfides, commonly represented as Li₂Sₓ.

Because these intermediates remain dissolved in the ether electrolyte, the upper-plateau reaction can maintain relatively effective contact between the active sulfur species, electrolyte, and conductive matrix.

The lower plateau completes sulfur conversion

Near 2.1 V, the higher-order polysulfides undergo further reduction toward lower-order species and ultimately Li₂S.

This stage provides a major portion of the cell’s capacity, but it involves the formation of a solid product rather than continued conversion entirely through soluble species.

Why Li₂S Creates Sluggish Kinetics

Li₂S is electronically insulating

Li₂S does not conduct electrons effectively. Once it deposits on carbon or another conductive cathode surface, it can cover the very sites needed for electron transfer.

The electrochemical reaction therefore becomes increasingly dependent on maintaining isolated, well-connected interfaces between the remaining active material, electrolyte, and conductive network.

Li₂S is insoluble in the electrolyte

Unlike soluble lithium polysulfides, Li₂S cannot simply dissolve and redistribute through the liquid electrolyte.

Its accumulation is consequently localized around reaction sites. As the deposit grows, it can form a passivating layer that separates unreacted sulfur-containing material from the electronic conduction network.

The active interface progressively deteriorates

The lower-plateau reaction requires simultaneous access to electrons, lithium ions, and electrochemically active sulfur species.

Li₂S precipitation consumes conductive surface area and can interrupt local electronic pathways. The result is higher interfacial resistance and slower conversion, particularly as the discharge proceeds toward its end.

Why the Problem Is Worse at Practical C-Rates

The reaction has less time to complete

At higher current, the cell must convert sulfur more rapidly. The rate of Li₂S formation can exceed the rate at which the cathode structure preserves usable reaction interfaces.

This causes increasing polarization and can leave sulfur-containing material unconverted before the discharge cutoff voltage is reached.

Voltage losses become more visible

As interfacial resistance rises, a larger portion of the applied driving force is lost inside the cell.

The measured voltage can therefore fall rapidly near the lower plateau, even when chemically active material remains in the cathode.

Cathode architecture becomes decisive

A poorly connected sulfur–carbon composite amplifies the problem because Li₂S can form on a limited number of accessible conductive sites.

A well-designed conductive matrix distributes reaction sites more effectively and helps retain electronic contact as solid discharge products accumulate.

What Testing Data Can Reveal

A depressed or shortened lower plateau indicates polarization

A lower plateau that is substantially depressed, shortened, or difficult to sustain commonly signals increased resistance during the final conversion process.

The behavior should be interpreted alongside capacity, rate capability, and impedance measurements rather than as an isolated voltage feature.

Rate testing distinguishes kinetic limitations

If the lower-plateau capacity decreases strongly as the C-rate increases, the result is consistent with insufficient reaction kinetics and growing transport or interfacial limitations.

Comparing voltage profiles at different rates helps separate intrinsic conversion sluggishness from broader cell-design limitations.

Cell preparation affects the apparent kinetics

Electrode mixing, coating, pressing, current-collector contact, and electrolyte wetting all influence the measured lower-plateau behavior.

Poor uniformity can create local regions with weak electronic or ionic access, making Li₂S passivation appear more severe than it would be in a better-connected electrode.

Understanding the Trade-offs

More conductive carbon is not automatically better

Increasing carbon surface area and conductivity can improve electronic access to sulfur and Li₂S-containing regions.

However, electrode design must still preserve adequate sulfur loading, electrolyte access, mechanical integrity, and practical energy density. Conductivity should be optimized as part of the complete electrode structure.

Excessive pressing can have mixed effects

Electrode pressing can improve particle contact and reduce electronic resistance.

But overly aggressive densification may compromise electrolyte penetration or restrict the pore structure needed for polysulfide transport. The goal is a continuous conductive network with sufficient ionic access, not maximum compaction.

Soluble intermediates also introduce competing losses

The lower plateau is governed primarily by Li₂S precipitation, but soluble polysulfides can migrate through the electrolyte and contribute to shuttle-related capacity loss.

This means a cell can suffer simultaneously from solid Li₂S passivation at the cathode and polysulfide transport to the lithium anode. Improving one mechanism does not automatically eliminate the other.

How to Improve Lower-Plateau Performance

Build a continuous conductive network

The sulfur host should provide reliable electronic pathways throughout the active electrode, including regions where Li₂S is likely to precipitate.

Conductive matrix structure, carbon surface area, and current-collector contact are therefore central processing variables.

Preserve accessible reaction interfaces

The cathode must retain contact among the conductive host, electrolyte, and sulfur-containing species as discharge progresses.

Uniform mixing, appropriate binder selection, controlled coating, and carefully optimized pressing help prevent isolated regions that become electrochemically inactive.

Evaluate performance under realistic conditions

Testing should include voltage profiles across relevant C-rates, lower-plateau capacity contribution, impedance evolution, and reproducible cell assembly conditions.

High-precision cycling and consistent electrode processing are essential because small structural differences can significantly affect apparent lower-plateau kinetics.

Making the Right Choice for Your Goal

The most effective solution depends on whether the priority is rate capability, capacity retention, or diagnostic accuracy.

• If your primary focus is high-rate discharge: Prioritize a highly connected conductive matrix and low-resistance current-collector interfaces so Li₂S precipitation does not rapidly isolate active material.
• If your primary focus is capacity utilization: Preserve sufficient ionic access and reaction-site availability throughout the electrode so the final Li₂S-forming conversion can proceed more completely.
• If your primary focus is cycle life: Combine cathode structures that limit Li₂S passivation with measures that reduce soluble-polysulfide migration and shuttle reactions.
• If your primary focus is reliable testing: Standardize electrode mixing, coating, pressing, electrolyte wetting, and cell assembly before comparing lower-plateau performance between formulations.

Understanding the 2.1 V plateau as a solid-product passivation problem provides the clearest path to improving lithium–sulfur cell performance.

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