A lithium–sulfur cathode discharges through two principal voltage plateaus. The upper plateau, typically 2.3–2.4 V vs. Li/Li⁺, corresponds to the reduction of elemental sulfur, S₈, into soluble high-order lithium polysulfides. The lower plateau, typically near 2.1 V, represents further reduction of polysulfides into insoluble lithium sulfide, Li₂S, with Li₂S₂ often appearing as an intermediate.
The two plateaus reflect a change from solid sulfur to soluble polysulfides, followed by conversion of those dissolved intermediates into solid, electronically insulating Li₂S. Their position, duration, and separation provide direct evidence of sulfur utilization and cathode conversion kinetics.
How the Discharge Reaction Proceeds
The overall cell reaction
At the lithium metal anode, lithium is oxidized:
[ \mathrm{Li \rightarrow Li^+ + e^-} ]
At the cathode, sulfur accepts lithium ions and electrons. The ideal overall reaction is:
[ \mathrm{S_8 + 16Li \rightarrow 8Li_2S} ]
The cathode reaction is not a single direct solid-state conversion. It proceeds through dissolved polysulfide intermediates and involves changes between solid, liquid, and solid phases.
Stage 1: Sulfur reduction and dissolution
At the beginning of discharge, cyclic S₈ is reduced at approximately 2.3–2.4 V vs. Li/Li⁺.
The sulfur rings open and form high-order lithium polysulfides, commonly represented as:
[ \mathrm{Li_2S_x,\quad 6 < x \leq 8} ]
These species are generally soluble in ether- or glyme-based electrolytes. This process produces the upper voltage plateau and marks the solid-to-liquid conversion of the sulfur cathode.
Stage 2: Polysulfide chain shortening
As discharge continues, the high-order polysulfides undergo further reduction and chemical disproportionation.
Their sulfur chains shorten progressively, producing medium- and lower-order polysulfides. This region can appear as a sloping transition or voltage drop between the two main plateaus rather than as a perfectly flat feature.
Stage 3: Conversion to insoluble products
Near 2.1 V vs. Li/Li⁺, lower-order polysulfides are reduced further.
The reaction forms insoluble species such as Li₂S₂ and ultimately Li₂S:
[ \mathrm{Li_2S_x \rightarrow Li_2S_2 \rightarrow Li_2S} ]
This produces the lower discharge plateau, which often supplies a substantial portion of the delivered capacity.
Stage 4: Li₂S nucleation and growth
Near the end of discharge, solid Li₂S precipitates within the cathode’s pores and conductive framework.
Because Li₂S is both insoluble and electronically insulating, its nucleation, growth, and electrical contact strongly influence the final capacity. A voltage decline below the main lower plateau may indicate increasing polarization as the active material becomes harder to access.
What the Two Voltage Plateaus Mean
Upper plateau: approximately 2.3–2.4 V
The upper plateau corresponds primarily to:
- Reduction of solid elemental sulfur.
- Ring opening of S₈.
- Formation of soluble high-order polysulfides.
- A solid-to-liquid conversion within the cathode.
Its duration and capacity contribution reflect how effectively the electrode enables sulfur dissolution, electron transport, and polysulfide formation.
Lower plateau: approximately 2.1 V
The lower plateau corresponds primarily to:
- Reduction of dissolved polysulfides.
- Formation of lower-order polysulfide intermediates.
- Precipitation of Li₂S₂ and Li₂S.
- A liquid-to-solid conversion within the cathode.
Its shape is particularly sensitive to precipitation kinetics, pore accessibility, electronic connectivity, and the ability of the cathode host to accommodate insulating discharge products.
Why the plateaus are not perfectly fixed
The reported values are typical rather than universal. Plateau positions can shift with:
- Current density or C-rate.
- Electrolyte composition and concentration.
- Sulfur loading and cathode porosity.
- Temperature.
- Electrode formulation and compression.
- Cell configuration and polarization.
Accordingly, plateau analysis should compare cells tested under identical conditions.
How to Interpret a Discharge Curve
The initial voltage decline
A modest decline from roughly 2.4 V toward 2.2 V commonly accompanies sulfur reduction and the onset of polysulfide dissolution.
A pronounced or unusually early decline may indicate increased polarization, poor wetting, insufficient electronic connectivity, or limited reaction kinetics.
The transition between plateaus
The transition region reflects the evolution from high-order to lower-order polysulfides.
Its position and sharpness can reveal how readily the electrolyte supports polysulfide transport and how effectively the electrode manages the associated liquid-phase intermediates.
The lower-plateau duration
A long, stable lower plateau generally indicates effective conversion of polysulfides into Li₂S and good utilization of the sulfur inventory.
A shortened or sloped lower plateau can indicate rapid passivation, poor conductive contact, excessive precipitation outside the conductive network, or restricted ion transport.
The end-of-discharge region
The final region is dominated by Li₂S deposition and increasing resistance.
Voltage collapse before the expected capacity is reached may indicate that Li₂S has blocked pores or disconnected sulfur-containing material from the electronic network.
What Testing Systems Should Measure
Plateau capacity
Separate the discharge capacity delivered on the upper and lower plateaus.
This helps determine whether a formulation is limited primarily during sulfur-to-polysulfide conversion or during polysulfide-to-Li₂S precipitation.
Plateau voltage and polarization
Track the average voltage and voltage variation of each plateau under controlled current conditions.
Greater polarization often signals higher internal resistance, slower conversion kinetics, poorer wetting, or loss of conductive pathways.
Plateau duration and stability
Measure how long each plateau persists and whether its voltage remains stable.
Changes in plateau duration over cycling can reveal active-material loss, increasing impedance, electrolyte degradation, or progressive cathode passivation.
Complementary cyclic voltammetry
Cyclic voltammetry typically shows two cathodic reduction features corresponding broadly to the upper and lower discharge reactions.
Comparing these peaks with galvanostatic plateaus helps distinguish changes in reaction potential, reversibility, and kinetic polarization.
Understanding the Trade-offs
Soluble polysulfides enable conversion but create the shuttle effect
Dissolution is an essential part of the normal Li–S reaction, but soluble polysulfides can migrate through the electrolyte to the lithium anode.
This polysulfide shuttle can cause self-discharge, parasitic reactions, low Coulombic efficiency, and apparent capacity fade.
Li₂S formation delivers capacity but can passivate the cathode
The final product, Li₂S, is electrochemically active in principle but is electronically insulating and difficult to reoxidize when deposited excessively or outside the conductive framework.
Cathode hosts therefore need sufficient conductivity, pore structure, and physical confinement without unnecessarily restricting electrolyte and lithium-ion transport.
Higher sulfur loading increases practical difficulty
Increasing sulfur loading can improve electrode-level energy density, but it also increases transport distances and makes uniform wetting and electronic access more difficult.
A plateau that looks acceptable at low loading may degrade substantially in a practical, high-loading electrode.
Apparent plateau changes do not identify a single failure mechanism
A shortened lower plateau could result from poor Li₂S nucleation, pore blockage, electrolyte depletion, increased impedance, or loss of active material.
Voltage data should therefore be combined with capacity analysis, impedance measurements, post-mortem characterization, and controlled cell fabrication.
Making the Right Choice for Your Goal
Use the plateau structure as a diagnostic framework rather than treating the discharge curve as a single performance number.
- If your primary focus is sulfur utilization: Compare the capacity and stability of both plateaus under identical test conditions.
- If your primary focus is cathode kinetics: Examine plateau polarization, voltage hysteresis, and cyclic-voltammetry peak separation.
- If your primary focus is capacity fade: Track changes in lower-plateau capacity, Li₂S precipitation behavior, and Coulombic efficiency over cycling.
- If your primary focus is practical cell design: Evaluate sulfur loading, electrolyte amount, conductive-host structure, electrode pressure, and polysulfide confinement together.
- If your primary focus is mechanism identification: Combine galvanostatic curves with CV, impedance measurements, and post-discharge structural analysis.
Understanding how each plateau maps to a specific phase transformation allows discharge testing to reveal not only how much capacity a lithium–sulfur cathode delivers, but also why it succeeds or fails.
Summary Table:
| Stage | Voltage Plateau | Reaction Description |
|---|---|---|
| 1 | 2.3–2.4 V | Reduction of S₈ to soluble high-order polysulfides (Li₂Sₓ, 6<x≤8) |
| 2 | Sloping transition | Polysulfide chain shortening to medium/low-order polysulfides |
| 3 | ~2.1 V | Reduction to insoluble Li₂S₂ and Li₂S |
| 4 | Below 2.1 V | Li₂S nucleation and growth, causing polarization and capacity fade |
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