A lithium-sulfur cell typically shows four discharge regions, grouped around two main voltage plateaus. The curve generally begins with a mild decline from approximately 2.4 to 2.2 V vs. Li⁺/Li, followed by a sharper voltage drop, a relatively stable lower plateau near 2.1 V, and a final decline as solid Li₂S formation approaches completion. Battery test systems monitor the voltage, delivered capacity, plateau duration, and endpoint behavior in each region.
The upper-voltage behavior reflects sulfur dissolution and formation of soluble polysulfides, while the lower plateau reflects conversion toward insoluble Li₂S₂ and Li₂S. The most useful evaluation is not voltage alone, but the relationship between each region’s voltage profile, capacity contribution, duration, and changes across cycling or operating conditions.
How the Li-S Discharge Curve Is Organized
Region I: Initial sulfur dissolution and reduction
Region I shows a relatively small voltage decline, typically from about 2.4 V to 2.2 V vs. Li⁺/Li. Solid octasulfur, S₈, begins dissolving and is reduced into soluble, high-order polysulfide species.
This region is commonly associated with the upper discharge plateau, often described broadly as the conversion of sulfur into soluble lithium polysulfides such as Li₂Sₓ.
Region II: Sharp voltage fall
Region II is characterized by a more pronounced voltage decrease. It represents continued polysulfide reduction, including conversion toward species such as Li₂S₄.
The voltage drop indicates that the cell is moving away from the favorable upper-plateau reaction and toward the lower-voltage conversion process. Polarization, changing species solubility, and electrolyte transport can all influence the slope and position of this region.
Region III: Lower discharge plateau
Region III is the relatively stable plateau near 2.1 V vs. Li⁺/Li. During this stage, soluble polysulfides are progressively converted into lower-order and ultimately insoluble products, principally Li₂S₂ and Li₂S.
This is the second major plateau of a conventional liquid-electrolyte Li-S cell. It often provides a substantial portion of the practical discharge capacity.
Region IV: Final Li₂S formation
Region IV occurs near the end of discharge, when reduction proceeds toward predominantly solid Li₂S. The voltage typically declines again as active sulfur is consumed, solid products accumulate, and reaction or transport resistance increases.
The end of this region is governed by the test’s lower cutoff voltage. Capacity measured beyond the main lower plateau should therefore be interpreted together with the cutoff condition and discharge rate.
What the Regions Mean Electrochemically
The upper and lower plateaus are broad electrochemical groupings
The four-region description provides useful resolution for analyzing the discharge curve, but Li-S behavior is also commonly summarized as two principal plateaus:
- An upper plateau near 2.3–2.4 V, associated with sulfur reduction to soluble high-order polysulfides.
- A lower plateau near 2.1 V, associated with conversion of polysulfides toward insoluble Li₂S₂ and Li₂S.
These descriptions are not contradictory. The four regions divide the overall curve into more detailed stages, while the two-plateau model emphasizes the dominant electrochemical features.
Dissolution and precipitation shape the voltage profile
Unlike a simple solid-state insertion reaction, sulfur reduction involves soluble intermediates. High-order polysulfides can dissolve in the electrolyte, while lower-order products and Li₂S become progressively less soluble and may precipitate within the cathode.
This phase evolution affects active-material utilization, electronic contact, ionic transport, and polarization. It also explains why the shape and duration of each voltage region are sensitive to electrolyte composition and cathode structure.
The discharge curve is also a diagnostic signature
A shortened upper plateau can indicate limited sulfur dissolution or poor reaction accessibility. A reduced lower-plateau capacity can point to incomplete polysulfide conversion, Li₂S passivation, transport limitations, or loss of electronic connectivity.
For this reason, researchers evaluate the individual regions rather than relying only on total discharge capacity or average voltage.
How Battery Test Systems Monitor the Regions
Voltage is recorded continuously during controlled discharge
A high-precision, multi-channel battery tester applies a defined discharge program—commonly constant current—and records cell voltage against time and capacity.
The system identifies the transitions between regions by tracking changes in voltage slope, plateau level, and the voltage at which each feature occurs. Measurements are generally reported relative to Li⁺/Li for laboratory Li-S cells.
Capacity is assigned to each voltage region
The tester integrates current over time to determine the capacity delivered in each region. Typical analysis compares:
- Capacity delivered during the upper-voltage region.
- Capacity delivered during the lower plateau.
- Capacity remaining in the final reduction region.
- Total discharge capacity to the selected cutoff voltage.
This reveals whether a formulation is improving sulfur utilization broadly or merely shifting capacity between reaction stages.
Plateau duration is measured as a performance indicator
Researchers monitor how long the upper and lower plateaus persist at a specified current. Plateau duration can be expressed as time, capacity, or both.
A longer, more stable plateau generally indicates more sustained reaction at that potential, but it should not be interpreted independently of current rate, sulfur loading, electrolyte quantity, and cutoff voltage.
Voltage stability and polarization are tracked
The tester evaluates the absolute plateau voltage as well as voltage variation during the plateau. A lower-than-expected plateau or a steepening voltage slope can indicate increased polarization or impaired mass transport.
Comparing the initial voltage, plateau voltages, transition points, and end-of-discharge behavior across channels helps distinguish chemistry effects from cell-to-cell variation.
Cycling tests reveal region-specific degradation
During repeated cycling, the system tracks how each region changes. Common indicators include shortening of plateau duration, loss of upper- or lower-region capacity, increased voltage hysteresis, and earlier arrival at the cutoff voltage.
These measurements help determine whether capacity fade is associated primarily with polysulfide loss, incomplete Li₂S conversion, active-material isolation, or growing impedance.
What Test Variables Should Be Controlled
Electrolyte composition
Electrolyte solvent ratios, lithium salts such as LiTFSI or LiFSI, and additives influence polysulfide solubility, transport, reaction kinetics, and interfacial stability.
Battery testing therefore compares the voltage regions under otherwise consistent conditions to determine whether an electrolyte improves plateau stability and active-material utilization.
Discharge rate
Higher current generally increases polarization and can compress or shift the characteristic regions. A formulation that performs well at a low rate may show reduced lower-plateau capacity or an earlier cutoff at higher rates.
Rate-capability testing should therefore examine both total capacity and the capacity contribution of each voltage region.
Temperature
Temperature affects electrolyte transport, reaction kinetics, and precipitation behavior. Temperature-controlled testing can show whether a voltage feature is limited by reaction kinetics or by diffusion and phase conversion.
Temperature comparisons are most meaningful when current, electrode loading, electrolyte quantity, and cutoff voltage are also controlled.
Cell construction and repeatability
Accurate interpretation requires consistent sulfur loading, electrolyte-to-sulfur ratio, separator placement, electrode alignment, sealing, and wetting. Poor assembly can distort the discharge curve and make a chemistry appear better or worse than it is.
Multi-channel testing is valuable because it supports parallel comparisons, but the channels are only comparable when the cells are fabricated and conditioned consistently.
Understanding the Trade-offs
Four regions improve diagnosis but complicate reporting
The four-region model provides more diagnostic detail than a simple two-plateau description. However, the exact boundaries between regions are not universal and can shift with current density, temperature, sulfur loading, electrolyte composition, and cell design.
Region labels should therefore be defined by an explicit voltage-slope or voltage-window criterion rather than treated as fixed thermodynamic boundaries.
Total capacity can conceal poor regional performance
A cell may deliver acceptable total capacity while losing a significant portion of either the upper or lower plateau. Total capacity alone cannot show whether sulfur conversion is balanced across the discharge process.
Regional capacity, plateau duration, and voltage stability should be reported together.
Liquid-electrolyte and solid-state Li-S cells differ
The four-region and two-plateau behavior described here is most representative of conventional liquid-electrolyte Li-S cells. Bulk solid-state sulfur cells can show a substantially different profile, including a single dominant plateau, because dissolved polysulfide intermediates are physically confined and solid-state diffusion becomes more limiting.
Results from these two cell architectures should not be compared without accounting for their different transport mechanisms.
Self-discharge can distort later measurements
Dissolved polysulfides can migrate toward the lithium anode during rest, causing parasitic reactions, voltage decay, and active-material loss. Consequently, the measured upper-plateau capacity may decrease after prolonged storage even if the nominal discharge protocol is unchanged.
For self-discharge studies, testers monitor open-circuit-voltage decay, upper-plateau capacity after defined rest periods, and, where applicable, steady-state shuttle current.
Making the Right Choice for Your Goal
Use the discharge curve as a set of region-specific diagnostic data, not merely as a single capacity-versus-voltage trace.
- If your primary focus is electrolyte development: Compare upper- and lower-plateau voltages, regional capacities, plateau durations, and voltage polarization under identical current and temperature conditions.
- If your primary focus is sulfur utilization: Quantify the capacity delivered in each region and determine whether the final Li₂S-forming stage reaches the intended cutoff without excessive polarization.
- If your primary focus is cycle life: Track region-specific capacity loss, plateau shortening, voltage hysteresis, and end-of-discharge shifts over repeated cycles.
- If your primary focus is self-discharge: Measure open-circuit-voltage decay during rest and compare the recovered upper-plateau capacity after different storage intervals.
- If your primary focus is rate capability: Repeat regional analysis across discharge currents to identify whether performance is limited by kinetics, electrolyte transport, or solid-product passivation.
A reliable Li-S evaluation combines precise voltage-region identification with capacity, plateau-duration, rate, temperature, and cycling data to explain not only how much energy the cell delivers, but why.
Summary Table:
| Region | Voltage Range (vs. Li+/Li) | Electrochemical Process | Monitoring Focus |
|---|---|---|---|
| I | ~2.4-2.2 V | Sulfur reduction to soluble high-order polysulfides | Upper plateau capacity, dissolution efficiency |
| II | ~2.2-2.1 V (sharp drop) | Continued reduction to lower-order polysulfides | Polarization, transition point |
| III | ~2.1 V (stable plateau) | Conversion to insoluble Li2S2/Li2S | Lower plateau capacity, duration, stability |
| IV | <2.1 V (declining) | Final Li2S formation, resistance increase | Cutoff condition, end-of-discharge capacity |
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