A research team including the University of Maryland and Vanderbilt University has developed a lithium-sulfur battery that draws more electrons from sulfur than conventional designs. By using chloride to widen the range of sulfur's reactions, the team raised both the capacity per gram of sulfur and the average voltage in experiments.
The results were published on September 10 in the peer-reviewed journal Nature Energy. The team also tested charging and discharging in a small pouch-type cell.
However, the eye-catching figure of "more than 1,700 Wh/kg" is not the performance of a finished battery. The value obtained for the electrode material, the value calculated from the weight of the cell components, and the durability confirmed in the pouch cell need to be viewed separately.
From two electrons to three per sulfur atom
In conventional lithium-sulfur batteries, sulfur shuttles between an oxidation state of 0 and −2 during charging and discharging, so two electrons per sulfur atom take part in the reaction.
Sulfur has long been seen as a promising cathode material because it is light and inexpensive. But the technology faces challenges including low operating voltage, slow reaction rates, and the "shuttle effect," in which polysulfides migrate to the anode side. Even if the material's capacity is increased, low voltage means the energy that can be extracted does not grow much.
The team added a reaction in which sulfur bonds with chlorine, designing a pathway that produces disulfur dichloride (S₂Cl₂) from lithium sulfide during charging.
In this reaction, sulfur is oxidized to +1, adding one more electron's worth of reaction to the conventional two-electron process. The reverse reaction occurs during discharge.
The key feature of this work is that it does not simply add chlorine to the cathode; it widens the range of redox reactions available to sulfur itself.
According to the paper's published abstract, in tests at 25°C and 0.2C, the average operating voltage rose from 2.05 V with the conventional approach to 2.54 V. Capacity based on the mass of sulfur also increased by 58%.
Because the energy a battery can deliver depends on both capacity and voltage, this mechanism raises both at once.
That said, the "58% increase" does not mean the energy stored in the whole battery rose by 58%. In a finished cell, the weight of the anode, electrolyte, separator, current collectors, and other parts is added on top of the cathode.
Supplying chloride while keeping reaction products at the cathode
Simply adding chlorine to the reaction is not enough to sustain stable repeated charging and discharging.
If the disulfur dichloride that forms dissolves into the electrolyte and moves to the lithium metal anode, the material needed for the next reaction at the cathode is lost. In addition, if chloride ions bind too strongly to lithium ions in the electrolyte, they become hard to use in the sulfur oxidation reaction.
The team used molecular simulations to study ion behavior and chose an electrolyte containing an ionic liquid, which makes it easier to secure free chloride ions available for the reaction.
At the same time, they set conditions that make it harder for the disulfur dichloride to dissolve into the electrolyte, so that it stays near the cathode.
According to the university's announcement, the team also used multiple spectroscopic methods to confirm that the chemical state of sulfur changes with charging and discharging.
Experiments under other conditions showed that changing the electrolyte composition alone is not sufficient.
Even when a different electrolyte containing LiCl was used at the cathode, the reaction that oxidizes sulfur to the higher-voltage state did occur. But the products dissolved into the electrolyte, and much of the active material could not be kept at the cathode.
In other words, what matters in this mechanism is not only supplying chloride. The products must also be kept near the cathode so they can be used again in the next discharge.
According to the paper's supplementary materials (Note 3), the chloride source is not limited to the electrolyte.
Lithium chloride (LiCl) built into the cathode replenishes chloride as it dissolves, and free chloride ions in the electrolyte mediate the reaction.
The team estimates that energy derived from the electrolyte itself accounts for only about 2–3% of the whole battery.
This is also important for understanding that the system is not one in which adding more electrolyte simply adds more capacity.
"More than 1,700 Wh/kg" is not the performance of a finished battery
The three prominent figures in the paper each refer to a different target.
The specific energy calculated at the electrode-material stage, the calculated value assuming the weight of the cell components, and the capacity retention of the small pouch cell cannot all be compared as the same "battery energy density."
| Figure | What it represents | Caveat |
|---|---|---|
| More than 1,700 Wh/kg | Electrode-level specific energy given in the paper's abstract | Not a measured value for a finished cell including the anode, electrolyte, and casing |
| 477 Wh/kg | Specific energy including the weight of cell components, calculated in Table 2 of the supplementary materials | A calculated value under set conditions such as areal capacity. The conventional lithium-sulfur battery used for comparison is 346 Wh/kg |
| 78% after 100 cycles | Capacity remaining after 100 charge-discharge cycles in a small single-layer pouch cell | A durability figure, not the Wh/kg of the pouch cell |
In other words, 1,700 Wh/kg and 477 Wh/kg were not measured under the same conditions. And what was confirmed for the pouch cell was a capacity retention of 78% after 100 cycles; it does not mean the pouch cell achieved 1,700 Wh/kg or 477 Wh/kg.
The figure of more than 1,700 Wh/kg is the electrode-level specific energy presented in the paper's abstract.
The 477 Wh/kg figure in Table 2 of the supplementary materials, on the other hand, was calculated by adding the weight of not just the cathode but also the lithium metal anode, separator, electrolyte, current collectors, and other components.
This calculation assumes an areal capacity of 4.00 mAh/cm². A conventional lithium-sulfur battery calculated under the same conditions comes to 346 Wh/kg, so the 477 Wh/kg figure indicates how much improvement may be possible under that design.
The university's announcement, meanwhile, does not give a measured Wh/kg for the small pouch cell.
In Table 2 of the supplementary materials, the calculation for this cathode assumes a loading of 6.23 mg/cm², an active-material fraction of 90%, and an electrolyte filling rate of 100%.
These are conditions set to derive the 477 Wh/kg figure; they do not mean the same conditions and performance were verified in an actual pouch cell.
The calculation also covers only the electrodes, separator, electrolyte, current collectors, and similar parts. It does not include the cell casing or the entire battery pack.
Practical use will require less electrolyte and longer life
Maintaining 78% of initial capacity after 100 cycles in a small pouch cell is a step beyond basic experiments using coin cells.
Vanderbilt University also explains that the prototype still faces challenges on the road to practical use.
The prototype uses a relatively small amount of sulfur in the cathode and more electrolyte than commercial batteries. It also uses lithium metal for the anode.
The results alone do not show whether the same reactions and performance can be maintained in larger cells with less electrolyte.
The lithium metal anode poses its own challenges.
Because lithium metal is repeatedly plated and stripped with each charge and discharge, the interface between the anode and the electrolyte tends to degrade. Even if the cathode achieves high capacity, that performance cannot be fully exploited if anode degradation ends the cell's life first.
This problem also appears in the experiments in the supplementary materials.
In a symmetric cell with lithium metal facing lithium metal using the original electrolyte, stable operation lasted only about 10 cycles, and after about 20 cycles the overpotential grew and the cell short-circuited.
The team therefore added another salt and lithium nitrate to the electrolyte. As a result, stable operation beyond 100 cycles was confirmed in the same symmetric-cell test.
This shows one way to stabilize the lithium metal anode, but it does not mean long-term life or safety has been secured in an actual pouch cell.
The study shows that widening the range of reactions available to sulfur may help address the low operating voltage that has long troubled lithium-sulfur batteries.
The next key question is whether the same performance can be maintained under more practical conditions, with more sulfur loaded on the cathode and less electrolyte.
If the whole cell's Wh/kg can be measured under those conditions, and capacity holds up after long-term cycling, it will become clearer how far this material-level result of high capacity and high voltage translates into real battery performance.