Although lithium batteries (LIBs) are promising energy storage devices, they cannot meet the growing demand for new electronic devices. Thus, lithium-sulfur (Li-S) batteries containing sulfur polymer cathode materials with tunable sulfur chain length and organic groups are of critical importance to enhance battery performance. This review discusses the advantages of sulfur-containing polymers in energy storage.
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Sulfur Polymer Cathode
Elemental sulfur is a promising cathode material with the highest theoretical value of 1675 mAh g-1, and sulfur-containing polymers have sulfur fragments attached to organic compounds. These types of polymers are classified into two classes.
One class of polymers has sulfur-containing linkages in the backbone or branch, and another class of polymer has sulfur-bearing groups like sulfur-based polybenzimidazoles and sulfur-containing polyurethanes and polyesters with sulfur-based groups.
Organosulfides are sulfur-containing polymers consisting of sulfur chains between aromatic rings or aliphatic segments. Using organosulfides as a sulfur polymer cathode enhances the specific capacity and energy density of Li-S batteries. The network of organic material in sulfur polymer cathodes buffers the expansion and contraction of cathode volume during the cycle and suppresses the transfer of lithium polysulfide from cathode to anode.
Moreover, altering the functional groups on organic material can tune the functionality of the polymers.
Limitations of Conventional Li-S Batteries
Although there are 30 different solid allotropes of elemental sulfur, the ring-crown sulfur-8 molecule (S8) is an active material for Li-S batteries. The total reaction in Li-S batteries, S8 + 16 Li++16e- ⇌ 8Li2S, reveals that elemental sulfur can transfer 16 molar electrons. Conventional Li-S batteries have high (~2.4 V) and low (~2.1 V) plateaus during discharging process. While a high plateau indicates the reduction of S8 into lithium polysulfide (Li2Sx), a low plateau indicates its reduction into Li2S2/Li2S.
The critical problem with the lithium polysulfides formed is the dissolution in ether electrolytes. Under electric field and concentration gradient, some polysulfides diffuse from the cathode to the lithium anode, and Li2S2/Li2S deposits on the anode. A part of the deposited Li2S2/Li2S irreversibly returns to the cathode.
The solid electrolyte interphase (SEI) results from an electrochemical reaction between the lithium anode and polysulfides causing anode deterioration. Further, the bulk sulfur delays the lithium transport and electrons. Therefore, only limited sulfur is used.
Sulfur Polymer Cathode in Li-S Batteries
In the case of Li-S batteries with a sulfur polymer cathode, the working mechanism depends on the length of the sulfur chain on sulfur-containing polymers. If the sulfur chain has more than six sulfur atoms, the working method is like that of elemental sulfur since the bond between the second S and the S attached to carbon on the backbone breaks to form Li2Sx (4 < x ≤ 8), which initially converts to low order Li2Sx (x ≤ 4) and finally to Li2S2/Li2S on continued reduction.
If the sulfur chain has less than six S atoms (S ≤ 6), the battery shows a single plateau, “solid-solid conversion” behavior, and when the sulfur chain has less than two sulfur atoms (S ≤ 2), the battery showed only one reduction peak in the cyclic voltameter (CV) curve.
The performance of a sulfur polymer cathode is due to the unique structure of sulfur-containing polymers, and the discharging production of the cathode is due to the multimicroporous structure and cross-linked network skeleton of sulfur-containing polymers.
The sulfur-containing polymers with a single discharging voltage plateau do not have the formation of dissoluble long-chain lithium polysulfides. The sulfur polymer cathodes have a “solid-solid reaction” mechanism, and the sulfur-containing polymers are converted to low-order organic lithium sulfide and Li2S directly.
Advantages of Sulfur Polymer Cathodes in an Electrochemical Cell
Sulfur polymer cathodes made of sulfur-containing polymers have organic groups that improve the plateau by tuning the voltage window of the electrochemical reaction. The sulfur-containing polymer increases the voltage window from 1.7–2.8 V to 1-3 V. This higher voltage is due to the presence of the electrophilic phenyl group, which can elongate the discharge voltage plateau. Additionally, lower LUMO is imparted to the polymers by nitrogen heterocycles. Thus, the organic groups on sulfur polymer cathodes have an advantage in increasing the average cell voltage of the electrochemical reaction.
According to the Nernst equation, the number of electrons transferred is directly proportional to the energy density in the batteries. Thus, sulfur-containing polymers with multiple S-S bonds realize the goal of multielectron transport during the lithiation process.
To summarize, Li-S batteries containing sulfur polymer cathodes will meet the current demand for batteries with high energy density. The unique structure and electrochemical performance of the Li-S batteries have gained much attention in recent years. Moreover, the presence of a tunable sulfur chain and various functional groups endow unique properties on the sulfur polymer cathode.
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References and Further Reading
Zou, R., Liu, W., & Ran, F. (2022). Sulfur-Containing Polymer Cathode Materials: From Energy Storage Mechanism to Energy Density. InfoMat. https://onlinelibrary.wiley.com/doi/full/10.1002/inf2.12319
Zhao, F., Li, Y., & Feng, W. (2018). Recent Advances in Applying Vulcanization/Inverse Vulcanization Methods to Achieve High‐Performance Sulfur‐Containing Polymer Cathode Materials for Li–S Batteries. Small Methods, 2(11), 1800156. https://doi.org/10.1002/smtd.201800156
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