Lithium-sulfur (Li-S) batteries are a promising next-generation energy storage technology due to their high theoretical energy density and low material cost. However, their commercialization faces challenges, particularly related to the stability of interfaces during cycling. Additives such as lithium nitrate (LiNO₃) and redox mediators play a critical role in enhancing performance by stabilizing these interfaces, mitigating parasitic reactions, and improving Coulombic efficiency.
### The Role of Additives in Stabilizing Interfaces
The electrolyte in Li-S batteries is a key component where dissolution and shuttling of lithium polysulfides (LiPS) occur. These soluble intermediates lead to active material loss, anode corrosion, and rapid capacity fade. Additives modify the electrolyte chemistry to suppress these issues by forming stable interfaces on both the lithium anode and sulfur cathode sides.
#### Lithium Nitrate (LiNO₃) as a Stabilizing Agent
LiNO₃ is one of the most widely studied additives for Li-S batteries. Its primary function is to form a protective passivation layer on the lithium metal anode, known as the solid electrolyte interphase (SEI). This layer prevents continuous side reactions between lithium and LiPS, which would otherwise deplete active material and reduce cycle life.
The mechanism involves the reduction of NO₃⁻ anions on the lithium surface, forming compounds such as Li₃N, LiNO₂, and Li₂O. These inorganic species create a dense and ion-conductive SEI that blocks LiPS reduction while allowing lithium-ion transport. Studies show that electrolytes containing LiNO₃ can increase Coulombic efficiency from below 90% to over 98%, significantly reducing capacity fade.
Additionally, LiNO₃ participates in the cathode-electrolyte interface by oxidizing sulfur species, which minimizes the accumulation of insoluble Li₂S₂/Li₂S on the cathode surface. This dual role—anode protection and cathode stabilization—makes LiNO₃ indispensable in conventional Li-S systems.
However, LiNO₃ has limitations. It undergoes gradual consumption during cycling due to its participation in SEI formation and reactions with lithium metal. Over time, its concentration depletes, leading to SEI degradation and renewed polysulfide shuttling. Strategies to mitigate this include optimizing LiNO₃ concentration or combining it with other additives to prolong its effectiveness.
#### Redox Mediators for Improved Reaction Kinetics
Redox mediators are soluble additives that facilitate the electrochemical conversion of LiPS at the cathode. They act as electron carriers, accelerating the oxidation of Li₂S₂/Li₂S back to higher-order polysulfides during charging. This is particularly important because Li₂S is electronically insulating and tends to passivate the cathode, leading to high overpotentials and incomplete active material utilization.
Common redox mediators include organic molecules like tetrathiafulvalene (TTF) and inorganic complexes such as iodine (I₂). These compounds have redox potentials that lie between the sulfur reduction potentials, enabling them to shuttle electrons efficiently. For example, TTF can oxidize Li₂S at lower voltages than the typical charging plateau, reducing energy losses and improving rate capability.
Redox mediators also mitigate the accumulation of inactive sulfur species by ensuring complete reconversion of Li₂S to S₈ during charging. This maintains cathode activity and prevents capacity loss over cycles. In systems with mediators, the charging overpotential can decrease by up to 200 mV, directly enhancing energy efficiency.
### Synergistic Effects of Additive Combinations
While LiNO₃ and redox mediators function independently, their combined use can yield synergistic improvements. For instance, LiNO₃ stabilizes the anode while a redox mediator optimizes cathode kinetics, addressing both major failure modes simultaneously.
One approach involves using LiNO₃ alongside iodine-based mediators. Iodine not only enhances sulfur conversion but also interacts with LiNO₃ to form a more robust SEI. The iodine-derived species, such as LiI, integrate into the SEI, further improving its ionic conductivity and mechanical stability. Such systems demonstrate extended cycle life with minimal capacity decay over hundreds of cycles.
Another strategy employs dual-functional additives that serve as both mediators and SEI modifiers. For example, organosulfur compounds like dimethyl disulfide (DMDS) can mediate sulfur redox reactions while decomposing to form sulfur-rich SEI layers on the anode. These layers are more flexible and resistant to polysulfide penetration compared to inorganic SEIs.
### Challenges and Future Directions
Despite their benefits, additives introduce complexities. LiNO₃ decomposition products may increase electrolyte viscosity, impairing ion transport at high currents. Redox mediators can sometimes migrate to the anode, where they participate in unintended side reactions. Careful optimization of additive concentrations and compatibility with other electrolyte components is necessary.
Future research focuses on developing next-generation additives with higher stability and multifunctionality. For example, non-consumable mediators that remain active throughout the battery’s lifespan or additives that self-replenish the protective layers could further enhance Li-S battery performance.
### Conclusion
Additives like LiNO₃ and redox mediators are essential for stabilizing interfaces in Li-S batteries. By forming protective SEI layers and facilitating efficient sulfur conversion, they address critical challenges such as polysulfide shuttling and anode corrosion. While individual additives offer significant improvements, their synergistic combinations present a pathway to achieving long-cycle-life, high-energy-density Li-S batteries. Continued innovation in additive chemistry will be pivotal for advancing this technology toward commercial viability.