In lithium-sulfur (Li-S) battery systems, gas evolution and electrolyte degradation present significant challenges to performance, safety, and cycle life. The polysulfide shuttle effect is a primary driver of these issues, leading to the generation of gaseous byproducts such as hydrogen sulfide (H₂S) and sulfur dioxide (SO₂), as well as decomposition of organic electrolyte solvents. These processes originate from complex electrochemical and chemical reactions at both the sulfur cathode and lithium metal anode. Understanding these mechanisms is critical for mitigating their effects without altering the fundamental chemistry of Li-S systems.

The sulfur cathode contributes to gas evolution through multiple pathways involving soluble lithium polysulfides (Li₂Sₓ, where x ranges from 4 to 8). During discharge, sulfur undergoes a series of reduction reactions to form long-chain polysulfides, which dissolve into the electrolyte. These intermediates participate in parasitic reactions that generate gaseous species. For instance, the chemical disproportionation of polysulfides can produce H₂S when reacting with trace water or proton donors in the electrolyte. Even minute quantities of water, at levels as low as 50 ppm, can trigger H₂S formation. Additionally, electrochemical reduction of polysulfides at the lithium anode can lead to further decomposition pathways, releasing SO₂ as a byproduct of incomplete reduction.

The lithium metal anode plays an equally critical role in gas generation and electrolyte degradation. Polysulfides that migrate to the anode surface undergo reduction to form insoluble Li₂S and Li₂S₂. These reactions consume active lithium and create passivation layers that increase cell impedance. More importantly, the reduction process can generate reactive intermediates that attack carbonate or ether-based electrolytes, leading to solvent breakdown. For example, in ether-based electrolytes, which are commonly used in Li-S systems, nucleophilic polysulfide species can initiate ring-opening reactions of cyclic ethers like 1,3-dioxolane (DOL), producing volatile organic compounds and contributing to pressure buildup within the cell.

Electrolyte solvent degradation is further exacerbated by the highly reducing environment at the lithium anode. Even in the absence of polysulfides, lithium metal reacts spontaneously with most organic solvents, forming a solid electrolyte interphase (SEI). However, the presence of polysulfides destabilizes this SEI, leading to continuous electrolyte consumption and gas evolution. Studies have shown that the combination of polysulfide reduction and lithium corrosion can result in gas generation rates exceeding 0.5 mL per ampere-hour during cycling, with H₂S and SO₂ constituting a significant portion of the evolved gases.

Quantitative analysis of gas evolution reveals distinct patterns during different stages of cycling. Early cycles typically show higher H₂S production due to the initial activation of sulfur and its reaction with residual moisture. As cycling progresses, SO₂ becomes more prevalent as the system accumulates more reduced sulfur species. The ratio of these gases depends on factors such as electrolyte composition, operating temperature, and charge-discharge rates. Elevated temperatures above 40°C accelerate both polysulfide shuttle and electrolyte decomposition, increasing total gas volume by as much as 300% compared to room temperature operation.

The polysulfide shuttle also indirectly promotes gas evolution by facilitating continuous redox cycling of sulfur species. This shuttle effect leads to overcharging at the cathode and under-deposition at the anode, creating localized high-potential conditions that degrade electrolyte solvents. Ether solvents, while more stable than carbonates in Li-S systems, still undergo oxidative decomposition at voltages above 3.5 V versus Li/Li⁺, producing CO₂ and other gaseous byproducts. This is particularly problematic in cells with unbalanced stoichiometry or during high-voltage charging.

Several strategies have been investigated to mitigate these effects without altering the fundamental Li-S chemistry. Electrolyte additives such as LiNO₃ are known to suppress polysulfide shuttle by forming a more stable SEI on the lithium anode, reducing both gas evolution and electrolyte consumption. The addition of 0.1 M LiNO₃ to the electrolyte has been shown to decrease H₂S generation by up to 80% by passivating the lithium surface against polysulfide reduction. Other approaches include the use of scavenger materials that selectively bind polysulfides, preventing their migration to the anode. Porous carbon interlayers or metal-organic frameworks incorporated into the cell design can physically trap polysulfides, reducing their concentration in the electrolyte and subsequent gas-forming reactions.

The impact of gas evolution extends beyond simple capacity fade. Accumulation of gaseous products increases internal cell pressure, which can lead to mechanical stress on cell components and potential safety hazards. H₂S, in particular, poses toxicity risks if cells are vented or breached. SO₂, while less toxic, can react with lithium to form Li₂SO₃, further depleting active materials. These factors necessitate careful consideration in battery management system design for Li-S applications, particularly in confined environments like electric vehicles or aerospace systems.

Electrolyte formulation plays a crucial role in determining the extent of gas evolution. Higher electrolyte molarity has been correlated with reduced gas generation, as concentrated electrolytes limit polysulfide solubility. A 4 M concentration of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in DOL/DME (1:1 v/v) has demonstrated up to 60% less gas production compared to standard 1 M solutions after 50 cycles. The choice of lithium salt also influences gas composition, with LiTFSI-based electrolytes producing less SO₂ than LiPF₆ alternatives due to differences in anion decomposition pathways.

Separator modification represents another avenue for controlling gas evolution. Conventional polyolefin separators allow unrestricted polysulfide migration, whereas functionalized separators with ionic selectivity or polysulfide-blocking layers can significantly reduce crossover. Ceramic-coated separators or those incorporating graphene oxide layers have shown promise in decreasing H₂S formation by physically limiting polysulfide transport while maintaining sufficient lithium ion conductivity.

Operational parameters must be optimized to minimize gas-related degradation. Lower cutoff voltages during charging (below 2.8 V versus Li/Li⁺) can prevent excessive oxidation of electrolyte components, while moderate discharge voltages (above 1.7 V) limit the formation of highly reactive short-chain polysulfides. Temperature control is equally critical, as maintaining cells below 30°C substantially slows the kinetics of gas-evolving side reactions.

The cumulative effect of these processes leads to a complex degradation signature in Li-S batteries. Post-mortem analysis of cycled cells typically reveals multiple degradation products, including lithium alkyl carbonates from solvent decomposition, Li₂S/Li₂S₂ from polysulfide reduction, and various sulfur-oxygen compounds from gas recombination. These byproducts collectively contribute to capacity fade through active material loss, electrolyte depletion, and increased cell impedance.

Understanding these mechanisms provides a foundation for developing more robust Li-S systems without fundamental chemistry changes. Future work may focus on advanced diagnostic techniques such as operando gas analysis coupled with electrochemical measurements to precisely quantify gas evolution rates under different operating conditions. Such data could inform predictive models for gas accumulation over the battery lifecycle, enabling better safety management in practical applications. While challenges remain, continued investigation of these phenomena will be essential for realizing the full potential of Li-S battery technology.