Gas evolution in battery systems presents significant challenges to performance, safety, and longevity. The mechanisms differ between sodium-ion (Na-ion) and lithium-ion (Li-ion) batteries due to variations in chemistry, electrode interactions, and electrolyte stability. Ester-based electrolytes, increasingly studied for their potential benefits in both systems, exhibit distinct behaviors when paired with Na-metal or Li-metal electrodes. Understanding these differences is critical for advancing battery technology.

In Li-ion batteries, gas generation primarily occurs through electrolyte decomposition at the electrode-electrolyte interface. Common ester-based electrolytes, such as ethyl methyl carbonate (EMC) or dimethyl carbonate (DMC), undergo reduction at the anode, forming gaseous byproducts like CO2, CO, and hydrocarbons. The presence of lithium metal exacerbates this process due to its high reactivity, leading to the formation of a solid electrolyte interphase (SEI) that is often unstable in ester solvents. The SEI layer in Li-metal systems tends to be non-uniform, allowing continuous electrolyte breakdown and gas evolution. Elevated temperatures accelerate these reactions, increasing gas pressure and the risk of cell swelling or rupture.

Na-ion systems, while sharing some similarities with Li-ion, exhibit key differences in gas evolution mechanisms. Sodium metal is less reactive than lithium, but its interaction with ester-based electrolytes still leads to notable gas generation. The SEI formed on Na-metal surfaces in ester solvents is typically more porous and less stable than in Li-metal systems. This instability results in persistent electrolyte reduction, releasing gases such as H2, CO2, and light hydrocarbons. The lower reduction potential of sodium compared to lithium means that ester solvents decompose at slightly different electrochemical windows, altering the composition and quantity of evolved gases.

Ester-based electrolytes demonstrate varying stability in Na-ion and Li-ion systems due to differences in solvation behavior. Sodium ions have a larger ionic radius and weaker Lewis acidity than lithium ions, leading to reduced solvation energy in ester solvents. This weaker interaction affects the decomposition pathways of the electrolyte. For example, in Li-ion systems, ester solvents tend to decompose into lithium alkyl carbonates and gaseous CO2, whereas in Na-ion systems, the decomposition favors sodium alkoxides and H2 evolution. The latter is particularly concerning due to the flammability of hydrogen and its potential to increase internal cell pressure rapidly.

Temperature plays a critical role in gas evolution for both systems. In Li-ion batteries, ester-based electrolytes exhibit increased decomposition rates above 60°C, with CO2 being the dominant gas product. Na-ion systems, however, show a higher propensity for H2 release even at moderate temperatures (40-50°C), attributed to the catalytic effect of sodium metal on solvent breakdown. This difference necessitates distinct thermal management strategies for each battery type.

The role of additives in mitigating gas evolution is another differentiating factor. In Li-ion batteries, additives like vinylene carbonate (VC) or fluoroethylene carbonate (FEC) improve SEI stability, reducing gas generation. These additives are less effective in Na-ion systems due to the differing electrochemical reduction pathways. Instead, sodium-specific additives, such as sodium bis(fluorosulfonyl)imide (NaFSI), have shown promise in stabilizing the SEI and minimizing gas production in ester-based electrolytes.

Mechanical stress also influences gas evolution differently in Na-metal and Li-metal systems. The softer nature of sodium metal leads to more significant electrode deformation during cycling, which disrupts the SEI and exposes fresh metal surfaces to the electrolyte. This continuous SEI reformation contributes to sustained gas generation. In contrast, lithium metal, while also prone to dendrite formation, forms a more mechanically resilient SEI in ester solvents, though it remains prone to cracking under high current densities.

A comparison of gas composition in both systems reveals further distinctions:

| Gas Species | Li-ion (Ester Electrolyte) | Na-ion (Ester Electrolyte) |
|-------------------|---------------------------|---------------------------|
| CO2 | Primary product | Secondary product |
| H2 | Minor component | Dominant product |
| Hydrocarbons (CxHy)| Moderate generation | Low generation |
| CO | Present in trace amounts | Rare |

The table highlights that Na-ion systems produce more H2, while Li-ion systems generate greater amounts of CO2. This divergence stems from the distinct reduction mechanisms of ester solvents on Na and Li surfaces.

Long-term cycling exacerbates gas evolution in both systems but follows different degradation patterns. In Li-ion batteries, gas accumulation tends to plateau after initial cycles as the SEI stabilizes, though localized lithium plating can reintroduce gas generation. Na-ion systems exhibit more linear gas accumulation over time due to the persistent instability of the SEI and the continuous exposure of fresh sodium metal. This behavior underscores the need for improved electrolyte formulations tailored to sodium chemistry.

Safety implications also vary. The higher H2 content in Na-ion systems poses a greater explosion risk if not properly managed, whereas Li-ion systems face challenges with CO2-induced pressure buildup. Cell design must account for these differences—Na-ion batteries may require enhanced venting mechanisms for hydrogen, while Li-ion batteries need robust pressure containment for CO2.

Efforts to suppress gas evolution focus on electrolyte engineering. In Li-ion systems, hybrid electrolytes combining esters with more stable solvents (e.g., ethers) have reduced gas generation. For Na-ion systems, high-concentration salt electrolytes or ionic liquid additives have shown efficacy in minimizing H2 production. However, these approaches often come with trade-offs in ionic conductivity or cost.

In summary, while both Na-ion and Li-ion systems with ester-based electrolytes face gas evolution challenges, the underlying mechanisms differ significantly. Sodium's weaker SEI-forming tendency and propensity for H2 production contrast with lithium's CO2-dominated gas generation. These distinctions necessitate tailored solutions in electrolyte formulation, additive use, and cell design to ensure safe and efficient operation. Future advancements will depend on deepening the understanding of interfacial chemistry in both systems to mitigate gas-related degradation.