Gas chromatography (GC) and Fourier-transform infrared spectroscopy (FTIR) are two analytical techniques widely used for studying gas evolution in battery systems. Both methods provide critical insights into battery degradation mechanisms, but they differ significantly in their principles, detection capabilities, and suitability for specific applications. Understanding their strengths and limitations is essential for selecting the appropriate tool for analyzing battery off-gassing products.

Gas chromatography separates chemical components in a gas mixture based on their interaction with a stationary phase within a column. The separated compounds are then detected, typically using a flame ionization detector (FID), thermal conductivity detector (TCD), or mass spectrometer (MS). GC excels in separating complex mixtures of volatile organic compounds (VOCs) and provides high sensitivity for trace analysis. In contrast, FTIR measures the absorption of infrared light by gas molecules, producing a spectrum that identifies functional groups and specific molecular vibrations. FTIR is particularly effective for detecting polar molecules and inorganic gases, offering real-time monitoring capabilities without the need for sample separation.

Detection limits vary significantly between the two techniques. GC with an FID can detect hydrocarbons at concentrations as low as parts per billion (ppb), making it ideal for trace VOC analysis. When coupled with a mass spectrometer (GC-MS), detection limits can reach sub-ppb levels, enabling the identification of minor degradation products. FTIR, however, typically has higher detection limits, often in the parts per million (ppm) range, but it performs exceptionally well for certain inorganic gases like hydrogen fluoride (HF), carbon dioxide (CO2), and sulfur dioxide (SO2). For example, FTIR can detect HF at concentrations as low as 0.1 ppm, whereas GC struggles with HF analysis unless derivatization techniques are employed.

Selectivity is another key differentiator. GC achieves high selectivity through chromatographic separation, allowing it to distinguish between structurally similar compounds such as ethylene, ethane, and propylene. This makes GC indispensable for studying hydrocarbon evolution in lithium-ion batteries, where even minor changes in gas composition can indicate specific degradation pathways. FTIR, on the other hand, identifies molecules based on their unique infrared absorption patterns. It excels in detecting polar and inorganic species, including water vapor, CO2, and HF, which are challenging for GC without additional sample preparation. However, FTIR can suffer from spectral overlap when analyzing complex mixtures, requiring advanced deconvolution algorithms for accurate quantification.

The suitability of each technique depends on the target degradation products. GC is the preferred method for analyzing organic volatiles such as ethylene, dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), which are common in electrolyte decomposition. It is also highly effective for studying thermal runaway events, where a wide range of hydrocarbons and fluorinated compounds are released. FTIR, meanwhile, is better suited for detecting inorganic gases like HF, which is a critical marker for electrolyte decomposition and electrode corrosion. FTIR’s ability to monitor multiple gases simultaneously in real time makes it valuable for in-situ studies of battery aging and failure mechanisms.

Scenarios where GC is preferred include trace VOC analysis, such as identifying electrolyte solvents and their decomposition products at very low concentrations. GC is also advantageous when studying gas evolution during formation cycling, where subtle changes in VOC profiles can indicate electrode-electrolyte interactions. However, GC has notable limitations, particularly its inability to directly detect inorganic gases like HF, CO2, or ammonia without derivatization or specialized detectors. Additionally, GC analysis is typically slower than FTIR due to the required separation step, making it less suitable for real-time monitoring.

FTIR’s strengths lie in its rapid, non-destructive analysis and its capability to detect a broad range of polar and inorganic species. It is particularly useful for studying battery systems where HF or CO2 evolution is of interest, such as in phosphate-based or high-voltage cathodes. FTIR can also monitor dynamic processes, such as gas release during overcharging or thermal abuse, providing immediate feedback on reaction pathways. However, FTIR is less sensitive than GC for hydrocarbons and may struggle with overlapping absorption bands in complex gas mixtures. Its reliance on infrared-active molecules also means it cannot detect homonuclear diatomic gases like hydrogen (H2) or nitrogen (N2).

In practical applications, the choice between GC and FTIR often depends on the specific research question. For comprehensive gas analysis, a combination of both techniques may be employed to leverage their complementary strengths. GC provides detailed VOC profiling, while FTIR captures inorganic and polar species that GC might miss. For example, in studying thermal runaway, GC can identify flammable hydrocarbons contributing to venting, while FTIR can simultaneously track HF release, offering a complete picture of the degradation process.

In summary, gas chromatography and FTIR spectroscopy each offer distinct advantages for battery gas analysis. GC is unmatched in sensitivity and selectivity for organic volatiles, making it indispensable for trace VOC studies. FTIR, with its real-time capabilities and proficiency in detecting polar and inorganic gases, is ideal for monitoring species like HF and CO2. The selection between these techniques should be guided by the target analytes, required detection limits, and the need for real-time data. Combining both methods can provide a more holistic understanding of battery degradation mechanisms, enabling better diagnostics and improved battery safety and performance.