Operando monitoring of gassing dynamics in lithium-ion batteries with optical fiber photothermal spectroscopy

Gaseous molecules are inherent byproducts of (electro-)chemical reactions in lithium-ion battery cells during both formation cycles and long-term operation. While monitoring gas evolution can help understand battery chemistry and predict battery performance, the complex nature of gas dynamics makes conventional mass spectrometry approaches insufficient for real-time detection. Here, we present a radically different methodology for operando analysis of gas evolution in lithium-ion batteries using optical fiber photothermal spectroscopy. By placing an optical hollow-core fiber inside the battery cell, evolved gases can rapidly diffuse into the hollow core of the fiber, enabling photothermal spectroscopy which precisely and selectively quantifies their concentrations without altering the internal operation of the cell. This approach facilitates identification of individual gaseous species, thereby allowing for further clarification (electro-)chemical reaction pathways. Collectively, we show that the evolution paths of C₂H₄ and CO₂ are closely associated with the formation of the solid electrolyte interphase, the selection of electrolyte salts, and the inclusion of specific additives. Significantly, we confirm for the first time the spontaneous formation of CO₂, which occurs exclusively in the presence of LiPF₆ salt. Beyond the scope of batteries, the methodology presented here offers substantial potential for broader applications, particularly in characterizing electrocatalytic processes, providing unmatched precision, accuracy, and scalability compared to existing analytical techniques.