In-Situ Monitoring of Li-Ion Batteries Formation and Cycling Behaviour Through Optical Approach

Lithium-ion batteries are widely used in various industries, powering applications from small portable electronics to electric vehicles. In battery production, cell formation is one of the most critical processes as it directly influences essential battery performance metrics such as rate capacity, lifespan and safety. However, the formation process is time-consuming, energy-demanding, and significantly contributes to the production cost. Optimizing this process is crucial, but its intricate internal mechanisms are not yet fully understood. In this work, embedded fiber Bragg grating (FBG) sensors were employed for in-situ and continuous monitoring of internal temperature and strain variations throughout the formation process. The sensors were calibrated for sensitivity to temperature and strain, enabling precise detection of internal changes. The FBG sensors successfully capture dynamic internal changes, with the first cycle exhibiting the largest wavelength shifts, corresponding to strain accumulation and temperature rise. By analyzing the wavelength shifts, and decoupled strain and temperature variations as functions of cell voltage and state of charge (SOC), distinct and cycle-dependent trends were revealed. These results demonstrate the utility of embedded FBG sensors as operando diagnostic tools, offering valuable insights into thermal and mechanical phenomena during formation.