In Operando Optical Analysis of Electrolyte Colour Change and Its Correlation to Capacity Fade in Li-Ion Cells at Elevated Temperatures

The existing body of research on battery state of health has identified various degradation modes for the electrolyte, yet very few studies have explored the role of electrolyte colour changes as a diagnostic tool for state of health (SOH). This study investigates the impact of elevated temperatures and its correlation with electrolyte colour changes and capacity fade during cycling. Specifically, the research examines whether cycling cells at elevated temperatures induces noticeable changes in electrolyte colour and whether these changes can be linked to the SOH of the cells. The methodology employs in operando optical sensors to monitor real-time colour shifts in the electrolyte, aiming to demonstrate a qualitative relationship between electrolyte colour change, degradation, thermal ageing, and capacity fade, laying the foundations for future quantitative assessment of the relationships identified. Our research builds upon these findings by offering a novel approach that integrates optical sensing to provide real-time visual evidence of electrolyte degradation and colour change during cell operation. The results demonstrate a clear relationship between elevated temperature, electrolyte colour change, and capacity fade, leading to accelerated degradation. This approach offers a new insight over traditional in exitu battery diagnostics, as it enables continuous in operando monitoring of electrolyte colour change and has the potential to unlock a detailed understanding of the chemical reactions and electrolyte breakdown during cycling.