Abstract
Improvements to battery performance, reliability and lifetime are essential to meet the expansive demands for energy storage. As part of this, continuous monitoring of the dynamic chemistry inside cells offers an exciting path to minimizing parasitic reactions and maximizing sustainability. Building upon recent fibre-optic/battery innovations, we report the use of operando infrared fibre evanescent wave spectroscopy to monitor electrolyte evolution in 18650 Na-ion and Li-ion cells under real working conditions. This approach enables identification of chemical species and reveals electrolyte and additive decomposition mechanisms during cycling, thereby providing important insights into the growth and nature of the solid–electrolyte interphase, the dynamics of solvation and their complex interrelations. Moreover, by directly embedding fibres within the electrode material, we demonstrate simultaneous observations of both the material structural evolution and the Na(Li) inventory changes upon cycling. This illuminating sensing method has the power to reveal the otherwise opaque chemical phenomena occurring within each key battery component.