Operando monitoring of internal gas pressure in commercial lithium-ion batteries via a MEMS-assisted fiber-optic interferometer

Operando monitoring the gas pressure inside the battery continuously during operation is essential to analyze its charge/discharge characteristics and safety performance. We propose a method for measuring lithium-ion (Li-ion) batteries' internal gas pressure based on a compact fiber-optic sensor with the Micro-Electro-Mechanical Systems (MEMS) at the fiber tip to form a Fabry-Perot (F–P) interferometer for high-sensitivity pressure measurement, together with an eliminated temperature crosstalk. It enables absolute gas pressure monitoring of commercial Li-ion batteries to present the gas production and pressure change. By eliminating the temperature effect, the volumetric gas pressure variation can directly reflect the variation in cell lattice volume. In this study, we have experimentally verified the method by measuring the internal gas pressure of commercial lithium iron phosphate (LFP) and ternary (NCM523) Li-ion batteries. The experiment results show that the technique can accurately monitor the absolute gas pressure and the gas pressure baseline reflects the gas production. It also demonstrates that the internal volumetric pressure is in good agreement with the cell lattice volume at different States of Charge (SOC) and charge/discharge rates. The method provides a promising approach for gas pressure monitoring of batteries and the study of cell lattice volume changes in an operando way.