Fibre optic sensor options for the assessment of lithium-ion pouch cell degradation and safety

The implementation of safer, high-energy-density lithium-ion batteries remains a societal concern. A mandatory criterion for the development of the worst-case scenario, the thermal runaway of the battery cell, is an increased local heat generation to activate the disastrous exothermic chain reaction. The associated degradation reactions lead to higher temperatures and pressures and consequently irreversible changes in the material system.

Early detection of these safety-critical conditions based on measurable phenomena is the foundation of any safety strategy. Presently used fault detection methods are based on a fixed operating window of the primary measured variables such as voltage, current and temperature. This often proves to be insufficient due to the complexity of possible fault progression and superposition of numerous degradation effects over the battery lifetime. The addition of further resilient measured variables with high local resolution contributes substantially to the differentiation and detection of degradation effects and battery defects. The addition of further resilient measured variables with high local resolution contributes substantially to the differentiation and detection of degradation effects and battery defects.

This study uses for this purpose surface-applied fibre optical sensors with inscribed Bragg fibre gratings on pouch cells to provide an in-operando spatial resolution of cell changes in the mechanical strain as well as temperature.

The high reproducibility of the sensor signals due to the method of applying the optical sensors to the surface enables the composition of the signal to be reliably determined. To be able to separate the coupled influence of mechanical strain and temperature, the sensors must be calibrated beforehand under isothermal conditions and during battery inactivity.

In addition to detecting abnormal conditions, this application of fibre optic sensors shows that these sensors can support the estimation of degradation state and deterioration rate through a statistical analysis of the gathered data.

Future investigations include the integration of fibre optic sensors into the anode of the battery to evaluate the mechanical performance of the battery and furthermore the parameterisation of the volume expansion of the electrode materials.