Refractive Index Measurement of Lithium Ion Battery Electrolyte with Etched Surface Cladding Waveguide Bragg Gratings and Cell Electrode State Monitoring by Optical Strain Sensors

Abstract

In this scientific publication, a new sensor approach for status monitoring, such as state of charge and state of health, of lithium ion batteries by using special Bragg gratings inscribed into standard optical glass fibers is presented. In addition to well-known core gratings, embedded into the anode of 5 Ah lithium ion pouch cells as a strain monitoring unit, the manufacturing of a surface cladding waveguide Bragg grating sensor incorporated into the cell’s separator, that is sensitive to changes of the refractive index of the surrounding medium, is demonstrated. On the basis of the experiments carried out, characteristics of the cell behavior during standard cyclization and recognizable marks in subsequent post-mortem analyses of the cell components are shown. No negative influence on the cell performance due to the integrated sensors have been observed; however, the results show a clear correlation between fading cell capacity and changes of the interior optical signals. Additionally, with the novel photonic sensor, variations in the electrolyte characteristics are determinable as the refractive index of the solution changes at different molar compositions. Furthermore, with the manufactured battery cells, abuse tests by overcharging were conducted, and it was thereby demonstrated how internal battery sensors can derive additional information beyond conventional battery management systems to feasibly prevent catastrophic cell failures. The result of the research work is an early stage photonic sensor that combines chemical, mechanical and thermal information from inside the cell for an enhanced battery status analysis.

Critical Review of Intelligent Battery Systems: Challenges, Implementation, and Potential for Electric Vehicles

“…It is shown that FBGs exhibit 28 % lower rise time compared to K-Type thermocouples, emphasizing their real-time monitoring capabilities [355]. Furthermore, it is suggested that FBGs are deployable within the jelly roll or even in the electrode coatings [336,[356][357][358][359]. The mechanical and temperature changes during operation can be precisely monitored in this position.…”

Fiber Optic Sensing Technologies for Battery Management Systems and Energy Storage Applications

“…Another work with FBGs measured the refractive index of the electrolyte as a result of the changes in conductive ion concentration [ 127 ]. Illustrated in Figure 9 , Nedjalkov et al introduced a novel fiber design: in addition to FBGs in the fiber core, an additional optical waveguide was inscribed within the cladding of the original fiber using the point-by-point method via a femtosecond laser to enhance its sensitivity to refractive index variation in battery ionic solutions.…”

“… Schematics of typical and sensitivity-enhancing optical waveguide structure [ 127 ]: ( a ) the original design where gratings are inscribed into the fiber core; ( b ) the self-compensating design that can enhance refractive index sensitivity by inscribing an additional waveguide in the cladding to divert a portion of the propagating light. …”

Operando Raman analysis of electrolyte changes in Li-ion batteries with hollow-core optical fibre sensors

“…Second, changes in Li + solvation will likely increase the refractive index of the electrolyte by typically 0.1% per Li + molar concentration, as was demonstrated with different salts and salt concentations. 67,68 This changes the interference conditions in the fibre and may affect the amount of Raman pump light propagating along the hollow-core fibre. 39 PVDF binder, and 5 wt.% Timcal C45 conductive carbon on 20 um thick aluminium foil.…”