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.

Commercial applications of Li-ion batteries for the electro mobility and stationary storage require high energy density, high rate capability and long cycle life of the battery. In this paper we have shown the improvement in energy density and rate capability of LiFePO4 composite cathodes by partially replacing conventional binders with poly-o-methoxyaniline (POMA) and tosylate doped poly(3,4-ethylenedioxythiophene) (PEDOT:p-Tos). Due to different redox properties of the used conducting polymers, POMA specifically improves the capacity to 14% as compared to conventional polyvinylidene difluoride (PVDF) and in case of PEDOT:p-Tos 83% capacity is retained at 5 C charging rate.

Understanding the degradation of graphite anodes plays a major role in the improvement of Li-ion batteries. Herein, we used the nondestructive character of lock-in thermography (LIT) to investigate the aging of graphite anodes in pouch cells. The aged cells show a typical ring pattern in lock-in thermography images, which can be explained by the help of Raman spectroscopy. We attribute this pattern to the different contact due to the formation of a gas during operation, which also leads to inhomogeneous degradation of the graphite electrode. Finally, we demonstrate the detection of two-dimensional variances on the sub-mm scale in the composition of the graphite anodes in pouch cells by LIT. For this purpose, we combined LIT measurements with Raman spectroscopy mapping to link differences in the heat emission to those in the structure of the graphite.