The increasing spread of lithium-ion batteries requires a better understanding of the cell state. Especially for systems that need a high reliability or possess a high-power output, cell-monitoring is mandatory. Current electrical methods like voltage metering or coulomb counting show a significant deviation from actual conditions. Therefore, alternative concepts for state-of-charge and state-of-health determination are necessary to get non-electrical information from the cell.
Here, the optical observation of the electrode is an interesting field of research as the optical change of the electrode materials during cycling can reveal their current state. For graphite a color change is well-known and can be observed by the implementation of refractive-index-sensitive optical fibers. For the lithium iron phosphate cathode, marker materials indium tin oxide or antimony tin oxide were added to achieve a noticeable change of brightness of the electrode. This effect was also observed with a fiber optical system utilizing the evanescent field of an etched optical fiber with a micro-sized cladding.
By this means, a cell with an optical fiber in each electrode was assembled and the cycling behavior of the optical transmission signal was monitored. The data shows a high correlation between optical signal and measured state-of-charge for both electrodes. This allows the state-of-charge determination with the help of an optical signal that is independent from electrical measurements. The additionally obtained cell information can be used to improve battery management systems and eventually achieve a saver and more efficient battery use.