In-situ electrode observation as an optical sensing method for battery state of charge

In this paper, we present a novel optical method to observe lithium iron phosphate (LFP) cathodes, where optical effects are usually suppressed by carbon additives. Our approach to utilize electrochromic markers in the cathode overcomes this restriction. During cell cycling, reflectance and color of the cathode were found to precisely correlate with state of charge. In addition, the spatial and temporal progression of ion intercalation has been made visible. Data gained from these experiments can be used to determine reaction kinetics and improve electrochemical battery models. This can directly benefit battery research as a novel method for materials characterization in the laboratory. As a next step, we propose a complementary sensor system for commercial battery cells, where an optical fiber is placed between cell electrodes as an evanescent field sensor. The cell-integrated fiber optic system has the potential to directly determine the state of charge and additionally act as a redundant safety level to prevent critical cell states.