Design guidelines to use optical fibers as state of charge sensors: an operando micro wide and small angle X-ray scattering study of an IR-fiber equipped smart Na-ion battery

Abstract

Smart batteries are equipped with sensors that monitor temperature, state of charge, and strain during utilization. These data estimate the battery's state of health, crucial to increasing the battery life by adapting battery use and triggering self-healing mechanisms. However, few studies are devoted to the reliability of sensors. This work aims to evaluate the accuracy of state-of-sodiation determination using an optical fiber. Along that line, an optical fiber is introduced inside a Na-ion battery and used to obtain local (<1 µm around the fiber) operando infrared (IR) spectra of the Prussian blue analog (PBA) positive electrode material during the first three cycles. The fourth cycle is performed while the battery is scanned with a micro X-ray beam (ESRF), allowing the measurement of wide and small-angle scattering (WAXS/SAXS) maps. IR, WAXS, and SAXS are used to determine the PBA material's state of sodiation (SoS). Qualitatively, the average SoS measured by all the methods has the same trend; however, quantitatively, the SoS measured by the fiber deviates from that obtained by electrochemical and WAXS methods. We investigate the origin of this deviation with WAXS mapping. First, the SoS at the fiber position is delayed by 5–20% for the cell studied in this work. Using preliminary simulations, we qualitatively reproduce this delay when the electrode is locally compressed by the fiber. Second, the sodiation of the PBA electrode is very heterogeneous. The heterogeneity cannot be captured by a single fiber integrating the IR spectra along its surface and length. These results indicate future directions to optimize the design of smart batteries: (1) introducing the fiber without modifying the electrode, (2) using simulation to predict the compression effect and calibrate the state of sodiation determination, and (3) developing spatially resolved measurements through fiber coating or using fiber nets.