Towards in-situ State of Health Monitoring of Lithium-Ion Batteries Using Internal Fiber-Optic Sensors
Authors: Wahl Markus Solberg , Lamb Jacob , Sundby Eirik , Thomas Peter James , Hjelme Dag Roar , Burheim Odne Stokke 
Published: July 2022

We demonstrate a fiberoptic sensor for measuring the lithium-ion concentration inside a lithium-ion battery (LiB), live during charge and discharge. The goal is to monitor the health of the battery based on the dynamic and state-of-charge dependent concentration of lithium ions in the electrolyte.
An important factor to improve the sustainability of LiBs is to prolong the usable lifetime of the batteries, as it directly reduces the environmental footprint per functional unit (e.g., per kilometer driven). Batteries lose capacity over time, both due to charging/discharging and to shelf ageing (calendar ageing). This ageing is caused either by i) unwanted chemical reactions, reducing the number of available lithium-ions or increasing the impedance, or ii) by physical changes that reduces the available lithium storage capacity of the electrodes.
Generally, ageing is studied through monitoring the operational performance (e.g., coulombic capacity, internal resistance), or through post-mortem investigations. Alternatively, advanced equipment such as thermal neutron imaging or X-ray tomography has been used to take snapshots of the internal composition of the battery [1,2]. In this study we propose to monitor and study the health of LiBs through internal measurements using fiber-optic sensors, specifically through live measurements of the internal lithium concentration.
The intrinsic properties of optical fibers (e.g., small, chemically inert, electrically insulating) make them ideal for internal sensing in the chemically harsh interior of a battery. Multiple sensors can also be produced on a single fiber, enabling multi-point or multi-parameter sensing with only one entry point in the cell. Alternatively, the whole fiber can be used as a sensor, giving a continuous measurement along the fiber. In this study, the lithium concentration is measured by immobilizing a lithium-sensitive fluorophore at the tip of an optical fiber.
Multiple fluorophores that exhibit increased fluorescence in the presence of lithium ions have been described in the literature [3–5]. Padilla et al. synthesized such a turn-on type fluorophore with a red-shifted absorption/emission spectrum to reduce UV photobleaching [5]. However, they did not demonstrate the Li-sensitivity in a setup suitable for internal health monitoring in LiBs, which will be done in this study using optical fibers.
The experimental setup used in this study can be seen in Figure 1a. The fluorophore (2-(2-hydroxyphenyl)-naphthoxazole, HPNO) is synthesized according to Padilla et al.
When the fluorophore (HPNO) binds a lithium-ion (see Figure 1b), the absorption edge redshifts enough to enable excitation at 405 nm. This increases the fluorescence intensity, which therefore becomes sensitive to the Li-ion concentration (Figure 1c). The fluorophore concentration used in this plot was 5 mM, with the Li-ion concentration varying from 0 to 14 mM. The sensitivity range therefore needs to be shifted to higher concentrations to reach typical conditions in LiBs (~1 M). The fluorophore shows minimal photo-bleaching, making the sensing principle suitable also in long term cycling studies.

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Figure 1