Fiber Optic Monitoring of Composite Lithium Iron Phosphate Cathodes in Pouch Cell Batteries

Abstract

Developing techniques for real-time monitoring of the complex and dynamic environment in lithium-ion batteries is crucial for optimal use of the cells and to develop the next generation of batteries. In this work, we demonstrate the use of fiber optic evanescent wave (FOEW) sensors for monitoring lithium iron phosphate (LFP) composite cathodes in pouch cells. The fiber optic sensors were placed on top of the LFP electrodes, and the pouch cells were found to cycle well with significantly improved electrochemical performance compared to fully embedded fibers in Swagelok cells. Galvanostatic, voltammetric, and pulsed current techniques demonstrated that the optical response correlated well with the capacity, and a clear difference in sensor response was seen when the sensors were placed at the surface of composite electrodes compared to fibers embedded in the cathode. The optical response from LFP at different rates was also investigated, but no apparent influence on intensity output was found even though polarization was observed in the voltage profiles at higher currents. It was also demonstrated that the electrolyte itself functioned as a fiber cladding and that the salt concentration in the electrolyte did not influence the optical signal. In addition, given the short penetration depth of the evanescent waves, the sensor response is most likely dominated by the surface conditions of electrode particles near the sensing region. These findings provide further insight into the application and performance of FOEW sensors integrated into batteries, as well as the possibility of developing low-cost fiber optic sensors for battery monitoring under working conditions.

Fiber Optic Sensors for Detection of Sodium Plating in Sodium-Ion Batteries

“…As a result, the evanescent wave is weakened and the electromagnetic energy in the optical fiber becomes attenuated. 19,23,24 In the sodium-ion battery cells, the electrolyte functions as a fiber cladding, 25,26 and the evanescent waves that form at the fiber core−electrolyte interface extend a short distance into the surrounding medium. As metallic sodium is deposited across the surface of the copper electrode or begins to plate on hard carbon, it is simultaneously deposited in the vicinity of the fiber optic sensor, strongly modulating the intensity output as the combination of sodium clusters and electrolyte forms a new lossy cladding.…”

Fiber Optical Detection of Lithium Plating at Graphite Anodes

“…As described in more detail elsewhere, [ 22 ] the original fiber cladding was removed before the incorporation of the fiber optical sensor in the pouch cell, hence the electrolyte and nearby graphite particles in the very close vicinity of the fiber will act as the new cladding. [ 16 ] In the optical fiber, light is guided via total internal reflection at the interface between the core and the cladding, and at each point of reflection, an evanescent wave is formed that penetrates into the cladding. Although the amplitude of the evanescent waves decreases rapidly from the interface, they still interact with the surrounding medium.…”

“…In those experiments, the modulation of the light at the sensing region of the fiber could also be linked to the oxidation and reduction of iron in the LFP. [15,16] Incorporation of optical fibers in batteries is still at a rather low technology readiness level and may not be trivial in commercial cells, but has the potential to provide a BMS with important information for the optimized use of a battery pack. In general, the knowledge of how cell chemistry modulates the light at the fiber/battery interface must also be improved.…”

Optical Fiber‐Based Gas Sensing for Early Warning of Thermal Runaway in Lithium‐Ion Batteries

“…Sealing of the pouch cell integrated with optical fiber has also been studied. [127,130] Raghavan et al [130] found that it is very likely to have voids at the entrance of the pouch cell (Figure 13d,e), and proposed a method to tightly seal the entrance with a thermal sealing film (Figure 13f).…”

“…Besides, it is important to assess the mutual inference between the optical fiber and the electrodes materials. A pouch cell with an inserted bare optical fiber evanescent wave sensor laying on the surface of cathode [ 127 ] showed a great stability over cycles for both the cell and the optical fiber. However, the stability of optical fiber sensor obviously degrades after having a coating layer.…”