**3. Critical parameters tracked in LIBs by OFS**

In the almost last 10 years, OFS was introduced and started to be used as a useful and precise tool to monitor critical key parameters inside and outside the LIB. Significant advantages instead of other sensing technologies can be reported comparatively with the electronic technology, in the monitorization of temperature (TCs, RTDs,) and strain (strain-gauges), regarding the low intrusiveness (fibers thickness of �125 microns), higher capacity of multipoint and multiparameter discrimination, and essentially the capability of to be embedded in their harsh electrochemical environment, by tracking in loco and real-time specific parameters with good accuracy and reliability, and without damage the batteries performances. Until now, it was almost impossible to access the internal behavior of these batteries in operation, to know how they behave in terms of physical and chemical performance.

This optical fiber technology presented above has been successfully integrated into battery sensing, allowing their smart sensing of LIB safety aspects, such as temperature and/or thermal gradients, strain, RI, and pressure variations, internal gassing evolution, electrochemical events (chemical reactions, and SEI composition), and their correlation with SoX battery indicators. This section divides and reports all studies presented in the literature since the first paper reported by Yang et al. in 2013, regarding the use of OFS to monitor all these parameters in LIB.

### **3.1 Temperature tracking**

Of all safety problems in LIBs, thermal runaway is a vital issue, which is reproduced by the fast increase of temperature. This rise produces heat energy at a rate faster than heat can be dissipated followed by a failure of the LIB internal separator components, resulting in local short circuits and critical situations, to their explosion [75]. Moreover, accumulated heat in the batteries takes worries of performance drops and safety risks. Temperature can affect the LIBs lifetime and energy, and therefore, it should be within an ideal range of temperature, to ensure better performance and long life, both for use and storage [17]. The ability to quantify and evaluate the mechanism of thermal runaway generated during the electrochemical processes which happen will create beneficial information regarding their behavior, as well as an active tool to promote their safety [76–78].

Typically, in the real context of the LIB, this parameter is monitored through external electronic devices, such as TCs and RTDs, by detecting just single points on their surface. Optical fiber sensing technology was used as an alternative method to realize multipoint external and internal temperature measurements on LIBs, during their operation, also performing in different types of LIBs, thermal gradients characterizations, and evolutions. Of the different types of OFS, the FBGs were the mainly used due to their inherent advantage of multipoint monitoring and fast response time.

In 2013, Yang et al. [19], integrated by the first time, FBGs in a coin LIB to measure real-time temperature changes during the battery's operation under normal and abnormal conditions. The FBG sensors exhibited good thermal responses to dynamic loading when compared with the TCs. Novais et al. [26], in 2016, presented the integration of four FBGs in lithium-ion cells for *in situ* and in-operando temperature monitoring during galvanostatic cycling at C-rates ranging from 1 to 8°C (**Figure 11A**). In the internal FBG locations, the fiber was covered by an exterior silica

#### **Figure 11.**

*A) Schematic diagram of internal and external FBG sensors positions [26]. B) Schematic of a LiB test setup with the location of TCs and FBGs. Copyrights 2016 and 2017.*

tube to free the sensors of any external stress promoted by the surrounding materials and, in this way, only detect temperature variations.

The internal sensors registered higher temperature variations at 8°C and in the center of the active area of the one-layer pouch cell. The authors concluded that the low invasiveness and high tolerance to the chemically aggressive environment make them a motivating option for integration into the LIBs. This study also contributes to the detection of a temperature gradient in real-time inside an LIB, thanks to the different locations of the sensors inside the battery. Nascimento et al., one year later [30], attached FBGs, all recorded in the same single fiber and TCs on a commercial LIB surface, to perform a comparative study between their signal responses (**Figure 11B**). The response rates were 4.88°C/min and 4.10°C/min for the FBG and TC, respectively. The results also demonstrate that the FBGs were able to sense temperature fluctuations with a 1.2 times higher response rate than the K-type TCs. The rise time obtained for the FBG was 28.2% lower than the TC, making the FBGs a better choice for the real-time temperature tracking on a LIB.

In 2017 and 2019, Nascimento et al. [29, 43] has developed two studies about the thermal distribution on a surface of a prismatic LIB by a network of five FBGs in a single fiber, to assess in real time and operation, the impact of different environmental conditions, temperature, and relative humidity, on batteries performance. These studies provided a real-time thermal mapping to elucidate which areas of the battery needed to be cooled faster when it was exposed to dry, temperate, and cold climates. Faster variations of voltage usually translated in higher temperature variations at the LiB surface, and this effect is evidenced when the LiB operates under abnormal conditions. After a pre-calibration step, the FBGs were calibrated to convert the wavelength shift peak to the correspondent temperature values based on their calculated sensitivity. These temperature values are tracked by following the FBGs peaks in the spectrum response. Complete temperature values of 30.0 0.1°C, 53.0 0.1°C, and 65.0 0.1°C were achieved on the top location (near electrodes) during the higher discharge rate, when exposed to the cold, temperate, and dry climates, respectively. The higher temperature shifts detected by the optical sensors in the temperate and dry environments are related to the superior performance of the LiB in terms of discharge capacity and power capabilities. This study demonstrates also which are the best environmental conditions to run the LiBs, in order to extend their lifetime and safety, and is also helpful for the next generation of batteries, showing which areas require faster cooling to reduce accumulated heat.

Bhagat's group, in 2018, performed three studies by embedding FBGs, in cylindrical LIBs, to monitor *in situ* and in-operando temperature variations. The sensors were resistant to the strain imposed during the battery instrumentation procedure and their harsh chemical environment. The results presented a temperature difference between the core and the can temperatures (monitored by K-type TC) of up to 6.0°C during the discharging process, while a temperature difference of 3.0°C was obtained during the charging process. The zones nearer to the anode presented a higher temperature during discharge while the location closer to the cathode performed higher temperature values during charge [38]. The authors demonstrate that FBGs produce reliable core temperature data, while their small mechanical profile allows for a low-impact instrumentation method [41, 42].

Nascimento et al., in 2018, proposed a network of 36 FBGs for real time, *in situ*, and operando multipoint monitoring of the surface temperature distribution on a pack of three prismatic LIBs, performing a spatial and temporal thermal mapping of all

*Tracking Li-Ion Batteries Using Fiber Optic Sensors DOI: http://dx.doi.org/10.5772/intechopen.105548*

**Figure 12.**

*Surface LIB pack thermal mapping performed by a network of 36 FBGs, during discharging at 1.4C [36]. Copyright 2018.*

pack interfaces (**Figure 12**). In total, four optical fibers were used to monitor all locations of the LIB pack. The results show that in general, a thermal gradient is identified from the top to the bottom surface locations. Due to the higher current density of the Li<sup>+</sup> ions near the positive tab collector, the presence of hot spots between two of the three batteries was identified [36].

Peng et al., in 2020 [53] and 2021 [55] proposed an OFS to monitor temperature variations in the external LIB electrodes, during cycling tests. The sensor consists of a metal ring and an FBG. The FBGs were gloved on the external electrodes, and PT100 sensors were also attached to the electrodes as a comparison measurement. The FBGs calibration test presents good linearity and high sensitivity. From the results, during all the cycles, the sensors placed on the positive electrodes recorded higher temperature variations instead of those on the negative electrodes. Even this year, Alcock et al. developed an accessible method to attach FBGs on cylindrical LIB surfaces to *in situ* thermal sensing. This study differs from the others by using a "guide-tube" to decouple the temperature and strain variations on the LIB surface [54]. Recently, Li et al. developed an optical fiber temperature sensor for battery temperature monitoring based on fluorescence intensity ratio technology [58]. In this study, β-NaYF4:Er3+/Yb3 + @NaYF4 nanoparticles were used to design the optical sensor in the fiber tip. After the fiber functionalization, this sensor was preliminary calibrated to temperature in function of their fluorescence intensity response to correctly convert their spectral response in temperature values during battery operation. The maximum relative sensitivity obtained by the optical sensor was 1.62% K<sup>1</sup> at 293 K, the temperature detection limit was within 0.5 K, and high-temperature changes were registered under a higher discharge rate.

#### **3.2 Strain tracking**

Along with the cycling processes of LIBs, strain evolution is also an important parameter to be tracked in order to identify possible cracks in their internal materials or the occurrence of some swelling in case of a wrong operation through a gasification production. In this way, OFS has also been recently used to monitor this parameter. From all studies reported so far, the FBGs were the sensors selected to perform this sensing. Li-ion pouch cell configuration is the most used in tests while coin cell configuration is only employed to demonstrate the preliminary tests.

In 2016, Bae et al. [27] developed two approaches to track strain and stress evolution in the graphite anode of a Li-ion pouch cell using FBGs. In one approach, the optical sensor was attached between the graphite anode and the separator, while in the other implanted approach, the sensor was embedded totally within the anode material. Measurements of strain and stress states of the graphite anode were run over cycling tests. Reproducible peak shifting in both attached and embedded FBGs was observed at different states of charge and discharge. Specifically, an embedded sensor that is completely surrounded by graphite particles simultaneously suffers accumulated longitudinal, as well as transverse strains associated with the expansion or contraction of the negative electrode. Additionally, the embedded FBG showed 3 higher sensitivity than the attached FBG sensor at 100% SoC. The process to detect and convert the FBG wavelength peaks to strain measurements is the same as used for temperature monitoring, through the strain sensitivities of each sensor and using free FBGs just to decouple temperature variations.

Peng et al., in 2019, have reported two papers regarding an external and novel strain sensor based on FBGs for LIBs [45, 47]. The structure of the strain sensor consisted of two FBGs, a sensitization structure and a protective cover, which contained two symmetrical lever mechanisms and an installation platform, in which the rotating pairs of levers were replaced by flexure hinges. Enhanced strain sensitivity of 11.55 pm/με was obtained, with good linearity and repeatability. From the cycling tests, the drift in strain is analogous to different C-rate charge-discharge cycles. The strain rises evidently close to the end of discharge with an evolution in the C-rate. However, the proposed sensor cannot be embedded inside the LIBs due to their bulky structure, providing higher invasiveness.

Rente et al., in 2021, reported the tracking of strain shifts, also through FBGs, on a surface of cylindrical LIB, under cycling tests [57]. In this study, a simple machinelearning algorithm based on dynamic time warping (DTW) was used to estimate the SoC of representative LIBs. The FBG data obtained were shown to be reliable and sufficiently reproducible to serve as the input for the DTW algorithm used. The use of a model train has proved to be very effective as a proof-of-concept study for future BMS, especially in electrical vehicles.

#### **3.3 SoX battery indicators tracking**

The SoX battery indicators are crucial factors reflecting the state of batteries, in which they are commonly estimated under the assistance of the evanescent wave sensors in LIBs. Additionally, the FBGs are combined with them to improve the sensing performance and used as parameters discrimination. In 2015, an integrated OFS technology for monitoring charge steps in LIB cells was studied by Alemohammad et al. [23]. The sensor consists of an optical fiber encapsulated inside a LIB with direct interaction with the cell electrochemical environment. The sensor operates on the basis of the changes in the optical properties of the LIB cell electrodes, that is, variations in optical absorption and reflection at different charge levels, that will change the spectral response in terms of wavelength and also optical power losses, showing the SoC in battery and providing information about aging and stabilization following charge/discharge cycles.

Ghannoum et al., reported in 2016, a reflectance study of commercial graphite anodes in LIB and the optical fiber evanescent wave spectroscopy of electrochemically lithiated graphite [28]. A substantial rise in the reflectance of the lithiated graphite in the near-IR band (750–900 nm) as a function of SoC and similar SoC tendency in the

transmittance when the fiber was embedded in the battery was observed. The same authors, one year later, developed the fabrication and integration of the OFS, by using similar sensing technology, into cylindrical LIBs as well as a Li-ion pouch cell [31]. The sensitivity of the sensor increased along with increasing the contact area of the sensor within the graphite anode and the optical fiber evanescent wave sensor integrated into the graphite anode demonstrated the potential use to track the both SoC and SoH of LIB, by correlating the optical data with the voltage and current signals of the LIBs.

Lao et al., in 2018, designed an innovative method, named TFBG-based SPR sensor, for *in situ* tracking the SoC of supercapacitors, for the first time [39]. This new configuration is based on a 50-nm-thick gold layer of high surface quality deposited on the TFBG. The FBG recording angle was 18° and an additional gold coating was deposited on the fiber end to achieve a single-ended sensor with interrogation in the reflection scheme (**Figure 13A**). The proposed plasmonic TFBG sensor was attached to one of the electrodes of the supercapacitor to monitor the electrochemical activity. The charge density and SoC measurement were demonstrated, and the results showed that the spectral response of the SPR mode of the TFBG was directly related to the charge density and the SoC of the supercapacitor. Basically, the wavelength peak of the TFBG and the SPR mode changes with the SoC level at which the supercapacitor is. Then, the variations of the charge density and the SoC during the cycling steps could be tracked by following the shifts of the place and the intensity of the reflection spectrum.

Modrzynski et al. [46] presented an SoC measurement technique based on an optical fiber sensing system, in 2019. In this system, two optical fibers were etched to increase the interaction between the light propagation inside the fiber core with the surrounding fiber environment, detecting in this way RI changes in real time. The fibers were integrated into both graphite anode and lithium iron phosphate with the addition of indium tin oxide cathode of a Li-ion pouch cell. The SoC was monitored in real time by simultaneously detecting the light transmission through both fibers. The results showed that the SoC correlated transmission behaved equally for both electrodes. However, diverse relaxation and wavelength-dependent behaviors were identified during the charge and discharge cycling steps. The study proved that the OFS process was able to estimate the SoC independent of the electrical measurement methods.

In 2020, Hedman et al. used an OFS based on evanescent waves for monitoring the charge/discharge cycles of lithium iron phosphate batteries in real time [49]. The sensor is fully embedded within the positive electrode in a customized Swagelok cell in both a reflection- and transmission-based OFS configuration. Both constant current cycling and cyclic voltammetry were employed to associate the optical spectrum

#### **Figure 13.**

*A) Electrochemical SPR sensing principle and experimental demonstration with a gold-coated TFBG sensor [39]. B) OFS principle. Multicolored light is guided through an optical fiber embedded in the electrode [59]. Copyright 2018, 2021.*

response with the cycling processes of LIBs. From the results, the optical signal correlates well with the SoC in the positive electrode in real time, and it is reproducible over various cycles. Furthermore, the optical signal detected does not rely on other usually estimated parameters in SoC estimation, such as current, voltage, and temperature. Rittweger et al., in 2021, present measurement results based on transmitted light intensities through the optical fiber as an indicator for the SoC (**Figure 13B**) [59]. The work also purposes to present an explanation of how to use the measured transmission intensity to decrease cross effects, such as temperature, pressure, or aging LIBs parameters. For that, a referencing methodology based on transmission intensities from light with different wavelengths is approached. Due to the reduced fiber cladding by a preliminary etching process, the light interacts with electrode material surrounding the fiber. So, transmission losses can be sensed, which depend on the lithium concentration in the electrode. From the results, the calculated transmission ratios are in good agreement with the SoC for various C-rates.

### **3.4 Electrochemical events tracking**

Electrochemical events, such as gassing production, electrode lithiation, and chemical changes of the electrolyte, are fundamental issues that enable the battery manufacturers to identify degradation mechanisms that currently limit the lifetime and capacity of these energy-storage systems.

In 2014, Lochbaum et al. measured the evolution of gaseous CO2 inside lithium-ion pouch cells during overcharge tests with optical fiber colorimetric sensors (the chemical sensing fiber used comprises a silica core surrounded by a fiber cladding, which is permeable to the chemical to be detected (analyte) and functionalized such that it changes its optical characteristics with analyte concentration) to examine the dynamics of electrolyte decomposition reactions [20]. For the ratiometric read-out principle used, the averaged intensity between 570 nm and 600 nm (CO2-sensitive band) was normalized by the averaged intensity between 800 nm and 820 nm (CO2-insensitive band). The results indicate a nonreversible gas evolution inside the LIBs during overcharge, in which the beginning of gas evolution is delayed in time relative to the overcharge condition.

Ghannoum et al., in 2017, presented the application of an innovative optical fiberbased sensing system for the lithiation of graphite within a lithium-ion pouch cell in real-time using a narrow-band spectrum concentrated around 850 nm [31]. For that, a polymer optical fiber was used and etched for the fiber core to directly interact with the surrounding materials. The main results show that the sensor signal can be correlated with the lithiation of graphite anode over multiple full and partial cycles.

More recently, in 2020, the same authors show an analysis of the interaction between the optical fiber evanescent wave sensor and the graphite particles within a LIB [50]. The proposed sensor was sensitive to lithium concentration at the surface of graphite particles; then, it was able to monitor the capacity fade of LIBs. In the same year, photonic crystal fibers were used by Miele et al. to monitor chemical changes within LIBs under real working conditions [52]. The technique used was based on optofluidic single-ring hollow-core fibers, which uniquely allow light to be guided at the center of a microfluidic channel. The signal analysis was performed by background-free Raman spectroscopy to identify early signs of battery degradation. From the results, the Raman peaks related to ethylene carbonate and the important

battery additive vinylene carbonate, offer a direct vision in the formation of the SEI, the main buffer layer that largely forms during its first electrochemical cycle, and whose stability is key to the longevity of the LIBs.

### **3.5 Simultaneous tracking of temperature, strain, pressure, and RI**

The main challenge in tracking critical parameters inside the LIB, such as thermal gradients, strain, pressure, and RI changes, is that due to its electrochemical environment, the LIB presents a very dynamic behavior. The temperature variation influences the thermal expansion of the materials that compose the LIB, promoting strain changes. The electrochemical behavior also promotes internal gassing production, which will affect the pressure variation and RI changes on the electrolyte. LIBs primarily employ liquid electrolytes to ensure rapid ion transport for high performance of the variation in the RI of the electrolytes is related to the variations in the conductive salt concentration. Thus, the RI shifts can be treated as an indicator of the degradation evolutions.

As some of the OFS are sensitive to more than one parameter simultaneously, they suffer from large cross sensitivity, such as strain, RI, and temperature. In this way, solutions to decouple these parameters should be considered by the researchers. As the LIBs are very complex systems with dynamic and diverse physical and electrochemical behaviors, in which many parameters are linked and correlated between them, such as temperature, strain, gas formation, and pressure, several studies were already reported by sensing and decoupling simultaneous parameters in LIB since 2013.

Sommer et al. have reported many studies concerning the use of FBGs to simultaneously decouple strain and temperature variations in LIBs [21, 22, 32, 33]. In 2014, the authors start by externally attaching LIB pouch cell FBGs to monitor additional informative cell parameters (strain and temperature) and using other FBGs as a reference to perform this parameters discrimination, as described by Rao et al. [66]. Two FBGs were employed in the experimental setup, one, bonded at two points to the surface of the pouch cell with epoxy, sensing both strain and temperature variations; while the other one, loosely attached to the cell skin with a heat-conducting paste, only detecting temperature variations. Several charge and discharge cycles were performed to examine the repeatability of the measured signals and compared with conventional strain and temperature sensors to verify the accuracy of these sensors. In 2015 [22], the same authors examined the excess volume change at the end of charge and the volume relaxation in the subsequent rest phase by monitoring the strain variations of externally attached FBGs of a lithium-ion pouch cell. The strain was instigated by the alteration of electrode volume, due to the constant Li<sup>+</sup> oscillation and intercalation from and to the positive electrode, and thermal expansion/contraction during cycling charge/discharge steps. A strain relaxation was observed at higher SoC levels, especially strain signal relaxed by 30% at an SoC level of 100%, and the ratio of Li<sup>+</sup> in the external electrode region to Li<sup>+</sup> in the internal electrode region was larger at a higher SoC level. The association between them was also explored at various room temperatures. It concluded that the residual strain increased with decreasing temperature for a certain SoC level, and the alteration between the residual strains was higher for superiors SoC levels.

In 2017, two-part papers about embedded fiber optic sensing for accurate internal monitoring of cell state in advanced BMS by monitoring temperature and strain shifts inside of a pouch cell LIB were developed by Raghavan et al. (part 1) and Ganguli et al. (part 2), belonging to the same research group [32, 33]. Part 1 focuses on the embedding method details and performance of LIBs. The seal integrity, capacity retention, cycle life, compatibility with existing module designs, and mass-volume cost estimates indicate their suitability for electric vehicles and other advanced battery applications. One of the two FBGs was enclosed in a special tubing to make it selectively sensitive to thermal variations alone. The tracked wavelength peak values of the "reference" FBGs in the tubing are subtracted from the total wavelength shift of the adjacent FBG sensor, which is sensitive to strain so that temperature variations are compensated. The second part focuses on the internal strain and temperature signals got under different conditions and their use for high-accuracy cell state estimation algorithms. In particular, the measured strain is used to estimate the battery capacity and predict the capacity up to 10 cycles.

Nascimento et al. have also reported many studies regarding the simultaneous decoupling temperature and strain variations in LIBs through FBGs and interferometric sensors (**Figure 14**). Different type of LIBs was tested on this discrimination. The prismatic and cylindrical configurations were tested externally and pouch cell configurations were tested both internally and externally [25, 37, 40, 44]. In 2015, FBGs were attached to the surface of a cylindrical LIB to track its thermal and strain fluctuations during charge and different discharge C-rates (**Figure 14A**). The tests were repeated twice for each discharge C-rate applied (0.25 C and 1.33 C). The FBG1 and FBG2 only measured temperature variations, while FBG3 was fixed to the battery edges and was subjected to strain and temperature variations. Temperature measurements made by the FBG2 sensor were used to compensate for thermal effects on FBG3, allowing in this way to measure the longitudinal strain variation along the battery length [25]. In 2018, a network of FBGs was attached at a prismatic LiB to sense its temperature and bi-directional (x- and y-directions) strain variations during normal charge and two different discharge C-rates (1.32 C and 5.77 C). The discrimination method used by the OFS was also the reference FBG method [66]. Maximum temperature variations were detected close to the positive electrode side, and higher

#### **Figure 14.**

*Temperature and strain discrimination by OFS in different LIBs configurations. A) Cylindrical B) Prismatic C) Pouch cell [25, 37, 44]. Copyrights 2015, 2018, 2019.*

strain values were sensed in the y-direction (**Figure 14B**). One year later, fiber optic hybrid sensors were embedded in a Li-ion pouch cell to internally monitor and simultaneously discriminate *in situ* and operando strain and temperature shifts in different locations (**Figure 14C**). The hybrid sensing network was constituted by FBGs and FP interferometers. Due to the different strain and temperature sensitivities attained by both types of optical sensors, it was possible to decouple the strain and temperature values by using the matrixial method. Galvanostatic cycles by using different C-rates were applied to correlate the temperature and strain signals with electrochemical processes in the LIB.

In 2017, Fortier et al. also tracked internal strain and temperature variations in the coin cell configuration [35]. However, how this decouple was performed is not explicit in the manuscript. The batteries were evaluated at a cycling C/20 rate, and the FBGs were placed between electrodes and separator layers, near the electrochemically active area. Results show a stable strain behavior within the cell and a near of 10.0°C difference was registered between the interior of the coin cell and room environment temperature over time during cycling steps.

In 2019, a novel-designed OFS, about self-compensating FBGs, to monitor the separator internal status of a LIB by detecting the RI of the battery electrolyte, was proposed by Nedjalkov et al. [48]. The proposed sensor consisted of two FBGs recorded of the same length but in different fiber layers (one on the core and the other near the surface of the cladding, by using a femtosecond laser system). The cladding, near the FBG region, was also softly etched to increase the sensitivity for RI variations. Between the surface FBG, an additional waveguide positioned at half the distance between the fiber core and cladding surfaces in the radial direction was integrated into the inner cladding at the same axial position. Both the influences of the longitudinal strain and temperature could be compensated with this arrangement, so the remaining variable of the measurement was the influence of the effective RI, which was relative to the reflected Bragg wavelength shift. The proposed FBG configuration was embedded centrally between two separator layers of a 5 Ah lithium-ion pouch cell. The results obtained, show that the optical signal was dominantly influenced by the effective RI of the battery electrolyte.

Huang et al., in 2020, published one study, about operando decoding of chemical and thermal events in commercial LIB, by discriminating and sensing temperature and pressure variations through FBGs and microstructured optical fibers (MOF) [51]. The sensing of different parameters was performed, thanks to the different sensitivities of both optical sensors at each parameter (temperature and pressure), in which the matrixial method was applied. These findings allowed to detect chemical events such as the SEI formation and structural evolution in the LIBs. The authors also demonstrate how multiple sensors are used to determine the heat generated by converting the optical data to heat flux values. In the last year, they also demonstrate the feasibility and diversity of TFBGs to operando access the chemistry and states of electrolytes [2]. They show how a single TFBG can simultaneously sense temperature and RI evolutions inside LIB, which is correlated with the chemical electrolyte behavior (see **Figure 15**). From the time-resolved RI signals, the feasibility of monitoring electrolyte deteriorations while accessing their turbidity via particulate-induced optical scattering and absorption was studied as well. These unraveled electrolyte characteristics by TFBG help to determine the electrochemical reaction pathways, being strongly correlated with the batteries' capacity loss.

#### **Figure 15.**

*Schematics and spectra of the TFBG used by Huang et al. [2]. A) TFBG inserted in the core of a cylindrical LIB. B) A gold layer was deposited on the end of TFBG to obtain the reflection probe. C) The temporal voltage, temperature, and RI of two cells during the first two formation cycles at C/10.*
