1. Introduction
Lithium-ion batteries (LiB) are extensively used as power sources for a wide range of electronic devices such as smartphones and laptops, ensuring optimized conditions in the perspective of power, energy, long cycle-life, and slow self-discharge [
1,
2]. Prismatic LiBs, characteristically used on smartphones, are more disposed to thermal and mechanical abuses from external actions.
The induced strain can be a question that affects the LiB stability and safety, making it the principal cause of material cracking and other forms of performance degradation [
3]. Analogous to other electrochemical energy storage systems, the chemical compositions of the active materials change under the charge/discharge processes, which induces strains in electrode particles and causes changes in LiB volume. The aim of thickness reduction of smartphones can be a problem for the users, because these new designs of the smartphone do not integrate a dually protective device which relieves at a set pressure, thus avoiding the overpressure of the LiB. Additionally, LiB companies pursue higher energy density and thinner devices at the cost of safety, which moves against the inherently safer design of a commercial LiB [
4].
In addition to strain, thermal runaway is also an essential issue, with impact in the global LiB performance, which is reproduced by the fast increase of temperature. The internal structures of LiBs are made-up of multiple layers, forming a ‘jelly roll’ structure, where each layer consists of the anode, cathode, electrolyte, and polymer film separators. Under abnormal conditions, such as temperature exceeding the separators melting point, mechanical deformation, or breakdown of the layered materials, an internal short circuit can happen. The hot spot generated by internal short circuit can ignite thermal runaway, leading to fire or explosion of the LiB [
5,
6].
The ability to quantify and evaluate the mechanism of strain and thermal runaway generated during the electrochemical processes that the batteries can operate will be beneficial information regarding their behavior as well as an active tool to promote their safety. The temperature and strain sensing of LiB is typically performed using thermocouples [
7,
8], pyrometers [
9], electro-mechanical sensors [
10], and 3D digital image correlation [
6]. Due to their ease in multiplexing, fast response, immunity to electromagnetic interference advantages when compared to electronic sensors [
11], recent works showed that fiber Bragg gratings (FBG) are an effective method to perform temperature and strain measurements in LiBs [
12,
13,
14,
15,
16].
In this work, through a sensing network of FBGs, quantitative temperature, bi-directional strain shifts, and correspondent longitudinal and transversal variations are provided, as a function of the respective voltage signal of a prismatic LiB, during different cycling protocols.
3. Results and Discussion
The temperature variations registered by the FBG sensors placed on the two sides of the battery are presented in
Figure 2 and
Figure 3. The positive and negative electrodes indicated in the Figures correspond to the sensors placed on the
y- and
x-directions, respectively. The cycling protocol, where the discharge rate of 1.32 C was applied, is shown in
Figure 2. Over the CC charge step (1), a maximum temperature variation of 1.29 ± 0.13 °C was measured by the sensors placed on the positive electrode side, whereas on the negative electrode side, the maximum temperature shift was of 0.86 ± 0.13 °C. Between the constant current (CC) charge/discharge steps, constant voltage (CV) steps (2) of 10 min and 15 min were selected to stabilize the temperature and relax the battery.
The starting of the CC discharge step (3) causes an instantaneous increase on the battery surface temperature, however, the maximum temperature shift was achieved only at the end of the discharge process. These fast and significant increases are related to the larger electrochemical reactions produced by the flow of Li+ ions on migration to the positive electrode. During all the discharge steps, two different moments of temperature variations can also be observed. The first moment occurs when the LiB voltage crosses the cut-off voltage, and the second between the cut-off and the end of the discharge process. The maximum temperature variations detected were very similar on both electrodes, around 11.50 ± 0.13 °C.
Figure 3 shows the cycling protocol where a discharge rate of 5.77 C was applied. Attending to the CC charge steps (1) and comparing with previous cycling protocol, the same range of values was registered by the FBG sensors located on the positive and negative electrodes. As expected, the main difference was in the temperature variations achieved during the CC discharge step (3). In all discharge steps, a monotonic increase can be observed, until the LiB reaches 1.95 V. A maximum temperature variation of 27.52 ± 0.13 °C was recorded on the positive electrode side by the FBG4. On the negative electrode side, an inferior Δ
T value of 27.25 ± 0.13 °C was obtained, however, this difference is not significant, taking the measurement error into account.
The bi-directional strain variations detected by the FBG sensors on the
x- and
y-directions, during the experimental cycling protocol with the discharge rate of 1.32 C, as well as their voltage signal, are presented in
Figure 4. Over the CC charge steps (1), mean strain variations of 122.49 ± 0.01 µε and 29.61 ± 0.01 µε were measured on the
y- and
x-directions, respectively, when the battery reached the 3.95 V. As already mentioned, during the 10 min CV step (2), between the CC charge (1) and CC discharge (3) steps, a relaxation occurs on the battery. This relaxation was higher on the
y-direction, translating into a strain diminishing of 25.58 ± 0.01 µε.
When the battery is subjected to the CC discharge step (3), at 1.32 C, three different behaviors can be observed on the two different directions. In the first one, until the LiB reached the 3.30 V, there is a sudden increase of strain. In the y-direction this increase is nearly two-times higher than the one measured in the x-direction. After this, a strain decreases of 7.39 ± 0.01 µε and 6.46 ± 0.01 µε was measured in the y- and x-directions, respectively, until the cut-off voltage was reached. This was followed by a fast increase of strain, as the battery starts to operate under abusive conditions, between 3.20 V and 1.95 V. In this case, the y-direction strain increased up to 213.53 ± 0.01 µε. On the x-direction, a 3.4-times lower strain value was recorded.
These values can be converted to length variations (ΔL), by multiplying the strain by the length or the width of the LiB on the y- or x-directions, respectively. Thus, maximum ΔL of 18.36 ± 0.05 µm and 1.86 ± 0.05 µm on the y- and x-directions were calculated, respectively. These length variations can be translated to a longitudinal and transversal variation in the battery of 0.021% and 0.006% in the y- and x-directions, respectively.
The same LiB was submitted to a different cycling protocol test, where a different discharge rate of 5.77 C was applied. This higher discharge rate was selected so that the battery would operate under abnormal conditions. However, the same charge rate was used to charge the battery at 3.95 V. The bi-directional strain variations obtained are presented in
Figure 5.
Comparatively to the cycling protocol presented previously and attending the CC charge steps (1) and CV steps (2), the same range of Δε were obtained, causing maximum ΔL of 10.53 ± 0.05 µm and 0.89 ± 0.05 µm in the y- and x-directions, respectively, and longitudinal and transversal shifts of 0.012% and 0.003%, respectively, on the battery.
When the battery is subjected to CC discharge steps (3), a monotonic increase can be observed in both directions. However, a different increasing time occurs, before and after the battery reached the cut-off voltage, following the fast voltage decrease. On the y-direction, a maximum strain variation of 593.58 ± 0.01 µε was measured and a correspondent ΔL of 51.05 ± 0.05 µm was calculated. This high elongation translated in a battery longitudinal variation of 0.060%. The sensors located in the x-direction followed a similar behavior as in the previous cycling protocol and detected lower strain variations. In this case, a maximum transversal variation of 0.022% was obtained.
In all cycles, it was observed that the maximum strain variations are registered when the battery discharge process ends. A relation between the different discharge rates which are applied and the longitudinal and transversal variations on the LiB can be performed by considering the battery dimensions and the ΔL values obtained in the two directions. For instance, the ratio of the LiB dimensions is 2.87, whereas dividing the longitudinal by the transversal variation measured over the higher discharge rate, a ratio of 2.73 is calculated. Making the same analogy but for the discharge rate of 1.32 C, a ratio value of 3.50 is obtained. This means that the expansion and contraction of the internal ‘jelly roll’ structure that constitutes the prismatic battery behaves differently on each charge/discharge process which is subject, and it is not proportional to the LiB dimensions.
Through a complete analysis of the cycling test, it is evident that, in both cases, the maximum values obtained for the temperature and strain variations agree with the sudden decrease of the voltage. Considering all strain and temperature values measured, different behaviors in terms of direction of expansion and temperature variations on the two sides of the battery were detected. These results are in good agreement with the ones found in the literature [
6,
9,
10].
The monitoring of the internal and external parameters during charge and discharge cycles, combined with battery management systems, seems to be adequate to improve overall safety. They also could be used to determine realistic operating conditions and geometries, towards more stable cell pack designs.
4. Conclusions
A network of FBG sensors was successfully used to simultaneously monitor temperature and bi-directional (x- and y-direction) strain in a prismatic rechargeable LiB, under an experimental cycling protocol with normal CC charge and different CC discharge steps (1.32 C and 5.77 C). When the battery was subjected to abnormal operating conditions, as fast discharge and operating below the cut-off voltage, it is evident that higher temperature and strain variations occur, which are promoted by the rapid Li+ transport between the positive and negative electrodes. Over the CC charge step, maximum temperature and strain variations were reached at the end of the process, with values of 1.29 ± 0.13 °C in the positive electrode side and 122.49 ± 0.01 µε and 29.61 ± 0.01 µε on the y- and x-directions, respectively.
During the CC discharge step, the higher strain and temperature values of 593.58 ± 0.01 µε and 27.52 ± 0.13 °C were respectively register for the higher discharge rate and when the LiB voltage was lower. These values also relate to the y-direction and positive electrode side and correspond to a battery longitudinal expansion of 0.060%. As expected, there is a deformation increase when the temperature also increases, due the thermal expansion of the materials that compose the battery. Thus, the internal structure of the battery is an important parameter to have in consideration and can influence the behavior of battery materials in terms of expansion and contraction over its operation.
The sensing network presented proved to be an effective, precise, alternative solution to real time monitor, multipoint and in operando temperature and bi-directional strain changes in the LiBs promoting their safety.