Using Cyclic Nanoindentation to Enhance Li-Ion Battery Safety

By Pritam RoyOct 18 2022Reviewed by Susha Cheriyedath, M.Sc.

In an article published in Applied Materials & Interfaces, researchers obtained the temperature distribution of a battery during the process of discharging at sub-zero temperatures while considering the separator's susceptibility to temperature variations.

Study: Temperature-Shift-Induced Mechanical Property Evolution of Lithium-Ion Battery Separator Using Cyclic Nanoindentation. Image Credit: ktsdesign/Shutterstock.com

There were three sets of separator samples that experienced different temperature shifts. Experimental evidence for the temperature-dependent decrease of the separator hardness and the elastic modulus was provided by multicycle depth-sensitive nanoindentation. Additionally, by extracting from the multicycle loading and unloading nanoindentation responses, the fluctuation trends of elastic modulus, hysteresis, and hardness response of the separator samples in terms of temperature were examined.

The temperature-dependent fluctuations in the separator's elastic modulus were examined by monitoring the thermostatic, heating, and cooling processes. In contrast, the nanoindentation tests confirmed that when a thermostatic operation followed heating or cooling, the influence of temperature fluctuations on the hardness demonstrated an attenuation trend. Also, the typical size effects dependent on the depth of the nanoindentation were evident in the fluctuation analysis of nanoindentation hardness as the function of temperature shifts.

Energy-dispersive X-Ray spectroscopy and X-Ray diffraction were used to characterize the elemental distribution and the temperature-induced residual stress, respectively. The determined evolution rule of temperature shift-induced mechanical characteristics of a separator made it easier to design separators optimally and offered the necessary data to improve the safety efficiency of a lithium-ion battery. 

Understanding the Make-up of a Lithium-Ion Battery

A lithium-ion battery (LIB), the preferred option for most electric vehicle batteries, has several remarkable advantages, including (i) high energy density, (ii) a lengthy service life, (iii) a low self-discharge rate, (iv) a high energy conversion rate (96%), and (v) the absence of a memory effect. Also, the commercial lithium-ion battery is sensitive to coupling effects, including physical, multi-mechanical, and thermal fields, under actual operating circumstances.

However, the usage of the lithium-ion battery in temperature-dependent environments is constrained by its decreased performance and micro failure mechanism at low and high temperatures. The safety profile of a lithium-ion battery is deteriorated by the internal short-circuit fault caused by mechanical, electrical, and thermal abuse.

Also, the thermal control and early fault identification of a lithium-ion battery are hampered by the ambiguity and uncertainty of internal short-circuit. Thus, a lithium-ion battery is prone to significant performance degradation problems, including limited poor cycling, capacity, lithium precipitation, rate performance, and imbalanced lithium intercalation, especially in frigid environments.

The internal short-circuit caused by lithium dendrites, the quick decrease in conductivity, the reduction in the activity of electrolyte movement, and the rate are indications that the lithium-ion battery separator performance is deteriorating. The safety of a lithium-ion battery is significantly impacted by the unpredictability of temperature-induced fluctuations in the separator's mechanical characteristics, which also hinders both the separator's best possible design and the lifespan of a lithium-ion battery.

Separators may be impacted by changes in ambient temperature or heat production within batteries during actual operating conditions. Such effects may impair battery performance and cause unforeseen thermal runaways. Numerous studies have been performed on modifying or optimizing the solvent component and electrolyte ratio, integrated with the microstructural characterization and synthesis of electrodes and their materials to reduce the application restrictions. These studies also revealed an increase in the range of operating temperatures of a lithium-ion battery.

By utilizing the battery charging and discharging system and the infrared temperature detection, the temperature variation of the separator could be monitored under various charging or discharging circumstances. The depth-sensing nanoindentation method is efficient for directly measuring the separator's micromechanical characteristics. This nanoindentation testing of separators can precisely assess the various levels of microdomain mechanical mistreatment through varied depth or cyclic loading modes.

Three temperature variations were applied to the separators in the current work. Five cycles of gradational nanoindentation loads were employed on the three different separator samples to achieve the load-depth responses. It was established that the temperature changes altered the separators' elastic modulus, hysteresis, and hardness response to varying degrees.

The evolution of the separator's performance as a result of temperature changes was later examined using various compositional and crystallographic characterization techniques. Thus, the experimental findings helped to clarify how temperature changes affected separator performance, which helped enhance the separator's mechanical–thermal coupling characteristics.

Proof-of-Concept Investigations

For the nanoindentation testing, cylindrical cells of NCR-18650B were employed. Employing a Power Focus BTC3100 cycler, the cells were cycled using a constant current constant voltage (CCCV) charge and constant current (CC) discharge configuration. The cyclic operation was carried out in a temperature-controlled incubator at 26.4 degree Celsius.

The temperature fluctuations of 0, -40, and 40 degrees Celsius were considered for ambient temperatures; -20, 20, and 60 degrees Celsius were the beginning temperatures, correspondingly. The temperature of the separators was changed in three steps. First, the separators were heated from -20 to 20 degree Celsius, followed by an isothermal procedure, and cooling from 60 to 20 degrees Celsius. Then, separator specimens were subjected to multicycle incremental loads using the nanoindentation technique. Finally, the load-depth response curves were produced to examine the differential variations in the separators' elastic modulus and surface hardness.

The investigations on the nanoindentation of three different separator specimens were conducted using a commercial nanoindenter with a Berkovich indenter. These nanoindentation studies were performed for each separator specimen using three-by-three dot matrices with dots spaced 20 micrometers apart.

Fifteen sets of five cyclic load-depth nanoindentation response curves of the separator specimens were generated using three temperature variations and five nanoindentation depths. During the continual loading and unloading procedures, every nanoindentation response curve displayed periodicity, characterized by unique peak loads with varied coordinates. The findings revealed that the nanoindentation depth was positively associated with the gradually decreasing or increasing load in the response curves with different temperature shifts.

According to the experimental results for 15 sets of five cyclic nanoindentation cycles, the mean value of the separators' maximum nanoindentation depth were 1655.54, 1933.18, and 1861.26 nanometers, respectively, which corresponded to temperature variations of 0, -40, and 40 degrees Celsius.

The phenomenon showed that temperature changes during the cooling and heating processes increased the plasticity of separators and impacted their ability to withstand plastic deformation. Additionally, the separator film size appeared to depend on the nanoindentation depth, as evidenced by the elastic modulus gradually decreasing as the nanoindentation load grows.

Significance of the Study

The current study studied the mechanical property development of a lithium-ion battery separator when exposed to depth-sensitive, multicycle nanoindentation in a variable-temperature environment. Temperature changes of 40, 0, and -40 degrees Celsius were applied to the separator samples removed from the cylindrical NCR-18650B cells. The three aspects of the observations were described.

The findings revealed that the thermostatic, heating and cooling operations impacted temperature changes on the separator's elastic modulus. Also, the indentation tests confirmed that when a thermostatic procedure followed cooling or heating, the effect of temperature changes on the hardness exhibited an attenuation trend. The nanoindentation hardness also showed the influence of nanoindentation depth at various temperature fluctuations.

Thus, the impacts of temperature-induced fluctuations on the mechanical characteristics of the separator, which were essential for the safe performance of a lithium-ion battery, were investigated experimentally. 

According to the experimental findings, temperature changes caused the separators' mechanical qualities to degrade. It showed that the separator was more likely to produce mechanical deformation, which caused dendrite puncture and a potential internal short-circuit fault when it was extruded by the active particles or lithium plating inside the battery. The findings of this research could aid in determining the best separator design to increase the safety of a lithium-ion battery. 

Reference

Wang, S et al. (2022). Temperature-Shift-Induced Mechanical Property Evolution of Lithium-Ion Battery Separator Using Cyclic Nanoindentation. ACS Applied Materials & Interfaces. https://pubs.acs.org/doi/10.1021/acsami.2c11680

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