EV Battery Pack Development

Image 1: Electric Vehicle Battery Pack

Improving the performance and safety of EV batteries is what a successful future for all looks like. Currently, lithium-ion batteries (LIB) are the most dominant EV battery on the market. LIB packs can be divided into four key components- including the cathode, anode, electrolyte, and separators. Understanding the thermophysical properties of battery pack components is fundamental for optimizing battery performance and avoiding catastrophic thermal runaway events. Analysis techniques including TGA, DSC, TMA, DMA, and thermal conductivity measurement can illustrate whether all battery components work in harmony and confidence to manufacturers and end users alike.

Thermogravimetric Analysis (TGA)

TGA is a technique that allows the observation of sample mass variation and composition in a set environment over a temperature range. Key parameters include mass changes caused by phenomena such as oxidation and thermal degradation, as well as glass transition. TGA is an excellent tool for insights into battery thermal stability. Shakoor et al studied the thermal stability of sodium hexafluoro ferrate, Na₃FeF₆, as a potential cathode (Shakoor, et al., 2012). A Setaram Themys TGA was used to measure the weight loss of the material when heated from room temperature to 500°C.

Figure 1: TGA curve for cathode material (Shakoor, et al., 2012)

Results demonstrate 2% mass is lost when the cathode is heated to 500°C. This indicates that when even heated to extreme temperatures (well above operating conditions), the cathode material does not decompose, and is thus thermally stable and safe.

Differential Scanning Calorimetry (DSC)

DSC measures the heat flow of an environment while it is held isothermal or increased or decreased in temperature. DSC can measure the heat capacity, the heat of reaction, and phase properties of a material such as melting temperature and glass transition. Heat capacity is an important parameter in the battery thermal management space, as it is a measure of the ratio of the heat added to an object to the affected temperature change. Loos et al. mapped the heat capacity of lithium iron phosphate (LiFePO₄), a cathode material often used in LIBs (Stefan Loos, et al., 2015).

Figure 2: Cp versus temperature curve of cathode (Stefan Loos, et al., 2015)

Dynamic Mechanical Analysis (DMA)

DMA measures a material’s response to stress and frequency as a function of time and temperature. One of the main parameters obtained via DMA testing is storage modulus- a measure of a material’s ability to store energy elastically. A higher storage modulus means a material will have a spring-like reaction to stress, while a lower storage modulus means it will deform and not bounce back.

Lufrana et al. used a Metravib DMA to measure the compressive stress-strain behavior of fuel cell membranes and were able to identify which candidate material exhibited the highest storage modulus over the operational temperature (Lufrano, Simari, Enotiadis, & Nicotera, 2022).

Figure 3: Storage modulus versus temperature of membranes (Lufrano, Simari, Enotiadis, & Nicotera, 2022)

Their four candidate materials (red, black, blue, and orange) were compared to Nafion 212. The materials exhibited a higher storage modulus, indicating they will retain their shape and resist deformation in response to the stresses they endure over their lifecycle. The study of these membranes in fuel cells is analogous to that of battery separators in EVs. Battery separators are used in EVs to help with thermal management and act as damping agents. The battery separators will be subject to a plethora of vibrations throughout their lifetime, and thus understanding their structural integrity is key. Should a separator material fail, an internal short circuit could develop and lead to a potential thermal runaway event.

Thermomechanical Analysis (TMA)

TMA examines a material’s deformation over temperature and time by measuring its shrinkage or expansion. Thermal stresses induced by the heating cycle of batteries in the charge-discharge cycle can lead to the expansion of cell components and cause accelerated degradation of materials. It is important to measure the expansion of EV battery materials over the operating temperatures to understand how the materials will deform over temperature cycles. Touloukian et al. studied the thermal expansion of aluminum over ambient temperature to 300°C with a Rigaku Thermo plus EVO2 TMA8311 (Touloukian, Kirby, Taylor, & Desai, 1975).

Figure 4: Thermal expansion plot for Al sheet (Touloukian, Kirby, Taylor, & Desai, 1975)

The sample demonstrates a linear thermal expansion with temperature. The maximum linear expansion was approximately 0.723%. Al is a common anode material for LIBs, and this result confirms the material is safe to use over a large temperature cycle without a worry of significant expansion. A large CTE in EV battery materials is undesirable, as it leads to mechanical degradation and capacity fade.

Thermal Conductivity

Thermal conductivity is an essential property to understand for EV battery thermal management. Choosing materials with high thermal conductivity can improve heat dissipation in a battery preventing overheating and eventually thermal runaway. C-Therm’s MTPS sensor can measure a wide range of materials; from solids, liquids, and pastes, so that the thermal conductivity of all battery components can be measured. The MTPS sensor can also measure materials under applied forces, to replicate real-world scenarios. C-Therm measured the thermal conductivity of thermal gap pad materials under various forces.

Figure 5: Gap pad thermal conductivity versus applied force

Thermal conductivity is increased as applied force is. The thermal conductivity with 2000 g applied force is approximately 25% greater than that under 75 g applied force. Measuring under true conditions can enhance our understanding of the material’s behavior which would have otherwise gone unnoticed.

Metal foils are another material commonly used in battery applications. Foils are a challenging material to measure since they often deform under their own weight. Their thinness also makes them invalid to test via transient methods, which fail to account for barriers in the through-plane direction. Since thermal conductivity is a key performance attribute for foils used in electronics, it is important in the design in quality control phases to characterize the thermal conductivity to ensure no impurities and variations are introduced.

Figure 6: TPS slab results for Al foil compared to the literature value

C-Therm’s TPS slab testing utility was used to measure the thermal conductivity of aluminum foil. The slab utility provides results within 0.9% of the expected literature value, confirming this method is accurate for thin foil applications.

Image 2: TPS slab with foil setup (bottom half)

TAL: Your EV Testing Experts

Thermal Analysis Labs (TAL) offers a full range of contract testing and expertise in TGA, TMA, DMA, DSC, and thermal conductivity measurement. TAL can perform any of the above-mentioned tests and more. For a full overview of our services, consult our 2023 contract testing catalog. Have a question? Reach out to one of our thermal analysis experts to start the discussion.

Bibliography

Lufrano, E., Simari, C., Enotiadis, A., & Nicotera, I. (2022). Sulfonated Polyether Ether Ketone and Organosilica Layered Nanofiller for Sustainable Proton Exchange Membranes Fuel. Applied Sciences.

Shakoor, R., Lim, S. Y., Kim, H., Nam, K.-W., Kang, J. K., Kang, K., & Choi, J. W. (2012). Mechanochemical synthesis and electrochemical behavior of Na3FeF6 in sodium and lithium batteries. Solid State Ionics, 35-40.

Stefan Loos, D. G.-H., Seidel, J., Hüttl, R., Wolter, A. U., Bohmhammel, K., & Mertens, F. (2015). Heat capacity (Cp) and entropy of olivine-type LiFePO4 in the temperature range (2 to 773)K. The Journal of Chemical Thermodynamics, 77-85.

Touloukian, Y. S., Kirby, R. K., Taylor, R. E., & Desai, P. D. (1975). Thermal Expansion on Metallic Elements and Alloys. Thermophysical Properties of Matter – the TPRC Data Series. Volume 12.

About the Author

Lauren Hunter, Technical Sales Specialist

Lauren works in technical sales with the SETARAM product line and is working towards her professional engineer designation. She holds a Bachelor of Science in Chemical Engineering with the Nuclear Power Option from the University of New Brunswick.