Researchers at the Gwangju Institute of Science and Technology in South Korea Say They've Uncovered the Key to Safer EV Batteries
Researchers at the Gwangju Institute of Science and Technology in South Korea say they have uncovered the key to safer electric vehicle batteries and energy storage devices. The Gwangju Institute of Science and Technology is a research-oriented university focused on science and technology fields.
More specifically, the scientists at the institute have shed light on critical changes in the thermal properties of energy storage devices during operation, paving the way towards better thermal management of electric vehicle batteries or other energy storage devices.
Effectively managing the thermal properties of energy storage devices like EV batteries is the key to avoiding thermal runaway, which can lead to a fire.
Much of the focus in the auto industry is the development of lithium-ion battery packs that can deliver longer ranges. While range is a big selling point for automakers, thermal management of an EVs battery pack is vital to the actual safety of the battery pack in an EV, as well as its occupants.
The researchers said they've uncovered key changes to the thermal properties of electric double-layer capacitors (EDLCs) during charging and discharging, which will help in the development of future thermal management strategies.
EDLCs, are also known as "supercapacitors" or "ultracapacitors" and help ensure quality and short-term reliability in power systems. EDLCs can charge and discharge energy very quickly and are becoming increasingly important to meet the battery storage needs of electric vehicles. Like a conventional capacitor that stores energy, the electricity in an EDLC is stored in the electrical field between separated plates.
Modern energy storage devices, such as supercapacitors and batteries, have highly temperature-dependent performance. If a device gets too hot, it becomes susceptible to a condition known as "thermal runaway".
Thermal runaway occurs when a battery cell's temperature rises rapidly, which can occur in just milliseconds. If this occurs, the energy stored in the battery is released very suddenly causing a chain reaction that creates extremely high temperatures inside a vehicle's battery pack of roughly 750 degrees Fahrenheit or 400 degrees Celsius.
Thermal runaway can result in explosions or fires. For automakers, adopting an optimum thermal management strategy is necessary for the safe operation of electric vehicles.
To do this, it is important to understand how an EV battery's thermal properties, like heat capacity (Cp), dynamically change during charging and discharging. Heat capacity is a substance's ability to store thermal energy. In scientific terms, It equals the amount of heat that must be added to one gram of the substance in order to raise its temperature by one Kelvin.
EV batteries tend to heat up when charging, so automakers like Tesla developed battery cooling systems. Tesla batteries for example, are cooled by a heat exchanger that circulates liquid coolant around the battery cells. This keeps them at the optimum temperature for the best performance.
As an EV is being charged, the battery begins to heat up as it begins to store energy. The average temperature of a fully charged Tesla battery is 104 °F (40 °C), making it more efficient for storing electricity, as colder temperatures affect battery performance and discharge rates.
The researchers from the Gwangju Institute of Science and Technology investigated the thermal properties of EDLCs as a technical foundation for thermal measurement. The team said it revealed significant new information in the process.
EDLCs are used to store energy from regenerative braking and to provide the necessary power for quick acceleration when the driver demands it. They can charge and discharge energy very quickly. The supercapicitors serve to increase the overall efficiency of an electric vehicle.
Using the 3ω hot-wire method, which is a scientific method used to measure the thermal conductivity of a material, the researchers were able to measure the change in heat capacity of EDLCs in real-time with a microscopic amount of electrode-electrolyte volume to test for the adsorption (where ions adhere to a conductive material) and desorption (where ions are drawn into a material), explained Prof. Jae Hun Seol, who led the study.
The research team conducted these experiments both under static conditions and during charging cycles. They found that the temperatures of the positive and negative electrodes in the experiment changed by 0.92% and 0.42% during charging, which corresponded to 9.1% and 3.9% reductions in their respective heat capacities, which in theory, can help reduce the chance of thermal runaway of a battery that could lead to a fire.
"According to thermodynamic theory, the ionic configuration entropy (a measure of randomness) of a system decreases during charging. This also affects the free energy of the system. Together, this leads to a decrease in Cp (heat capacity)," explained Prof. Seol. The team also varied the concentration of the potassium hydroxide electrolyte, to see how it affects EDLC performance.
The researchers found that the EDLC displayed maximum capacitance, which is the ratio of the electric charge stored on a capictor's conductor to a difference in its electric potential, along with a reduction in heat capacity when the electrolyte concentration was increased. The research team attributed the variations in the concentration of the electrolyte and how it affects ionic mobility, which is the speed at which ions move through an electric field.
"An important aspect of this study is that charging and discharging also alters the heat capacity (Cp) of EDLCs," said Prof. Seol. "These findings will extend our understanding of the underlying thermal physics of EDLCs. Indeed, these results can be considered a major step towards future effective thermal management strategies, which will create safer and more reliable energy storage devices."
The findings of the study were first made available online in February. The full report will be published in Volume 188, Issue 122632 of the "International Journal of Heat and Mass Transfer" on June 1.
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