Battery temperature needs to be regulated in operation. Now, a shape memory alloy-based thermal regulator is shown to be able to automatically switch between thermally insulating and conducting states depending on temperature, demonstrating a new paradigm for thermal management of batteries.
Batteries form a key component for enabling sustainable transportation across all sectors, from passenger vehicles to heavy-duty trucking1. Enormous effort has pushed current Li-ion cells towards their theoretical limits through both material and architecture engineering. Implementation of these cells in battery packs provides additional opportunities for system optimization and more importantly, weight and volume reduction, an important requirement for electric vehicles. Thermal management is required to ensure battery packs remain within the operating-temperature window (–20 °C to 45 °C). High temperature increases the rate of electrochemical processes and thus battery capacity, but thermal runway can lead to catastrophic events such as the fires seen in hoverboards and other devices using Li-ion batteries1. Low temperature, on the other hand, usually has a negative impact on capacity, power and efficiency. The requirement to ensure a tight temperature window is a new paradigm in thermal management in contrast to merely removing heat in electronics such as microprocessors and light-emitting diodes. Writing in Nature Energy2, Chris Dames and colleagues from the United States and China demonstrate a novel thermal regulator (Fig. 1a) that passively targets this thermal sweet spot by switching heat dissipation on and off, thus improving performance and safety, and making us contemplate how thermal switches could improve other power technologies.
Improvement in volumetric and gravimetric energy density of Li-ion batteries is crucial for a variety of applications such as heavy-duty trucking, light commercial vehicles, and aviation3. Packing burden for battery packs, which represents the weight for the thermal-management systems, module hardware, battery jackets, and other non-cell inactive materials, can contribute an additional 50% to the cell mass3. Approaches towards better thermal management can provide both safety as well as volume and weight benefits. Ultrafast charging, an important goal highlighted by the US Department of Energy4 for future electric vehicles (EVs), only magnifies thermal-management requirements, as parasitic heat losses could be 2–3 times that of normal charging. Next-generation battery chemistries such as lithium–sulfur and lithium–air batteries suffer from additional overpotential losses that further increase the thermal burden5. Thus, managing thermal issues without adding non-cell mass to the thermal-management system is crucial. Most EVs today use an active thermal-management system with a liquid coolant, yet there is a keen interest in lowering the number of active control systems for EV battery packs in order to reduce their total weight and vulnerability to cyberattack.
Thermal regulators control temperature with a temperature-dependent switch that promotes heat flow in the ON state (high thermal conductance), and inhibits heat flow in the OFF state (low thermal conductance). The switch ratio (SR) defines the ratio of ON conductance to OFF conductance and is thus the major performance metric for thermal regulators. Experimental demonstrations of reversible thermal switches near room temperature are limited; a subset is summarized in Fig. 1b. In the work published in Nature Energy, Dames and colleagues create an unprecedented SR of over 2,000 by combining the exceptionally high thermal contraction and expansion that accompanies a well-known phase transition in shape memory alloys (SMAs), with a novel engineered thermo-mechanical gating mechanism that amplifies the thermal structural change to control the physical gap between the heat sink and the battery. Specifically, the researchers use the SMA as a wire that lays perpendicular to the direction of gap closure. The wire transfers its length change to act along the axis of gap closure by using 90° bends (Fig. 1a). In this way, the wire runs along the perimeter of the battery, giving it the length needed for contraction to close a macroscopic gap, as well as the contact area and small thermal capacitance needed for it to switch quickly (~1s) to keep pace with the battery temperature change. In most environments, the regulator defaults to the OFF state allowing the battery’s internal heat generation to raise its temperature to within the desired window for high performance (efficient kinetics and ion transport). At a threshold temperature set by the phase-transition temperature of the SMA, the regulator switches to the ON state, allowing excess heat to be dumped until the battery cools down.
Dames and colleagues demonstrate their thermal regulator in two ways. In the validation experiment, the thermal regulator closes a vacuum gap between two heat-flow meters (steel bars with embedded thermocouples) to measure the ON and OFF conductance. A record SR of 2,020 is possible as the conductance in the off state is extremely low (~1 Wm–2 K–1) due to the vacuum gap. For comparison, the conductance of the same 0.5 mm gap filled with air is ~50 Wm–2 K–1. They next move on to a practical demonstration where the regulator controls the temperature of a Panasonic NCR18650PF Li-ion cell during discharge. Using an SMA with a phase transition temperature in the range of 10–30 °C maintains the battery temperature within the desired operating window and a 300% higher discharge capacity is achieved for cold surroundings compared to a baseline thermal-management system. Meanwhile, the battery does not exceed the maximum allowable temperature when operated in hot surroundings.
In Fig. 1b, we summarize the speculative operating windows of various thermal switches. These windows might ultimately impact their applicability to regulate battery temperature. The optimal operating temperature, shown by the shaded grey region, is determined by two competing effects: energy in each cycle and longevity of the battery pack. Energy derived from the cell increases as temperature increases, due to reduced impedance losses while cycle life is enhanced as temperature is decreased due to lower parasitic reaction rates6. We consider the optimal operating-temperature range for the NCR18650PF cell as the window over which the energy derived is within 90% of the maximum derivable energy and the cycle life is greater than 1,000 under a 1C discharge rate (full discharge in 1 hour) and C/2 charge rate. The required switching speed would be determined by the self-heating rate associated with the battery size, chemistry, and the parasitic reactions involved. Co-optimization of the battery chemistry and switching method could lead to significant benefits in reducing the packing burden.
This demonstration by Dames and team offers hope for improved battery performance and safety and also promises opportunities for thermal switches in other power technologies including pyroelectrics, electrocalorics and thermoelectrics7,8,9. In the terminology of transistors, their thermal switch is temperature gated, which is ideal for temperature regulation in batteries. Electric10,11,12, magnetic13, hydration14, and electrochemical15 gating mechanisms that provide added control, needed for power technologies, have also been investigated in the literature. For example, pyroelectric materials, which generate power based on changes in temperature with time, need thermal switches to generate transient temperature changes from ubiquitous time-invariant heat sources. In all cases the SR is key, but for energy conversion, as opposed to thermal regulation, the parasitic energy cost of switching will also need to be considered. Opportunity exists, as the high switching speeds (<<1s) needed to enhance the power output of pyroelectrics or the cooling power of electrocalorics have not yet been achieved with high SR.
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Malen, J.A., Viswanathan, V. Regulating hot and cold. Nat Energy 3, 826–827 (2018). https://doi.org/10.1038/s41560-018-0265-2