Like a blonde girl and her porridge, a lithium-ion battery performs best when the temperature range is right—not too hot or too cold. But this is a huge limiting factor when it comes to using a lithium-ion battery in an electric In many places, cars (EVs) vary greatly in temperature. Lithium-ion batteries do not perform well under extreme high or low temperatures, which is a barrier to the transition to a wider use of electric vehicles. The authors of the study pointed out that ' 51 metropolitan areas in the United States, 20 regions typically experience extremely cold days below -18 ° C (0 ° F), while summer temperatures in 11 regions (including the top 20) often exceed 38 ° C (100 ° F) There are certainly similar temperature changes in major urban areas around the world, and this also hinders the use of electric vehicles as a potential renewable energy transportation solution.
However, in a recent paper published in Nature Energy, a team of researchers at the University of California, Berkeley, reported a new invention that is expected to effectively reduce thermal extremes when used with lithium-ion batteries. The impact of their paper, entitled 'Efficient Lithium-Ion Battery Thermal Management with a Passive Interface Thermal Regulator Based on Shape Memory Alloy,' Contemporary Operational Detailing of the Words of Landscape and Environmental Temperature Changes in Different Areas, But Also Other confounding factors, such as the new Ma Steel's three-burner battery, are further complicated by the fast charging and thermal management strategies in the dry ash recovery process. They point out that traditional linear thermal elements are often unable to take into account both extreme heat and cold conditions, while others are possible. Solutions, such as controlled fluid circuits, do not provide high enough on/off contrast, not to mention cost and weight considerations when used with electric vehicles. Their solution is 'a fluid-free, passive heat The regulator can stabilize the temperature of the battery in extreme environments of high and low temperatures. ' In the absence of any power or logic, the thermal regulator is based on The ground battery temperature switches its thermal conductivity and provides the required thermal function to maintain heat during cold and promote cooling during heat.
To achieve this, their passive thermal regulator design draws on two key nonlinear features in the existing thermal regulator concept. The first characteristic is the solid-state phase transition, which exhibits good response to temperature changes. Mutant, but does not achieve a sufficiently high switching ratio (SR) - the switching state thermal conductivity - which is the main performance indicator of the thermal regulator. The second feature is the opening and closing of the thermal interface, its SR It is much higher, but relies on differential thermal expansion between the two materials. When the interfacial gap between the materials is closed, it exhibits a strong nonlinear thermal conductance. However, since the thermal expansion effect here is relatively weak, this design requires An oversized heat regulator body to complete the opening and closing of the gap.
Although the previous examples sound complicated, their solution—which embodies two aspects of solid-state phase transitions and interfacial thermal contact conductance—is very simple. To achieve their design goals, researchers use Nitinol. Shape memory alloy (SMA). Nickel-titanium alloy is a flexible nitinol wire that runs around the edge of the top thermal regulator plate. The ends of the SMA wire correspond to each of the thermal regulators. The corner, connected to a bottom heat sink, called the Thermal Interface Material (TIM). The top and bottom plates are controlled by a set of four-way springs that create a 0.5 mm air gap between the top and bottom plates and hold the SMA wire under tension. This defines the thermal insulation disconnection state.
When the battery is heated, the SMA begins to shrink and pull the two plates closer due to the phase change. The thermal conductivity is very low until the two plates are in contact, at which point the force of the shrink line is greater than the reaction force of the bias spring, TIM The plate (bottom) contacts the thermal regulator plate (top) and begins to dissipate heat; this condition defines the ON state. The prototype model described in this paper reveals the nature of the passive interface thermal regulator.
To verify the basic principles of this concept for SMA wire and biased springs, the author of the study established a model and tested it in a vacuum chamber, using two thermocouple stainless steel bars as heat sources and hot dragging these corresponding top and bottom Plate, respectively. In the experiment, the thermal isolation in the OFF state proved to be very good, confirmed by a very large temperature discontinuity at the interface and a small temperature gradient measured at each stainless steel bar. When the rod temperature exceeds the SMA transition temperature, the gap closes and the TIM (lower rod) begins to heat up. The authors note that this conversion process is completed quickly in about 10 seconds and the SR record is achieved at 2070:1. They point out that nickel Titanium memory alloy wires must be pre-conditioned under high stress loads to produce a stable, repeatable response over many cycles.
With the establishment of proof of concept, the researchers began to demonstrate this concept in practice. Two Panasonic 18650PF LIBs were sandwiched between aluminum plates and tested in an environmental chamber. The design here uses a similar thermal regulator design to accommodate the battery. The size of the bracket, which requires a longer SMA wire length and a gap of about 1 mm between the upper and lower plates. In addition, in order to meet high levels of performance, the aerogel layer is used to isolate the wires, and the parallel heat channels of the springs and LIBs themselves are Important. To compare performance, the researchers also provided two standard linear models, 'always OFF' and 'always ON', which included replacing the SMA with stainless steel wires, which were configured between the two plates. Constant gap or constant contact.
Under the experimental conditions, from -20 ° C (4 ° F; very cold) 45 ° C (114 ° F; very hot), the thermal regulator performs well, and the climate warms rapidly from -20 ° C (4 ° F) 20 ° C (68 ° F) due to the heat retention of the battery through the air gap and increase the battery's uable factor is caused by three factors. At the other extreme, the thermal regulator also performed very well, transitioning to the country at 45 ° C (113 °F) then the temperature rise of the lyrics is limited to 5 ° C (9 ° F). In testing this thermal regulator set in the 1000 on / off cycle, the investigators found that the state performance is only slightly reduced (8.5% reduction battery The capacity is -20 ° C '4 ° F '), while the performance in the country remains unchanged.
As the authors of the study point out, when using the standard 'always on' thermal management method, the cost of their thermal regulators is minimal, which already includes a TIM heat sink. SMA and bias The additional mass of the spring is less than 1 gram, and the cost of Nitinol is about $6. 'The demonstration by a battery module consisting of a commercial 18650 lithium-ion battery shows that this thermal regulator only saves the self-produced heat of the battery. It can increase the capacity of cold weather by more than three times. ' At the same time, it can prevent the module from overheating even at a high temperature of 2 degrees Celsius.
