LR03 battery

New invention could help LR03 battery in extreme weather conditions

Like Goldilocks and her porridge, lithium-ion batteries (LIBs) perform best when they are in the right temperature range (not too hot, not too cold). But this is a huge limiting factor when using LIBs in electric vehicles (EVs), where temperature variations are drastic. LIBs don’t perform well in extremes of high or low temperatures, a barrier to their wider adoption by EVs. As the study’s authors point out, “of the 51 metropolitan areas in the United States, 20 areas routinely experience extreme cold in the winter, with outdoor temperatures below -18°C (0°F), while 11 areas (including the previous 20) regularly experience summer temperatures exceeding 38°C (100°F)”. Similar temperature variations exist in major urban areas around the world, and again, this is a barrier to EVs becoming a potential renewable energy transportation option.

However, in a recent paper published in the journal Nature Energy, a team of researchers at the University of California, Berkeley, reports a new invention that, when used with LIBs, can effectively mitigate the effects of extreme temperatures. The paper, titled "Efficient thermal management of Li-ion batteries with a passive interfacial thermal regulator based on a shape memory alloy," details the actual operation of lithium-ion batteries in terms of ambient temperature variations at different locations, and also takes into account multiple factors, such as fast charging and discharging batteries, which also complicate thermal management strategies. The researchers point out that traditional thermal linear components cannot usually handle both hot and cold extremes at the same time, and other potential solutions, such as controlled fluid loops, do not provide a high enough on/off contrast, not to mention cost and weight considerations for electric vehicle use. The researchers' solution is "to use a fluid-free, passive thermal regulator to stabilize battery temperature in hot and cold extreme environments. In the absence of any power source, the thermal regulator can switch its thermal conduction based on the local battery temperature and provide the required thermal function, retaining heat when cold and promoting cooling when hot." To achieve the desired effect, their passive thermal regulator design borrows two key nonlinear features of existing thermal regulator concepts. The first of these features, solid-state phase change, exhibits good abruptness to temperature changes, but fails to achieve a sufficiently high switching ratio (SR), the ratio of thermal conductivity between the on/off states, which is the main performance indicator of thermal regulators. The second feature, the opening and closing of the thermal interface, has a high SR, but relies on the differential thermal expansion between the two materials. When the interface gap between the materials closes, it exhibits strong nonlinear thermal conductivity. However, because the thermal expansion effect here is relatively weak, this design requires an oversized thermal regulator body to complete the opening and closing of the gap.

Although the previous example sounds complicated, the method to achieve the solid-state phase change and the conductive aspects of the interface thermal contact is very simple. To achieve the researchers' design goals, the researchers relied on a shape memory alloy (SMA) - a flexible nickel/titanium alloy wire made of nickel-titanium alloy, wrapped around the periphery of the top thermal regulator plate where the LIB is placed. The two ends of the SMA wire, corresponding to each corner of the thermal regulator, are connected to the bottom heat sink plate, called thermal interface material (TIM). The top and bottom plates are held relative to each other by a set of four bias springs, creating a 0.5mm air gap between them and also holding the SMA wire in tension. This is known as the thermal isolation OFF state.

As the battery heats up, the SMA undergoes a phase change and begins to contract, bringing the two plates closer together until the two plates touch, at which point the force of the wire contraction is greater than the opposing force of the bias springs, and the TIM (bottom) remains in contact with the thermal conditioning plate of the battery (top), beginning to dissipate heat; this condition is known as the ON state. The prototype model described here is the essence of a passive interface conditioner.

To verify the basic principles of this concept regarding the SMA wire and bias springs, the researchers built a model and tested it in a vacuum chamber using two thermocouple stainless steel rods as heat sources and a heat sink - these correspond here to the top and bottom plates, respectively. In this experiment, the thermal isolation in the OFF state proved to be excellent, as evidenced by the very large temperature discontinuity at the interface and the small temperature gradient measured in each stainless steel rod. However, when the upper rod temperature exceeds the SMA transition temperature, the gap closes and the TIM (lower rod) begins to heat up significantly. The authors note that the switching process here occurs rapidly, in about 10 seconds, and that an SR of 2.070:1 is achieved. They note that the Nitinol SMA wires must be preconditioned at higher stress loads before they can be relied upon to produce a stable, repeatable response through multiple cycles.

With proof of concept established, the researchers set out to demonstrate the concept in practice, with two Panasonic 18650PFLIBs sandwiched between aluminum plates and tested in an environmental chamber. A similar thermal regulator design was used here, modified to accommodate the size of the battery in the holder, with a longer SMA wire length and a gap of about 1mm between the top and bottom plates. In addition, isolation of the parallel thermal paths of the wires and springs, as well as the LIBs themselves, from the aerogel blanket was critical to meet the high level of performance. To compare performance, the researchers also provided two standard linear models, “always off” and “always on,” where the SMA wires that maintain a constant gap or constant contact between the two plates are replaced with stainless steel wires.

The thermal regulator performed well under experimental conditions ranging from -20°C (-4°F; very cold) to 45°C (114°F; very hot), ramping quickly from -20°C (-4°F) to maintain temperatures around 20°C (68°F), increasing the battery’s service factor by a factor of three due to increased cell heat generation due to the retained air gap. At the opposite extreme, the thermal regulator also performed well, transitioning to the ON state at around 45°C (113°F), after which the temperature rise in the LIB was limited to 5°C (9°F). After testing the thermal regulator through 1,000 on/off cycles, the researchers found that OFF-state performance was slightly degraded (8.5% reduction in battery capacity at -20°C [-4°F]) while ON-state performance remained unchanged.

As the study authors note, their thermal regulator is low cost when using standard “always-on” thermal management approaches, which already include a TIM heat sink. The added mass of the SMA and bias spring is less than 1 gram, and the cost of the nitinol wire is about $6. "Demonstrations using battery modules composed of commercial 18650 lithium-ion cells show that the thermal regulator can more than triple the cold-weather capacity by simply retaining the heat generated by the cells themselves, while also preventing the module from overheating, which can occur even at high 2C discharge rates," the researchers concluded.

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