Longer Life Implantable Batteries

Longer Life Implantable Batteries
Longer Life Implantable Batteries - The new type of battery is completely depleted in a few days in the accelerated photo series. The color of the battery cell darkens as it depletes, because the new "catholyte" substance inside the battery cell undergoes a chemical transformation, turning it into a reddish compound. Credits: Courtesy of photo scientists

With a new strategy, pacemakers and other medical devices, long-range drones and remote sensors will need fewer battery replacements.

Rechargeable lithium-ion batteries have been the main focus of battery development for the past few decades, used in everything from electric cars to portable devices, and have improved significantly in price and capacity. However, despite their critical importance in many important uses such as implantable medical devices such as pacemakers, non-rechargeable batteries have not made much progress during this time.

Now, MIT researchers have found a technique to increase the energy density of these primary or non-rechargeable batteries. They claim that for a given level of power or energy capacity, this technique can provide up to a 50% increase in lifespan or a proportional reduction in size and weight, while also increasing safety at little or no additional expense.

In a paper published in the Proceedings of the National Academy of Sciences, MIT Kavanaugh Postdoctoral Fellow Haining Gao, graduate student Alejandro Sevilla, associate professor of mechanical engineering Betar Gallant, and four other researchers from MIT and Caltech suggested that the typically inactive battery electrolyte for energy distribution explains the new findings, which include replacing it with a material that is active.

According to Gallant, any improvement in the battery life of pacemakers or other medical implants can significantly affect a patient's quality of life because battery replacement for these devices requires surgery. Because primary batteries have about three times the capacity of rechargeable batteries for a given size and weight, they are used for such important applications.

Because of the difference in capacity, primary batteries are “necessary for applications where charging is not possible or impractical,” according to Gao. The new materials operate at body temperature, making them suitable for use in medical implants.

Applications can also include sensors in shipment monitoring devices to ensure that temperature and humidity requirements are properly maintained throughout the shipping process, for example, for food or drug shipments, in addition to implantable devices with further development to ensure batteries operate efficiently at cooler temperatures.

They can also be applied to remotely operated air or submarine vehicles that need to be kept ready for deployment for extended periods of time.

Pacemakers' batteries usually last five to ten years, and much less if they have to perform high-voltage tasks such as defibrillation. However, according to Gao, the technology for these batteries is thought to have matured, and "no significant progress has been made in basic cell chemistry during the previous 40 years."

The electrolyte is the material that sits between the two electrical poles of the battery, the cathode and the anode, and allows charge carriers to pass from one side to the other. This new type of electrolyte is key to the team's breakthrough. By using a new liquid fluorine, scientists have discovered that they can combine some of the functions of cathode and electrolyte into a single compound known as the catholyte.

While there are other substances besides this new compound that could theoretically play a similar catholyte role in a high-capacity battery, Gallant explains that these substances have lower inherent voltages, unlike the substances that remain in a typical pacemaker battery or CFx. Extra capacity will be wasted due to voltage mismatch. Because the total output of the battery cannot be more than the sum of the two electrode materials. But Gallant says with the new substance, “one of the key advantages of our fluorinated liquids is that their voltages align extremely well with the CFx.”

The liquid electrolyte of a standard CFx battery is very important because it allows charged particles to move freely between the electrodes. But according to Gao, "these electrolytes are actually chemically inactive, so they're essentially dead weight." This shows that the main active component of the battery, the electrolyte, contains approximately 50% of inactive material. However, Gao claims that with the new design using fluorinated catholyte material, the amount of dead weight can be reduced to roughly 20%.

According to Gallant, the new cells also offer safety advantages over other types of proposed chemicals, as their formulations are free of dangerous and corrosive catholyte elements. He also adds that preliminary tests have shown that the batteries have a stable shelf life of more than one year, a very important attribute for mainstream batteries.

The team has yet to experimentally achieve the full 50% increase in energy density predicted by their calculations. According to Gallant, they showed a 20% improvement, which would be a significant improvement for many applications. Although the cell's design has not yet been fully optimized, the researchers are able to predict the cell's performance based on the performance of the active material. When scaled up, “we can see that the predicted performance at the cell level could be about 50% higher than the CFx cell,” he says. The team's next goal is to reach this level experimentally.

Sevilla, a PhD candidate in mechanical engineering, will focus on this task next year. “I was brought into this research to try to understand some of the limitations of why we weren't able to achieve the maximum imaginable energy density,” he says. “My role has been to try to understand the shortcomings that underlie the reaction,” the author says.

According to Gao, one of the key advantages of the new material is that it can be easily incorporated into existing battery manufacturing processes by simply replacing one material with another. Gao reports that preliminary discussions with manufacturers support this relatively simple change. Gao claims that the basic raw material currently used for other reasons has already been scaled up for production and is priced similarly to the raw materials currently used in CFx batteries. According to him, the price of batteries made from the new material will likely be similar to batteries currently on the market.

The team has already filed a patent for the catholyte, and they predict medical uses will likely be the first to be commercialized, perhaps with a full-scale prototype being tested in real devices in about a year.

According to the researchers, future uses for the new materials could include extending the lifespan of devices such as remotely readable smart gas or water meters or EZPass transponders. It may take longer to generate power for drones or underwater vehicles because they will need more power. Batteries can be used for equipment used in remote locations such as oil and gas drilling rigs, as well as devices lowered into wells to monitor conditions.

source: news.mit.edu


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