Automatic sprinklers, lighting, and security systems have been common in homes and offices for several decades. However, in recent years they have become increasingly efficient at anticipating user needs and optimizing their performance due to their ability to communicate information over wireless networks. Known as the Internet of Things (IoT), this emerging phenomenon describes how interconnected devices analyze data with each other without human intervention to improve convenience.
And as the IoT and robotics industries continue to grow and evolve, the demand for smaller, higher-energy-density batteries to power these connected devices will increase. This has led to the development of micro batteries, miniature batteries that can power small IoT devices such as sensors, smart wearables, drones and small robots.
cents todaymet withJames Pikul, Assistant Professor at theFaculty of Mechanical Engineering and Applied MechanicsinsideSchool of Engineering and Applied Sciencesto discuss the challenges engineers face in manufacturing the next generation of micro batteries and how the development of new configurations can enable smaller dimensions and greater energy storage.
Probleme beim Downsizing
Pikul's laboratory develops and works to improve micro batteries. A major hurdle in battery design for tiny devices is that batteries suffer from energy storage and performance deficiencies as their dimensions are reduced, explains Pikul. "Energy scales with volume, which is length times length times length, which is length in cubes," he says.
“If you want to make a battery 10 times smaller in every dimension, it now has 10 times 10 times 10 (thousand times) less energy. That is one of the biggest challenges.”
The packaging materials used to house the batteries are also a major barrier to scaling.
"The electrochemical reactions that batteries use to generate electricity are sensitive to contamination, so they have to be enclosed in hermetically sealed packaging material," says Pikul.
He explains that packaging materials need to be of a certain thickness to prevent leakage and contamination, but as batteries get smaller, the thickness of the materials can add unwanted bulk and bulk. Pikul compares this limitation to packing a laptop instead of a phone with the same packaging materials.
"In terms of overall volume, the box-to-laptop ratio is much smaller than the box-to-phone ratio if the box is the same thickness," he says. "Making really small packages, using battery materials that store a large amount of energy per unit volume (energy density), and creating new architectures that optimally organize these materials in 3D space is the way we're trying to go." to overcome these problems.”
Unpacking the energy storage problem
In 2019, as part of a project funded by DARPA's Short-Range Independent Microrobotic Platforms (SHRIMP) initiative, Pikul and his collaborators began to address some of the limitations of packaging to develop small, insect-sized robots that could be used in search and rescue operations .
He and his colleagues published aPapier2021 in the magazineadvanced materialsentitled "A near-package-free design paradigm for lightweight, high-performance, and energy-dense micro-primary batteries." They proposed a new way to reduce the size of packaging material by incorporating it into battery components known as current collectors. "So our arrangement uses the current collectors to transport electrons quickly while avoiding contamination of the battery," says Pikul. "That way, instead of having two materials each doing one thing, we have this material that does two things."
These batteries also used a new cathode architecture that increases their energy density. The cathode is the part of the battery that accepts electrons during electrochemical reactions, says Pikul. "We made this new type of cathode, which we call 'fully dense', which means that the material that accepts electrons occupies almost the entire volume of the cathode, so it stores more energy per unit volume."
He compares the old cathodes to the type his team developed, describing how a bowl of small potatoes occupies a different volume than a bowl of a large hunk of corned beef. “In small potatoes, the arrangement is not as dense, which means that, for example, water has more room to penetrate, unlike corned beef, which is too dense for water to penetrate. Can you tell I just had lunch? he jokes
Pikul and his team found that this fully dense cathode not only offers an advantage in terms of higher energy density, but also improves the battery's power density.
"Typically when making batteries, there's a trade-off between energy density and power density, where energy density is the amount of juice in the tank and power density is the speed of your car, or how fast you can charge your car," he says. "By developing this fully dense array, we were able to utilize another quality similar to corned beef: 'muscle fibers'."
He describes the muscle fibers or granules as a type of channel through which ions can flow much faster. Battery makers have traditionally preferred the baby potato-shaped arrangement because ions had to travel a shorter distance between particles than when traversing the cross-section of a large, dense piece of corned beef, he says. However, with their cathode arrangement, the team found a way to align the grains so that the ions can move quickly in specific directions. This allowed the batteries to take advantage of the higher energy density (corned beef) architecture while maintaining high power density.
Go one step further
In another SHRIMP project, Pikul worked with the same group and longtime collaborator Paul V. Braun of the University of Illinois Urbana-Champaign, who led the research. Once again, the team combined the fully dense cathodes with packaging material, but their goal was to increase the voltage of the micro-batteries so the tiny robots could travel farther on a single charge.
"If you remember the energy density or the amount of energy you have in the tank for a single ride, let's say it's about 100 gallons, and that's the amount of energy I can store," says Pikul. “If I have to do a lot of rides now, I would have to fill up the tank many times. But with batteries, topping up the tank degrades performance and so I get less utility and efficiency every time I fill up.”
Pikul explains that this is a common feature in battery-powered devices with long lifecycles, such as cell phones. They need to be recharged over and over again, but this reduces the amount of energy they can deliver with repeated recharges and uses. "Usually the way to get around this is to limit the amount of energy it can draw so it doesn't go to zero every time, with the idea of having something left over to prevent wear and tear from repeated dumping." So most batteries only use 50 gallons per discharge instead of the full 100 gallons.”
What Pikul and his colleagues were able to achieve in this SHRIMP research published in the journalCell Reports Physical Sciences, is to use some more energy stored in the cathodes used for battery materials. Pikul notes that humans don't typically do this because it sacrifices the ability to charge the battery, which would make it impossible for a device you use every day like a phone, but would be ideal for robots or drones that do it only need to be used once. For example, a robot that explores and collects information in environments that are too dangerous or too big for humans to traverse.
"Since we only use the battery once, we can get as much energy out of a single performance as possible."
In a newer onelearn, Pikul and his colleagues faced similar challenges in making batteries for insect-sized robots, but they approached the battery architecture in a different way. Instead of a lithium-ion battery, they used zinc-air electrodes and worked on ways to reduce the size of a battery component known as the electrolyte, which is like a bridge between the positive and negative poles.
"Metal-air batteries have been around for a while and are commonly used in hearing aids," says Pikul. "Metal-air batteries are unique in that they use the air around us as a cathode, making them much lighter and able to store more energy per unit mass."
However, he cautions that this ability is a double-edged sword, as engineers must now be concerned with controlling the interaction between the precise chemistry inside the battery and the open environment, which can lead to electrolyte problems.
Efforts to harness these high energy densities have focused primarily on improving the electrodes rather than the electrolyte, explains Pikul. So he and his team set out to create a new type of slimmer hydrogel electrolyte solution that could also withstand outdoor air pollution. System. "People typically use potassium hydroxide as an electrolyte, but it tends to react with CO2 in the air," he says.
"So we took a step back and looked at the conventional chemistry behind the problem and proposed an electrolyte solution that contains the best elements of potassium hydroxide, namely its ability to be very thin compared to other solid electrolytes and how good it is at moving." . ions, but avoid contamination by adding a few more compounds at different concentrations.”
Pikul clarifies that this electrolyte solution is not a panacea for metal-air batteries and that there is still a long way to go before these batteries become commonplace in larger devices such as mobile phones. However, he says it still paves the way for the development of micro batteries for small robots and drones.
"For our use cases where the robot or device needs to run at full capacity for a few weeks or days, the ways we can reduce size and increase throughput present new opportunities for these devices," says Pikul.