By Dr. Daniel Choi, Associate Professor of Mechanical and Materials Engineering, Khalifa University of Science and Technology
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Batteries are ubiquitous in our daily lives: they power everything from cell phones to cars, homes, aircraft and even spaceships. They are critical in enabling the next generation of electric vehicles and large-scale stationary energy storage. Yet, despite rapid advances in battery performance, battery applications are also advancing rapidly and this is creating the need for even higher performing batteries.
What is the future of battery development? Can we make better batteries that can store more energy, safely?
I am leading a team of researchers at Khalifa University looking to achieve just that in an affordable and scalable manner by advancing a paper-thin, flexible lithium-ion battery with all the energy density and safety features necessary for space applications, but which will also have huge implications for robotics and emerging Internet of Things (IoT) technologies in the post-Fourth Industrial Revolution world.
In most batteries, a liquid electrolyte allows electric charge, or ions, to flow between the electrodes of a battery. When ions are collected at either electrode, energy is stored and when ions are released, electrons flow as well to provide electricity
In current battery designs, energy limits are set by the number of ions each electrode can hold – this determines the battery’s energy density – which is the result of the chemical reactions taking place within the electrodes and at the electrode and electrolyte interfaces. With lithium-ion, today’s most commonly used rechargeable battery chemistry, the upper limits for energy density are somewhere around five times the current performance for realistic designs. In other words, the energy density of today’s lithium-ion battery chemistries can be increased significantly but is nonetheless limited.
Another limiting factor for conventional lithium-ion batteries is the issue of flammability. Non-solid electrolytes are not particularly stable at very high temperatures. These flammable electrolytes can cause batteries to explode. There are a few reasons a battery bursts into flames: most commonly, too much heat or bad battery design makes the electrolytes react in such a way that an uncontrolled positive feedback loop called ‘thermal runaway’ occurs and leads to a fire.
Solid electrolytes improve a lithium-ion battery’s thermal and mechanical stability at high temperatures, greatly reducing the risk of explosion. However, it is harder for ions to move through solids than through liquids, which makes a solid electrolyte an interesting materials science and engineering challenge.
My team has come up with a novel battery design that addresses both of these performance limitations: A lithium battery that employs thin-film electrodes made from carbon nanotubes-based nanocomposites sandwiching a thin-film polymer-based solid electrolyte, which is designed for enhancement of ion transport. Essentially, our battery is a flexible, lightweight, ultra-thin sheet with a myriad of potential applications.
Carbon nanotubes (CNTs) are tiny cylindrical tubes made of tightly bonded carbon atoms, measuring just one atom thick. A CNT’s cylindrical and porous shape contributes to its large surface area, which allows it to potentially hold more ions and maintain better contact with the electrolyte that current electrodes.
The super-thin CNT-based nanocomposite anodes and cathodes mean our battery can also be super-thin and functional. The flexibility of the nanocomposite material means it can be shaped to fit any odd or underutilized space, and the large surface area means any heat produced is easily dissipated.
Our research stemmed from space application considerations, which meant we had to develop the technology appropriate for a space-faring craft. Spacecraft have important requirements including lowering the weight of the battery and ensuring safety – no one wants a battery failure in orbit. Current batteries in spacecraft comprise much of the weight of the overall craft, using very heavy materials (metal and ceramic) for the electrodes and current collectors, so reducing the weight was our primary concern.
The new battery weighs less than 20 percent of the weight of a traditional battery and while you’d expect a large trade-off in energy, for the same volume of a traditional battery, ours offers approximately 90 percent the same energy. A patent resulting from our research is now pending with the US Patents Office.
Spacecraft have all sorts of left-over space into which our batteries could be easily folded. You could layer them into areas traditionally reserved for rectangular, blocky batteries in the next Mars rover, or layer them on unmanned aerial vehicle (UAV) surfaces to maximize space and power available to said UAV. Our ultra-thin batteries can also expand functionality for a broad range of electronic products, including sensors and other technologies at the heart of IoT systems.