A research team at Georgia Institute of Technology has designed a new metal for solid-state batteries that enables operation at lower pressures. While lithium metal is often used in these batteries, the team – the research group of Matthew McDowell, professor and Carter N Paden Jr distinguished chair in the George W Woodruff School of Mechanical Engineering and the School of Materials Science and Engineering – discovered that combining lithium with softer sodium metal results in improved performance and novel behaviour.
Lithium-ion batteries power everything from electric cars to laptops to leaf blowers. Despite their widespread adoption, lithium-ion batteries carry limited amounts of energy, and overheating can lead to safety concerns. Consequently, for decades, researchers have sought a more reliable battery.
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Solid-state batteries are less flammable and can hold more energy, but they often require intense pressure to function. This requirement has made them difficult to use in applications, but the new research could change that.
Lithium-ion batteries have been the industry standard because they combine compact size, reliability and longevity. However, they contain a liquid electrolyte that helps lithium ions move in the battery but is also flammable. In solid-state batteries, this electrolyte is a solid material that’s less flammable. The challenge is that when the battery is used, the lithium metal changes its shape, potentially losing contact with the solid electrolyte, which degrades performance. A common way to ensure the metal doesn’t lose contact is to apply high pressure to these batteries.
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‘A solid-state battery usually requires metal plates to apply this high pressure, and those plates can be bigger than the battery itself,’ McDowell said. ‘This makes the battery too heavy and bulky to be effective.’
The researchers, led by Georgia Tech research scientist Sun Geun Yoon (pictured above), sought a solution. The solid-state batteries would still require some pressure to function, but they found that by also using a softer metal, less pressure is required. The researchers decided to pair the commonly used lithium metal with a surprising element: sodium.
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‘Adding sodium metal is the breakthrough,’ McDowell noted. ‘It seems counterintuitive because sodium is not active in the battery system, but it’s very soft, which helps improve the performance of the lithium.’
How soft can sodium be? In a controlled environment, a person could stick their gloved finger into sodium metal and leave an imprint.
To understand the enhanced performance of their battery, the researchers borrowed a concept from biology called morphogenesis. This concept explains how tissues or other biological structures evolve based on local stimuli. Morphogenesis is rarely seen in materials science, but the researchers found that the combination of lithium and sodium behaves according to this concept.
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McDowell’s research group has been working on applying morphogenesis to battery materials as part of a project funded by the Defense Advanced Research Projects Agency in collaboration with several other universities. Their battery is among the first viable demonstrations of this concept –the sodium deforms readily at the low pressures needed for solid-state batteries to function.
The possibilities of a viable, smaller solid-state battery are vast. Imagine a phone battery that could last much longer or an electric vehicle that could drive 800 kilometres between charges. With this in mind, McDowell and his team have filed for a patent for the battery system.
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While solid-state batteries still have some way to go before commercial use, results such as these could mean that solid-state batteries can compete with lithium-ion. McDowell’s lab continues to experiment with other materials to further improve performance.
The research has been published in Science.