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Solid-state sodium ion batteries are much safer than conventional lithium-ion batteries, which pose a fire and explosion hazard, but their performance has been too small to offset the safety benefits. The researchers announced Friday that they have developed an organic cathode that significantly improves stability and energy density.
The performance improvement, reported in the newspaper Joule, is linked to two main conclusions:
- The resistive interface between the electrolyte and the cathode, which usually forms during the cycle, can be reversed, extending the life of the cycle.
- The flexibility of the organic cathode allowed it to maintain intimate contact at the interface with the solid electrolyte, even when the cathode is dilated and contracted during the cycle.
Yan Yao, an associate professor of electrical engineering and computer science at the University of Houston and corresponding article author, said the organic cathode – called PTO, for pyrene-4,5,9, 10-Tetraone – offers unique advantages over previous inorganic cathodes. But he said the underlying principles are just as important.
"We have discovered for the first time that the resistive interface that forms between the cathode and the electrolyte can be reversed," Yao said. "This can contribute to the stability and longevity of the cycle." Yao is also senior researcher at the Texas Center for Supraconductivity at UH. His research group focuses on green and sustainable organic materials for energy production and storage.
Yanliang "Leonard" Liang, an assistant professor in the UH Department of Electrical and Computer Engineering, said the reversibility of the interface was key, allowing the semiconductor battery to reach a higher energy density without sacrificing cycle life. Normally, the ability of a semiconductor battery to store energy is interrupted when the cathode-resistive electrolyte interface is formed; reversing this resistance allows the energy density to remain high during cycling, he said.
Lithium-ion batteries with their liquid electrolytes are able to store relatively large amounts of energy and are commonly used to power the tools of modern life, from cell phones to hearing aids. But the risk of fire and explosion has sparked increased interest in other types of batteries, and a solid-state sodium ion battery offers the promise of increased security at a lower cost.
Xiaowei Chi, Yao Group's postdoctoral researcher, said the major challenge was to find a solid electrolyte as conductive as the liquid electrolytes used in lithium-ion batteries. Now that sufficiently strong solid electrolytes are available, solid interfaces remain a challenge.
A problem raised by a solid electrolyte: it struggles to maintain intimate contact with a traditional rigid cathode while it expands and contracts during the cycle of the battery. Fang Hao, a PhD student working in Yao's group, said the organic cathode was more flexible and could stay in touch with the interface, improving the cyclist's life. The researchers said the contact remained stable for at least 200 cycles.
"If you have a reliable contact between the electrode and the electrolyte, you will have a great chance of creating a high performance solid state battery," said Hao.
Besides Yao, the authors include the co-first authors, Hao and Chi, Liang, Ye Zhang and Hui Dong, all of UH; Rong Xu and Kejie Zhao from Purdue University; and Hua Guo, Tanguy Terlier and Jun Lou from Rice University. Most of this work was funded by the Advanced Research Projects Agency of the United States Department of Energy (ARPA-E).
Source of the story:
Material provided by University of Houston. Original written by Jeannie Kever. Note: Content can be changed for style and length.
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