Stanford Students Disprove Long-held Li-Ion Battery Design Theory

A graduate student and an undergraduate student at Stanford University have debunked a long-held belief about lithium-ion battery development, creating the potential for the future design of batteries with more power, greater capacity, and faster charge and discharge capabilities.

Graduate student Yiyang Li and undergraduate Sophie Meyer led a collaborative effort to design experiments that disproved an assumption that's been a common belief among battery designers for 20 years. That assumption is that while lithium-ion batteries needed a substance called carbon black to function, the precise amount of that material had not been considered crucial to overall performance.

"Our research demonstrated that isn't true," said Meyer, in a press release on the Stanford website. Li, a PhD student who plans to become a professor, supervised Meyer over two years of experiments and now the undergraduate plans to pursued a master's degree in engineering

The two were part of a 10-person research team led by William Chueh, an assistant professor of materials science and engineering. The team published an article in the journal Advanced Materials that explains how carbon black-previously considered secondary in importance--was actually critical to overall battery performance. They also came up with new design rules for better batteries.

Li explained to Design News in an interview how the team came up with its discovery.

"A Li-ion battery requires an ion and an electron to travel from one side of the battery (the negative electrode, or anode) to the positive side (or cathode)," she said. "It was long believed that the transport of electrons is very fast, but the transport of ions is very slow. However, we show that the transport of electrons under typical conditions is also very slow."

Li said this is the case because the electrode is made of a large number of individual battery particles that can store lithium atoms to store energy. They collect electrons through an "electron highway" made up of carbon black, she said.

"However, most particles are not directly connected to the carbon black, so they can only collect their electrons from their neighboring particles," she told us. "One way to think about this is that, if not every battery particle is connected to the carbon black highway, then it's as though the electrons must travel on 'local roads' from one particle to the next. It was previously assumed that traveling on 'local roads' from one particle to another, is fast, but we show that it's certainly not the case."

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The significance of the work for future battery design is that researchers developing batteries now will have to carefully consider how to deliver electrons to the particles, not only the ions, Li said.

"If we want a battery that can cycle quickly, we need to think about how to deliver the electrons to the battery particles," she said. "Our approach, by adding more carbon black, was just to test a hypothesis. This approach will not work in real batteries because we are replacing battery particles that can store energy with carbon black particles that don't store any energy, making the batteries heavier and more expensive. However, we believe this will provide motivation to find better ways to deliver electrons."

The next step for the research is to build a model that will take into account the need to deliver electrons, Li said. The team also will try to directly visualize the location of the carbon black highway to see if the particles close to the carbon black indeed charge first. "Right now, we cannot see the carbon black, only the battery particles, and we had to infer the relationship based on the size of the particles," she said.

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Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 15 years. She has lived and worked as a professional journalist in Phoenix, San Francisco and New York City. In her free time she enjoys surfing, traveling, music, yoga and cooking. She currently resides in a village on the southwest coast of Portugal.