Lithium-ion batteries used to power electric buses and vacuum cleaners typically consist of billions of lithium iron phosphate nanoparticles. The battery materials studied in this paper can also be used to store wind and solar energy in the power grid. Image credit: Greg Stewart / SLAC National Accelerator Laboratory
In the past three decades, through the research on the upgrade of lithium-ion batteries that use lithium-ion reciprocating cycles to charge and discharge, the smaller devices can run faster and significantly extend their service life.
Now, X-ray experiments from the US Department of Energy's SLAC National Accelerator Laboratory and Lawrence Berkeley National Laboratory have shown that the theory of lithium ion operation is more complicated than imagined. By correcting inherent assumptions about materials, we will help us improve battery design, which will lead to a new generation of lithium-ion batteries.
An international research team led by Professor William Chueh of the SLAC Stanford Institute of Materials and Energy Science published their findings today in Nature Materials.
Martin Bazant, a professor at the Massachusetts Institute of Technology and another person in charge of the study, said: "Previously, their research was a bit like blindness. You can see that the materials work well, and certain additives seem to help, but you ca n’t know for sure Know where lithium ions are in this process. You can only try to develop a theory and apply it back from measurement. With new instruments and measurement technologies, we begin to have a more rigorous scientific understanding of how these things actually work. "
'Popcorn effect'
Anyone riding an electric bus or using other power tools could benefit from their research. It can be used in start-stop devices for automobiles with internal combustion engines, and wind and solar storage applications in the power grid. A better understanding of this and other similar materials can lead to faster charging, longer lasting and more durable batteries.
When the lithium-ion battery is charged and discharged, lithium ions flow from the liquid solution into the solid storage. But once it enters the solid, lithium rearranges, sometimes splitting the material into two different stages, just like oil and water separate when mixed together. This leads to what Professor Chueh calls the "popcorn effect." Ion condensing into hot spots will ultimately shorten battery life.
In this study, the researchers used two X-ray technologies to explore the inner workings of lithium-ion batteries. At SLAC's Stanford Synchrotron Radiation Light Source (SSRL), they reflected X-rays from a lithium iron phosphate sample to show their atomic and electronic structures and let them understand how lithium ions move in the material. In the Advanced Light Source (ALS) at Berkeley Labs, they used X-ray microscopy to magnify the process, allowing them to map lithium ion concentration over time.
Head upstream
Previously, researchers thought lithium iron phosphate was a one-dimensional conductor, meaning that lithium ions can only travel in one direction through most materials, just like salmon swim upstream.
But in sifting through their data, the researchers noticed that lithium ions moved on the surface of the material in completely different directions than previous models. It was as if someone threw a leaf on the surface of the stream and found that the direction of water flow was completely different from the direction in which the salmon was swimming.
They collaborated with Saiful Islam, a professor of chemistry at the University of Bath, UK, to develop computer models for system simulation. These studies revealed that lithium ions move in two other directions on the surface of the material, making lithium iron phosphate a three-dimensional conductor.
"It turns out that these additional pathways are problematic for the material, which will cause popcorn-like behavior to fail, and if lithium ions can move more slowly on the surface of the material, it will make the battery more uniform. This Is the key to developing higher performance and longer lasting batteries. "Professor Chueh said.
New field of battery engineering
Although research on lithium iron phosphate has existed for the past two decades, it was not until a few years ago that we could study it at the nanoscale and during battery operation.
"This explains how the important characteristics of this material have been ignored for a long time. With the advent of new technologies, people will always find some new and interesting materials that will make you think about them differently." Li Yiyang said that he has done postgraduate and postdoctoral experiments at Stanford University and SLAC.
Li Yiyang said: "We have discovered and developed some of the best bulk materials, and at the same time, we are seeing that lithium-ion battery technology is still evolving at a very significant rate: they are getting better and better. We are building an important knowledge base that can be added to the battery engineer's toolkit, which will help them develop better materials. "
Across different scales
To follow up on this research, researchers will continue to combine modeling, simulation, and experimentation, and try to learn more about the basic issues of battery performance at different lengths and time scales through facilities such as SLAC's Linac coherent light source or LCLS. They use LCLS, which can detect single ion transitions that occur on time scales, with speeds up to a trillionth of a second.
Professor Chueh said, "One of the obstacles to the development of lithium-ion battery technology is the length and time range involved. Key processes can occur instantly or over many years. The path of research is that these processes need to be mapped to atomic motion "At SLAC, we are studying all battery materials, and we are convinced that by combining modeling and experimentation, we can ultimately help us understand more of the basics of battery operation."