The first hours of a lithium-ion battery’s life largely determine just how well it will perform. In those moments, a set of molecules self-assembles into a structure inside the battery that will affect the battery for years to come.
This component, known as the solid-electrolyte interphase or SEI, has the crucial job of blocking some particles while allowing others to pass. The structure has been an enigma for scientists who have studied it for decades. Researchers have tapped multiple techniques to learn more but never — until now — had they witnessed its creation at a molecular level.
Knowing more about the SEI is a crucial step on the road to why lithium-ion batteries work and to creating more energetic, longer-lasting and safer lithium-ion batteries.
The SEI is the secret
Why lithium-ion batteries work?
The solid-electrolyte interphase is a very thin film of material that doesn’t exist when a battery is first built. Only when the battery is charged for the very first time do molecules aggregate and electrochemically react to form the structure, which acts as a gateway allowing lithium ions to pass back and forth between the anode and cathode. Crucially, the SEI forces electrons to take a detour, which keeps the battery operating and makes energy storage possible.
It’s because of the SEI that we have lithium-ion batteries at all to power our cell phones, laptops, and electric vehicles.
The team found that one layer of the structure, next to the anode, is thin but dense; this is the layer that repels electrons but allows lithium ions to pass through. The outer layer, right next to the electrolyte, is thicker and mediates interactions between the liquid and the rest of the SEI. The inner layer is a bit harder and the outer later is more liquidy, a little bit like the difference between under-cooked and overcooked oatmeal.
The work, published on January 27, 2020, in Nature Nanotechnology, was performed by an international team of scientists led by researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory and the U.S. Army Research Laboratory. Corresponding authors include Zihua Zhu, Chongmin Wang and Zhijie Xu of PNNL and Kang Xu of the U.S. Army Research Laboratory.
Reference: “Real-time mass spectrometric characterization of the solid–electrolyte interphase of a lithium-ion battery” by Yufan Zhou, Mao Su, Xiaofei Yu, Yanyan Zhang, Jun-Gang Wang, Xiaodi Ren, Ruiguo Cao, Wu Xu, Donald R. Baer, Yingge Du, Oleg Borodin, Yanting Wang, Xue-Lin Wang, Kang Xu, Zhijie Xu, Chongmin Wang and Zihua Zhu, 27 January 2020, Nature Nanotechnology.