Solid electrolyte interphase is a secret semiconductor

For decades, researchers have assumed that the inevitable filmy build-up on the electrodes inside rechargeable lithium-ion batteries is a driver of performance loss. Now, they know that view is backward.

The build-up of mossy or tree-like lithium metal deposits on battery electrodes is not the root cause of performance loss, but rather a side effect of it. This was revealed by the first direct measurements of the electrical properties at the boundary between the solid electrode and the liquid electrolyte inside a rechargeable battery.

The study, led by a research team at the US Department of Energy’s Pacific Northwest National Laboratory (PNNL), shows that the so-called solid electrolyte interphase (SEI) is not an electronic insulator, as previously thought, but instead behaves like a semiconductor. This research solves a long-standing mystery of how the SEI functions electrically during battery operation.

These findings, reported in a paper in Nature Energy, have direct implications for designing longer-lasting batteries by tuning the physical and electrochemical properties of the liquid electrolyte, which is often referred to as the blood supply of an operating battery.

“A higher rate of electrical conductance induces a thicker SEI with intricate solid lithium forms, ultimately leading to inferior battery performance,” said Chongmin Wang, a PNNL laboratory fellow and battery technology expert who co-led the study.

Researchers focus on this SEI layer, which is thinner than a sheet of tissue paper, because of its out-sized role in the performance of rechargeable lithium-ion batteries. This filmy mosaic selectively permits charged lithium ions to cross during discharge and controls movement of the electrons that supply the battery’s power.

When batteries are new, the SEI forms on the first charging cycle and ideally remains stable during the battery’s expected lifespan. But a look inside an aging rechargeable battery often reveals a substantial build-up of solid lithium on the negative electrode. Battery researchers have always assumed that this build-up causes the performance losses, but they’ve never known for sure because of their inability to make measurements to test cause and effect.

Wang, along with colleagues at PNNL, Texas A&M University and Lawrence Berkeley National Laboratory, solved this problem by developing a new technique to directly measure electrical conduction across the SEI in an experimental system. This technique involves combining transmission electron microscopy with nanoscale manipulation of microfabricated metal needles inside the microscope. Using the technique, the  researchers measured the electrical properties of the SEI layer that formed on either copper or lithium metal exposed to four different types of electrolytes.

The group’s measurements revealed that as voltage increases in the battery, the SEI layer in all cases leaks electrons, making it semi-conductive.

Once they had recorded this semiconductor-like behavior, which had never been directly observed previously, the researchers wanted to understand which components of the chemically complex SEI are responsible for the electron leakage.

“We found that the carbon-containing organic components of the SEI layer are prone to leaking electrons,” Xu said. The researchers concluded that minimizing the organic components in the SEI would allow batteries to have a longer useful life.

“Even slight variations of the rate of conduction through the SEI can result in dramatic differences in efficiency and battery cycling stability,” Wang added.

This story is adapted from material from Pacific Northwest National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.