Adapted from this PNNL press release
For decades, researchers have assumed that the inevitable filmy buildup on electrodes inside rechargeable batteries is the driver of performance loss. Now, we know that view is backward.
The buildup of mossy or tree-like structured lithium metal deposits on battery electrodes is not the root cause of performance loss, but rather a side effect. The first direct measurement of the electrical properties at the boundary between the solid electrode and the liquid electrolyte inside a rechargeable battery is reported recently in Nature Energy.
The study, led by Foundry users from the Department of Energy’s Pacific Northwest National Laboratory, shows that the so-called solid electrolyte interphase (SEI) is not an electronic insulator, as previously thought, but instead behaves like a semiconductor. The research solves the long-standing mystery of how SEI functions electrically during battery operation.
The findings have direct implications for designing longer-lasting batteries by fine-tuning the physical and electrochemical properties of the liquid electrolyte, which is often referred to as the blood supply of an operating battery.
Researchers focused on this SEI layer, which is thinner than a sheet of tissue paper, because of its outsized role in battery performance. This filmy mosaic selectively permits charged lithium ions to cross during discharge and controls movement of electrons that supply the battery’s power.
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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 substantial buildup of solid lithium on the negative electrodes. Battery researchers have assumed that this buildup causes the performance losses. Part of the reason for this guess work has been an inability to make measurements to test cause and effect.
The research team solved this problem by developing a new technique to directly measure electrical conduction across the SEI in an experimental system. The team combined transmission electron microscopy with nanoscale manipulation of microfabricated metal needles inside the microscope. The researchers then measured the electrical properties of the SEI layer formed on either a copper or lithium metal with four different types of electrolytes.
Read the full press release