For decades, scientists have believed that the inevitable film buildup on rechargeable battery electrodes is responsible for performance degradation. But a recent study in the United States found that there are actually other reasons behind the performance decline. A research team from the U.S. Department of Energy's Pacific Northwest National Laboratory (PNNL) has found that the accumulation of lithium metal deposits in mossy or tree-like structures on battery electrodes is not the root cause of performance degradation, but rather a side effect.
Their latest findings were recently published in the journal Nature Energy.
Their results show that the so-called solid electrolyte interface (SEI) is not an electronic insulator as previously thought, but behaves like a semiconductor. The SEI acts like a guardian, allowing lithium ions to move freely in and out of the anode. Scientists have long focused on studying this SEI layer, which despite being thinner than a piece of paper, plays a huge role in battery performance.
SEI forms during the first charge cycle when the battery is still new and ideally remains stable for the expected life of the battery. But looking inside an aging rechargeable battery usually reveals a large buildup of solid lithium on the negative electrode. Battery researchers believe this buildup leads to performance degradation. But it has previously been impossible to measure cause and effect.
In their latest research, they solved this problem by developing a new technique to directly measure the conductivity of SEI in experimental systems. The team combined transmission electron microscopy with nanoscale manipulation of microfabricated metal needles within the microscope. The researchers then measured the electrical properties of SEI layers formed on copper or lithium metal using four different types of electrolytes.
In doing so, they solved a long-standing mystery about how SEI functions in battery operation. The team's measurements showed that as the cell voltage increased, the SEI layer leaked electrons in all cases, making it semiconducting. In addition, the carbon-containing organic components of the SEI layer easily leak electrons and shorten battery life.
"Higher conductivity leads to thicker SEI and complex solid lithium forms, ultimately leading to poorer battery performance," said Chongmin Wang, a PNNL laboratory researcher and battery technology expert who co-led the study.
At this point, the researchers concluded that minimizing the organic components in the SEI would result in a longer battery life.
"Even small changes in the conduction rate through the SEI can lead to huge differences in efficiency and battery cycling stability," Wang added.