Scientists at the DOE’s Pacific Northwest National Lab have uncovered a “root” cause of the growth of needle-like structures – known as dendrites and whiskers – that plague lithium batteries and can cause short circuit, failure, and even fire.
Dendrites are tiny, rigid tree-like structures that can grow inside a lithium battery. Their needle-like projections are called whiskers. They can pierce the separator inside a battery and increase unwanted reactions between the electrolyte and the lithium, speeding up battery failure.
Dendrites and whiskers are holding back the widespread use of lithium metal batteries, which have higher energy density than their commonly used lithium-ion counterparts. Universities and research teams around the world are working on solving the dendrite problem through various methods. The PNNL team hopes their discovery will lead to new ways to prevent dendrite growth by manipulating the battery’s ingredients. The results were published online in Nature Nanotechnology.
The team found that the origin of whiskers in a lithium metal battery lies in the solid-electrolyte interphase (SEI), which is a film where the solid lithium surface of the anode meets the liquid electrolyte. The scientists pinpointed ethylene carbonate, an indispensable solvent added to the electrolyte to enhance battery performance, as the culprit.
A dendrite begins when lithium ions start to clump, or “nucleate,” on the surface of the anode. The structure grows slowly as more and more lithium atoms glom on. The team found that the energy dynamics on the surface of the SEI push more lithium ions into the slowly growing column. Then, suddenly, a whisker shoots forth.
The level of ethylene carbonate directly correlates with dendrite and whisker growth. The more of the material the team put in the electrolyte, the more the whiskers grew. The researchers experimented with the electrolyte mix, changing ingredients in an effort to reduce the growth of dendrites. Some changes, such as the addition of cyclohexanone, prevented the growth of dendrites and whiskers.
“We don’t want to simply suppress the growth of dendrites; we want to get to the root cause and eliminate them,” said lead researcher Chongmin Wang. “We drew upon the expertise of our colleagues who have expertise in electrochemistry. My hope is that our findings will spur the community to look at this problem in new ways.”