Researchers at the U.S. Dept. of Energy’s (DOE) Argonne National Laboratory say they’ve identified a major culprit in capacity fade of high-energy lithium-ion batteries. With the cause identified, the next step is to figure out ways of curing the fading problem.
The capacity of a Li-ion battery is tied directly to how many lithium ions can shuttle back and forth between the two terminals of the battery as it charges and discharges. This shuttling is enabled by certain transition metal ions, which change oxidation states as lithium ions move in and out of the cathode. But as the battery cycles, some of these ions — most notably manganese — get stripped out of the cathode material and end up at the battery anode.
Once near the anode, the metal ions interact with a region of the battery called the solid-electrolyte interphase. The interphase forms because of reactions between the highly reactive anode and the liquid electrolyte that serves as a medium for carrying the lithium ions back and forth. For every electrolyte molecule that reacts and becomes decomposed in a process called reduction, a lithium ion becomes trapped in the interphase. As more and more lithium gets trapped, the capacity of the battery drops.
Some molecules in this interphase are incompletely reduced, meaning that they can accept more electrons and tie up even more lithium ions. When the manganese ions become deposited into this interphase they are efficient at catalyzing reactions with the incompletely reduced molecules, trapping more lithium ions in the process. The more manganese ions present, the more lithium ions trapped.
“There’s a strict correlation between the amount of manganese that makes its way to the anode and the amount of lithium that gets trapped,” said study coauthor and Argonne scientist Daniel Abraham. “Now that we know the mechanisms behind the trapping of lithium ions and the capacity fade, we can find methods to solve the problem.”
Abraham published the findings along with coauthors James Gilbert and Ilya Shkrob in the Journal of The Electrochemical Society.