Lithium ion batteries, such as those used in cellphones and laptops, have a finite lifespan. Engineers refer to the diminished battery performance that comes with use as “capacity fade.” That translates into the amount of charge a battery can supply decreasing with repeated use. Capacity fade is why a cell phone battery that used to last a whole day will, after a few years, last only a few hours.
Up until now, the reasons behind capacity fade were not fully known. But researchers at Argonne National Laboratory recently identified a major culprit in capacity fade of high-energy lithium-ion batteries. In Li-ion batteries—the kind used in laptops, smartphones, and plug-in hybrid vehicles—the capacity is tied directly to the amount of lithium ions that can be shuttled back and forth between the battery’s two terminals as it is charged and discharged.
This shuttling is made possible by certain transition metal ions, which change oxidation states as lithium ions move in and out of the cathode. However, as the battery is cycled, some of these ions (most notably, manganese) get stripped out of the cathode material and end up at the battery’s anode.
Once near the anode, these metal ions interact with a region of the battery called the solid-electrolyte interphase, which forms due to reactions between the highly reactive anode and the liquid electrolyte that carries the lithium ions back and forth. For every electrolyte molecule that reacts and decomposes in a process called reduction, a lithium ion gets trapped in the interphase. As more and more lithium gets trapped, the battery’s capacity decreases.
Some molecules in this interphase are not completely reduced, meaning they can accept more electrons and tie up even more lithium ions. These molecules are like tinder, awaiting a spark.
Manganese ions deposited into this interphase act like a spark igniting the tinder: These ions are efficient at catalyzing reactions with incompletely reduced molecules, trapping more lithium ions in the process.
“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 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.”