With the potential to hold many times more energy than graphite, silicon is a tantalizing candidate for scientists working on the next generation of lithium batteries. The problem is that silicon doesn’t hold up well to the stress of battery cycling, and researchers have gained new insight into why, plus clues to how to avoid this rapid deterioration.
Scientists working to integrate silicon into lithium-ion batteries hope to incorporate or completely replace graphite as a component of the anode, which has the potential to store up to 10 times more energy. The problem is that as the battery charges and discharges, silicon expands and can cause the anode to crack and eventually destroy the battery.
For years, scientists at Pacific Northwest National Laboratory have worked to address this problem, including using silicon with special nanostructures, combining it with solid electrolytes, forming silicon sandwiches or wrapping the material in graphene.
Now, a new understanding of why silicon anodes fail so quickly could greatly help bolster their stability.
As the battery cycles, lithium ions move back and forth between the anode and the other electrode, the cathode, through a liquid electrolyte. When these ions enter the silicon anode, they push silicon atoms to one side, which is what causes the anode to swell to three or four times its size. Then when the lithium ions leave again, they create holes that cause the battery to fail quickly.
The researchers used a modified transmission electron microscope to record the molecular activity of lithium batteries with silicon anodes as they were charged and discharged. This showed that as the departing lithium ions create these vacancies, they evolve into larger and larger gaps, which the liquid electrolyte then rushes into. This has the effect of distorting a key structure at the edge of the anode – the solid electrolyte interphase, which is seen to penetrate the anode and form where it should not. The end result of this is the creation of a “dead zone” that makes the anode inoperable.
From the observations so far, it is clear that to solve the problem of silicon, a hard shell must be formed to isolate the silicon from the liquid electrolyte, and there are two ways to do this. One method is to “temporarily” form a hard shell on the silicon when the battery is initially operating, which requires adjusting the composition of the liquid electrolyte to allow the formation of this smart shell. Or a smart coating can be applied to the silicon to isolate the silicon from the contacting liquid electrolyte.