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Garnet makes ideal electrolyte separator material

Posted: 24 Oct 2014 ?? ?Print Version ?Bookmark and Share

Keywords:LLZO? electrolyte? battery? lithium?

Novel chemistries and materials for improving a battery's density and lifespan may just be the modern day philosopher's stone researchers are desperately finding, with the ever-present complaints on portable energy storage's life falling short of expectations.

So far, researchers have been seeking to improve a battery's energy density by using a pure lithium anode, which offers the highest known theoretical capacity, and an aqueous electrolyte that can speedily transport lithium. But scientists at Oak Ridge National Laboratories (ORNL) believe that the key is using an electrolyte separator to shield the lithiumand a cubic garnet material called LLZO might just be the ideal material for it.

The team used scanning transmission electron microscopy to take an atomic-level look at LLZO. They found the material to be highly stable in a range of aqueous environments, making the compound a promising component in new battery configurations.

LLZO under electron microscope

LLZO maintains its structural stability even after immersion in a range of aqueous solutions. (Source: ORNL)

"Many novel batteries adopt these two features [lithium anode and aqueous electrolyte], but if you integrate both into a single battery, a problem arises because the water is very reactive when in direct contact with lithium metal," said ORNL postdoctoral associate Cheng Ma, first author on the team's study published in Angewandte Chemie. "The reaction is very violent, which is why you need a protective layer around the lithium."

Battery designers can use a solid electrolyte separator to shield the lithium, but their options are limited. Even the primary separator of choice, known as LAPT or LISICON, tends to break down under normal battery operating conditions.

"Researchers have searched for a suitable solid electrolyte separator material for years," said ORNL's Miaofang Chi, the study's lead author. "The requirements for this type of material are very strict. It must be compatible with the lithium anode because lithium is reactive, and it also has to be stable over a wide pH range, because you can have an alkaline environmentespecially with lithium air batteries."

The researchers used atomic resolution imaging to monitor structural changes in LLZO after the samples' immersion in a range of aqueous solutions. The team's observations showed that the compound remained structurally stable over time across neutral and extremely alkaline environments.

"This solid electrolyte separator remains stable even for a pH value higher than 14," Ma said. "It gives battery designers more options for the selection of aqueous solutions and the catholyte." Catholyte is the portion of the electrolyte close to the cathode.

In lithium-air batteries, for instance, researchers have previously tried to avoid the degradation of the separator by diluting the aqueous solutions, which only makes the battery heavier and bulkier. With this new type of solid electrolyte separator, there is no need to dilute the aqueous electrolyte, so it indirectly increases the battery's energy density.

Higher-energy batteries are in demand for electrified transportation and electric grid energy storage applications, leading researchers to explore battery designs beyond the limits of lithium-ion technologies.





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