While purity is often associated with better material performance, some naturally occurring crystal defects at the nanoscale could be leveraged for better energy storage materials.

A study led by Cornell University used an X-ray nanoimaging process, offering an unprecedented view of solid-state electrolytes - a central part of the solid-state battery. The imaging process unveiled previously undetected and unknown crystal defects and dislocations, which are naturally occurring crystallographic imperfections. Furthermore, these defects could be used for the next generation of energy storage materials.

Researchers present their findings in the article "X-ray Nanoimaging of Crystal Defects in Single Grains of Solid-State Electrolyte Li7-3xAlxLa3Zr2O12," appearing in the American Chemical Society journal Nano Letters, April 29. The study's lead author is Yifei Sun,  a doctoral student at Cornell.

A Diagram of a Solid State Battery
(Photo: Luca Bertoli via Wikimedia Commons)

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Investigating Defects at the Nanoscale

For the past few decades, a part of materials science has investigated tiny defects in metals and their relationship to the material's properties. With technology allowing for more sophisticated and precise imaging tools, researchers can now explore how naturally occurring defects affect different materials even at the nanoscale. This includes materials used for energy storage.

Researchers led by an assistant professor Andrej Singer, David Croll Sesquicentennial Faculty Fellow in Cornell's Department of Materials Science and Engineering, used synchrotron radiation to uncover defects in the nanoscale for battery materials that were previously undetectable with conventional imaging methods like electron microscopy.

Particularly, researchers focused on solid-state electrolytes because these materials have the potential to replace existing liquid and polymer electrolytes found in lithium-ion batteries as a more efficient and safer alternative. A disadvantage of using liquid electrolytes is its susceptibility to spiky dendrites forming between its anode and cathode end, which increases the risks of shorting the battery or worse, causing it to explode.

Meanwhile, solid-state electrolytes are not flammable, although they still require further study to be practically usable. One, these materials don't conduct lithium ions as well as fluids, which also makes maintaining contact between terminals more difficult. Second, there is the problem of keeping these solid-state electrolytes extremely thin because failure to do so would result in a bulkier battery, negating any advantages in energy storage capacity.

Advantageous Defect

According to Singer, in a news release from the Argonne National Laboratory, a possible solution to make solid-state electrolytes feasible is in its crystal defects. He explains that a similar phenomenon in metals is the presence of defects at the nanoscale that could facilitate ionic diffusion, causing the ions to be transferred at a faster rate. Additionally, crystal defects could also help prevent fracture in these materials, making them less prone to breakage.

Singer's team worked with Nikolaos Bouklas, co-author of the report and an assistant professor from Cornell's Sibley School of Mechanical and Aerospace Engineering, as well as with researchers from Virginia Tech led by Feng Lin, co-senior author of the paper. Lin's team also helped synthesize the sample, a garnet crystal structure: lithium lanthanum zirconium oxide (LLZO), which contains pieces of aluminum introduced at various concentrations, at the nanoscale, in a process called doping - a similar process used in introducing "impurities" in semiconductor materials.


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