Single-crystal technology holds promise for next-generation lithium-ion batteries

A single crystal with a high Nickel content created by the PNNL team. Credit: PNNL

A promising technology being developed by major battery manufacturers has become even more attractive thanks to researchers who have looked at one key barrier to better and longer-lasting lithium-ion batteries in an unprecedented way.

Scientists from the U.S. Department of Northwest Pacific national energy laboratory report new data on how to make a single-crystal, Nickel-rich cathode hardier and more efficient. The team's work on the cathode, one of the most important components of lithium-ion batteries that are now widely used in electric vehicles, is published in the December 11 issue of the journal Science .

Researchers around the world are working to create batteries that provide more energy, last longer, and are cheaper to produce. Improved lithium-ion batteries are crucial for the wider adoption of electric vehicles.

There are many problems. The simple appearance of the battery belies its complexity, and for the device to work properly, it is necessary to control the complex molecular interactions within it. Constant chemical reactions take their toll, limiting battery life and affecting its size, cost, and other factors.

Prospects for a cathode with a high Nickel content: high energy intensity

Scientists are working on ways to save more energy in cathode materials by increasing the Nickel content. Nickel is on the drawing Board of lithium-ion battery manufacturers mainly because of its relatively low cost, wide availability, and low toxicity compared to other key battery materials such as cobalt.

"Cathode materials with a high Nickel content have real potential to store more energy," said Jie Xiao, author of the paper and head of PNNL's battery research program. "But large-scale deployment was a problem."

Although Nickel is promising, it can cause problems with batteries in large quantities. The more Nickel there is in the lattice of the material, the less stable the cathode is. High Nickel content can increase undesirable side reactions, damage the material, and make it difficult to store and handle.

The use of all the advantages of a larger amount of Nickel in the minimization of defects is a problem.

Currently, the most common cathode with a high Nickel content is in the form of polycrystals-aggregates of many nanocrystals in one larger particle. They have advantages for faster energy storage and discharge. But sometimes polycrystals break down during repeated Cycling. This can cause a large part of the surface to be exposed to the electrolyte, which will accelerate undesirable chemical reactions caused by high Nickel content and gas release. This permanent damage causes the Nickel-rich cathode battery to fail faster and raises safety concerns.

A collection of Nickel-rich crystals. Credit: Andrea Starr / Pacific Northwest national laboratory

Mono crystals, ice cubes, and lithium-ion batteries

Scientists like Xiao are trying to circumvent many of these problems by creating a single-crystal cathode with a high Nickel content. PNNL researchers have developed a process for growing high-performance crystals from molten salts-sodium chloride, table salt-at high temperatures.

What is the advantage of a single crystal over a polycrystalline material? Consider keeping your food cool during the hike. A solid block of ice melts much more slowly than the same amount of ice in the form of small cubes; a block of ice is more resistant to damage from higher temperatures and other external forces.

It is the same with Nickel-rich cathodes: a collection of small crystals is much more vulnerable to the environment than a single crystal under certain conditions, especially when the Nickel content is high, since Nickel tends to cause undesirable chemical reactions. Over time, with repeated battery cycles, the aggregates eventually get crushed, destroying the cathode structure. This is not such a big problem when the amount of Nickel in the cathode is less; in such conditions, a polycrystalline cathode containing Nickel provides high power and stability. However, the problem becomes more obvious when scientists create a cathode with a large amount of Nickel - a cathode that is really rich in Nickel.

Cathodic micro-cracks are reversible, preventable

The PNNL team found one reason why a single-crystal cathode rich in Nickel fails: this is due to a process known as crystal sliding, when the crystal begins to break down, resulting in microcracks. They found that sliding is partially reversible under certain conditions, and suggested ways to avoid damage altogether.

"With this new fundamental understanding, we will be able to prevent sliding and microcracks in a single crystal. This is different from damage in polycrystalline form, when the particles are crushed in an irreversible process, " Xiao said.

It turns out that microcracks are based on sliding movements inside the layers of the crystal lattice. Layers move back and forth like cards in a deck when they are shuffled. Sliding occurs as the battery is charged and discharged-the lithium ions leave and return to the cathode, each time slightly straining the crystal. Over many cycles, repeated sliding leads to the formation of microcracks.

Xiao's team learned that the process can partially reverse due to the natural actions of lithium atoms, which create stresses in one direction when the ions enter the crystal lattice, and in the opposite direction when they leave. But these two actions do not completely exclude each other, and over time, microcracks will occur. This is why single crystals eventually fail, even though they don't break down into small particles like their polycrystalline counterparts.

The team applies several strategies to prevent slipping. Researchers found that using the battery at a normal voltage of about 4.2 volts minimizes damage while remaining within the normal range of lithium-ion batteries for electric vehicles. The team also predicts that keeping the single crystal size below 3.5 microns can prevent damage even at higher voltages. And the team is exploring ways to stabilize the crystal lattice to better adapt to the arrival and departure of lithium ions.

The group estimates that a single-crystal cathode with a high Nickel content provides at least 25 percent more energy than lithium-ion batteries used in modern electric vehicles.

Now, PNNL researchers led by Xiao are working with Albemarle Corporation, a major manufacturer of specialty chemicals and one of the world's leading manufacturers of lithium for electric vehicle batteries. As part of a collaboration funded by the US Department of energy, a team of researchers will investigate the effect of advanced lithium salts on the characteristics of single-crystal cathode materials with a high Nickel content, demonstrating the process on a kilogram scale.

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