Scientists at the US Department of Energy’s Pacific Northwest National Laboratory (PNNL) report new findings about how to make a single-crystal, nickel-rich cathode hardier and more efficient. The team’s work on the cathode appears in the journal Science.

High-energy nickel (Ni)–rich cathode will play a key role in advanced lithium (Li)–ion batteries, but it suffers from moisture sensitivity, side reactions, and gas generation. Single-crystalline Ni-rich cathode has a great potential to address the challenges present in its polycrystalline counterpart by reducing phase boundaries and materials surfaces. However, synthesis of high-performance single-crystalline Ni-rich cathode is very challenging, notwithstanding a fundamental linkage between overpotential, microstructure, and electrochemical behaviors in single-crystalline Ni-rich cathodes. We observe reversible planar gliding and microcracking along the (003) plane in a single-crystalline Ni-rich cathode. The reversible formation of microstructure defects is correlated with the localized stresses induced by a concentration gradient of Li atoms in the lattice, providing clues to mitigate particle fracture from synthesis modifications.

—Bi et al.

A single nickel-rich crystal created by the PNNL team. (Image courtesy of Jie Xiao | Pacific Northwest National Laboratory)

Researchers are working on ways to store more energy in the cathode materials by increasing nickel content. Nickel offers relatively low cost, wide availability and low toxicity compared to other key battery materials, such as cobalt.

Nickel-rich cathode materials have real potential to store more energy. But large-scale deployment has been a challenge.

—Jie Xiao, corresponding author

While nickel holds great promise, in high amounts it can pose problems in batteries. The more nickel in the material’s lattice, the less stable the cathode. High nickel content can increase unwanted side reactions, damaging the material and making storage and handling very difficult.

Currently the most common nickel-rich cathode is in the form of polycrystals—aggregates of many nanocrystals in one larger particle. These carry advantages for storing and discharging energy faster. However, the polycrystals sometimes break down during repeated cycling.

This can leave much of the surface area exposed to electrolyte, accelerating unwanted chemical reactions induced by high nickel content and generating gas. This irreversible damage results in a battery with a nickel-rich cathode that fails faster and raises safety concerns.

Researchers are trying to sidestep many of these problems by creating a single-crystal, nickel-rich cathode. An aggregate of small crystals is much more vulnerable to its surroundings than a single crystal under certain conditions, especially when there’s high nickel content, since nickel is prone to induce unwanted chemical reactions.

Over time, with repeated battery cycles, the aggregates are ultimately pulverized, ruining the cathode’s structure. That’s not so much a problem when the amount of nickel in the cathode is lower; under such conditions, a polycrystalline cathode containing nickel offers high power and stability. The problem becomes more pronounced, though, when scientists create a cathode with more nickel—a cathode truly rich in nickel.

The PNNL team discovered one reason why a single-crystal, nickel-rich cathode breaks down: It’s due to a process known as crystal gliding, where a crystal begins to break apart, leading to microcracks. They found that the gliding is partially reversible under certain conditions and have proposed ways to avoid the damage altogether.

The PNNL researchers have developed a process to grow high-performance crystals in molten salts—sodium chloride, common table salt—at high temperature.

With the new fundamental understanding, we will be able to prevent the gliding and microcracks in the single crystal. This is unlike the damage in the polycrystalline form, where the particles are pulverized in a process that is not reversible.

—Jie Xiao

Gliding motions within the crystal’s lattice layers are at the root of microcracks. The layers move back and forth, like cards in a deck as they’re shuffled. The gliding occurs as the battery charges and discharges—lithium ions depart and return to cathode, straining the crystal ever so slightly each time. Over many cycles, the repeated gliding results in microcracks.

Xiao’s team learned that the process can partially reverse itself through the natural actions of the lithium atoms, which create stresses in one direction when the ions enter the crystal lattice and in the opposite direction when they leave. But the two actions don’t completely cancel each other out, and over time, microcracks will occur. That’s why single crystals ultimately fail, though they don’t break down into small particles like their polycrystalline counterparts.

The team is pursuing several strategies to prevent the gliding. The researchers have discovered that operating the battery at a common voltage—around 4.2 volts—minimizes damage while still within the normal range of lithium-ion batteries for electric vehicles. The team also predicts that keeping the size of a single crystal below 3.5 microns may avoid damage even at higher voltages. And the team is exploring ways to stabilize the crystal lattice to better accommodate the arrival and departure of lithium ions.

The team estimates that the single-crystal, nickel-rich cathode packs at least 25% more energy compared to the lithium-ion batteries used in today’s electric vehicles.

Now, PNNL researchers led by Xiao are working with Albemarle Corporation, a major specialty chemical manufacturing company and one of the world’s leading producers of lithium for electric vehicle batteries. In a collaboration funded by DOE, the team will research the impacts of advanced lithium salts on the performance of single-crystal nickel-rich cathode materials by demonstrating the process at kilogram scale.

The work was funded by DOE’s Office of Energy Efficiency and Renewable Energy’s Vehicle Technologies Office. Part of the research was done using the National Synchrotron Light Source II at Brookhaven.

Resources

• Yujing Bi, Jinhui Tao, Yuqin Wu, Linze Li, Yaobin Xu, Enyuan Hu, Bingbin Wu, Jiangtao Hu, Chongmin Wang, Ji-Guang Zhang, Yue Qi, Jie Xiao (2020)“Reversible planar gliding and microcracking in a single-crystalline Ni-rich cathode” Science doi: 10.1126/science.abc3167