Researchers solve key problem with sodium-ion batteries for electric vehicles and grid energy storage

Lithium-ion batteries have long dominated the market as the go-to power source for electric vehicles. They are also increasingly being considered for the storage of renewable energy for use on the electricity grid. However, with the rapid expansion of this market, lithium supply shortages are expected over the next five to ten years.

“Sodium-ion batteries are emerging as an attractive alternative to lithium-ion batteries due to the greater abundance and lower cost of sodium,” said Gui-Liang Xu, a chemist at the U.S. Department of Health’s Argonne National Laboratory. energy (DOE).

To date, the commercialization of such batteries faces serious obstacles. In particular, the performance of the sodium-containing cathode decreases rapidly with repeated discharging and charging.

An Argonne team has made significant progress in solving this problem with a new sodium oxide cathode design. It is closely based on an earlier Argonne design for a lithium-ion oxide cathode with high energy storage capacity and a proven long lifespan. The research is published in the journal Nature Nanotechnology.

A key feature of both designs is that the microscopic cathode particles contain a mixture of transition metals, which may include nickel, cobalt, iron or manganese. It is important to note that these metals are not uniformly distributed within the individual cathode particles. For example, nickel appears in the heart; around this core are cobalt and manganese, which form a shell.

These elements serve different purposes. The manganese-rich surface gives the particle its structural stability during charge-discharge cycles. The nickel-rich core provides high energy storage capacity.

However, during testing of this design, the energy storage capacity of the cathode gradually decreased over the course of the cycle. The problem was caused by cracks forming in the particles during cycling. These cracks formed due to the stresses appearing between the envelope and the core of the particles. The team sought to eliminate this constraint before cycling by refining its method of preparing the cathodes.

The precursor material used to start the synthesis process is a hydroxide. In addition to oxygen and hydrogen, it contains three metals: nickel, cobalt and manganese. The team made two versions of this hydroxide: one with the metals distributed along a gradient from core to shell and, for comparison, another with the three metals distributed evenly throughout each particle.

To form the final product, the team heated a mixture of a precursor material and sodium hydroxide to 600°C, held it at that temperature for a selected period of time, then cooled it to temperature ambient. They also tried different heating rates.

Throughout this processing, the team monitored structural changes in the particles’ properties. This analysis involved the use of two DOE Office of Science user facilities: the Advanced Photon Source (beamlines 17-BM and 11-ID) at Argonne and the National Synchrotron Light Source II (beamline 18-ID ) at DOE’s Brookhaven National Laboratory.

“Using the X-ray beams from these facilities, we could determine real-time changes in particle composition and structure under realistic synthesis conditions,” said Wenqian Xu, a scientist at the Argonne beamline.

The team also used Argonne’s Center for Nanoscale Materials (CNM) for additional analyzes to characterize the particles and the Argonne Leadership Computing Facility’s (ALCF) Polaris supercomputer to reconstruct the X-ray data into detailed 3D images. The CNM and ALCF are also DOE Office of Science user facilities.

Initial results revealed no cracks in uniform particles, but cracks forming in gradient particles at temperatures as low as 250°C. These cracks appeared at the core and at the core-shell boundary and then moved towards the surface. Obviously, the metal gradient caused significant stresses leading to these cracks.

“As we know that gradient particles can produce cathodes with high energy storage capacity, we wanted to find heat treatment conditions that would eliminate cracks in gradient particles,” said Wenhua Zuo, a postdoctoral fellow at Argonne.

The rate of temperature rise was found to be a critical factor. Cracks formed at a heating rate of five degrees per minute, but not at a slower rate of one degree per minute. Testing in small cells with cathode particles prepared at a slower speed maintained their high performance for more than 400 cycles.

“Preventing cracks during cathode synthesis pays big dividends when the cathode is subsequently charged and discharged,” said Gui-Liang Xu. “And although sodium-ion batteries do not yet have sufficient energy density to power vehicles over long distances, they are ideal for urban driving.”

The team is now working to remove nickel from the cathode, which would further reduce costs and be more sustainable.

“The outlook looks very good for future sodium-ion batteries, with not only low cost and long life, but also energy density comparable to that of the lithium iron phosphate cathode currently found in many lithium batteries -ion,” said Khalil Amine, an Argonne professor emeritus. Companion. “This would result in more durable electric vehicles with good range.”