Argonne scientists have advanced sodium-ion batteries by preventing cracks in the cathode particles during the synthesis process, making them a cost-effective and sustainable alternative to lithium-ion batteries. This research first appeared in Nature Nanotechnology.
To date, there has been a serious roadblock to commercialization of sodium-ion batteries. In particular, the performance of the sodium-containing cathode rapidly declines with repeated discharge and charge.
A team at Argonne has made important strides in resolving this issue with a new design for a sodium-ion oxide cathode. It is closely based on an earlier Argonne design for a lithium-ion oxide cathode with proven high energy storage capacity and long life.
“The prospects seem 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 now in many lithium-ion batteries”, says Khalil Amine, Argonne Distinguished Fellow and co-corresponding author
A key feature of both designs is that the microscopic cathode particles contain a mix of transition metals, which could include nickel, cobalt, iron or manganese. Importantly, these metals are not uniformly distributed in individual cathode particles. As an example, nickel appears at the core; surrounding 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 cycling. The nickel-rich core provides high capacity for energy storage.
In testing this design, however, the cathode’s energy storage capacity steadily declined during cycling. The problem was traced to the formation of cracks in the particles during cycling. These cracks formed due to strain arising between the shell and core in the particles. The team sought to eliminate that strain before cycling by fine-tuning their method of cathode preparation.
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 in a gradient from core to shell and, for comparison, another with the three metals evenly distributed throughout each particle.
To form the final product, the team heated up a mixture of a precursor material and sodium hydroxide to as high as 600 degrees Celsius, maintained it at that temperature for a select period, then cooled it to room temperature. The researchers also tried different heat-up rates.
During this entire treatment, the team monitored the structural changes in the particle properties. This analysis involved 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.
The team also used the Center for Nanoscale Materials (CNM) at Argonne for additional analysis to characterize the particles and the Polaris supercomputer at the Argonne Leadership Computing Facility (ALCF) to reconstruct the X-ray data into detailed 3D images. The CNM and ALCF are also DOE Office of Science user facilities.
The initial results revealed no cracks in the uniform particles, but cracks forming in the gradient particles at temperatures as low as 250 degrees C. These cracks appeared at the core and the core-shell boundary and then moved to the surface. Clearly, the metal gradient caused significant strain leading to these cracks.
“Since we know that gradient particles can produce cathodes with high energy storage capacity, we wanted to find heat treatment conditions that will eliminate the cracks in the gradient particles”, says Wenhua Zuo, an Argonne postdoctoral appointee
The heat-up rate proved a critical factor. Cracks formed at a heat-up rate of five degrees per minute, but not at a slower rate of one degree per minute. Tests in small cells with cathode particles prepared at the slower rate maintained their high performance for over 400 cycles.
“Preventing cracks during cathode synthesis pays big dividends when the cathode is later charged and discharged. And while sodium-ion batteries do not yet have sufficient energy density to power vehicles over long distances, they are ideal for urban driving”, says Gui-Liang Xu, co-corresponding author.