Sodium-ion oxide cathodes made from transition metal core-shell particles — a nickel-rich core surrounded by a cobalt- and manganese-rich shell — were studied.
"The manganese surface provides the particle with structural stability during cyclic charge-discharge," the laboratory reports. "The nickel core provides high energy storage capacity."
The energy storage capacity gradually decreased during cyclic processes, when cracks began to form in the cathode particles due to deformation occurring between the shell and core in the particles.
Further research showed that these cracks form deep inside the particles, rather than, as expected, on their surface. This also revealed a way to prevent crack formation.
These two statements suggest the vast research efforts needed for this discovery, which, among other equipment, required the use of two of the world's most powerful synchrotrons and a supercomputer ranked in the top 50 in the world: the Advanced Photon Source from Argonne, the National Synchrotron Light Source II from Brookhaven National Laboratory, and the Polaris supercomputer from Argonne.
"Preventing crack formation during cathode synthesis yields significant dividends when the cathode is later charged and discharged," said chemist Guo-Liang Xu from the Argonne laboratory.
Deceptively simple, crack-resistant cathode particles that did not lose their capacity after 400 cycles were the result of modifying the heat treatment used to fabricate them.
Core-shell cathode particles are made by heating hydroxide precursors at up to 600°C.
When heated at 5°C/min, cracks began to form at the core-shell boundary at only 250°C, but at an extremely slow heating rate — 1°C/min — the particles were stronger.
"Prospects look very promising for future sodium-ion batteries, which will not only be cheaper and more durable, but also have energy density comparable to lithium iron phosphate cathodes currently used in many lithium-ion batteries. This will lead to electric vehicles with good range," noted battery development team leader at Argonne, Khalil Amin.
Future work in the laboratory will include attempts to reduce costs and enhance durability by removing nickel from the cathode.