Cui’s innovation is a cathode made of nanoparticles, each a tiny sulfur nugget surrounded by a hard shell of porous titanium dioxide, like an egg yolk in an eggshell. Between the yolk and shell, where the egg white would be, is an empty space into which the sulfur can expand. During discharging, lithium ions pass through the shell and bind to the sulfur, which expands to fill the void but not so much as to break the shell. The shell, meanwhile, protects the sulfur-lithium intermediate compound from electrolyte solvent that would dissolve it.
Previous attempts to make sulfur cathodes using bare sulfur or simply coated particles could not prevent the dramatic reduction of energy-storage capacity as the lithium-sulfur intermediate compounds (polysulfides) created during charging broke free and dissolved away. (Credit: Zhi Wei She, Stanford University)
Each cathode particle is only 800 nanometers (billionths of a meter) in diameter, about one-hundredth the diameter of a human hair.
“It basically worked the first time we tried it,” Cui said. “The sulfur cathode stored up to five times more energy per sulfur weight than today’s commercial materials.
“After 1,000 charge/discharge cycles, our yolk-shell sulfur cathode had retained about 70 percent of its energy-storage capacity. This is the highest performing sulfur cathode in the world, as far as we know,” he said. “Even without optimizing the design, this cathode cycle life is already on par with commercial performance. This is a very important achievement for the future of rechargeable batteries.
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