
About five years ago, said the university, Professor Gerbrand Ceder calculated that lithium ions in lithium iron phosphate cathodes should move very quickly, but measurements showed they were actually moving slowly in cells.
“If transport of the lithium ions was so fast, something else had to be the problem,” said Ceder.
Further calculations showed that lithium ions were restricted by limited access to crystal channels in battery materials.
“If a lithium ion at the cathode surface is directly in front of a tunnel entrance, there’s no problem: it proceeds efficiently into the tunnel,” said the Institute. “But if the ion isn’t directly in front, it is prevented from reaching the tunnel entrance because it cannot move to access that entrance.”
Ceder and student Byoungwoo Kang have created a surface structure by coating the cathode with a lithium ion conductor – lithium phosphate glass – to allow the ions to move quickly around the outside of the cathode, much like a ring road around a town. “When an ion travelling along this reaches a tunnel, it is instantly diverted into it,” said MIT.
Using their new processing technique, the two went on to make a small battery that could be fully charged or discharged in 10 to 20 seconds – compared with the six minutes it takes for a reference cell to charge or discharge.
Ceder claims tests show the modified cells will also have a longer cycle life.
The work was supported by US government funding and the process has already been licensed by two companies.
It is described in the journal Nature this week.
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