A new rechargeable battery technology could double the output of current lithium ion cells, an advance that promises to extend electric vehicle ranges and the time between mobile phone charges.
By using a ceramic, solid-state electrolyte, engineers at the University of Michigan have harnessed the power of lithium metal batteries without the historic issues of poor durability and short-circuiting. Their breakthrough could lead to longer-lasting drop-in replacements for lithium ion batteries.
"This could be a game-changer - a paradigm shift in how a battery operates," said Jeff Sakamoto, a U-M associate professor of mechanical engineering who led the work.
The first rechargeable lithium metal batteries contained combustible liquid electrolytes. Furthermore, lithium atoms that moved between the electrodes tended to build dendrites on the electrode surfaces, eventually shorting the battery and igniting the electrolyte.
Lithium ion batteries followed, replacing lithium metal with graphite anodes, which absorb the lithium and prevent dendrites from forming. This increased safety at the cost of energy density.
According to U-M, graphite anodes in lithium ion batteries hold one lithium ion for every six carbon atoms, giving it a specific capacity of approximately 350mAh/g. The lithium metal in a solid-state battery has a specific capacity of 3,800mAh/g.
Current lithium ion batteries have a total energy density around 600Wh/L at the cell level. In principle, solid-state batteries can reach 1,200Wh/L.
To solve lithium metal's combustion problem, U-M engineers created a ceramic layer that stabilises the surface by preventing the build-up of dendrites, which allows batteries to harness the energy density and high-conductivity of lithium metal without the inherent dangers of fire or degradation over time.
"What we've come up with is a different approach - physically stabilising the lithium metal surface with a ceramic," Sakamoto said. "It's not combustible. We make it at over 1,800oF in air. And there's no liquid, which is what typically fuels the battery fires you see. You get rid of that fuel, you get rid of the combustion."
In earlier solid-state electrolyte tests, lithium metal grew through the ceramic electrolyte at low charging rates, causing a short circuit. U-M researchers are said to have overcome this with chemical and mechanical treatments that provide a pristine surface for lithium to plate evenly, effectively suppressing the formation of dendrites or filaments. Not only does this improve safety, it enables a dramatic improvement in charging rates, Sakamoto said.
"Up until now, the rates at which you could plate lithium would mean you'd have to charge a lithium metal car battery over 20 to 50 hours [for full power]," Sakamoto said. "With this breakthrough, we demonstrated we can charge the battery in three hours or less.
"We're talking a factor of 10 increase in charging speed compared to previous reports for solid state lithium metal batteries. We're now on par with lithium ion cells in terms of charging rates, but with additional benefits. "
Repeatedly exchanging ions between the cathode and anode produces visible degradation.
In tests on the ceramic electrolyte no visible degradation was observed after long term cycling.
"We did the same test for 22 days," said Nathan Taylor, a U-M post-doctoral fellow in mechanical engineering. "The battery was just the same at the start as it was at the end. We didn't see any degradation. We aren't aware of any other bulk solid-state electrolyte performing this well for this long."
The group's findings are published in the Journal of Power Sources.
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