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The study in the American Chemical Society's ACS Applied Energy Materials describes a previously unknown mechanism by which lithium gets trapped in batteries, thus limiting the number of times it can be charged and discharged at full power.
[...] The Rice lab of chemical and biomolecular engineer Sibani Lisa Biswal found a sweet spot in the batteries that, by not maxing out their storage capacity, could provide steady and stable cycling for applications that need it.
Biswal said conventional lithium-ion batteries utilize graphite-based anodes that have a capacity of less than 400 milliamp hours per gram (mAh/g), but silicon anodes have potentially 10 times that capacity. That comes with a downside: Silicon expands as it alloys with lithium, stressing the anode. By making the silicon porous and limiting its capacity to 1,000 mAh/g, the team's test batteries provided stable cycling with still-excellent capacity.
[...] The team led by postdoctoral fellow Anulekha Haridas tested the concept of pairing the porous, high-capacity silicon anodes (in place of graphite) with high-voltage nickel manganese cobalt oxide (NMC) cathodes. The full cell lithium-ion batteries demonstrated stable cyclability at 1,000 mAh/g over hundreds of cycles.
Some cathodes had a 3-nanometer layer of alumina (applied via atomic layer deposition), and some did not. Those with the alumina coating protected the cathode from breaking down in the presence of hydrofluoric acid, which forms if even minute amounts of water invade the liquid electrolyte. Testing showed the alumina also accelerated the battery's charging speed, reducing the number of times it can be charged and discharged.
There appears to be extensive trapping as a result of the fast lithium transport through alumina, Haridas said. The researchers already knew of possible ways silicon anodes trap lithium, making it unavailable to power devices, but she said this is the first report of the alumina itself absorbing lithium until saturated. At that point, she said, the layer becomes a catalyst for fast transport to and from the cathode.
"This lithium-trapping mechanism effectively protects the cathode by helping maintain a stable capacity and energy density for the full cells," Haridas said.
More information: Anulekha K. Haridas et al, ALD-Modified LiNi0.33Mn0.33Co0.33O2 Paired with Macroporous Silicon for Lithium-Ion Batteries: An Investigation on Lithium Trapping, Resistance Rise, and Cycle-Life Performance$, ACS Applied Energy Materials (2019). DOI: 10.1021/acsaem.9b01728
(Score: 4, Informative) by The Mighty Buzzard on Monday January 27 2020, @04:44PM
They're saying that graphite-anode batteries have ~400mAh/g, silicon-anode batteries could potentially have 10x that (~4,000mAh/g), but that making the silicon porous (which is essentially using less silicone, thus having a less effective anode) allows them to achieve more than twice the energy storage density (~1,000mAh/g) without sacrificing the longevity that graphite brings to the table.
tl;dr Batteries with twice the charge per cubic inch that don't wear out any faster.
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