Engineers at Rice University’s Brown School of Engineering have combined a macroporous silicon anode and alumina-coated NMC cathode to provide stable cyclability by means of capacity-limited charging. Their Si-NMC full-cell design with alumina passivation on cathode has exhibited a stable capacity of 1000 mAh/g, with 1.2 times higher energy density than the Si-NMC full-cell without cathode passivation.
A paper on their work is published in ACS Applied Energy Materials.
Illustration courtesy of the Biswal Lab
The study 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.
Maximum capacity puts a lot of stress on the material, so this is a strategy to get capacity without the same degree of stress. 1,000 milliamp hours per gram is still a big jump.—Sibani Lisa Biswal
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, ALD), 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.—Anulekha Haridas
Co-authors are Rice graduate students Quan Anh Nguyen and Botao Farren Song, and Rachel Blaser, a research and development engineer at Ford Motor Co.
Ford’s University Research Program supported the research.
Anulekha K. Haridas, Quan Anh Nguyen, Botao Farren Song, Rachel Blaser, and Sibani Lisa Biswal (2020) “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 doi: 10.1021/acsaem.9b01728