The current generation of lithium-ion batteries uses graphite as an anode, which has a relatively low capacity and could potentially be replaced with a silicon anode that has a higher capacity and low environmental impact. This is a highly promising direction for research – yet elusive, as batteries with silicon anodes with a large particle size tend to have shorter lives, generally less than 50 cycles.
But when researchers try using nanoparticles of silicon, aluminum and bismuth, they find that these nano-sized alloy anodes still suffer short cycle lifes and high cost. Now, however, a team of researchers from the University of Maryland and the US Army Research Laboratory may have found a new approach to fixing this degradation problem: the electrolyte.
These researchers have made an electrolyte that forms a protective layer on silicon; this layer is stable and resists the swelling that normally occurs in silicon anode particles. The new electrolyte – rationally designed with underlying principles in place – gives the anode particles room for the silicon to swell inside the protected layer. The researchers report their work in a paper in Nature Energy.
"Our research proves that it is practical and possible to stably cycle silicon, aluminum and bismuth particles as lithium ion battery anodes, simply with a rationally designed electrolyte, which has been regarded as unachievable before," said Ji Chen from the Department of Chemical and Biomolecular Engineering at the University of Maryland and a lead author of the paper.
"The energy density of the battery is determined by the electrodes, while the performance of the battery is critically controlled by the electrolytes. The designed electrolytes enable the use of micro-sized alloy anodes, which will significantly enhance the energy density of the battery, " said Xiulin Fan, a co-first author from the University of Maryland, and now a professor at Zhejiang University in China.
"Current efforts by combination of molecular modeling and experimental provided a clear path to a new direction to rationally design the electrolytes that enable long cycle life for high capacity silicon anodes opening a path to developing high energy batteries for a warfighter, " said Oleg Borodin, a collaborator from the Army Research Laboratory.
The newly designed electrolyte forms a protective layer over the silicon nanoparticles that make up the anode in a lithium-ion battery. Image: University of Maryland.
Current electrolyte design for silicon anodes aims to form a uniform polymer layer called the solid electrolyte interface (SEI) on the anode; this layer is flexible and strongly bonds with silicon. Unfortunately, the strong bonding between the polymer SEI and the silicon anode forces the SEI to experience the same volume change as the anode particles when they swell, leading both the particles and the SEI to crack during battery operation.
"After extensive research on silicon electrodes, the battery community has reached a consensus that the micro-sized silicon anodes cannot be used in commercial lithium-ion batteries," said Chunsheng Wang, a professor of chemical and biomolecular engineering at the University of Maryland. "We successfully avoided the SEI damage by forming a ceramic SEI that has a low affinity to the lithiated silicon particles, so that the lithiated silicon can relocate at the interface during volume change without damaging the SEI. The electrolyte design principle is universal for all alloy anodes, and opens a new opportunity to develop high energy batteries."
Challenges still remain for the commercialization of the electrolyte; for example, the voltage window of 4.2V still needs to be expanded, Wang said.
