Among the biggest complaints inhibiting growth in the electric vehicle market is the limited lifespan and range of lithium-ion batteries. Consumers fear being stranded far from home with long wait times at recharging stations. A promising area of research has focused on layer-structured metal oxide cathodes. Specifically, a material known as lithium nickel manganese cobalt oxide, shortened to NMC811, has been the subject of intense study as a cathode material due to its low cost and high energy capacity.
The rub with NMC811 is it suffers from cell performance degradation during cycling as a result of oxygen release (the process of completely charging and then draining a battery is a cycle). The released oxygen can also oxidize electrolytes by generating gases and other undesirable byproducts that can eventually cause safety hazards, such as fires.
New research published in Small has reported a promising solution to increasing the lifespan of lithium-ion batteries (and earned a spot on the back cover ). Led by the University of Arkansas, the researchers applied nanoscale coatings of zirconium sulfide to prefabricated NMC811 cathodes by means of atomic layer deposition. The sulfide coating, just two billionths of a meter thick, helped capture the released oxygen (or "scavenged" it as the researchers write) by transforming the coating from a sulfide into a sulfate. That is to say, the additional oxygen transformed the coating from ZrS2 to Zr(SO4)2.
The conversion has proven to be very effective at protecting the battery electrolyte from decomposition. In addition, the resultant sulfate coating further inhibits undesirable reactions, stabilizes the interface between NMC811 and the electrolyte, suppresses microcracking and maintains the structural stability of the NMC811 cathode.
Performance Benefits
Consequently, the sulfate coated NMC811 cathode has demonstrated extraordinary performance. How extraordinary? Without the coating, bare NMC811 cathodes will survive for approximately 200 cycles. The new coating increased the cycling performance of NMC811 cathodes to more than 1,000 cycles. Furthermore, the coating/cathode combination helped the battery retain 60% of its charge after 1,300 cycles.
The project is sponsored by the U. S. Department of Energy. The principal investigator is Xiangbo "Henry" Meng, an associate professor in mechanical engineering at the U of A. Meng first discovered sulfides are a novel class of coatings that could convert into sulfates in-situ in battery cells. He describes such coatings as "robust, clean and antioxidative protective layers on battery cathodes." So far, Meng has verified such sulfide-sulfate conversions with a variety of sulfides (such as Li2S, ZrS2, Al2S3, ZnS and Cu2S). The research is still on-going.
Ultimately, this work advances understanding of interface engineering while paving a new technical pathway for commercializing NMC811 cathodes. Such technologies can be applied to the wide range of cathodes currently used in our cell phones and laptops to extend their lifetime and improve their safety.
Kevin Velasquez, a Ph.D. student in the Meng Nano & Energy Lab , was first author on the paper and tested cathode coatings in the lab using a coin cell, which are commonly used in low power electronics like key fobs, watches and calculators. Meng is the corresponding author on the paper and oversaw all research.
Meng's research focuses on synthesis of new inorganic, organic, and hybrid nanomaterials in precisely controllable modes at the atomic and molecular level, and development of high-performance energy-storage battery systems. To date, Meng has four patents issued, 15 patents pending, and six more intellectual property disclosures, five of which are related to sulfide coatings.
Co-authors on paper included Jiyu Cai, Taohedul Islam, Hua Zhou, Wenquan Lu, Fumiya Watanabe and Yuzi Liy. Islam is a postdoctoral fellow at the U of A, while Cai, Lu, Zhou, and Kiu are all affiliated with the Argonne National Laboratory. Watanabe is affiliated with University of Arkansas, Little Rock.
Meng noted that several large tech companies were interested in the results, and would work with the Argonne National Laboratory to test the coatings on different batteries.
