CAREER: Structural Implications of Anion Redox in Li-Rich Sulfide Cathodes for Li-ion Batteries

Part 1: Non-Technical Summary

Batteries are energy storage devices that convert chemical energy to electrical energy and are critical technologies to realizing a fully electrified energy infrastructure. The rechargeable batteries that store the most energy per unit mass are lithium ion (Li-ion) batteries. Li-ion batteries store electrons in solid materials composed of transitions metals, like nickel and cobalt, and anions (negatively charged ions), usually oxygen.

As electrons are introduced into (removed from) the material, the transition metal is reduced (oxidized) and Li cations are incorporated (removed) so the net charge of the material remains neutral. In conventional materials, the anions are bystander atoms that only act to hold the transition metals together. With this CAREER award, supported jointly by the Solid State and Materials Chemistry program in NSF’s Division of Materials Research and the Electrochemical Systems program in NSF’s Division of Chemical, Bioengineering, Environmental and Transport Systems, the principal investigator and her research group at the California Institute of Technology investigate the possibility to store electrons on both the transition metals and the anions in the material to increase the number of electrons that can be stored.

Instead of oxygen, which is very difficult to oxidize, materials with sulfur anions called sulfides are prepared and studied. They change the physical orientation of the anions relative to the transition metal atoms to determine how the material’s structure affects anion oxidation. The research advances the fundamental understanding of anion oxidation and reduction allowing for the design of new materials with high energy densities that leverage both the transition metals and the anions to store electrons.

Further, the research uses materials that have abundant resources to promote domestic supply chains of energy storage devices. As part of this CAREER award, the principal investigator also integrates the results of her work and other sustainability-related concepts into the General Chemistry course and develops new outreach modules for high school outreach that are focused on the implementation of research-based inquiry.

Part 2: Technical Summary

Conventional Li-ion battery cathodes store charge via intercalation chemistry in metal oxides that contain problematic metals like Co and Ni. Intercalation chemistry relies on transition metal redox to provide charge compensation for the (de)intercalation of mobile Li+ and yields, at most, one electron per transition metal. With this CAREER award, the principal investigator studies mechanisms beyond intercalation chemistry, namely anion redox in Li-rich metal sulfides.

Anion oxidation in sulfides can yield very high energy densities thanks to the high capacities, despite the low voltage. To develop a fundamental understanding of anion redox in Co- and Ni-free Li-rich sulfides the research is focused on the structural aspects that both control anion redox and are a consequence of anion redox. One approach is to introduce cation vacancies to provide structural flexibility and promote formation of persulfides.

Sulfur oxidation usually causes the formation of S-S bonds. It is hypothesized that the absence of cations allows the material to distort to form the S-S bonds. However, the making and breaking of bonds could slow down the charge storage processes, so other efforts include tuning the crystal chemistry and structure to prevent the formation of persulfides and stabilize holes in the S p bands.

Additionally, the principal investigator and her research group study the ability of stacking sequences and anion sublattice arrangements to direct anion redox. The materials are characterized using state-of-the-art tools both in-house and at national facilities. The results of the study are digested into vignettes accessible to freshman students in Caltech's General Chemistry course to showcase the direct ties between pioneering research and fundamental chemistry concepts.

Plans also include developing, testing, and eventually publishing outreach modules based on investigative learning environments with a theme of battery chemistry research questions.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.