Mixed Ion Electron Conductor (MIEC) Cascade Electrodes for High Density Energy Storage in Li2O2

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The lithium (Li)-air battery, with its potential energy density close to 1,700Wh/kg, is a promising battery solution for electric vehicles and renewable energy storage. In addition, light-weight and low-volume energy storage is crucial for a broad range of mobile power supply needs. This project will characterize a number of chemical processes that are relevant to storing energy using a reversible reaction between lithium and oxygen. The lithium-air battery system has the potential to be significantly lighter than conventional lithium-ion batteries of similar capacity. Current Li-air technology suffers from low efficiency and energy capacity. Battery cell design and operating conditions can be modified in ways to increase performance, but these conditions create a number of new complications, particularly with regard to materials durability. This project addresses a battery using solid materials (in contrast to conventional cells using organic liquid electrolytes). The research will determine the chemical nature and physical properties of various requisite solid-solid interfaces that can influence the viability of this battery design. The research will also provide fundamental knowledge of materials that are applicable to safer solid-state designs for conventional Li-ion batteries as well. Several educational outreach efforts will also be undertaken in this project. The PI will engage in a research experience for teachers (RET) program for local community college instructors for them to gain direct exposure to energy research and to incorporate related concepts into their curricula. Several undergraduate research internships will also be provided, and a series of educational web-modules related to batteries will be created.Li-O2 batteries have received recent attention due to their high theoretical energy density. To date these devices are still regarded as impractical due to the poor conducting character of the Li2O2 product (resulting in self-limiting discharge), as well as parasitic reactions with electrolyte solvents. This project will explore chemistries associated with a novel approach to raise the conductivity of Li2O2 through adjustments to operating conditions and doping. The approach utilizes an all-solid-state cell architecture. There is presently very little knowledge of the growth mechanisms and polarization behavior for the Li-O2 redox system under these conditions. The fundamental research will characterize charge carrier mobility, interfacial stability, and dopant chemistry with the aim of understanding how to engineer reversible, energy-dense storage systems based on Li-O2 redox. A key aspect of this work is a multi-faceted methodology for characterization of 'buried interfaces' between interacting solid materials, which are notoriously difficult to access. The project will utilize a combination of in-situ XPS, impedance spectroscopy, and cross-sectional aberration-corrected electron microscopy to characterize the properties of these interfaces and their evolution as a function of time, temperature, and polarization. While emphasis is placed on characterizing interfacial chemistries between Li2O2 and solid electrochemical materials, the methods and systems studied will yield insight that is transferrable to problems of broader interest in solid state ionics, particularly in the growing field of solid state Li-ion batteries.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.