CAREER: Electronic Transport in Sulfide-Based Lithium Solid Electrolytes

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NON-TECHNICAL SUMMARYLithium-ion batteries have succeeded in powering portable electronics, but they face the challenges of safety and energy density for applications in electrification transportation and grid storage. Solid-state batteries that utilize nonflammable solid electrolytes (SEs) are being considered as a promising approach for safe and high-energy storage systems. As an electrochemical energy storage system, a lithium-ion battery works in a way that only ions shuttle between cathode and anode through an electrolyte inside the battery while electrons flow through the external circuit during charge and discharge. The electrolyte should have a high ionic conductivity but low electronic conductivity to ensure that the electrons cannot travel directly inside the battery to cause self-discharge. Significant efforts have been devoted to improving the ionic conductivity of battery SEs. However, the electronic conductivity, which has important implications in the life-time, energy density, and cycling stability of solid-state batteries has rarely been studied. This CAREER proposal aims to study the mechanisms of electronic transport in sulfide-based Li solid electrolytes. The ultimate goal is to gain critical scientific insights for designing electronically insulating SEs for high-performance solid-state batteries. The multi-disciplinary research provides multiple opportunities for training graduate and undergraduate researchers. The project involves curriculum development and pedagogical innovations for teaching charge transport in solids, and includes multiple outreach activities to K-12 students, particularly those from historically and traditionally underrepresented groups. The proposed research is expected to be impactful with respect to the maintenance and advancement of the US battery technology leadership and achievement of the Nation’s strategic goal of full decarbonization by 2050. The education efforts are also expected to be helpful for STEM education and workforce development in the field of energy storage. TECHNICAL SUMMARYThis hypothesis-driven proposal aims to understand the mechanisms of electronic transport in sulfide-based lithium solid electrolyte (SEs), including binary Li2S-P2S5, Li10GeP2S12, and Li6PS5Cl. The research goals of the project are to determine the intrinsic electronic conductivity of Li SEs, reveal their voltage dependence, and identify the dominant causes and charge carriers for the electronic conduction in Li SEs. These goals are achieved through a combination of experimental and theoretical approaches, including synthesis of ceramic SEs with controlled composition, crystallinity, and microstructure, advanced and in-situ electrochemical measurements based on the Hebb-Wagner approach, and theory of defect equilibria and transport for minority carriers in mixed ionic and electronic conductors. The proposed research is expected to provide critical insights to understand defect chemistry and transport of electronic carriers in sulfide-based Li SEs and establish design principles of electronically insulating SEs for high-energy-density, long-calendar-life solid-state batteries. This proposal aims also to implement an innovative approach for the introduction of “solid state ionics (SSI)” – i.e., transport and reactions of ionic and electronic defects in solids – to students in the field of electrochemical energy storage. SSI play a critical role in the discovery and domination of lithium-ion batteries, but the instruction of SSI-related topics has been primarily focused on oxide-based materials in the context of fuel cells for energy conversion applications. Another educational goal for the proposal is to develop a learning module with the aid of virtual reality technology for teaching three-dimensional, tortuous, and anisotropic charge transport in solids.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.