(Digital Presentation) In-Depth Analysis of Interfacial Processes between Lithium Metal and Polymer Electrolyte Using Electrochemical Impedance Spectroscopy and Distribution of Relaxation Times

Abstract

Rechargeable solid-state lithium metal batteries are highly promising systems beyond lithium-ion technology, potentially affording high gravimetric and volumetric energy densities. Despite substantial progress in this field, significant challenges remain, including interfacial changes upon cell operation and achievement of reversible lithium inventory. Here, polymer electrolytes may offer flexible solutions and operational safety, providing compromises among conflicting demands of mechanical robustness and sufficient ionic conductivity, though advancements and tailored design of future polymer materials of functional layers and cell designs require input from insights into interfacial processes, characteristic charge transfer rates and associated resistances at electrode|electrolyte interfaces, respectively. In this work, electrochemical impedance spectroscopy (EIS) is therefore carefully exploited for the analysis of relevant charge transfer processes in case of introduced polymer electrolytes[1], invoking a distribution of relaxation times (DRT) approach. The obtained complex permittivity and conductivity of the materials upon variation of temperature as well as explicit modification of the thin lithium metal electrodes with artificial polymer layers yield characteristic data of the evolution of interfacial resistances and the corresponding charge transfer reactions, in this way demonstrating the significant diagnostic strength of EIS/DRT analysis for future developments. In addition, operando EIS/DRT analysis is done upon plating of lithium metal, complemented by 7Li solid-state NMR spectra acquired at various states during lithium deposition, thereby revealing occurring species and their microstructures based on the 7Li NMR chemical shifts. In summary, EIS/DRT analysis is utilized as powerful diagnostic technique, highlighting how to exploit observable trends of characteristic parameters at electrode|electrolyte interfaces for future design of polymer-based materials as well as high performance cell concepts. [1]Chiou et al., Journal of Power Sources 538 (2022) 231528 Figure 1