Understanding and Improving Electrode|Polymer Electrolyte Interfaces for High-Voltage Lithium Metal Batteries

Abstract

Lithium-ion batteries (LIBs) have transformed modern society as a ubiquitous power source for applications ranging from portable electronics to electric vehicles and grid-scale energy storage. However, conventional LIBs, which rely on flammable non-aqueous liquid electrolytes (LEs), are approaching their performance limits and pose significant safety concerns, prompting the need for safer, higher-energy-density alternatives. At the forefront of advancements, solid-state batteries (SSBs) incorporating solid electrolytes (SEs) are being actively studied due to their potential to support high-capacity anodes, compact cell designs, and enhanced safety. While substantial progress has been made in developing SEs with high ionic conductivity, wide electrochemical stability window (ESW), and suitable mechanical properties, critical challenges remain in understanding the complex phenomena occurring at electrode|electrolyte interfaces, where electrochemical reactions take place and ultimately determines cell performance. Among SEs, polymer electrolytes (PEs) have attracted considerable interest due to their flexibility and malleability, which promote intimate contact at electrode|electrolyte interfaces. Despite their practical relevance, detailed interactions at these interfaces in SSBs are not fully understood. Herein, we utilize multi-modal characterization techniques to elucidate interfacial dynamics in PE-based SSBs and demonstrate a strategy to improve cell performance. Our findings reveal a critical distinction between morphologically conformal and ionically conformal interfaces, underscoring that physical conformity alone does not ensure effective ionic transport across the interface. In Chapter 1, we present a brief overview of LIBs and their conventional LEs. We then introduce the concept of SSBs and SEs, with particular emphasis on PEs, lithium metal batteries (LMBs), and the molecular ionic composites (MICs) central to this study. Here we further explore the critical challenges associated with electrode|electrolyte interfaces in PE-based LMBs. Finally, we discuss recent advancements in characterization techniques that enable the investigation of these buried interfaces. In Chapter 2, we investigate interfacial phenomena in high-voltage LMBs using MICs. Through advanced characterization techniques, we reveal that heterogeneities at electrode|electrolyte interfaces deplete the ionically conductive phases of MICs, contributing to cell failure. We demonstrate that interphase engineering can mitigate these effects, providing a pathway toward more stable PEs-based SSBs. Our study reveals a critical discrepancy between morphological and ionic conformality at interfaces in PEs-based SSBs. In Chapter 3, we demonstrate MICs as a tunable electrolyte platform for enabling high-voltage LMBs. We highlight how compositional tailoring enhances electrochemical stability and cycling performance, while also achieving mechanical properties suitable for free-standing membranes without the risk of leakage. In Chapter 4, we build upon our previous work and introduce a tailored MIC electrolyte to improve ionic conformality at electrode interfaces. Using spatially resolved characterization techniques, we directly visualize interfacial heterogeneities and evaluate their impact on electrochemical performance. Our approach effectively mitigates interfacial heterogeneity, leading to enhanced interfacial stability and improved cycling performance in SSBs. In Chapter 5, we summarize the results of our work and propose perspectives of future research directions.