Designing electrolytes for lithium metal batteries: from quasi- solid to hybrid solid-state systems

The development of solid-state electrolytes (SSEs) represents one of the most promising routes toward safer and higher-performance lithium metal batteries (LMBs). Conventional liquid electrolytes offer excellent ionic transport but are not suitable for coupling with lithium metal, suffering from safety issues, poor interfacial stability, and uncontrollable lithium dendrite growth. This thesis work aims to explore, design, and optimize electrolyte systems capable of combining high ionic conductivity, mechanical robustness, and interfacial stability, following a rational progression from liquid-containing to fully solid materials. The work began with the study of in situ electro-initiated polymerization of quasi-solid electrolytes based on poly(1,3-dioxolane) (PDOL). However, the ionic conductivity remained limited and the lithium interface stability was unsatisfactory, motivating the transition to systems offering higher performance. The second stage focused on gel polymer electrolytes (GPEs) incorporating localized high-concentration electrolytes (LHCEs) within the polymer matrix. The polymerization of a fluorinated acrylate-based monomer and a crosslinker led to mechanically robust and highly conductive GPEs, allowing efficient liquid retention, enhanced ionic conductivity, and improved interfacial stability. However, the presence of residual solvent leaves open questions regarding safety. The third system investigated was a solvent-free blend polymer SSE composed of PVDF-HFP, Jeffamine®, and LiTFSI. The development and optimization relied on a data-driven approach combining logistic regression and machine learning to identify formulations offering a balance between mechanical stability and ionic transport. The optimized blend demonstrated good experimental agreement with model predictions. Finally, hybrid solid-state electrolytes (HSSE) combining inorganic and polymer phases were developed. A sulfide-based electrolyte combined with a thin Jeffamine®–LiTFSI interlayer enabled stable Li//Li cycling and demonstrated the potential of interfacial engineering. Each electrolyte system developed in this thesis exhibits distinct advantages and trade-offs, highlighting that progress depends not on maximizing a single property, but on harmonizing functional features to achieve practical solid-state battery technologies.