The project “LiSi – Lithium‑Solid‑Electrolyte Interfaces” was carried out by the Institute for Energy and Climate Research (IEK) at Forschungszentrum Jülich under the Basic Electrochemistry programme (IEK‑9). It was funded by the Federal Ministry of Economics and Climate Protection and ran from 1 April 2019 to 30 September 2022. The authors, Prof. Dr. Rüdiger‑A. Eichel, Dr. Hans Kungl and Dr. Chih‑Long Tsai, are responsible for the content of the report. The work was performed within a consortium, with the research centre acting as the lead partner and coordinating the collaboration with other institutions that contributed materials, characterization facilities and expertise in polymer chemistry and ceramic synthesis.

The scientific aim of the project was to identify the key factors that limit lithium‑ion conductivity in polymer‑ceramic hybrid solid electrolytes and to use this knowledge to design improved hybrid electrolytes for future lithium‑ion batteries. The main limiting factor identified was the transition resistance at the interface between the ceramic and polymer phases. This resistance originates from the way the polymer is bound to the ceramic surface and from the structural and chemical conditions at the interface. A systematic analysis of these interfacial properties was therefore essential. The project investigated the correlation between the characteristics of the ceramic and polymer components, their surface states, and the resulting lithium‑ion conductivity of the hybrid electrolyte under battery operating conditions, including the relevant electrochemical potentials.

Three fundamental design concepts for ceramic‑polymer electrolytes were examined: (i) layered ceramic‑polymer structures, (ii) ceramic‑in‑polymer composites, and (iii) polymer‑in‑ceramic composites. In the layered design, the polymer layer mainly serves as a protective barrier against reactions between the ceramic electrolyte and the electrode, and lithium ions cross the interface only once. In ceramic‑in‑polymer composites, the overall conductivity can be enhanced by embedding high‑conductivity ceramic particles in a polymer matrix; this improvement depends on the ability of lithium ions to transfer from the polymer into the ceramic particles, which requires a low interfacial resistance. In polymer‑in‑ceramic composites, continuous lithium‑ion pathways are created within a sintered ceramic scaffold infiltrated with polymer; this approach can yield high intrinsic conductivity but may sacrifice mechanical flexibility. Across all three designs, the project found that tailoring the surface chemistry and morphology of the ceramic particles and the polymer network is crucial for minimizing interfacial resistance and achieving high overall conductivity. The literature review included reports of both significant improvements and reductions in conductivity upon adding ceramic particles, underscoring the importance of interface engineering.

The experimental work focused on the synthesis and surface modification of garnet‑type solid electrolytes, specifically Li₆.₆Ga₀.₂LaZrO₁₂ (GaLLZO) and Li₆.₄₅Ga₀.₀₅La₃. The modified surfaces were characterized by advanced techniques to assess their structural, chemical, and electrochemical stability. Although the report does not provide explicit conductivity values, it documents the systematic approach to reducing interfacial resistance and outlines design rules for future hybrid electrolytes that combine high ionic conductivity with mechanical robustness and electrochemical stability. The findings provide a foundation for the next generation of solid‑state lithium‑ion batteries and demonstrate the importance of interdisciplinary collaboration between materials synthesis, surface chemistry, and electrochemical testing.