Ceramic-in-Polymer Composite Solid Electrolyte Enabled by Metal–Sulfur Interactions with Enhanced Li-Ion Conductivity

Ceramic-in-polymer composite solid electrolytes (SEs) show great potential for meeting the high-performance requirements of all-solid-state batteries (ASSBs) due to the combined benefits of easy processability, tunable Li-ion conductivity, wide electrochemical window, and facile interfacial contact with the Li-metal anode. However, their Li-ion conductivity remains lower than that of the pure ceramic phase, which can be attributed to the highly resistive ceramic/polymer interphase. In this paper, we introduced sulfur-containing functional groups through a less-explored metal/sulfur interaction strategy, enabling simultaneous modification of the polyethylene glycol diacrylate (PEGDA) polymer scaffold and the Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZO) ceramic surface. We elucidated the nature of metal/sulfur interactions, i.e., the preferential coordination interaction between Zr and sulfur, as well as electron-transfer reactions from sulfur to Zr and La atoms. In addition, we unraveled the mechanisms of metal/sulfur interaction-enabled in situ photopolymerization of the PEGDA scaffold and developed a layer-by-layer method that exploits metal/sulfur interactions for manufacturing sulfur-modified LLZO-in-PEGDA composites. This dual-modification strategy effectively promotes Li-ion transport at both LLZO/LLZO and LLZO/PEGDA interphases, resulting in enhanced ionic conductivity and lower activation energy. As a result, the LLZO-in-PEGDA composite exhibited a high conductivity of 5.1 × 10^-4 S cm^-1, exceeding the vendor-reported conductivity of pure LLZO. In addition, the sulfur-modified LLZO-in-PEGDA composites exhibited improved toughness and stretchability, suggesting the potential dual role as a protective layer for electrode materials. The metal/sulfur-interaction-enabled dual modification offers a promising strategy that can be broadly applied to the rational design of ceramic/polymer composite materials.