Oxygen-Vacancy-Mediated Triple Synergy in Gd-Doped Ceria Fillers Enables Dendrite-Free Solid-State Lithium Metal Batteries.

Solid-state polymer electrolytes (SPEs) suffer from a persistent trade-off between sluggish ion transport and unstable electrode-electrolyte interface. Herein, a synergistic "polymer grafting-inorganic defect" strategy is proposed to simultaneously address both bottlenecks. A low-crystallinity poly(vinylidene fluoride) (PVDF) matrix is first obtained via lithium sulfonate grafting (denoted as PVDF-STFE), which disrupts polymer chain packing and provides additional Li+ hopping sites. Into this matrix, we incorporated gadolinium-doped ceria (GDC) fillers rich in intrinsic oxygen vacancies. These vacancies act as strong Lewis acid sites that perform three coordinated functions: (i) anchoring TFSI- anions to promote salt dissociation, raising the Li+ transference number to 0.84; (ii) further disrupting polymer chain ordering to reduce crystallinity, lowering the activation energy for ion transport; and (iii) homogenizing interfacial Li+ flux to suppress dendritic growth. Control experiments using pristine CeO2, structurally identical but vacancy-deficient, confirm that oxygen vacancies, not mere physical filling, dominate these synergistic effects. The composite electrolyte comprising PVDF-STFE and GDC (denoted as PS-GDC) delivers a high room-temperature ionic conductivity (8.91 × 10-4 S cm-1), a wide electrochemical window over 5.0 V, and enables dendrite-free Li deposition as evidenced by post-cycling scanning electron microscopy (SEM). Quantitative X-ray photoelectron spectroscopy (XPS) reveals the formation of a LiF-enriched (65.11%) and parasitic-poor (24.02% Li-O species) solid electrolyte interphase. Consequently, the assembled LiFePO4||Li cell retains 71.4% of its initial capacity after 400 cycles at 0.5 C and demonstrates stable operation up to 5 C. This work establishes a defect-mediated interfacial paradigm for designing a robust composite SPE.