Composite solid-state electrolytes have received significant attention due to their combined advantages as inorganic and polymer electrolytes. However, conventional ceramic fillers offer limited ion conductivity enhancement for composite solid-state electrolytes due to the space-charge layer between the polymer matrix and ceramic phase. In this study, we develop a ferroelectric ceramic ion conductor (LiTaO₃) as a functional filler to simultaneously alleviate the space-charge layer and provide an extra Li⁺ transport pathway. The obtained composite solid-state electrolyte comprising LiTaO₃ filler and poly (vinylidene difluoride) matrix (P-LTO15) achieves an ionic conductivity of 4.90 × 10⁻⁴ S cm⁻¹ and a Li⁺ transference number of 0.45. The polarized ferroelectric LiTaO₃ creates a uniform electric field and promotes homogenous Li plating/stripping, providing the Li symmetrical batteries with an ultrastable cycle life for 4000 h at 0.1 mA cm⁻² and a low polarization overpotential (~50 mV). Furthermore, the solid-state NCM811/P-LTO15/Li full batteries achieve an ultralong cycling performance (1400 cycles) at 1 C and a high discharge capacity of 102.1 mAh g⁻¹ at 5 C. This work sheds light on the design of functional ceramic fillers for composite solid-state electrolytes to effectively enhance ion conductivity and battery performance.

The poor contact and side reactions between Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) and lithium (Li) anode cause uneven Li plating and high interfacial impendence, which greatly hinder the practical application of LATP in high-energy density solid-state Li metal batteries. In this work, a multifunctional ferroelectric BaTiO₃ (BTO)/poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene) (P[VDF-TrFE-CTFE]) composite interlayer (B-TERB) is constructed between LATP and Li metal anode, which not only suppresses the Li dendrite growth, but also improves the interfacial stability and maintains the intimate interfacial contact to significantly decrease the interfacial resistance by two orders of magnitude. The B-TERB interlayer generates a uniform electric field to induce a uniform and lateral Li deposition, and therefore avoids the side reactions between Li metal and LATP achieving excellent interface stability. As a result, the Li/LATP@B-TERB/Li symmetrical batteries can stably cycle for 1800 h at 0.2 mA cm⁻² and 1000 h at 0.5 mA cm⁻². The solid-state LiFePO₄/LATP@B-TERB/Li full batteries also exhibit excellent cycle performance for 250 cycles at 0.5 C and room temperature. This work proposes a novel strategy to design multifunctional ferroelectric interlayer between ceramic electrolytes and Li metal to enable stable room-temperature cycling performance.