A novel colossal polarization architecture generated at the fast Li-ion conductor–dielectric interface is proposed. A fast Li-ion conductor, La0.57Li0.29TiO3 (LLT), is utilized as the conductive core material, while the ferroelectric barium titanate (BaTiO3; BT) was coated onto the LLT as the shell layer via a liquid phase reaction. After densification of the composites, the shell BT fully transforms into the interdiffusion layer, i.e. (Ba, Li, La)TiO3 (BLLT). The BLLT effectively hinders Li diffusion, leading to a colossal polarization at the core–shell interface. Consequently, the optimized BT–LLT achieves dielectric characteristics at 1 kHz with permittivity of 3.28 × 104.