To improve the sinterability, a new LATP-x wt% LiBiO 3 composite solid electrolyte was fabricated by adding LiBiO 3 to a NASICON-type Li 1.5 Al 0.5 Ti 1.5 (PO 4 ) 3 (LATP) solid electrolyte synthesized by the sol–gel method. The results showed that the liquid phase formed by the LiBiO 3 additive promoted grain growth during the sintering process, which led to structural stability and an increased relative density of LATP. When the LiBiO 3 addition amount was 2 wt%, the relative density was 94%, and the bulk ionic conductivity was 2.91 × 10 −4 S cm −1 . In addition, to improve the interfacial instability between the solid electrolyte and lithium metal, we prepared a PPS/LATP-LiBiO 3 /PPS composite solid electrolyte with a sandwich structure using a polymer PVDF-HFP-PEO-SN (PPS) cladding method to construct an LFP/SSCEs/Li all-solid-state battery. The electrical property test results showed that the all-solid-state battery still had 92% discharging capacity and nearly 100% coulombic efficiency after 50 cycles at 25 °C and 0.1 C.