Solid-state batteries (SSBs) with Li-ion conductive electrolytes made from polymers, thiophosphates (sulfides) or oxides instead of liquid electrolytes have different challenges in material development and manufacturing. For oxide-based SSBs, the co-sintering of a composite cathode is one of the main challenges. High process temperatures cause undesired decomposition reactions of the active material and the solid electrolyte. The formed phases inhibit the high energy and power density of ceramic SSBs. Therefore, the selection of suitable material combinations as well as the reduction of the sintering temperatures are crucial milestones in the development of ceramic SSBs. In this work, the co-sintering behavior of Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) as a solid electrolyte with Li-ion conductivity of ≥0.38 mS/cm and LiFePO₄ with a C-coating (LFP) as a Li-ion storage material (active material) is investigated. The shrinkage behavior, crystallographic analysis and microstructural changes during co-sintering at temperatures between 650 and 850 °C are characterized in a simplified model system by mixing, pressing and sintering the LATP and LFP and compared with tape-casted composite cathodes (d = 55 µm). The tape-casted and sintered composite cathodes were infiltrated by liquid electrolyte as well as polyethylene oxide (PEO) electrolyte and electrochemically characterized as half cells against a Li metal anode. The results indicate the formation of reaction layers between LATP and LFP during co-sintering. At T_s > 750 °C, the rhombohedral LATP phase is transformed into an orthorhombic Li_1.3+xAl_0.3−yFe_x+yTi_1.7−x(PO₄)₃ (LAFTP) phase. During co-sintering, Fe³⁺ diffuses into the LATP phase and partially occupies the Al³⁺ and Ti⁴⁺ sites of the NASICON structure. The formation of this LAFTP leads to significant changes in the electrochemical properties of the infiltrated composite tapes. Nevertheless, a high specific capacity of 134 mAh g⁻¹ is measured by infiltrating the sintered composite tapes with liquid electrolytes. Additionally, infiltration with a PEO electrolyte leads to a capacity of 125 mAh g⁻¹. Therefore, the material combination of LATP and LFP is a promising approach to realize sintered ceramic SSBs.