Garnet-type Li₇La₃Zr₂O₁₂ (LLZO) ceramics has been considered as an ideal solid-state electrolyte for Li metal cells because of its high ionic conductivity and relatively stable interface with Li. However, it is electrochemically incompatible with some high-voltage cathodes, e.g. LiCoO₂. In this work, a nanoscale Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) fast ion conductor was coated on LiCoO₂ (only 1 wt% LATP), bringing obviously enhanced interfacial compatibility with a composite electrolyte composed of Al, Nb-codoped LLZO and polyethylene oxide (PEO). A free-standing, flexible and ultrathin (20 μm) electrolyte membrane was successfully fabricated by a facile and scalable route, even though with a high ceramics content (67 wt%). Quasi-solid-state coin and pouch-type Li cells were assembled with the LATP-coated LiCoO₂ cathode, free-standing composite electrolyte and Li anode, together with soft interface modification by in-situ polymerization. The cells show stable cycling due to combined factors of enhanced electrode/electrolyte compatibility, ultrathin nature of the electrolyte membrane and the in-situ built soft interface. The pouch cells can be cycled for 300 cycles at 0.3 C and 60 °C with 80% retention. The pouch cells can endure abuse tests of bending, cutting and nail penetration. At a practical LiCoO₂ loading of 3 mAh cm⁻², the Li|LiCoO₂ pouch cell still shows stable cycling with 90% retention after 100 cycles at 60 °C (0.2 C charge/0.5 C discharge). This work provides a practical method to fabricate high-performance solid-state Li cells.