We prepared thick and dense-crystalline LiNi1/3Co1/3Mn1/3O2 (NMC) composite films at room temperature that can work well as cathodes in all-solid-state battery cells. The thick films were fabricated by aerosol deposition using NMC powder (D50 = 10.61 μm) as a source material. Commercially-obtained NMC powder did not form films at all on silicon wafer substrates, and cracking of the substrates was observed. However, a few tens of nanometer coating with amorphous niobium oxide resulted in the deposition of 7 μm-thick crystalline dense composite films. The films were successfully fabricated also on Li+-conductive glass-ceramic sheets with 150 μm in thickness, and all-solid-state batteries were fabricated. The solid-state battery provided a cathode-basis discharge capacity of 152 mAh g−1 (3.0–4.2 V, 0.025 C, 333 K) and repeated charge–discharge cycles for 20 cycles.