Si nanoparticles with independently controlled size and oxygen content have been produced by plasma spraying physical vapor deposition followed by the retarded oxidation. These nanoparticles are used as the negative electrode of all-solid-state batteries with sulfide solid electrolyte, and the influence of size and oxygen content on battery performance has been analyzed. The cells containing Si nanoparticles smaller than 150 nm with the oxygen content x in SiO x smaller than 0.1 have attained relatively high capacity and a good stable cyclability simultaneously after 50 cycles. This could be due to the formation of unique and uniform synaptic-like Si network with small Si nanoparticles within the electrode maintaining a firm contact with the Cu foil, which contrasts to large lateral crack formation for the cell with large Si particles.