Nickel phosphide particles were synthesized by thermal decomposition of a nickel precursor in a mixed solution of trioctylphosphine and trioctylphosphine oxide. The crystal phase and morphology of samples prepared by changing the solvents, the amount of trioctylphosphine as a phosphorus source, the reaction temperature, and the nickel precursor were characterized using X-ray diffraction and transmission electron microscopy. Spherical Ni5P4 particles with diameters of 500 nm were obtained using nickel acetylacetonate as a nickel precursor at 360 degrees C for 1 h in trioctylphosphine oxide. NiP2 particles with diameters of 200-500 nm were obtained using nickel acetate tetrahydrate at 360 degrees C for 5 h in trioctylphosphine oxide. All-solid-state cells were fabricated using NiP2 particles as an active material and 80Li(2)S center dot 20P(2)S(5) (mol %) glass ceramic as a solid electrolyte. The Li-In/80Li(2)S center dot 20P(2)S(5)/NiP2 cell exhibited an initial discharge capacity of 1100 mAh g(-1) at a current density of 0.13 mA cm(-2) and retained a discharge capacity of 750 rnAh g(-1) after 10 cycles.