A reactor for coating particles includes a stationary vacuum chamber to hold a bed of particles to be coated, a vacuum port in an upper portion of the chamber, a chemical delivery system configured to inject a reactant or precursor gas into a lower portion of the chamber, a paddle assembly, and a motor to rotate a drive shaft of the paddle assembly. The lower portion of the chamber forms a half-cylinder. The paddle assembly includes a rotatable drive shaft extending through the chamber along the axial axis of the half cylinder, and a plurality of paddles extending radially from the drive shaft such that rotation of the drive shaft by the motor orbits the plurality of paddles about the drive shaft.