The rate capability of lithium cells containing a nano-FeS2 pyrite sample was compared to that of micrometer-sized FeS2 under an active material loading equivalent to that of commercial lithium battery electrodes. The nano-FeS2 sample had an average particle size of 0.5 mum and each particle consisted of nano-FeS2 crystals on the order of 50 mum. The rate capability of nano-FeS2 was superior to that of the FeS2 electrodes with an average crystal size of 10 mum but no improvement was found relative to the FeS2 electrodes having an average crystal size of 1 mum. The rate capability of commercial Li/FeS2 batteries may be limited by the ion conductivity of the electrolyte when FeS2 samples have crystal sizes smaller than 1 mum.