Pulverized FeS2 (pyrite) gives different discharge test results with as-received FeS2 electrodes. The as-received FeS2 electrode shows three voltage plateaus during the discharge test. However, the ball-milled FeS2 electrode shows two voltage plateaus. To interpret this result, the effect of FeS2 particle size on electrochemical reactions is investigated by unit cell discharge tests, SEM and XRD. As a result, it is found that the transition reaction product (Li2 + xFe + xS2) of FeS2 explains the difference. The as-received FeS2 reacts according to three reaction steps (FeS2 → Li3Fe2S4 → Li2 + xFe1 + xS2 → LiFe2S4). However, ball-milled FeS2 reacts without the Li2 + xFe1 + xS2 stage. In this study, this result is explained by the difference in electrochemical reaction mechanism. The as-received FeS2 has a larger radius than the ball-milled FeS2. Therefore, the lithium ion has to diffuse into the FeS2 unreacted core, and Li2 + xFe1 + xS2, the transition reaction product of as-received FeS2, is formed during this stage.
