FeS2 synthesized by hydrothermal method was used for sodium batteries for the first time. A large initial discharge capacity of 771 mAh g− 1 and a high reversible capacity of 521 mAh g− 1 were obtained. The reaction mechanism of electrochemical de-sodiation (charge) was investigated after the initial discharge. Our results have demonstrated that the composition and structure of electrode after the charging to 3.0 V could not recover its original state of FeS2 and a new phase of NaFeS2 was revealed. This should be responsible to the large irreversible capacity in the first cycle and the high discharge and charge plateaus after the initial cycle.