The new electrochemical reaction mechanism of Na/FeS2 cell at ambient temperature

•We have fabricated the FeS2 particle by hydrothermal process.•We have examined the electrochemical behaviors of FeS2 with sodium.•We have clarified the charge reaction mechanism of Na/FeS2 cell for the first time.

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.