Pyrite FeS2 for high-rate and long-life rechargeable sodium batteries

Abstract

High-performance rechargeable Na/FeS ₂ batteries showing only the intercalation reaction are obtained by selecting a NaSO ₃CF ₃/diglyme electrolyte and tuning the cut-off voltage to 0.8 V.

It is desirable to develop electrode materials for advanced rechargeable batteries with low cost, long life, and high-rate capability. Pyrite FeS ₂, as an easily obtained natural mineral, has been already commercialized in primary lithium batteries, but encountered problems in rechargeable batteries with carbonate-based electrolytes due to the limited cycle life caused by the conversion-type reaction (FeS ₂ + 4M → Fe + 2M ₂S (M = Li or Na)). Herein, we demonstrate that FeS ₂ microspheres can be applied in room-temperature rechargeable sodium batteries with only the intercalation reaction by simultaneously selecting a compatible NaSO ₃CF ₃/diglyme electrolyte and tuning the cut-off voltage to 0.8 V. A surprisingly high-rate capability (170 mA h g ⁻¹ at 20 A g ⁻¹) and unprecedented long-term cyclability (∼90% capacity retention for 20 000 cycles) has been obtained. We suggest that a stable electrically conductive layer-structured Na _xFeS ₂ was formed during cycling, which enables the highly reversible sodium intercalation and deintercalation. Moreover, 18650-type sodium batteries were constructed exhibiting a high capacity of ∼4200 mA h (corresponding to 126 W h kg ⁻¹ and 382 W h L ⁻¹) and a capacity retention of 97% after an initial 200 cycles at 4 A during charge–discharge. This shows that the production of rechargeable sodium batteries with FeS ₂ microspheres is viable for commercial utilization.