Understanding Shuttling Effect in Sodium Ion Batteries for the Solution of Capacity Fading: FeS2 as an Example

Pyrite (FeS2) has been regarded as one of the most promising electrode materials for sodium ion batteries owing to high theoretical capacity and low cost. However, the short cycle life of FeS2 electrode in sodium storage hampers its further development. Although some researchers focused on the mechanism of capacity fading in Na/FeS2 battery, such as irreversible crystal transform, terrible volume change, and so on, whether and how the shuttling effect of FeS2 electrode exists in half-cell or full-cell systems still remain so far. In this work, the shuttling effect on sodium ion batteries with FeS2 as electrode is investigated systematically in both half-cell and full-cell systems and it is confirmed the shuttle of Fe element and polysulfides simultaneously. These shuttling effects were influenced significantly by some factors. In detail, excessive voltage will cause oxidation for shuttled Fe and polysulfides and side reactions often take place at low voltage; carbonate electrolyte can react with polysulfides; stable solid electrolyte interface could prevent the loss of elements. According to these influence factors, as an example, a designed nano FeS2@C composite was synthesized to limit the shuttling effects. As a result, in a large operating voltage range of 0.01–3 V, it shows high capacity (about 600 mAh g–1) and good cyclic performance with ether-based electrolyte.