Criticality of solid electrolyte interphase in achieving high performance of sodium-ion batteries

Sodium-ion batteries (SIBs) have shown tremendous application prospects for large-scale energy storage system in recent years due to the abundant natural sodium resources. Notably, the ideal SEI film on the anode of SIBs plays an indispensable role in realizing excellent electrochemical performance. In this work, we report the CoSe2/Fe3C composite derived from Prussian blue analogue as an anode of SIBs, which forms a favorable SEI film within ether-based electrolytes and exhibits excellent electrochemical performance of ultrahigh initial Coulombic efficiency (96.4 % at 2 A/g) as well as long-term cycling life (412.4 mAh/g at 10 A/g over 2000 cycles). We unveil that the favorable SEI layer formed within ether-based electrolytes is rich in inorganics, and thin and dense in microstructure, all of which promote Na+ transport. The formed SEI also has a high Young’s modulus to resist the volume variation of active materials during cycling. Moreover, the theoretical molecular orbital analysis suggests that the SEI formation process with ether-based electrolytes is driven by the preferential decomposition of sodium salt prior to the solvent decomposition. Overall, this work underscores the significance and origin of forming a friendly SEI layer to achieve rapid and stable sodium storage.