Ionic‐Conducting and Robust Multilayered Solid Electrolyte Interphases for Greatly Improved Rate and Cycling Capabilities of Sodium Ion Full Cells

Abstract The energy storage performance of sodium‐ion batteries has been greatly improved by pairing ether‐based electrolytes with high‐capacity alloy‐type anodes. However, the origin of this performance improvement by a unique electrode/electrolyte interface has yet to be explored. To understand such results, herein, the deterministic and distinct interfacial chemistries and solid electrolyte interphase (SEI) layers in both the ether‐ and ester‐based electrolytes are described, as verified by post mortem, in‐depth X‐ray photoelectron spectroscopy, and electron energy loss spectroscopy analyses, employing a hierarchical Bi/C composite anode as the model system. In the ether‐based electrolyte, fast sodium‐ion storage kinetics and structural integrity are achieved due to the highly ionic‐conducting and robust multi‐layered SEI consisting of an inner dense bismuth‐containing inorganic and outer polyether layer. No drastic capacity decay is observed over 1000 cycles and high capacity retention of 89% is achieved, increasing rates from 0.1 to 10 A g −1 . The benefit of this SEI is confirmed to demonstrate a high energy density of 162 Wh kg −1 in a full‐cell and a high areal capacity of 3.3 mAh cm −2 even at a high mass loading of 11 mg cm −2 , which is nearly equivalent to the loading of commercial lithium‐ion battery anodes.