Stable Sodium-Based Batteries with Advanced Electrolytes and Layered-Oxide Cathodes

Sodium-ion batteries offer a promising alternative to the more expensive, resource-limited lithium-ion batteries, in particular to accommodate the growing demand for large-scale energy storage. One of the main challenges for sodium-ion batteries, however, is the poor electrolyte stability, which leads to rapid capacity fade during cycling. Recent advances in the lithium-ion-battery field have expanded our understanding of electrolyte compositions and stability, paving the way for better sodium-ion-battery electrolytes. Two of the most promising new classes of electrolytes are evaluated herein with a sodium layered-oxide cathode, for the first time: a localized high-concentration electrolyte (LHCE) composed of sodium bis­(fluorosulfonyl)­imide, dimethyl ether, and tetrafluoropropyl ether and a “highly fluorinated” electrolyte (HFE) composed of 20% fluoroethylene carbonate with a lithium difluorophosphate additive. With a combination of electrochemical and post-mortem characterization techniques, the stability of each electrolyte is assessed with the O3-type Na­(Ni_0.3Fe_0.4Mn_0.3)­O₂ cathode and sodium metal anode. Both electrolytes significantly improve the surface and bulk stability of the cathode, but only the LHCE has a meaningful improvement on sodium metal stability. For the purpose of developing a long-lasting, sodium-ion full cell, both classes of electrolyte show great promise.