Highly Reversible Sodium Ion Batteries Enabled by Stable Electrolyte-Electrode Interphases

The sodium ion battery (NIB) is a promising alternative technology for energy storage systems because of the abundance and low cost of sodium in the Earth's crust. However, the limited cycle life and safety concerns of NIBs hinder their large-scale applications. Here, we report a nonflammable localized high concentration electrolyte (sodium bis(fluorosulfonyl)imide-triethyl phosphate/1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (1:1.5:2 in molar ratio)) for highly reversible NIBs. By using a cryo-transmission electron microscope, it was found that an ultrathin (3 nm) and robust interphase layer formed on the cathode surface can block transition metal dissolutions and minimize surface reconstructions of the cathode. The inorganic-rich solid electrolyte interphase formed on the hard carbon (HC) surface minimized undesirable reactions between HC and the electrolyte. These stable interphases enabled high Coulombic efficiency and long-term stable cycling of the HC anode and the NaCu1/9Ni2/9Fe1/3Mn1/3O2 cathode. The insights obtained in this work can be used to further improve the cycling stability and safety of rechargeable batteries.