Ultraconcentrated Sodium Bis(fluorosulfonyl)imide- Based Electrolytes for High-Performance Sodium Metal Batteries

Department of Energy Engineering (Battery Science and Technology)Rechargeable Na metal batteries have gained great recognition as a promising candidate for next-generation battery systems, largely on the basis of the high theoretical specific capacity (1165 mAh g−1) and low redox potential (−2.71 V versus the standard hydrogen electrode) of Na metal, as well as the natural abundance of Na and the similarities between these batteries and lithium batteries. Much effort has been dedicated to improving the electrochemical performance of rechargeable Na batteries through the development of high-performance cathodes, anodes, and electrolytes. Nevertheless, the practical application of Na metal batteries is quite challenging because the high chemical and electrochemical reactivity of Na metal electrodes with organic liquid electrolytes leads to low Coulombic efficiencies and limited cycling performance. Severe electrolyte decomposition at the Na metal electrode results in the formation of a resistive and non-uniform surface film, leading to dendritic Na metal growth. To control the Na metal electrode–electrolyte interface for high performance Na metal batteries, considerable efforts have been made to find electrolyte systems that are stable at the Na metal electrode. Using fluoroethylene carbonate (FEC) as an electrolyte additive for in situ formation of an artificial solid electrolyte interphase (SEI) layer could stabilize the anode- electrolyte interface. However, the FEC-derived SEI acted as a resistive layer, impeding the sodiation–desodiation process and reducing the reversible capacity of the anodes. Finding new electrolyte systems that are stable at the Na metal electrode and possess high oxidation durability at high-voltage cathodes is necessary for the development of high-performance Na metal batteries. Very recently, there are some papers which introduced significant breakthroughs in lithium battery electrolytes. It is reported that improving the cycling efficiency of lithium plating/stripping and suppressing the formation of dendritic lithium metal is possible by using highly concentrated electrolytes, even at high current densities. And it is also reported that highly concentrated electroltyes can inhibit the dissolution of transition metals out of the 5 V-class LiNi0.5Mn1.5O4 (LNMO) electrode material and the corrosion of the Al current collector at high voltage conditions. After reading these papers, I thought that applying this highly concentrated electrolyte system to sodium metal batteries could be the solution for improvements in the electrochemical performance of Na metal anodes coupled with high-voltage cathodes. In this study, an ultraconcentrated electrolyte composed of 5 M sodium bis(fluorosulfonyl)imide in 1,2-dimethoxyethane will be introduced for Na metal anodes coupled with high-voltage cathodes. Using this electrolyte, a very high Coulombic efficiency of 99.3% at the 120th cycle for Na plating/stripping is obtained in Na/stainless steel (SS) cells, with highly reduced corrosivity toward Na metal and high oxidation durability (over 4.9 V versus Na/Na+) without corrosion of the aluminum cathode current collector. Importantly, the use of this ultraconcentrated electrolyte results in substantially improved rate capability in Na/SS cells and excellent cycling performance in Na/Na symmetric cells without the increase of polarization. Moreover, this ultraconcentrated electrolyte exhibits good compatibility with high-voltage Na4Fe3(PO4)2(P2O7) and Na0.7(Fe0.5Mn0.5)O2 cathodes charged to high voltages (>4.2 V versus Na/Na+), resulting in outstanding cycling stability (high reversible capacity of 109 mAh g−1 over 300 cycles for the Na/Na4Fe3(PO4)2(P2O7) cell) compared with the conventional dilute electrolyte, 1 M NaPF6 in ethylene carbonate/propylene carbonate (5/5, v/v).ope