Enhancing the Cycling Stability of Sodium Metal Electrodes by Building an Inorganic–Organic Composite Protective Layer

Owing to the natural abundance of sodium resources and their low price, next-generation batteries employing an Na metal anode, such as Na–O₂ and Na–S systems, have attracted a great deal of interest. However, the poor reversibility of an Na metal electrode during repeated electrochemical plating and stripping is a major obstacle to realizing rechargeable sodium metal batteries. It mainly originates from Na dendrite formation and exhaustive electrolyte decomposition due to the high reactivity of Na metal. Herein, we report a free-standing composite protective layer (FCPL) for enhancing the reversibility of an Na metal electrode by mechanically suppressing Na dendritic growth and mitigating the electrolyte decomposition. A systematic variation of the liquid electrolyte uptake of FCPL verifies the existence of a critical shear modulus for suppressing Na dendrite growth, being in good agreement with a linear elastic theory, and emphasizes the importance of the ionic conductivity of FCPL for attaining uniform Na plating and stripping. The Na–Na symmetric cell with an optimized FCPL exhibits a cycle life two times longer than that of a bare Na electrode

Tuning Two Interfaces with Fluoroethylene Carbonate Electrolytes for High-Performance Li/LCO Batteries

Various electrolytes have been reported to enhance the reversibility of Li-metal electrodes. However, for these electrolytes, concurrent and balanced control of Li-metal and positive electrode interfaces is a critical step toward fabrication of high-performance Li-metal batteries. Here, we report the tuning of Li-metal and lithium cobalt oxide (LCO) interfaces with fluoroethylene carbonate (FEC)-containing electrolytes to achieve high cycling stability of Li/LCO batteries. Reversibility of the Li-metal electrode is considerably enhanced for electrolytes with high FEC contents, confirming the positive effect of FEC on the stabilization of the Li-metal electrode. However, for FEC contents of 50 wt % and above, the discharge capacity is significantly reduced because of the formation of a passivation layer on the LCO cathodes. Using balanced tuning of the two interfaces, stable cycling over 350 cycles at 1.5 mA cm–2 is achieved for a Li/LCO cell with the 1 M LiPF6 FEC/DEC = 30/70 electrolyte. The enhanced reversibility of the Li-metal electrode is associated with the formation of LiF and polycarbonate in the FEC-derived solid electrolyte interface (SEI) layer. In addition, electrolytes with high FEC contents lead to lateral Li deposition on the sides of Li deposits and larger dimensions of rodlike Li deposits, suggesting the elastic and ion-conductive nature of the FEC-derived SEI layer