Department of Energy Engineering (Battery Science and Technology)Alkali metals such as lithium (Li) and sodium (Na) have been considered as an ideal anode for rechargeable batteries in next generation. Li metal has the high specific capacity of 3860 mAh g-1 with reduction potential of -3.04 (versus standard hydrogen electrode) and low density of 0.534 g cm-3. And Na metal has the high specific capacity of 1165 mAh g-1 with reduction potential of -2.71 V (versus standard hydrogen electrode). Nevertheless, the practical application of Na metal and Li metal batteries is quite challenging because the high chemical and electrochemical reactivity of Na metal and Li metal electrodes with organic liquid electrolytes leads to low Coulombic efficiencies and limited cycling performance. Severe electrolyte decomposition at the reactive metal electrode results in the formation of a resistive and nonuniform surface film, leading to dendritic metal growth. To control the Na metal and Li metal electrode???electrolyte interfaces for high performance Na metal and Li metal batteries, considerable efforts will be made to find electrolyte systems that are stable at the metal electrode. Moreover, the underlying mechanism of electrolytes at the electrode???electrolyte interface will be clearly elucidated through electrochemical method and characterization of the electrode???electrolyte interface.
I) For the electrolytes of Na metal batteries, degradation mechanism of Na metal in conventional carbonate???based electrolyte is studied. To mitigate the parasitic reaction between Na metal anode and electrolyte, fluoroethylene carbonate (FEC) is employed as additive and solvent in electrolyte to construct the protective layer on Na metal anode. The underlying mechanism of FEC at the electrode???electrolyte interface is clearly demonstrated by 13C nuclear magnetic resonance, X-ray photoelectron spectroscopy, in???situ differential electrochemical microscopy and in???situ optical microscopy.
II) For the electrolytes of Li metal batteries, fluorinated compounds can be employed as interface modifiers to extend the applicable voltage range of ether-based electrolytes, which are commonly used in Li metal batteries with charging cut-off voltages of lower than 4 V (vs. Li/Li+). In particular, we reveal that 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether promotes the construction of a solid electrolyte interphase as a shield accommodating the destructive stress induced by Li plating and stripping on the Li metal anode, while FEC makes the interface of the Ni-rich cathode electrochemically robust and prevents severe intergranular cracking of the cathode during pre-cycling. Thus, this study provides a promising method of tackling the reductive and oxidative decomposition of labile ether-based electrolytes and allows one to enhance the electrochemical performance of Li metal anodes and Ni-rich cathodes.ope
