Effect of Fluoroethylene Carbonate Electrolytes on the Nanostructure of the Solid Electrolyte Interphase and Performance of Lithium Metal Anodes

The mechanism for the performance enhancement of lithium metal electrodes by fluoroethylene carbonate (FEC) is revealed. Electrolytes containing FEC, 1.2 M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7, vol) with 10% FEC (mass %) and 1.2 M LiPF6 in FEC, improve the electrochemical performance of both Li parallel to Li and Cu parallel to LiFePO4 cells compared to the baseline electrolyte, 1.2 M LiPF6 in EC:EMC (3:7, vol). Ex situ surface analysis of lithium metal electrodes after the initial plating demonstrates that the solid electrolyte interphase (SEI) generated from FEC containing electrolytes is similar to the SEI generated from the baseline electrolyte, yet the corresponding Coulombic efficiencies are markedly different. Electron microscopy investigations reveal the presence of a unique SEI containing nanostructured LiF particles for the lithium electrode plated from the 1.2 M LiPF6 in FEC electrolyte. The presence of the nanostructured LiF particles correlate with the improved cycling performance, suggesting that the morphology of the SEI is as important as the composition of the SEI.