Objective: Lithium metal anode is considered as the "holy grail" of anode materials due to its high theoretical specific capacity and low electrode potential. However, the growth of lithium dendrites seriously hinders the practical application of lithium metal anode. In this work, the effect of heptafluorobutyric anhydride (HFAA) as an electrolyte additive on the electrochemical performance of lithium metal anodes was investigated. The mechanism of the function of HFAA additives in lithium metal anode was investigated by theoretical calculations, cyclic voltammetry (CV) tests, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS).
Methods: The LAND battery test system was used to test the charge and discharge performance of battery. CV testing was done on a CHI 660 electrochemical workstation with a scan rate of 10 mV/s and a scan range from -0.5 to 3 V. Copper was used as the study electrode and lithium was used as the reference electrode and counter electrode. The Hitachi S-4800 SEM was used for morphological characterization. The components on the surface of the lithium flakes were analyzed with a Thermo Fisher Scientific K-Alpha XPS instrument. Theoretical calculations were performed using the B3PW91 method in Gaussian 09 software.
Results: Theoretical calculations showed that HFAA was easier to obtained electrons than the solvent in the electrolyte, and thus HFAA was preferentially reduced on the negative surface, generating a solid electrolyte interface (SEI) film. The CV test showed that there was a clear reduction peak at 1.75 V when 3% (by mass) HFAA additive was added. Without the HFAA additive, such reduction peak was not observed at the same potential. This indicates that HFAA decomposed at 1.75 V and formed a film on the negative electrode, which was consistent with the results of theoretical calculations. The Li/Li symmetric cell can cycle stably for 480 h in the electrolyte containing 3% HFAA additive at a current density of 0.5 mA/cm2 and a capacity of 1 mAh/cm2. The SEM test results showed that the surface morphology of the lithium anodes was loose and porous after cycling in the electrolyte without additives. In the electrolyte with HFAA additives, the surface of lithium was flat and dense, and no lithium dendrites appeared. The XPS tests showed that the electrolyte with HFAA additives generated more organic/inorganic composite SEI films containing LiF and -CO2Li components on the lithium surface than the electrolyte without HFAA additives, which could effectively inhibit the growth of dendrites and improve the cycling performance of lithium metal batteries. The test results of Li/LiFePO4 cells showed that the capacity retention rate of the cell without additive was 73% after 98 cycles (initial capacity of 109.1 mAh/g), whereas the cell with 3% HFAA additive still had 92.8 mAh/g capacity after 300 cycles, with a capacity retention rate of 83%.
Conclusion: In this work, HFAA was introduced to carbonate electrolyte as an electrolyte additive. The test results showed that the HFAA additive could preferentially reduce on the lithium anode surface to generate more organic/inorganic composite SEI films containing LiF and -CO2Li components, which effectively inhibited the growth of lithium dendrites and improved the cycle performance of lithium metal batteries. In the electrolyte containing 3% HFAA, the Li/Li symmetric cell was able to cycle stably for 480 h (0.5 mA/cm2, 1 mAh/cm2), and the capacity retention rate of the Li/LiFePO4 cell was 83% after 300 cycles at a current density of 5 C.