Suppressed lithium dendrite growth in lithium batteries using ionic liquid electrolytes: Investigation by electrochemical impedance spectroscopy, scanning electron microscopy, and in situ 7Li nuclear magnetic resonance spectroscopy

In this work, the formation of lithium dendrites in lithium/Li4Ti5O12 battery cells is studied using different experimental techniques. Electrochemical impedance spectroscopy is presented as a tool to investigate non-invasively the dendritic growth on a lithium metal surface in lithium/Li4Ti5O12 cells during numerous discharging/charging cycles. Scanning electron microscopy is used for visual inspection of the dendrite formation. In situ 7Li nuclear magnetic resonance spectroscopy is sensitive to quantitative changes at the lithium metal surface. Application to symmetrical lithium/lithium battery cells allows for a careful comparison of the investigated electrolytes. All these experimental methods provide consistent results. It is demonstrated that the growth of lithium dendrites is significantly correlated to the electrolyte employed. All electrolytes based on the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)azanide (EMIM-TFSA) show reduced dendrite growth in comparison to the standard electrolyte for Li-ion batteries, lithium hexafluorophosphate (LiPF6) in ethylene carbonate/dimethyl carbonate (EC/DMC). LiPF6 in EMIM-TFSA and LiPF6 in EMIM-TFSA/propylene carbonate suppress lithium dendrites most efficiently.