In situ Observation and Evaluation of Electrodeposited Lithium by Means of Optical Microscopy with Alternating Current Impedance Spectroscopy

Electrodeposited lithium was evaluated by ac impedance measurement and optical microscopy in two solutions containing LiC10₄; a propylene carbonate (PC) solution and a blended solution consisting of PC and 1,2-dimethoxyethane (DME) (volume ratio = 1:1). The ac impedance data were analyzed in the higher frequency region, where the charge-transfer resistance (R^ct) of Li⁺+ e~ #=* Li can be evaluated. The value of R_ct was always larger in the PC electrolyte solution than in the PC-DME(1:1) electrolyte solution, and R_ct depended on the lithium deposition charge. Microscopic observation indicated that lithium deposited more uniformly in PC-DME (1:1) than in PC solution. The results of morphological evaluation of the electrodeposited lithium using optical microscopy corresponded to the variations in the value of R_ct obtained from ac impedance measurements, showing that the behavior of R^ct accurately represented the morphological state. The cy–cleability of the lithium anode was investigated by charge–discharge testing. The cycle efficiency was worse in PC than in PC–DME (1:1) solution; this seems to be related to the morphological properties of the lithium deposits.