Effect of Current Density on Nucleation and Morphology of Lithium during Electrodeposition in Ionic Liquids

Lithium metal is very attractive anode material but the dendrite formation during charging prevents the batteries with lithium metal anode from wide commercialization.  Our group has focused on some room-temperature ionic liquids as electrolytes from the viewpoint of high stability at the negative potential of Li/Li⁺.  In the present study, the morphology of electrodeposited lithium in the ionic liquids was investigated by ex situ SEM observations.  The effect of current density on lithium dendrite formation was figured out.

     The electrolytes are prepared by addition of 10 wt.% of Li[Tf₂N] to [C₃mpip][Tf₂N], [C₄mpyr][Tf₂N], and [N_6,1,1,1][Tf₂N] (see Figure).  Two-electrode cells were fabricated, using nickel foils as working electrodes and lithium foils as counter electrodes.  Lithium metal was electrodeposited with the current density from 5 to 200 μA cm^-2 for total coulomb amount of 3 C cm^-2.  The morphology of electrodeposited lithium on each working electrode was observed by ex situ SEM after washing with dimethylcarbonate (see Figure).

     For the cell with [C₃mpip][Tf₂N], when the current density was higher than 50 μA cm^-2, the needle like deposits were observed and the deposits were apparently thought to have grown from a semisphere-like nuclei.¹  When the current density was higher than 50 μA cm^-2 for the cell with [N_6,1,1,1][Tf₂N], and was higher than 100 μA cm^-2 with [C₄mpyr][Tf₂N], the needle like deposits were also observed and the deposits were thought to have grown from a semisphere like nuclei.¹

     The results for the lithium deposit morphology obtained from each cell with three ionic liquids were discussed with lithium-ion diffusion coefficient of the electrolytes, and we concluded as follows;  The deposits were better distributed with decreased size when the current density was increased in the current range where the deposition was charge-transfer controlled.  With higher current density where the deposition was diffusion controlled, deposits were more distributed but large and dendritic.  The current density dependency of the lithium deposition behavior from ionic liquid are very similar to the case with noble metal deposition from aqueous solution, which is due to the high stability of the ionic liquid used in this study at the negative potential of Li/Li⁺.

[1] H. Sano, et al., ECS Electrochem. Lett., to be submitted.