Nucleation and Growth Behavior of Electrodeposited Lithium in Ionic Liquid

Metallic lithium is a promising anode candidate for rechargeable battery; however, the dendrite formation during charging should be suppressed to improve safety and cycleability.  Many researches have been reported from the viewpoint of the control of the surface film on lithium metal¹.  Ionic liquids nowadays attract much attention from electrochemists, some of which have excellent stability at highly cathodic states (such as Li/Li⁺).  Our group has focused on their unique features and utilized these ionic liquids as electrolyte with lithium metal anodes.  In the present study, in order to determine the condition for the non-dendritic lithium deposition, we investigated the electrodeposition behavior during nucleation and growth state in ionic liquids based on overpotential and Li-ion diffusion.

   10 wt.% of Li[Tf₂N] was added to [C₃mpip][Tf₂N], [C₄mpyr][Tf₂N], and [N_6,1,1,1][Tf₂N] as the electrolytes.  Lithium metal was electrodeposited on nickel substrates at current densities ranging from 5 to 200 μA cm^-2 for total charge amount of 0.1 and 3 C cm^-2.  The morphology of electrodeposited lithium was observed by SEM.

   For the nucleation state, we focused on the case with charge amount of 0.1 C cm^-2.  The deposits were better distributed when the current density was increased (Figure 1).  Additionally, the deposits were better distributed when the electrolyte viscosity was increased.  In these conditions, an increased overpotential was confirmed.  This is believed to lead to a wide distribution of nucleation sites due to decreased critical radii of the nuclei².  On the contrary, for the growth state, we focused on the case with the charge amount of 3 C cm^-2.  More deposits were more dendritic when the current density was increased (Figure 2) and also when the electrolyte viscosity was increased.  This result can be rationalized by considering that the lithium ion concentration extremely decreased at the electrode surface, since Li-ion diffusion is slower than the decrement in the concentration of Li-ion at the electrode surface³.  These tendency is common for noble-metal electrodeposition.  The reason why these tendency is also observed for less noble metals such as lithium is considered to be that the ionic liquid used in this study is very stable at the highly cathodic state and therefore less effect of native film formation on the deposition.

1.  D. Aurbach, E. Zinigrad, Y. Cohen, H. Teller , Solid State Ionics, 148, 405 (2002).

2.  H. Sano, H. Sakaebe, H. Senoh, H. Matsumoto, J. Electrochem. Soc., Submitted.

3.  H. Sano, H. Sakaebe, H. Senoh, H. Matsumoto, J. Electrochem. Soc., 161, A1236 (2014).