Lithium dendrite formation is one of the largest obstacles against commercialization of secondary lithium metal battery¹.  Our group has been focused on the morphology determination factors of lithium deposition from the view point of electrochemistry^2-5.  Ionic liquids with high stability at the very cathodic potential as metallic lithium potential can be used to avoid the SEI formation and its effect on electrodeposition behavior.  In this study the lithium electrodeposition behavior in ionic liquid was examined changing holding temperature in order to control the electrochemical properties such as charge-transfer resistance of lithium redox and the ion diffusion coefficient in the electrolyte.

   An ionic liquid, [C₄mpyr][Tf₂N] (N-methyl-N-butyl-pyrrolidinium bis(trifluoromethanesulfonyl)amide), was selected as a model electrolyte-base, which has high stability at the very cathodic potential such as lithium redox potential².  The electrolyte was prepared by adding 10 wt.% of Li[Tf₂N] to [C₄mpyr][Tf₂N] in dried air.  The cell was fabricated using Ni foil as working electrode and Li foil as counter electrode, in an Air-filled glove box.  Lithium electrodeposition was conducted with current density of 200 μA cm^-2 for charge amount of 0.1 C cm^-2 at the temperature ranged from 25 to 55 °C.  The Ni working electrode was observed by SEM after dismantling the cell and washing the electrode with dimethylether. 

   The number of the deposits on the working electrode tends to decrease with increase in holding temperature⁶.  The number of the deposits per unit area, in other words, nuclear density is thought to be affected by overpotential of working electrode.  The overpotential is thought to be proportional to charge-transfer resistance.  To confirm this hypothesis the charge-transfer resistance was obtained though impedance spectroscopy measurement at each temperature and the result imply the hypothesis is to be the case.  At the meeting site, the growth behavior of electrodeposited lithium and lithium ion diffusion coefficient at each temperature will be also presented, and the ideal electrochemical properties for dendrite suppression will be discussed.

References:

1.   D. Aurbach et al., Solid State Ionics, 148, 405 (2002).
2.   H. Sano et al., J. Power Sources, 196, 6663 (2011).
3.   H. Sano et al., Electrochemistry, 80, 777 (2012).
4.   H. Sano et al., Chem. Lett., 42, 77 (2013).
5.   H. Sano et al., J. Electrochem. Soc., 161, A1236 (2014).
6.   H. Sano et al., 82th Annual Meeting of the Electrochemical Society of Japan, Abstr., 1J34 (2015).