Ionic Transport in Non-Aqueous Electrolyte Solutions -(i) Experimental Measurements of Conductivity and Viscosity of MgX2 (X=ClO4
-, TFSA-) Dissolved in PC and GBL
Authors: Sano Hikaru , Uchida Satoshi , Ozaki Hiroyuki , Kiyobayashi Tetsu 
Published: December 2020

Transport phenomena in electrolyte solutions is one of the most interesting topics in the solution chemistry. The mechanism is not fully understood, nonetheless. Furthermore, the physiochemical properties such as conductivity and viscosity are very important to design the electrolyte used in batteries. For the Mg battery, which is attracting much attention from the viewpoint of avoiding rare-element-based material, the electrolyte contains divalent Mg2+ ion as the charge carrier. The physiochemical properties of electrolytes for Li battery has been studied well.[1-4] In this study, in order to elucidate the transport phenomena in the electrolyte solutions containing divalent Mg2+ ion, the conductivity and viscosity of the electrolyte were experimentally investigated.
MgX2 (X=ClO4
-, TFSA- (TFSA: bis(trifluoromethanesulfonyl)amide)) and propylene carbonate (PC) and gamma-butyrolactone (GBL) are selected as the salt electrolytes and the solvents. The electrolyte solutions were prepared so as for the number density of ions, C to range 0.01 to 0.1, where C is defined as the molar fraction of ions (cations and anions) in the system. For example, when A mole of LiX is added to B mole of the solvent Y, the number density is

C = 2 A / (2 A + B).
When A mole of MgX2 is dissolved in B mole of the solvent Y, the number density is

C = 3 A / (3 A + B).
In the experiment, the viscosity of the electrolyte was measured using cone-type viscometer. The conductivity of the electrolyte was investigated as follows: The electrolyte was set in a custom-made four-electrode cell, which has two disc-shaped electrodes for current flow, and two short wire electrodes for voltage sensing. The position of the four electrodes are fixed in the cell. The electrochemical ac impedance of the cell was measured at open circuit in the range of 100 kHz to 1 Hz with alternating voltage of 10 mV.
As the number density of ions increases, the viscosity of Mg electrolyte increases more steeply than that of Li electrolyte. The cause of the steeper increase in viscosity probably comes from the greater interaction of the divalent cations with anions and solvent.
The relationship between the conductivity σ and the number density of the salt C of the magnesium electrolyte have similar tendency to that of Lithium electrolyte and other electrolyte containing only monovalent ions. That is, while σ increases at low concentrations due to the increase in the number of charge carriers, it starts to decrease after a maximum σ, σmax,Mg. The cause of the decrease in σ is thought to be the association of ions. The number density C*Mg at which σmax,Mg occurs is smaller than C*Li and C*Na at which σmax, Li and σmax,Na occur. The cause of this results also comes from the greater interaction of the divalent cations with anions and solvent. The theoretical implications are discussed in other (probably next) paper by our coauthors.

References

[1] K. Kuratani, N. Uemura, H. Senoh, H. T. Takeshita, and T. Kiyobayashi, J. Power Sources, 223, 175–182 (2013).
[2] K. Kuratani, I. Kishimoto, Y. Nishida, R. Kondo, H. T. Takeshita, H. Senoh, and T. Kiyobayashi, J. Electrochem. Soc., 163, H417–H425 (2016).
[3] H. Ozaki, K. Kuratani, and T. Kiyobayashi, J. Electrochem. Soc., 163, H576–H583 (2016).
[4] H. Ozaki, K. Kuratani, H. Sano, and T. Kiyobayashi, J. Chem. Phys., 147, 034904 (2017).