Study on Structure and Electrochemical Properties of Zn-Sulfolane Based Electrolyte Solution
Authors: Kagaya Kento , Watanabe Hikari , Shitanda Isao , Itagaki Masayuki 
Published: December 2024

The zinc secondary batteries (ZIBs) are promoting as one of the next-generation batteries using zinc with a high theoretical capacity as a negative electrode. However, the dendrite growth due to repeated charging and discharging are disadvantages. Deep eutectic solvents (DESs), which are obtained by mixing two or more compounds consisting of a hydrogen bond acceptor and a hydrogen bond donor, have suppressed dendrite growth.[1] Mixing Zn (ClO4)2・6H2O (solid) and sulfolane (SL, melting point 27.5 ℃) yields a DES and the DES prevents dendrite growth. However, the whisker-like Zn deposition is observed at high current density, suggesting that the DES cannot inherently prevent dendrite growth.[2] On the other hand, Dokko et al. have reported that the Li+ hopping conduction occurs in SL based super-concentrated electrolyte solution, suppressing concentration polarization.[3] Liquid structure plays a crucial role in the Li+ hopping conduction. In this work, to develop a DES that prevents dendrite growth by suppressing the concentration polarization, we prepared the SL based DESs consisting of various anions.
Zn(ClO4) DES was prepared by mixing Zn (ClO4)2・6H2O and SL. We also prepared the DES with the anion changed to Zn (TfO)2. The liquid structures of Zn-SL DES were observed using a laser Raman spectrophotometer NRS-4500. The chronoamperometry of the Zn-symmetric cell was measured at room temperature. An electrochemical measurement system (SP-150, Bio-logic) was used for the electrochemical measurement.
In the Raman spectrum of neat SL, a Raman band assigned to the scissoring vibration of SO2 is observed at 567 cm-1. This Raman bands appears at 571 cm-1 and 574 cm-1 in the Raman spectrum for Zn (ClO4)2 DES and Zn (TfO)2 DES, respectively. In addition, the FWHM of the band of Zn (TfO)2 DES is larger than that of Zn (ClO4)2 DES. In high concentration Li-SL electrolyte, Li+ interacts not only with SL but also with anions to form the contact ion pairs (CIP) and aggregates (AGG) in which the anion cross-links Li+. As these liquid structure form, the Raman band shifts to the high wavenumber and broadens. Therefore, the CIP and AGG may be easily formed in Zn (TfO)2 DES. This is expected to suppress concentration polarization and dendrite growth. In the case of migration-controlled Zn2+ transport, the current should increase with increasing the applied voltage. For the Zn (ClO4)2 DES, there was no increase in current density of the symmetric cell with increasing applied voltage. In contrast, for the Zn (TfO)2 DES, the current value increased continuously with increasing applied voltage, reaching a high value of 10.8 mA cm-2 at 600 mV. In addition, despite the low conductivity of Zn (TfO)2 DES, the high current density was observed upon polarization. We believe that this is due to the rapid transport of zinc by forming CIP and AGG. However, for Zn (TfO)2 DES, the current decay due to concentration polarization is observed at large polarization. To suppress the concentration polarization, Zn2+ concentration was increased and then the chronoamperometry measurements was conducted again. In the Zn (TfO)2 DES, there was no increase in the current density of the symmetric cell with increasing the applied voltage. On the other hand, the current density of the symmetric cell with Zn (ClO4)2 DES suddenly increased from 300 mV. This may be due to a change in electrode area caused by the formation of dendrites.
４．References
[1] Funian Mo, Guojin Liang, Qiangqiang Meng, et. al. Energy Environ. Sci., 706-715 (2019)
[2] Xidong Lin, Guodong Zhou, Matthew J. Robson, et. al. Adv.Funct.Mater. 2109322 (2021)
[3] Kaoru Dokko, Daiki Watanabe, et. al. J.Phys.Chem. B, 122, 10736–10745, (2018)