Optimization of Segmentation of Electrolyte to Enhance Polarization Behaviors and Cycling Performance of Zinc Rechargeable Battery
Authors: Morimitsu Masatsugu , Tomoda Kakeru , Komatsu Yuriko , Tada Ririka 
Published: December 2024

Zinc is one of the promising materials for a next-generation rechargeable battery, because it is cheap, abundant, and highly safe with the low redox potential which is attractive to make the cell voltage of aqueous secondary battery high (e.g., 1.73 V of Zn-Ni and 1.65 V of Zn-Air), while the zinc anode for secondary uses is still suffering from the issues on dendrite growth and non-uniform redistribution of zinc and zinc oxide, resulting in an internal short circuit and a poor capacity retention. Against the issues, one of the authors of this paper has reported a new concept of zinc anode, Segmentation of Electrolyte (SoE) which is to segment the space of the electrolyte between the zinc anode and the cathode, and this results in a stable performance for 5,500 charge-discharge cycles or more at 1 C rate [1-3]. In case of KOH solutions containing zincate ions that are the intermediate of the redox reaction between zinc and zinc oxide, SoE gives the limited space for zincate ions, within which they can move free but the non-uniformization of the spatial concentration distribution is suppressed. As a result, the reaction distribution on the zinc anode during charge and discharge is made more uniform than that without SoE. In this paper, we report the optimization of SoE for zinc-nickel secondary battery, in which the effects of the size of segmentation on the voltage performance during charge and discharge and the capacity retention with the cycle are contained.

The zinc anode with SoE was fabricated by attaching a PTFE sheet with an open area to one side of a zinc foil, where the size of the SoE area was varied between φ5mm and φ22mm. The zinc anode and the nickel cathode were placed opposite each other in an acrylic container, and the distance between the two electrodes was controlled in the range of 0.5 mm to 1 mm depending on the thickness of the PTFE sheet, by which the charge and discharge reactions of the zinc anode were limited within the open area of PTFE sheet. A zinc foil of 300 μm thickness was used as the zinc anode, which was also served as a current collector. The segmented reaction space was filled with a 7 mol/L KOH solution saturated with zinc oxide, and a constant current was applied to the cell at room temperature. The charge-discharge voltages and the potential of the zinc anode as referred to the zinc wire quasi-reference electrode were measured.

The cell with the SoE area of φ5mm operated without no significant change in the cell voltage and the anode’s potential over 100 cycles or more and the anode showed no dendrite on the surface observed by SEM after the operation. However, the SoE area strongly affected the cycling performance in other cases of φ8mm，φ11mm，φ16mm，and φ22mm, in which the stable charge-discharge cycles were shortened with the increase in the SoE area. In such cases, the cell voltage and the anode’s potential indicated large fluctuation during charge, suggesting that the internal short circuit by zinc dendrite was made and broken repeatedly. From the results, we obtained the relationship between the SoE area and the number of cycles until the first internal short circuit occurs, in which it was clearly indicated that the possibility of the dendrite formation depends on the SoE area. Thus, the optimization of the size of SoE can enhance the cycling performance of the zinc anode. Similarly, we examined the effects of the distance between the anode and the cathode, which is another factor to determine the reaction zone of SoE, on the charge-discharge characteristics, and the results will be also presented in this paper.

This work was supported by JSPS KAKENHI Grant Number 23K2345.

Ref.

[1] M. Morimitsu, Patent No. US-11652204-B2 (2023).

[2] M. Morimitsu, 2022 International Battery Seminar & Exhibit, Orlando, USA, March 2022.

[3] M. Morimitsu, The 12th Battery Safety Summit, Tysons Corner, USA, October 2022.