High Strength Hydrogel Enables Dendrite-Free Zn Metal Anodes and High-Capacity Zn-MnO2 Batteries
Authors: Zhu Ruijie , Kitano Sho , King Daniel , Zhu Chunyu , Aoki Yoshitaka , Habazaki Hiroki 
Published: July 2022

Introduction

Rechargeable aqueous zinc-ion batteries (RAZIBs) have some inherent advantages such as intrinsic safety, low-cost and theoretically high energy density, making them a current topic of interest. The problem is, dendritic growth of zinc (Zn) metal during electrodeposition occurs whatever alkaline electrolyte or neutral electrolyte, which will break the separator and cause a short-circuit inside the battery. Unless a solution can be found to effectively limit the growth of dendrites, the real-world application of RAZIBs will be nowhere in sight.
In this report, by using a glass fiber modified double network hydrogel consists of poly(2-acrylamido-2-methylpropanesulfonicacid)/polyacrylamide (PAMPS/PAM) as semi-solid-state electrolyte (coded as DNGF), we found the formation of Zn dendrites can be greatly alleviated. We attribute the elimination of Zn dendrites to a modified mechanical suppression effect. In addition, a Zn-MnO2 battery with high areal capacity (4.9 mAh cm-2) worked stably for 500 cycles with the help of DNGF.

Experimental

The obtained DNGF composite was prepared by the following steps: a piece of glass fiber contained PAMPS (PAMPS-GF) was soaked in a 3 mol L-1 acrylamide solution for 1 day, in order to thoroughly swell the PAMPS-GF skeleton with AM solution. 0.01 % N,N'-Methylenebisacrylamide (MBAA) was used as crosslinker. The AM-swollen PAMPS-GF was transferred to a glovebox and was polymerized by a 12 h UV irradiation.
The high-capacity Zn-MnO2 battery was assembled by placing a Zn metal foil, a DNGF separator that was swollen by 2M ZnSO4 electrolyte (additive 0.2 M MnSO4) and an MnO2 cathode that consists of α-MnO2 nanowire and carbon black in a Swagelok cell. The cut-off voltage was set as 0.8 V - 1.8 V.

Results and discussion

The structure of DNGF is depicted in Figure 1a, the second network PAM entangled with the first-network PAMPS while the glass fiber is dispersed in the structure. The DNGF can sustain a stress of 6.9 MPa at a strain of 80%, which is 10 times higher than the PAM gel (Figure 1b). As shown in Figure 2, when DNGF is used as separator in a Zn-Zn symmetric cell, the electrodeposition of Zn crystal becomes uniform and flat. Here, we attribute the suppression of Zn dendrites to a modified mechanical suppression effect. Unlike the case in lithium-ion batteries, in which a stiff polymer separator can effectively prevent the uncontrollable growth of dendrites, the growth of Zn dendrites generally cannot be suppressed by using a polymer separator due to the shear modulus of conventional polymer cannot meet the demanded value (1.8 times higher than the anode metal).[1] The use of DNGF does not stop the upward growth of Zn crystal through the normal mechanical suppression effect, but it delays the growth of Zn dendrites and forces the deposited Zn crystals flat and uniform. As shown in Figure 3, when DNGF was used in a high-capacity Zn-MnO2 battery to protect the cell from short-circuit, the cell worked stably for 500 cycles. The success in the DNGF protected Zn-MnO2 batteries gives us an important information: even within real-world application requirements (e.g. electrode capacity > 4 mAh cm-2), using polymer with excellent mechanical properties as a separator still can suppress the growth of zinc dendrites via the modified mechanical suppression effect.
[1] X. Zhang, A. Wang, X. Liu and J. Luo, Acc. Chem. Res., 2019, 52, 3223-3232

Figure 1