“Isolated zinc” in aqueous zinc batteries can cause negative electrode inactivation

On February 16, 2023, Beijing time, the Alex Robertson research team of the University of Warwick and the team of Professor Peter Bruce of the Department of Materials of the University of Oxford published a paper entitled “Decoupling, quantifying, and restoring aging-induced Zn anode losses in rechargeable aqueous.” zinc batteries”.

This achievement quantitatively studies the capacity decay process of zinc metal anode in aqueous zinc battery under standing conditions, and discovers and explains the inactivation phenomenon of anode caused by “zinc isolation”. By predepositing a quantitative zinc metal anode of 4 mAh/cm2, Professor Bruce’s team found that the reversible capacity of the zinc anode after standing was greatly reduced: the average coulomb efficiency of non-stationary samples was 99.2%, the average coulomb efficiency dropped to 74.4% after 24 hours of standing, and the coulomb efficiency after standing under some test conditions was even less than 30%. Most of this capacity loss comes from the “zinc isolation” formed during the stationary process: the hydrogen produced by zinc corrosion gradually accumulates on the surface of the negative electrode and isolates part of the zinc metal from the electrolyte, making it impossible to completely peel off during discharge. Professor Bruce’s team used XCT to observe the formation process of “isolated zinc” in situ and proved that by removing hydrogen on the negative electrode surface, “isolated zinc” can be activated again.

The corresponding authors are Shengda D. Pu, Xiangwen Gao, Peter G. Bruce and Alex W. Robertson. The first authors are Shengda D. Pu, Bingkun Hu, and Zixuan Li. The communication units are the University of Oxford and the University of Warwick.

Aqueous zinc batteries based on neutral and weakly acidic electrolytes have the advantages of low cost and safety, and have better cycling performance than alkaline electrolytes, and are considered to be strong competitors for the next generation of large-scale energy storage systems. However, the cycle stability of zinc anode has always been one of the bottlenecks in its commercialization. In recent years, many literature reports the occurrence of various attenuation problems such as zinc dendrite, corrosion, hydrogen evolution and other problems in zinc metal anodes under long cycle or standing conditions, but few literature has done systematic quantitative research on the capacity attenuation of zinc anodes. In most of the existing aqueous zinc battery literature, thicker zinc sheets are used as the negative electrode of the battery in the test of full battery long cycles (conventional 0.25 mm thick zinc sheets correspond to a negative surface load of 150 mAh/cm2). Such excessive negative loading and extremely low positive and negative material capacity ratio (N:P ratio) lead to the capacity loss of the negative electrode in the long cycle or stationary test of the battery, which is not reflected in electrochemistry. Therefore, the capacity attenuation problem of zinc anode is often overlooked.

In this work, Professor Bruce’s team quantitatively studied the change of zinc metal anode capacity in aqueous zinc batteries under standing conditions. By pre-depositing a quantitative zinc metal anode of 4 mAh/cm2 on a titanium current collector (current condition of 10 mA/cm2) and standing it at room temperature for different times (Figure 1a), the team found that the pre-deposited zinc metal anode had high reversibility under non-static conditions (average coulombic efficiency 99.2%). However, the average coulomb efficiency of samples dropped to only 74.4% after 24 h and 65.8% after 5 days of standing (Figure 1b, c). Under small current test conditions (deposition & peel current of 1 mA/cm2, surface load of 4 mAh/cm2), the negative electrode capacity loss after standing is higher, and the coulombic efficiency of the sample after standing for 24 hours is less than 30% (Figure 1c).

Figure 1: The reversible capacity of a zinc metal anode predeposited at 4 mAh/cm2 decreases with a longer resting time. The lower the deposition & peeling current, the more reversible capacity attenuation of the negative electrode due to standing.

Initially, based on previous literature, Professor Bruce’s team speculated that these negative electrode capacity decay stemmed from the continuous corrosion and consumption of zinc metal during the standing process, and the formation of by-products such as zinc hydroxide (ZHO & ZHS). However, the team found that only a small amount of by-products were generated on the surface of the zinc anode after a long period of standing (Figure 2b), and after quantification by in situ gas phase mass spectrometry (Figure S8), the team found that the negative electrode capacity loss caused by corrosion byproducts accounted for only 20% of the total capacity loss after standing (Figure 2a). The team also found through XRD that a large amount of zinc metal remained on the titanium current collector and battery separator after the battery was fully discharged (Figure 2c). This shows that part of the zinc metal anode is not consumed due to corrosion after standing, but for some reason, this part of the metal can not be stripped during discharge, resulting in the capacity loss of the negative electrode. By mass spectrometry quantification, the research team found that these unstripped zinc metals caused about 80% of the anode capacity loss after standing (Figure 2 d-e). At the same time, the team found that when the surface load of the battery negative electrode increased, the amount of zinc metal that could not be stripped after standing also increased (Figure 2f).

Figure 2: Capacity attenuation of zinc anode after standing and its main causes.

So how is this unpeelable zinc formed? By observing the deposition, standing and peeling process of zinc anode in pouch cells in situ by XCT, Professor Bruce’s team found that the formation of “isolated zinc” is caused by hydrogen evolution of zinc anode during the standing process: after zinc metal deposition, some hydrogen bubbles gradually form on its surface. As the resting time increases, these bubbles gradually grow and clump, forming larger bubbles and covering part of the surface of the zinc metal. When discharged at the end of standstill, these zinc metals that are covered and surrounded by bubbles cannot be peeled off because they lose contact with the electrolyte, thus forming a “zinc isolation” and thus a loss of capacity (Figure 3 a-h). The team also found that under the test conditions of low pressure and long standing time, the formation of air bubbles on the surface of the negative electrode was also larger, resulting in more “zinc isolation” and capacity attenuation (Figure 3 i-l).

Figure 3: In situ observation of the formation of “isolated zinc” by XCT.

Finally, Professor Bruce’s team proved that removing the hydrogen accumulated on the surface of the zinc anode can allow the “isolated zinc” to be infiltrated by the electrolyte again, thereby activating them again to restore most of the negative electrode capacity loss caused by standing.

Figure 4: Negative capacity loss due to standing is restored by removing hydrogen accumulated on the negative electrode surface.

In this work, a systematic quantitative study of the anode capacity attenuation of aqueous zinc batteries in the stationary state was carried out, and the formation of “isolated zinc” was found and observed in situ, and the mechanism of zinc anode inactivation caused by “isolated zinc” was explained. This study highlights the harm caused by hydrogen evolution to the capacity attenuation of the negative electrode in the aqueous zinc battery system, which provides insight for the future research and design of anode of aqueous zinc battery. (Source: Science Network)

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