Stable Ammonium-Ion Batteries in Diluted Electrolyte By Manipulation of H-Bonding Network Via Ethylene Glycol
Authors: Fei Huifang , Passerini Stefano , Varzi Alberto 
Published: February 2024

Aqueous rechargeable batteries (ARB) arose considerable interest for stationary energy storage due to low cost, inherent safety, and environmental friendliness.[1] Besides aqueous lithium-ion batteries, various kinds of aqueous batteries have been proposed, utilizing relatively inexpensive charge carriers either based on metal (Na+, K+, Ca2+, Mg2+, Zn2+, Al3+, etc.) or non-metal cations (H3O+, NH4
+
etc.).[2] Among non-metallic ion batteries, aqueous ammonium-ion batteries (AAIBs) distinguish themselves by working in near neutral or mild acidic environment, refraining from severe electrode corrosion.[3] However, like other ARB systems, the energy density of AAIBs is limited by the narrow electrochemical stability window of the aqueous electrolyte. In addition, NH4
+ ions hydrolyse in water resulting in decreased pH of the electrolyte which promotes the hydrogen evolution reaction (HER).[3] This means that increasing salt concentration may not be an effective strategy to enlarge the stability window of the electrolyte. In fact, according to a previous report, the cathodic stability of 25 m ammonium acetate (NH4Ac) is even slightly inferior than that of 1 M NH4Ac.[4] Developing alternative strategies to inhibit HER in diluted electrolytes is of uttermost importance.

In diluted aqueous electrolytes, the continuous and complicated H bond network among water molecules favours fast ionic motion but also causes severe HER. Here, we propose ethylene glycol (EG) as electrolyte component to manipulate the H bond structure in diluted (1 m) NH4Ac solutions. FTIR results are used to study the H-bonding network as function of the EG:water ratio. The diluted aqueous electrolyte containing EG show no freezing point and wider electrochemical window. As a proof-of-concept, full cells composed of Prussian blue derivatives, PTCDI anode, and 1 m NH4Ac with the addition of EG are tested to verify the effectiveness of EG during battery operation. Improved cycling performance and higher coulombic efficiencies are demonstrated compared to those including 1 m NH4Ac in water.

References

[1] Wang, Y.; Wang, T.; Dong, D.; Xie, J.; Guan, Y.; Huang, Y.; Fan, J.; Lu, Y.-C. Enabling high-energy-density aqueous batteries with hydrogen bond-anchored electrolytes. Matter
2021. DOI: 10.1016/j.matt.2021.10.021.

[2] Zheng, R.; Li, Y.; Yu, H.; Zhang, X.; Ding, Y.; Yan, L.; Li, Y.; Shu, J.; Su, B. L. Ammonium Ion Batteries: Material, Electrochemistry and Strategy. Angew Chem Int Ed Engl
2023, e202301629. DOI: 10.1002/anie.202301629.

[3] Tian, Z.; Yin, J.; Guo, T.; Zhao, Z.; Zhu, Y.; Wang, Y.; Yin, J.; Zou, Y.; Lei, Y.; Ming, J.; et al. A Sustainable NH4+ Ion Battery by Electrolyte Engineering. Angew Chem Int Ed Engl
2022, 61 (51), e202213757. DOI: 10.1002/anie.202213757.

[4] Holoubek, J. J.; Jiang, H.; Leonard, D.; Qi, Y.; Bustamante, G. C.; Ji, X. Amorphous titanic acid electrode: its electrochemical storage of ammonium in a new water-in-salt electrolyte. Chem Commun (Camb)
2018, 54 (70), 9805-9808. DOI: 10.1039/c8cc04713h.