Key Takeaways & Executive Findings
- •• Three zwitterions with identical quaternary ammonium cations but distinct anionic groups (carboxylate, sulfonate, phosphate) show distinct impact in aqueous zinc ion batteries. • The zwitterion with phosphate group (MPC) uniquely promotes oriented Zn (002) plane deposition and provides pH buffering capability, effectively suppressing dendrite formation and side reactions. • The Zn//NaVO full cell with MPC additive delivers an ultralong cycling life of 4000 cycles with an exceptionally low capacity decay of 0.014% per cycle. • The Zn//Zn cell with MPC additive in ZnSO4 electrolyte exhibits an ultralong lifespan over 5000 hours, demonstrating the synergistic effect of quaternary ammonium and phosphate groups.
Abstract
Zwitterionic materials with covalently tethered cations and anions have great potential as electrolyte additives for aqueous Zn-ion batteries (AZIBs) owing to their appealing intrinsic characteristics and merits. However, the impact of cationic and anionic moieties within zwitterions on enhancing the performance of AZIBs remains poorly understood. Herein, three zwitterions, namely carboxybetaine methacrylate (CBMA), sulfobetaine methacrylate (SBMA), and 2-methacryloyloxyethyl phosphorylcholine (MPC), were selected as additives to investigate their different action mechanisms in AZIBs. All three zwitterions have the same quaternary ammonium as the positively charged group, but having different negatively charged segments, i.e., carboxylate, sulfonate, and phosphate for CBMA, SBMA, and MPC, respectively. By systematical electrochemical analysis, these zwitterions all contribute to enhanced cycling life of Zn anode, with MPC having the most pronounced effect, which can be attributed to the synergistic effect of positively quaternary ammonium group and unique negatively phosphate groups. As a result, the Zn//Zn cell with MPC as additive in ZnSO4 electrolyte exhibits an ultralong lifespan over 5000 h. This work proposes new insights to the future development of multifunctional zwitterionic additives for remarkably stable AZIBs.
1. Introduction
Given the abundant resources and high theoretical capacity (820 mAh g−1) of Zn, aqueous Zn ion batteries (AZIBs) have garnered significant attention for large-scale energy storage [1–4]. Unfortunately, the commercialization of AZIBs still suffers from the poor reversibility of Zn anodes during plating/stripping. On the one hand, the tip effect results in an inhomogeneous distribution of Zn2+ flux at the electrolyte/electrode interface, inducing uneven Zn deposition [5–7]. Uncontrollable Zn dendrite deposits can increase the risk of internal short circuits. On the other hand, the continuous hydrogen evolution reaction (HER) leads to pH fluctuation engendering the formation of insulated Zn4(OH)6SO4·xH2O by-products [8–10]. Importantly, these side reactions and by-products aggravate the irregularity of the electric field and Zn2+ flux at the interface, further causing disordered Zn dendrites and providing more reaction sites for undesired HER [11].
Up to now, significant endeavors, including Zn anode modification, artificial interface layers, separator designing and electrolyte optimization, have been adopted to circumvent above issues [12–15]. Among these strategies, electrolyte optimization has been widely employed as an effective and practical solution to stabilize Zn anodes. Typically, functional additives for AZIBs can be divided into two types based on their different functions. Parts of those were used to adsorb onto the Zn surface and contribute to uniform zinc ion transportation [16–18]. The other parts were employed to optimize the solvation structure of Zn2+, thereby inhibiting the long-lasting detrimental water molecule-induced parasitic reactions [19–21]. Although some progress has been made, there are still several key issues: (1) single-functional additives are insufficient to simultaneously regulate the deposition behavior of Zn2+ and optimize the solvation structure of hydrated Zn2+ at the same time; (2) insoluble organic additives rely on the use of expensive organic Zn salts to improve the solubility in H2O, which seriously increases the cost; (3) some additives even have a certain toxicity, leading to serious environmental and economic problems.
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Biao Wang, Chaohong Guan, Qing Zhou, Yiqing Wang, Yutong Zhu, Haifeng Bian, Zhou Chen, Shuangbin Zhang, Xiao Tan, Bin Luo, Shaochun Tang, Xiangkang Meng, Cheng Zhang (2025). Screening Anionic Groups Within Zwitterionic Additives for Eliminating Hydrogen Evolution and Dendrites in Aqueous Zinc Ion Batteries. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01826-w
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Frequently Asked Questions
What are the three zwitterionic additives studied in this paper?
The three zwitterionic additives are carboxybetaine methacrylate (CBMA), sulfobetaine methacrylate (SBMA), and 2-methacryloyloxyethyl phosphorylcholine (MPC). They share the same quaternary ammonium cation but differ in their anionic groups: carboxylate, sulfonate, and phosphate, respectively.
How does the MPC additive improve the performance of aqueous zinc ion batteries?
MPC, with its phosphate group, uniquely promotes oriented Zn (002) plane deposition and provides pH buffering capability. This effectively suppresses dendrite formation and side reactions, leading to an ultralong cycling life of over 5000 hours in Zn//Zn cells and 4000 cycles in full cells with low capacity decay.
What is the significance of the anionic group in zwitterionic additives?
The anionic group plays a crucial role in determining the additive's effectiveness. The study shows that the phosphate group in MPC provides superior performance compared to carboxylate and sulfonate, due to its synergistic effect with the quaternary ammonium group in regulating Zn deposition and buffering pH.
What are the main challenges in aqueous zinc ion batteries addressed by this research?
The research addresses the issues of Zn dendrite growth and hydrogen evolution reaction (HER), which lead to poor reversibility and side reactions. These problems cause inhomogeneous Zn deposition and pH fluctuations, resulting in by-product formation and reduced battery performance.
What is the potential impact of this work on the development of AZIBs?
This work provides new insights into the design of multifunctional zwitterionic additives, highlighting the importance of anionic group selection. The findings could guide the development of more stable and efficient aqueous zinc ion batteries for large-scale energy storage.
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