Key Takeaways & Executive Findings
- •• • Symmetric Zn||Zn cells with PQ-7 additive sustain 2117 h of cycling at 5 mA cm−2 and 1 mAh cm−2, a 15-fold life extension versus additive-free cells (approximately 141 h). This directly addresses the industrial requirement for >2000 h operation in stationary storage, reducing battery replacement frequency and total cost of ownership. • • Zn||Ti cells achieve Coulombic efficiency above 98% after 240 cycles, indicating that the additive suppresses HER and Zn corrosion to a level compatible with commercial viability. This metric is critical because CE below 99.9% in full cells typically leads to rapid capacity fade; the observed stability suggests that PQ-7 mitigates irreversible byproduct accumulation. • • Zn||MnO2 full batteries retain 92.1% capacity after 1000 cycles at 1 C and 80% after 1000 cycles at 5 C. The high-rate retention (80% at 5 C) demonstrates that the additive does not impede Zn2+ transport kinetics, a common failure mode for bulky surfactant additives, and supports fast-charging applications. • • The additive functions via competitive adsorption: PQ-7 replaces H2O and [Zn(H2O)6]2+ at the Zn surface, shielding water and homogenizing Zn2+ flux. This dual mechanism simultaneously addresses dendrite growth and HER, two degradation modes that are typically tackled separately, simplifying electrolyte formulation and reducing system complexity.
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Abstract
Aqueous zinc-ion batteries (ZIBs) are a low-cost, high-safety alternative to lithium-ion batteries for grid-scale energy storage, but their commercial viability is constrained by zinc dendrite growth and the hydrogen evolution reaction (HER) on the Zn anode, which cause low Coulombic efficiency (CE), short cycle life, and capacity fade. This study introduces polyquaternium-7 (PQ-7), a cationic surfactant, as a multifunctional electrolyte additive. Experimental and theoretical analyses reveal that PQ-7 adsorbs at initial tip sites on the Zn anode, shielding H2O molecules and inhibiting HER. Competitive adsorption with Zn2+ mitigates the tip effect, promoting uniform Zn deposition over dendritic growth. Consequently, symmetric Zn||Zn cells with PQ-7 achieve stable cycling for over 2117 h at 5 mA cm−2 and 1 mAh cm−2, a 15-fold increase over additive-free cells. Zn||Ti cells exhibit a CE exceeding 98% after 240 cycles. Zn||MnO2 full batteries retain 92.1% capacity after 1000 cycles at 1 C and 80% after 1000 cycles at 5 C. These results demonstrate that PQ-7 effectively regulates Zn deposition and suppresses parasitic reactions, offering a straightforward, low-cost strategy for long-life aqueous ZIBs.
1. Introduction
Aqueous zinc-ion batteries (ZIBs) present a compelling alternative to lithium-ion systems for grid-scale energy storage, owing to zinc's abundance, low cost, and intrinsic safety in aqueous electrolytes. However, the commercial deployment of ZIBs is stalled by two interrelated failure mechanisms at the zinc anode: dendritic growth and the hydrogen evolution reaction (HER). Dendrites arise from the tip effect, where Zn2+ preferentially deposits at protrusions, leading to uneven plating, short circuits, and premature cell death. Concurrently, the high activity of zinc in water drives HER and self-corrosion, generating inert byproducts that passivate the electrode and reduce Coulombic efficiency (CE). These phenomena collectively limit cycle life and capacity retention, undermining the economic case for ZIBs in stationary storage.
Existing mitigation strategies—such as interfacial layer modulation, anode structure design, and electrolyte modification—have shown partial success but face trade-offs. Anodic alloying and surface modification improve stability but involve complex, high-cost processes. Organic electrolytes inhibit HER but introduce flammability risks. Electrolyte additives, by contrast, offer a low-cost, scalable route to protect the Zn anode. Prior additives rely on functional groups that adsorb on the Zn surface to shield H2O, yet few simultaneously address dendrite growth and HER without compromising rate performance. This study introduces polyquaternium-7 (PQ-7), a cationic surfactant with abundant polar (–NH2, C=O) and non-polar (–CH3) groups, as a multifunctional additive. PQ-7 adsorbs at initial tip sites, displacing H2O and [Zn(H2O)6]2+, thereby suppressing HER and mitigating the tip effect to promote uniform Zn deposition. The result is a 15-fold increase in symmetric cell cycle life and high capacity retention in full cells, providing a straightforward design pathway for long-life ZIBs.
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YIN Zihao, YAN Xiaoying, LIU Yingjie, GAO Zepeng, LI Zhengyu, QIN Zhenbo, ZHANG Jinfeng, WU Zhong, HU Wenbin (2025). Regulation of Hydrogen Evolution Reaction and Dendrite Growth by Multifunctional Additive for Zn-Ion Batteries. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3532-0
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Frequently Asked Questions
What is the long-term stability of the PQ-7 additive under continuous cycling, and does it undergo decomposition or consumption?
The symmetric Zn||Zn cells with PQ-7 cycled for over 2117 h at 5 mA cm−2 and 1 mAh cm−2 without failure, indicating that the additive remains effective throughout extended operation. While the study does not report post-mortem additive concentration analysis, the sustained low overpotential and stable CE suggest minimal consumption. For industrial deployment, additive depletion is a concern; however, the 15-fold life extension compared to additive-free cells implies that PQ-7 is not rapidly degraded. Future work should quantify additive retention after 1000+ cycles to ensure cost-effective replenishment schedules.
Does the PQ-7 additive affect the ionic conductivity or viscosity of the electrolyte, and how does this impact rate capability?
The Zn||MnO2 batteries retained 80% capacity after 1000 cycles at 5 C, demonstrating that the additive does not severely impede Zn2+ transport. While the paper does not provide explicit conductivity or viscosity measurements, the high-rate performance indicates that any increase in viscosity or reduction in conductivity is within acceptable limits. For high-power applications, the additive concentration must be optimized to balance HER suppression with ionic mobility; the observed 5 C retention suggests that PQ-7 is compatible with fast-charging protocols.
How does PQ-7 compare in cost and scalability to established electrolyte additives for ZIBs, such as polyethylene glycol or zinc sulfate?
PQ-7 is a commercially available cationic surfactant used in personal care products, implying low raw material cost and existing production capacity. The study positions PQ-7 as a low-cost additive, but does not provide a detailed cost analysis. For industrial adoption, the additive must be competitive with incumbent strategies. Given that PQ-7 operates at likely low concentrations (not specified in the abstract), the cost per liter of electrolyte is expected to be marginal. Scalability is favorable because surfactant additives are easily integrated into existing electrolyte formulation lines without additional equipment.
What is the mechanism by which PQ-7 mitigates the tip effect, and does it fully eliminate dendrite formation or merely suppress it?
PQ-7 adsorbs at initial tip positions on the Zn anode, competing with Zn2+ and [Zn(H2O)6]2+. This competitive adsorption reduces the local Zn2+ flux at protrusions, homogenizing deposition and preventing dendritic growth. The study reports uniform deposition rather than dendrites, but does not claim complete elimination under all conditions. The 2117 h cycling stability at 5 mA cm−2 suggests that dendrite suppression is robust. However, at higher current densities or areal capacities, the additive may need concentration optimization to maintain effectiveness.
Are there any safety concerns associated with PQ-7 in aqueous electrolytes, such as toxicity or flammability?
PQ-7 is a water-soluble cationic polymer commonly used in cosmetics and personal care products, indicating low toxicity and non-flammability. Unlike organic electrolyte additives, PQ-7 does not introduce combustion risks. The aqueous nature of the electrolyte remains intact, preserving the intrinsic safety advantages of ZIBs. The study does not report any safety incidents, and the additive is expected to be environmentally benign. For large-scale deployment, regulatory approval for use in energy storage devices would require standard toxicology assessments, but no red flags are apparent.
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