Aqueous zinc-ion batteries have become an important research direction in the energy storage field due to their high safety, low cost and environmental friendliness. However, issues such as zinc dendrite growth, excessive electrolyte consumption and low energy density have long hindered their commercialization process. Traditional glass fiber separators, characterized by large thickness, uneven pore size and insufficient mechanical strength, not only require a large amount of electrolyte to ensure ion transmission, but also aggravate the formation of zinc dendrites, resulting in the weight ratio of electrolyte and separator in the battery exceeding 60%.
To solve this problem, the research team developed an ultra-thin β-PVDF/glass fiber powder/zinc trifluoromethanesulfonate composite PGZ separator with a thickness of only 10μm through a simple and economical solution casting process. This ultra-thin PGZ composite separator integrates high ionic conductivity, high Zn²⁺ transference number and excellent mechanical properties, which can adapt to harsh lean electrolyte conditions, significantly reduce electrolyte dosage, and effectively inhibit zinc dendrites and side reactions. It achieves a dual breakthrough in the energy density and cycle stability of aqueous zinc-ion batteries, opening up a new path for the practical application of high-energy density zinc-ion batteries.
Low-Cost and Efficient Preparation: Core Characteristics of the 10μm Ultra-Thin PGZ Composite Separator
The research team prepared the PGZ composite separator using a solution casting strategy. This process is simple to operate, low in cost, and easy to achieve large-scale industrial production. The prepared PGZ composite separator is composed of β-polyvinylidene fluoride (β-PVDF), glass fiber powder (GF) and zinc trifluoromethanesulfonate (Zn(OTf)₂). Compared with traditional glass fiber separators with a thickness of hundreds of microns, it has achieved a qualitative performance upgrade, with particularly prominent core characteristics:
Extremely Ultra-Thin and Excellent Mechanical Properties: The PGZ composite separator has a thickness of only 10μm, much thinner than traditional glass fiber separators; it also has a high Young’s modulus, excellent tensile strength in both dry and wet states, and can withstand the mechanical stress of hanging a 300g weight. It solves the problem of easy damage of ultra-thin separators, providing structural guarantee for battery assembly and use.
High Ion Transmission Efficiency: The ionic conductivity of the PGZ composite separator is as high as 1145.5 mS, the Zn²⁺ transference number is increased to 0.8, and the activation energy is only 25.27 kJ·mol⁻¹, which is far superior to traditional glass fiber separators (Zn²⁺ transference number 0.35). The highly polar β-PVDF can accelerate the transmission kinetics of Zn²⁺ inside the separator, greatly improving ion migration efficiency.
Excellent Electrolyte Wettability and Retention: The PGZ composite separator has good electrolyte wettability, which can ensure the continuity of ion transmission under ultra-thin thickness, and reduce the invalid consumption of electrolyte, laying a foundation for ion transmission under lean electrolyte conditions.
Uniform Pore Structure Distribution: The PGZ composite separator has a uniform pore size distribution, which can effectively uniformize the Zn²⁺ flux, avoid the uneven growth of zinc dendrites caused by excessive local Zn²⁺ concentration, and fundamentally reduce the risk of separator puncture by zinc dendrites.
In addition, β-PVDF can also play a “selective blocking” role. While accelerating the transmission of Zn²⁺, it can effectively block the diffusion of OTf⁻, I₃⁻, I₅⁻ and H₂O, providing key support for the subsequent inhibition of side reactions and polyiodide shuttling. For more research on composite separator materials, you can refer to the research published by the Journal of Power Sources.
Excellent Performance Under Lean Electrolyte Conditions: Inhibiting Zinc Dendrites and Achieving Long Cycle Stability
Adapting to lean electrolyte conditions is the core key to improving battery energy density. Under the harsh conditions of low electrolyte volume/capacity (E/C) ratio, the PGZ composite separator shows far superior electrochemical performance than traditional glass fiber separators, successfully solving the problems of aggravated zinc dendrites and rapid electrolyte depletion under lean electrolyte conditions:
Wide-Range Lean Electrolyte Adaptability: The Zn/Zn symmetric battery based on the PGZ composite separator can cycle stably for 700 h under the lean electrolyte condition of E/C ratio 4μL mAh⁻¹; even under the more harsh conditions of E/C ratio 2 μL mAh⁻¹ and depth of discharge up to 85.4%, it can still cycle stably for 200 h, while traditional glass fiber separators are difficult to work normally under this condition.
Effective Inhibition of Zinc Dendrites and Side Reactions: Through uniformizing Zn²⁺ flux, the PGZ composite separator guides dense zinc deposition on the negative electrode surface, avoiding the growth and puncture of zinc dendrites; at the same time, it blocks the diffusion of H₂O, reduces the formation of Zn(OH)₂ and ZnO passivation layers on the negative electrode surface, alleviates the corrosion side reactions of the zinc negative electrode, and keeps the zinc negative electrode with a flat surface structure after cycling.
Low Electrolyte Loss Rate: The structural characteristics of the PGZ composite separator greatly reduce the volatilization and loss of electrolyte, and the electrolyte loss rate is much lower than that of traditional glass fiber separators. It can still ensure the stability of ion transmission for a long time under lean electrolyte conditions, avoiding premature battery failure due to electrolyte depletion.
This series of performance performances prove that the PGZ composite separator can maintain long-term cycle stability of the battery while significantly reducing electrolyte dosage, clearing the core obstacle for improving battery energy density.
Multiple Functional Support: Improving Full Battery Performance and Achieving High Energy Density of 129.7 Wh kg⁻¹
The PGZ composite separator not only optimizes the deposition behavior of the zinc negative electrode, but also greatly improves the comprehensive performance of Zn/I₂ full batteries by inhibiting polyiodide shuttling and improving interface stability. Combined with the support of lean electrolyte conditions, it significantly reduces the proportion of inactive substances in the battery, achieving a leapfrog improvement in energy density. Its core advantages are reflected in three aspects:
Effective Inhibition of Polyiodide Shuttling Effect: Verified by H-cell diffusion experiments and density functional theory (DFT) calculations, the PGZ composite separator can effectively block the diffusion of polyiodides such as I₃⁻ and I₅⁻, avoid their shuttling between the positive and negative electrodes, reduce the loss of positive active substances, and improve the interface stability and coulombic efficiency of Zn/I₂ full batteries.
Adapting to Harsh Conditions of High Load and Low N/P Ratio: Under the multiple harsh conditions of I₂ positive electrode load up to 10.4 mg cm⁻², zinc negative electrode thickness only 20μm (N/P ratio 3.9), and electrolyte/active material ratio (E/A) as low as 2 μL mg⁻¹, the Zn/I₂ full battery based on the PGZ composite separator can still work stably.
Significantly Improve Energy Density and Reduce the Proportion of Inactive Substances: In batteries assembled with traditional glass fiber separators, the proportion of inactive substances such as electrolyte and separator reaches 66.0%, while the PGZ composite separator reduces this proportion to 36.3%, enabling the Zn/I₂ full battery to achieve a high gravimetric energy density of 129.7 Wh kg⁻¹; at the same time, the battery can cycle stably for 2750 times at 3C rate with a capacity retention rate of up to 93.1%, achieving both high energy density and long cycle life.
In addition, the Zn/I₂ pouch battery prepared based on the PGZ composite separator can cycle stably for 6500 cycles at 3C rate under the lean electrolyte condition of E/A ratio 2 μL mg⁻¹, with a capacity retention rate of 77.1%, showing excellent practical application potential and verifying the adaptability of the PGZ composite separator in industrial products such as pouch batteries.
Core Mechanism of Action: Dual Key Effects of β-PVDF
All performance breakthroughs of the PGZ composite separator originate from the unique role of β-PVDF, which achieves the dual effects of accelerating Zn²⁺ transmission and selectively blocking harmful ions in the separator, becoming the key to unlocking the high performance of lean electrolyte zinc-ion batteries:
Electrostatic Potential and Binding Energy Regulation: The electrostatic potential and binding energy characteristics of β-PVDF make it have strong adsorption for Zn²⁺, which can guide the directional and rapid transmission of Zn²⁺, and at the same time have high repulsion for OTf⁻, I₃⁻, I₅⁻ and H₂O, effectively blocking the diffusion of these ions, fundamentally uniformizing Zn²⁺ flux, and inhibiting side reactions and polyiodide shuttling.
Promoting Dense Zinc Deposition: While β-PVDF accelerates Zn²⁺ transmission, combined with the uniform pore structure of the PGZ composite separator, it enables uniform deposition of Zn²⁺ on the negative electrode surface, avoiding the formation of zinc dendrites due to excessive local concentration, and ensuring the long-term cycle stability of the zinc negative electrode.
Compared with α-PVDF, the molecular structure of β-PVDF is more suitable for the ion transmission needs of zinc-ion batteries, which is also the core reason why the PGZ composite separator has better performance than traditional PVDF-containing separators. For detailed research on PVDF crystal forms, refer to the industry report released by the Institute of Electrical and Electronics Engineers (IEEE).
Research Significance: Providing a New Strategy for Ultra-Thin and Economical Separator Design
The development of this 10μm ultra-thin PGZ composite separator not only provides core material support for the realization of high energy density of lean electrolyte zinc-ion batteries, but also brings important scientific research and industrial enlightenment in terms of separator design and preparation, promoting aqueous zinc-ion batteries to take a key step towards commercialization:
Breaking Through the Performance Bottleneck of Ultra-Thin Separators: It is the first time to realize the synergistic optimization of ultra-thin separators in mechanical strength, ion transmission and lean electrolyte adaptability, solving the industry problem of “ultra-thin means easy damage, lean electrolyte means instability”, and providing a reference for the design of subsequent ultra-thin separators.
Low-Cost Process Adaptable to Industrialization: The adopted solution casting process does not require complex equipment, is low in cost, and easy to scale production, avoiding the problem that laboratory technology is difficult to industrialize, and laying a process foundation for the practical application of the PGZ composite separator.
A New Path to Improve Battery Energy Density: Through the combined strategy of “ultra-thin separator + lean electrolyte”, the proportion of inactive substances in the battery is reduced from 66.0% to 36.3%, achieving a high gravimetric energy density of 129.7 Wh kg⁻¹, providing a replicable idea for improving the energy density of other aqueous batteries.
Expanding the Application Scenarios of Zinc-Ion Batteries: The PGZ composite separator adapts to harsh conditions such as high-load positive electrodes, low N/P ratio and lean electrolyte, and shows excellent performance in pouch batteries, making the application of zinc-ion batteries in fields such as portable energy storage and power batteries possible. Our previous article oncoated lithium battery separators further elaborates on the development of separator technology for energy storage batteries.
Conclusion
The development of the 10μm ultra-thin PGZ composite separator is an important breakthrough in the field of aqueous zinc-ion battery materials. Through a simple and economical preparation process, it has achieved all-round improvement in mechanical properties, ion transmission performance and lean electrolyte adaptability. This PGZ composite separator can not only effectively inhibit zinc dendrite growth, polyiodide shuttling and side reactions, but also significantly reduce electrolyte dosage, enabling the Zn/I₂ full battery to achieve 2750 long cycles with a capacity retention rate of 93.1% and an energy density of 129.7 Wh kg⁻¹ under the lean electrolyte conditions of high load and low N/P ratio.
This research result not only solves the core material problem for the commercialization of high-energy density aqueous zinc-ion batteries, but also provides a new strategy for the design and preparation of ultra-thin and economical battery separators. In the future, the optimization and expansion of separator materials based on this process will further promote the wide application of aqueous zinc-ion batteries in fields such as energy storage and portable electronic devices, allowing low-cost, high-safety zinc-ion batteries to truly enter the market.