Poly(vinylidene fluoride), abbreviated as PVDF, mainly refers to vinylidene fluoride homopolymer or copolymer of vinylidene fluoride and other small amounts of fluorine-containing vinyl monomers. It has the characteristics of both fluororesins and general resins. In addition to good chemical corrosion resistance, high temperature resistance, oxidation resistance, weather resistance, and radiation resistance, it also has special properties such as piezoelectricity, dielectricity, and thermoelectricity.
Severe zinc dendrites and side reactions have greatly restricted the development of aqueous zinc-ion batteries. As a key component of the battery, the structure and properties of the separator play an important role in achieving stable zinc plating/stripping behavior. Uncontrollable zinc deposition may pierce the separator and cause a short circuit in the battery. Therefore, the development of high-quality separators with special structural and functional designs helps to guide the uniform deposition of zinc, thereby improving the cycle life of the battery.
Polydopamine modification is an excellent surface modification method that has been proven to effectively improve the electrochemical, thermal, and mechanical properties of the battery when applied to the separator system. The rich functional groups contained in polydopamine can form coordination bonds with metal ions, promote the synergistic effect and uniform diffusion of ions, and have great application potential in the preparation of aqueous zinc-ion battery separators. A polydopamine-functionalized PVDF (PVDF@PDA) nanofiber membrane was designed as a separator for aqueous zinc-ion batteries.
The PVDF@PDA separator has excellent mechanical strength and good zinc affinity. It promotes the formation of Zn-O and Zn-N coordination bonds through specific functional groups (-OH and -NH-), uniform zinc ion flux, and achieves dendrite-free zinc deposition. In addition, the separator effectively inhibits vanadium shuttle through the separator by forming V-O coordination bonds, further protecting the zinc negative electrode. Therefore, the Zn/NH4V4O10 battery using PVDF@PDA separator shows better electrochemical performance than ordinary glass fiber (GF) separator. The design of this separator provides a new strategy to promote the practical application of aqueous zinc-ion batteries.
The study describes the process of preparing PVDF@PDA membrane based on electrospinning and polydopamine modification, and further characterizes its morphology and structural components by combining scanning, infrared, Raman and other analytical methods. The thickness of the prepared PVDF@PDA membrane is 170μm, which is about one-fourth of the thickness of the GF membrane (640μm), and has excellent hydrophilicity and tensile strength.
The practicality of the prepared membrane was further verified by full-cell electrochemical testing, and button cells were assembled using GF membrane and PVDF@PDA membrane with NH4V4O10 positive electrode materials. After being equipped with PVDF@PDA membrane, the rate performance and cycle stability of the battery were greatly improved. At a current density of 5 A g-1, the capacity retention rate after 1000 cycles was 92.3%. And after 300 cycles at a current density of 2 A g-1, the surface of the Zn negative electrode remained smooth and flat. The Zn/NH4V4O10 battery was charged to 1.4 V for the first cycle. Vanadium was detected on the surface of the Zn negative electrode using the GF separator, but not on the surface of the Zn negative electrode using the PVDF@PDA separator, indicating that the PVDF@PDA separator effectively inhibited the shuttle of vanadium through the separator.
The analysis of XPS spectra and density functional theory (DFT) calculation results was studied. It can be found that the -OH and -NH- functional groups have strong zinc affinity, which will form Zn-O and Zn-N coordination bonds, making the zinc ion flux uniformly distributed, guiding the uniform deposition of zinc, and achieving effective inhibition of zinc dendrites. At the same time, there is a strong interaction between the -OH functional group and vanadium, forming a V-O coordination bond, which is conducive to inhibiting the shuttle of vanadium.
In order to further prove the effect of PVDF@PDA separator on zinc deposition, the deposition morphology of the assembled Zn/Ti asymmetric battery and the morphology of the Zn/Zn symmetric battery after cycling were scanned and analyzed. It can be found that serious agglomerations and protrusions with glass fibers attached to the surfaces of the GF diaphragm, Ti substrate and Zn substrate appeared. However, the deposition on the surfaces of the Ti substrate and Zn substrate using the PVDF@PDA diaphragm was dense and uniform, and the electron probe results also proved that the distribution of zinc elements on the surface was more concentrated and uniform, which more intuitively showed the regulatory effect of the PVDF@PDA diaphragm on zinc ions.
