First author: Zou Peichao, Lin Ruoqian
Corresponding authors: Xin Yulin, Lin Ruoqian, Xu Kang
Communications units: University of California, Irvine, Brookhaven National Laboratory, NASA Jet Propulsion Laboratory, U.S. Army Research Laboratory
First, the research background
Because of its high safety, high energy density, environmental friendliness and low cost, aqueous zinc-ion batteries have attracted widespread attention from researchers in recent years. However, due to the interfacial side reactions between the electrolyte and zinc metal, as well as the irregular morphological evolution of zinc anode, surface "chalking" and "dead zinc" formation, the reversibility of zinc metal anode still needs to be improved. Among them, in the process of zinc metal deposition, due to the hydrogen evolution reaction (HER) caused by water decomposition in the electrolyte and the precipitation of alkaline salt by-products induced by local strong alkalinity, not only serious corrosion and passivation of the zinc metal negative electrode, but also continuous consumption of the electrolyte, resulting in battery flatulence failure. However, in aqueous electrolytes, water decomposition and the interfacial side reactions it causes are difficult to avoid. This is mainly because the aqueous electrolyte tends to easily form a hydrated ionic solvation layer and a hydrophilic negative electrode-electrolyte interface.
Under ideal conditions, the construction of a negative electrode-electrolyte interface that conducts zinc ions without water molecules can effectively reduce the hydrodegradation of the negative electrode surface and the corrosion and passivation of the negative electrode surface caused by it. To achieve these goals, the strategies reported so far have focused on introducing (artificial) SEI layers on the surface of zinc metals, but this strategy inevitably reduces the interface reaction kinetics and increases the complexity of electrode preparation. How to construct a hydrophobic negative electrode-electrolyte interface in a water-based battery system without introducing an (artificial) SEI layer is still a problem. One possible scheme is to regulate the electrolyte formulation to achieve hydrophobicity directly in the electrolyte; Achieving hydrophobicity in aqueous solution systems seems impossible.
Second, a brief introduction to the research work
Recently, the team of Professor Xin Yulin (corresponding author) of the University of California, Irvine, Dr. Lin Ruoqian (corresponding author) of NASA's Jet Propulsion Laboratory, and Dr. Xu Kang (corresponding author) team of the US Army Research Laboratory, realized a new water electrolyte structure with local hydrophobic properties for the first time by introducing a new diluent of "cation-philic" and "anion-phobic" in the aqueous electrolyte. The hydrophobic aqueous electrolyte concept was validated in the aqueous ZnSO4-H2O system, where the new diluent introduced was a small molecule DMF with a single polar functional group (other small molecules such as NMP and unit alcohols are also suitable). Although the above small molecules have been reported in aqueous electrolytes, their local hydrophobic properties that promote formation have not been discussed. It is worth mentioning that dmF molecules are more easily adsorbed on the surface of the Zn metal anode than H2O molecules; The DMF molecule also has a strong hydroxide reactivity, so that the local hydrophobic electrolyte can further consume the hydroxide produced by water decomposition, thereby inhibiting the deposition of alkaline salt by-products on the negative surface, eliminating the formation of a passivation layer on the negative surface, and improving the interfacial reaction kinetics. The above aqueous electrolyte with local hydrophobic and interfacial alkaline elimination functions was applied to zinc metal batteries, and the extremely high zinc deposition/dissolution reversibility was achieved (the average coulomb efficiency of 1000 cycles reached 99.72%). The zinc-ion capacitors and zinc-ion all-batteries constructed have excellent long-cycle stability at -10oC and low N/P ratio. The article was published in the top international journal Nano Letters. Dr. Peichao Zou and Dr. Ruoqian Lin are the co-first authors of this article.
3. The core content expression part
Figure 1. Schematic diagram of electrolyte solvation structure, local hydrophobic formation mechanism and negative electrode-electrolyte interface.
In the aqueous ZnSO4 electrolyte, the DMF diluent molecules added had a single and identical Zn2+ affinity site and a water molecule coordination site; Therefore, when the diluent is coordinated with Zn2+, the diluent molecule no longer has the ability to coordinate with the hydrogen bond with the water molecule, thereby transforming the original hydrophilic diluent molecule into a hydrophobic molecule (Figure 1c-f). Thus, when the diluent molecule participates in the Zn2+ solvation process, the formed solvated molecules/clusters exhibit strong local hydrophobicity at the end close to the diluent (blue region in Figure 1g). During zinc metal deposition, when this solvated molecule/cluster moves to the negative surface, it promotes the formation of a water-poor interface layer (Figure 1b).
In addition, since the DMF molecule has a strong hydroxide reactivity, the DMF molecule adsorbed on the surface of the Zn anode can consume the hydrogen oxide produced by water decomposition, thereby inhibiting the precipitation of basic salt by-products. At the same time, since the products after the DMF reaction with the hydroxide are soluble ions and gases, the decomposition of the DMF reaction does not cause other products to form on the surface of the zinc anode (Figure 1b); This inference coincides with XPS and TEM experimental analysis. The elimination of the passivation layer on the surface of the zinc anode helps to improve its interfacial reaction kinetics.
Figure 2. The Zn2+-philic and SO42-phobic characteristics were confirmed by Raman, ATR-FTIR, HR-MS, NMR, XAS, PDF and DFT.
In order to explore the effect of DMF introduction on the structure of ZnSO4 electrolyte in the water system, the researchers first used Walden plot (Figure 2a) to confirm that the introduction of DMF reduces the dissociation of ZnSO4 and promotes the formation of ion contact pairs (CIP); This is consistent with the conclusions reached by Raman's fitting (Figure 2c). Further, the movement of featured signals in the Raman, ATR-FTIR, and NMR spectra shows that DMF has a strong affinity with Zn2+ (Figures 2b, d, f). The ion mass spectrometry test directly confirmed that DMF can participate in the Zn2+ solvation process and cannot coordinate with SO42( Figure 2e). In addition, DFT, XAS and PDF tests have shown (Figure 2g-i) that dmF molecules have stronger binding to Zn2+ than water molecules, which is conducive to reducing the activity of water molecules in the solvation layer.
Figure 3. ZnSO4-H2O-DMF electrolyte can inhibit HER, inhibit the formation of interfacial by-products, and promote the smoothing of Zn deposition morphology.
Using XRD characterization (Figures 3a, b), the researchers observed that the use of the ZnSO4-H2O-DMF system electrolyte can greatly eliminate the formation of zinc metal negative surface by-products (ZHS). At the same time, LSV testing (Figure 3c) and in situ photospectoscopic observation (Figure 3d, e) show that the introduction of DMF favors inhibition of the occurrence of HER on the negative surface. In addition, SEM and TEM 3D imaging analysis showed that the Zn metal deposited in the ZnSO4-H2O-DMF electrolyte was more uniform and flat. In contrast, Zn metals deposited in conventional ZnSO4-H2O electrolytes exhibit disordered, flaky morphology (Figure 3f-k).
Figure 4. ZnSO4-H2O-DMF electrolyte promotes uniform, high-density, flat Zn-shaped nuclei and growth.
In addition to regulating the structure of the electrolyte, the introduction of DMF can also change the dynamics of Zn metal-shaped nuclei and its evolutionary morphology. To this end, the researchers first studied the response current curve of the Zn metal anode under different overpotentials (Figure 4a, b). Tests have shown that zinc metal-shaped nuclei in the ZnSO4-H2O-DMF electrolyte have extremely short 2D diffusion times (<5s), indicating that their nucleuses are more uniform and dense (Figure 4c). This inference is also confirmed by in situ SEM characterization (Figure 4d, e). In addition, electrochemical tests have shown that when the nucleus-forming process is not dominated by mass transfer (i.e., at very small current densities), the introduction of DMF simultaneously reduces the zinc-shaped nucleus/overgrowth potential (Figure 4g), thereby promoting uniform, flat zinc metal deposition.
Figure 5. Zn deposition/peel efficiency and full battery long cycle stability test under normal temperature and low temperature conditions and different zinc salt conditions.
By introducing cationophilic and anophobic DMF diluent molecules into the aqueous ZnSO4 electrolyte, the work is achieved without introducing a negative electrode SEI layer, while inhibiting water decomposition, inhibiting the deposition of interfacial by-products, and promoting homogeneous and highly dense form nuclei. Therefore, the constructed local hydrophobic electrolyte can effectively improve the efficiency and stability of the zinc metal cycle. Among them, at room temperature, the average Coulomb efficiency of Zn metal deposition/stripping 1000 laps is as high as 99.72% (Figure 5a); Zn//Zn symmetrical battery can be stable for more than 1100 hours at 3mA cm-2 (Figure 5b); Zn//AC full battery has a capacity retention rate of 80.6% after 5000 cycles (Figure 5e). At the same time, since DMF has a low freezing point (-64 oC), the introduction of DMF can also broaden the operating temperature window of the electrolyte. For example, Zn//Cu, Zn//Zn, and Zn//AC batteries all exhibit excellent cycle stability at -10 oC (Figures 5c, d, f). In addition, the concept of "locally hydrophobic aqueous electrolyte" was also validated in the zinc acetate (Zn(Ac)2) system (Figures 5g, h).
The final core conclusion
The role of DMF in ZnSO4-H2O-DMF electrolyte can be summarized as follows:
- In conventional electrolyte formulation design, the introduced functional molecules typically act as (i) additives, (ii) cosolvents, or (iii) diluents. These functional molecules either have an affinity with both cations and anions in solution (e.g., co-solvents) or do not have an affinity with both anions and cations (additives and conventional diluents). DMF, on the other hand, has a strong affinity with Zn2+ in the ZnSO4-H2O system, and does not have an affinity with SO42-. In addition, when DMF is coordinated with Zn2+, it no longer has the ability to coordinate with H2O molecules, thereby promoting the formation of local hydrophobicity.
- DMF molecules can participate in the formation of Zn2+ solvation layer, thereby reducing the content of active water molecules in the solvation layer.
- Compared with the water molecules in the solvation layer, the DMF molecules in the solvation layer can bind to Zn2+ more, thereby reducing the activity of the water molecules in the solvation layer.
- Compared with free water molecules, free DMF molecules are easier to adsorb on the zinc negative electrode surface, thereby reducing the content of free water on the negative surface.
- Free DMF molecules can be hydrogen bonded with free water molecules and water molecules in the solvation layer, further reducing the activity of water molecules adsorbed in the free water and solvation layers on the surface of the zinc anode.
- DMF has strong hydroxide reactivity, which helps to inhibit the deposition of by-products on the surface of the negative electrode.
In summary, the work is the first to report and validate the possibility of introducing hydrophobicity into aqueous electrolytes, proposing a new concept of "hydrophobic aqueous electrolyte" (hydrophobic aqueous electrolyte). This new electrolyte structure design provides a new guiding idea for improving the reversibility of metal anode in water-based batteries. In addition, the work also proposes a new diluent concept of "cation-philic" and "anion-phobic", which is fundamentally different from conventional cosolvent and diluent, and is expected to inspire the design of new non-aqueous electrolyte structures.
4. Literature details
Localized Hydrophobicity in Aqueous Zinc Electrolytes Improves Zinc Metal Reversibility, Nano Letters. 2022, DOI: 10.1021/acs.nanolett.2c02514
https://doi.org/10.1021/acs.nanolett.2c02514