Exploring the Electrolyte Additives For Aqueous Zinc Ion Batteries

Abstract

To greatly improve the ecological environment and achieve the target of carbon neutrality around 2050s, it is significant to develop a kind of low-cost convenient-eco-friendly-green assemble-disassemble-recycle next-generation energy system. It is urgent to develop electrical energy storage which could achieve discharge-charge quickly and effective long life-span of advanced all-size devices and electrics. Rechargeable aqueous zinc batteries have been considered as a promising candidate for large-scale energy storage due to the low cost, intrinsic safety, low toxicity, the abundance of materials, and the unique features of zinc: good conductivity (5.91 microohm), low redox potential (-0.7626 V vs. standard hydrogen electrode, in acidic solution, 298.15 K), high gravimetric capacity (821 mAh g-1) and high volumetric capacity (5855 Ah L-1 compared to 2061 Ah L-1 for Li anode). Nevertheless, state-of-the-art techniques of ZIBs are far from satisfactory of industry standard due to the difficult ability to improve the poor reversibility (caused by evolution of hydrogen, the metal dendrite and by-products, etc.) of Zn anode, matching with the dissolution of cathode materials (such as shuttle effect), resulting in the obvious decline in battery performance. In the acidic electrolytes, HER inevitably happens and limits the coulombic efficiency (CE) of half-cell or full-cell, significantly influencing on the capacity. Further, the evolution of hydrogen makes the environment instability and annoys the hydroxyl ion (OH-) localized concentration around the surface of zinc anode, which accelerates the corrosion reaction of zinc and form the by-products (e.g., Zn4SO4(OH)6·nH2O). The compound acts as an isolation between zinc anode and electrolyte, not only deteriorate the ability to ion-electron diffusion but also have the side effects on the reversibility of zinc plating/stripping. And the evident Zn dendrite growth has a close relationship with the Coulombic efficiency (CE) during battery cycling and the battery lifespan. In the electrolytes, Zn2+ ion could form the solvation structure like a chelate structure, which is surrounded by six H2O molecules. These strong bonds make the Zn firmly riveted from desolvation and deposition more difficult, like a high energy barrier. In conclusion, it is necessary to develop strategies for modulating the kinetics of Zn electrodeposition so as to obtain a smoothly homogeneous reversible Zn nucleation. Therefore, one of the strategies is to add the electrolyte additives to minimize the electrolysis of water (e.g., electrolyte decomposition) while suppressing the dendrite growth and parasite reactions as well as controlling regulation of the solvation structure which significantly affect the aqueous Zn batteries. In the first case, the effects of electrolyte additive, dimethyl sulfone or (methylsulfonyl)methane (MSM) on the electrochemical performance of Zn/MnO2 was studied. The results show the additive promotes the deposit pattern of Zn ion, intending the dendrite-free Zn plating/stripping highly reversible reaction. Meanwhile, the hydrophilic ability of metal face was optimized by introducing the additive, accelerating the Zn-ion diffusion at the Zn anode/electrolyte interface. Benefiting from these effects, the capacity and cycling life of Zn/MnO2 batteries were improved to some certain degree. In the second case, Zn metal plating/stripping mechanism was thoroughly explored in 2M ZnSO4 electrolyte, demonstrating that the poor performance was ascribed to the formation of a by-product Zn4SO4(OH)6·5H2O flakes, hydrogen evolution reaction, and extremely grievous pulverization dendrite growth. To suppress the dendrite growth and parasitic reactions, a kind of rust remover and reactive organic diluent, Gamma-butyrolactone (C4H6O2, GBL) was introduced to be a new electrolyte additive into ZnSO4 electrolyte for dendrite-free ZIBs. A small volume of GBL (1% solvent) in 2M ZnSO4 could markedly improve the performance of Zn plating/stripping behavior under different current density (4200 h for 1 mAh cm-2 at 1 mAh cm-2; 1170 h for 10 mAh cm-2 at 10 mA cm-2; and 140 h for 20 mAh cm-2 at 20 mA cm-2). The high average CE of MnO2-cathodes and a high plating/stripping average CE of 99.7% for Zn anodes demonstrate that the problem of MnO2 dissolution and dendrite Zn growth have been effectively suppressed. Experiment and theoretical calculations confirmed that GBL could assist to help changing the solvation structure of Zn2+, alleviating the H2O activation and prohibit the by-product to some extent. Additionally, Zn metal surface was inclined to absorb GBL other than H2O, which is in favor of hoisting the nucleation overpotential and guiding the uniform deposition. It is noteworthy that 2M ZnSO4-GBL electrolyte is nonflammable, and conveniently stable, which is promising for next generation green and high-performance Zn-ion batteries. And the outstanding performance was attributed to the i) the nanoarchitecture that provides electrochemically large active sites, leading to the high energy storing performance and ii) the enhanced ionic transport from the hierarchically interconnected 2D-Zn nanostructures. In the third case, an important chemical raw material, α-pyrrolidone, which can be used as solvent and intermediate of organic synthesis. The effects of α-pyrrolidone on the electrochemical performance of Zn/MnO2 was studied. The results show the additive promotes the deposit pattern of Zn ion, intending the dendrite-free Zn plating/stripping highly reversible reaction. Meanwhile, the hydrophilic ability of metal face was optimized by introducing the additive, accelerating the Zn-ion diffusion at the Zn anode/electrolyte interface. Benefiting from these effects, the capacity and cycling life of Zn/MnO2 batteries were improved to some certain degree. In the future work, I would try more organic molecules as electrolyte additives such as Aspartame, Betaine, and Glucurolactone, etc. In summary, the chaos of low CE and terrible reversibility of Zn electrode cause of the dendrite growth, hydrogen evolution reaction, by-products and metal corrosion in mild electrolyte severely hinder the further development of aqueous Zn-based batteries at the industry standard. I believe this doctoral research could provide a fundamental understanding of Zn electrode interplay with aqueous-organic media and presents a potential and perspective direction to develop and open up the electrolyte additives in purpose of making advanced aqueous Zn-ion batteries more closely to be commercialized.