Gelatin/PEDOT: PSS Modified Manganese Dioxide Cathode in Aqueous Rechargeable Zinc-ion Batteries

Abstract

The global energy crisis and greenhouse effect have become important problems in the 21st century, and researchers have paid attention to renewable energy, including hydro, wind, solar, and nuclear energy sources. However, some of them are not always available. Energy storage systems become critical for their availability, and batteries are most commonly used to this end. Although lithium-ion batteries take up most of the portable battery market, it still has safety, cost, and environmental concerns. A battery with high energy density, high safety, cost efficiency, and environmental friendliness is strongly in demand. The aqueous rechargeable Zn-MnO2 battery has attracted many researchers’ attention due to its high energy density, high safety, low cost, and low toxicity. However, the cycling performance of this battery is not very satisfactory because of the Mn dissolution problem. Therefore, two effective strategies were proposed and conducted in this project to overcome the Mn dissolution issue and improve the electrochemical performance of the battery. First, the ε-MnO2 material was synthesized by an innovative, safe, and cost-effective co-precipitation method. Compared to the commercial MnO2 material with poor electrochemical performance and the MnO2 material synthesized by dangerous hygrothermal methods in labs, this new co-precipitation method is simple, safe, and cheap. More importantly, the ε-MnO2 material exhibits an excellent specific discharging capacity of 330 mAh/g at 50 mA/g when applied in the aqueous rechargeable Zn-MnO2 battery. However, the cyclability of the battery is not outstanding with only 20 cycles at 50 mA/g and 200 cycles at 500 mA/g at 80% capacity retention. To improve the battery’s cycling performance, gelatin was coated on top of the ε-MnO2 cathode by spin coating. Gelatin can confine Mn2+ near the cathode through hydrogen bonding and van der Waal’s force, thereby effectively controlling the Mn dissolution and improving the battery cyclability to 1000 cycles with 80% capacity retention at 500 mA/g, which is a remarkable improvement in the cycling performance of the aqueous rechargeable Zn-MnO2 battery. Another strategy was conducted to improve battery cyclability. Since PEDOT: PSS is a highly conductive polymer with a negatively charged deprotonated sulfonyl group, it could attract Mn2+ through ionic interactions, which can regulate the Mn dissolution problem. Benefiting from the PEDOT: PSS coating on the ε-MnO2 cathode, the battery showed a good cycling performance of 250 cycles at 500 mA/g and 50 cycles at 50 mA/g with both 80% capacity retention. These strategies have increased the cyclability of the aqueous rechargeable Zn-MnO2 battery up to 1000 cycles with 80% capacity retention, which is a dramatic enhancement of the cycling performance. They not only opened new ways for other researchers to further develop the battery performance, but also accelerate the application progress of aqueous rechargeable Zn-MnO2 batteries.