Development of High Energy Density Zn-ion Hybrid Supercapacitors and Mn-based Zn-ion Batteries

高能量密度鋅基水系複合超級電容器及錳基鋅離子電池的研發

Student thesis: Doctoral Thesis

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Award date27 Aug 2021

Abstract

Recently, batteries, which can make use of clean and renewable energy, have become one of the most powerful competitors of non-renewable resources. So far, different kinds of batteries have been developed, such as monovalent (Li, K, Na) and/or multivalent (Mg, Ca, Zn, Al) elements based batteries. Among all these different kinds of batteries, lithium-ion batteries (LIBs) are the most popular and extensively used ones in our daily life due to their durability, high energy density, and high discharge voltage. However, the shortage of lithium resources, high cost of production, and toxic electrolytes limit the further development of LIBs. Besides, the inflammable organic electrolytes may cause fire disasters when short out or overload occurs. The development of aqueous Zn-based devices, such as zinc ion batteries (ZIBs) and Zn-ion hybrid supercapacitors (ZHSs) seems to provide an effective method to solve this problem. The non-toxic and non-flammable ZnSO4 aqueous electrolyte, as well as the stable Zn anode, have paved the way for realizing safe energy storage devices. Moreover, the production cost is significantly reduced due to the richness of Zn resources, low cost of ZnSO4 salt, and simple fabrication process of aqueous devices. Furthermore, the ionic conductivity of aqueous electrolytes (≈1 S cm-1) is 2 orders of magnitude higher than nonaqueous electrolytes (1−10 mS cm-1), which leads to a better rate performance for both aqueous batteries and supercapacitors. 

Although supercapacitors normally exhibit extremely high power density and also stability, their low energy density greatly limits their applications. Integrating an EDLC (electrochemical double-layer capacitors)-type cathode with a Zn anode to construct a Zn hybrid supercapacitor can solve this problem, benefitting from the large gravimetric and volumetric energy density offered by Zn anode. Moreover, Zn anode can be used as active material and current collector simultaneously, further improving the overall energy density of the device. In chapter 2, a Zn/aMEGO (chemically activated microwave exfoliated graphene oxide) hybrid supercapacitor is constructed and ultrastability of 80,000 cycles is achieved, benefitting from the high Zn stripping/plating efficiency in 3M Zn(OTF)2 electrolyte and the excellent stability of aMEGO. Besides, the Zn stripping/plating behavior and efficiency in typical Zn-based aqueous electrolytes (i.e., ZnCl2, Zn(NO3)2, ZnSO4, Zn(CH3COO)2 and Zn(CF3SO3)2) were investigated, and results showed that the 3 M and 4 M Zn(CF3SO3)2 electrolytes demonstrated the best Zn stripping/plating efficiency, benefiting from the strong interaction between water molecules and CF3SO3- anions.

Even ZHSs can achieve extremely high life stability, their energy density is still unsatisfying when compared with ZIBs. Among the different kinds of cathode materials for ZIBs, manganese dioxide attracts much attention due to its unique advantages such as high discharge voltage and capacity, but it also has some problems when it comes to practical applications. In fact, the most obvious issue for manganese dioxide is Mn2+ dissolution from Mn3+ disproportionation caused by the Jahn-Teller effect during the charge/discharge processes, which will cause degradation of capacity in the long time run. One effective method to solve this problem is introducing reversible Mn2+ ion oxidation deposition, which can be realized by broadening the working window of aqueous ZIBs. In chapter 3, an aqueous Zn/MnO2 battery with a 0.8-2.4 V working voltage window is successfully fabricated through using LiTFSI electrolyte additive and cathode activation process, and the redeposition of dissolved Mn2+ is achieved in this widened voltage window, which also greatly improves the stability of cathode. Besides, the addition of LiTFSI can effectively suppress hydrogen evolution, thus greatly improve the reversibility of Zn stripping/plating processes.

Recently, the Mn4+/Mn2+ redox reaction has become a hot topic in aqueous Zn/MnO2 batteries because it involves two-electron transfer during the charge/discharge processes and can greatly enhance the capacity of MnO2 compared with the traditional one-electron transfer reaction. However, in most cases, strongly acidic electrolytes or high charging potential is needed to achieve the complete dissolution/redeposition of MnO2, which sets a high standard of equipment and also requires a large working window of electrolytes. In chapter 4, the Mn4+/Mn2+ redox reaction in mild conditions can be achieved through Cu doping, and the cycle performance can be significantly enhanced because the dissolution/redeposition mechanism can avoid irreversible phase transformation. Detailed characterizations demonstrate that in discharge, Cu0 is generated and catalyzes the further reduction of Mn4+ and Mn3+ to dissoluble Mn2+ below 1.1 V, accompanied by the formation of Cu-containing ZHS (Zn4SO4(OH)6·5H2O). Whereas in the charging process, Mn2+ in the electrolyte will react with Cu-containing ZHS to form CMO again.

This thesis work provides useful insights into the study of high energy density ZHSs and Mn-based ZIBs and gives many meaningful suggestions for the development of new-type energy storage devices.

    Research areas

  • Supercapacotor, Zn ion battery, MnO2