Zn-based electrochemical energy storage devices

  • Jie Yang

Student thesis: Phd


As a new and promising type of energy storage device in terms of safety, cost and environmental compatibility, Zn-based electrochemical storage devices such as zinc-ion batteries and zinc ion hybrid supercapacitors are investigated in terms of the fundamental mechanism and electrochemical performance. Firstly, MnO2 prepared by a hydrothermal method was used to investigate reaction mechanism in aqueous Zn-MnO2 batteries. Direct evidence was provided of the importance of Mn2+ in this type of battery by using a bespoke cell. Without pre‐addition of Mn2+, the cell exhibited an abnormal discharge–charge profile, meaning it functioned as a primary battery. By adjusting the Mn2+ content in the electrolyte, the cell recovered its charging ability through electrodeposition of MnO2. Additionally, a dynamic pH variation was observed during the discharge–charge process, with a precipitation of Zn4(OH)6(SO4)⋅5H2O buffering the pH of the electrolyte. Contrary to the conventional Zn2+ intercalation mechanism, MnO2 was first converted into MnOOH, which reverted to MnO2 through disproportionation, resulting in the dissolution of Mn2+. The charging process occurred by the electrodeposition of MnO2, thus improving the reversibility through the availability of Mn2+ ions in the solution. Secondly, the voltage range and self-discharge phenomena have been systematically investigated in aqueous zinc ion hybrid supercapacitors by using cyclic voltammetry and galvanostatic charge-discharge method, which leads to better understanding of this hybrid device. The upper cut‐off voltage is limited to 1.8 V (vs. Zn2+/Zn) due to the oxidation of carbon surface functional groups or oxidation of the carbon bulk. The mechanism of the zinc ion hybrid supercapacitor involves the simultaneous adsorption/desorption of ions on the activated carbon cathode and zinc ion plating/stripping on the Zn anode. Constructed by the high capacity of the Zn metal negative electrode, neutral aqueous electrolyte and activated carbon positive electrode, this hybrid supercapacitor has demonstrated excellent electrochemical performance including high specific capacitance (308 F g−1 at 0.5 A g−1 and 110 F g−1 at 30 A g−1), good cycling stability (10,000 cycles with 95.1% capacitance retention) and a high energy density 104.8 Wh kg−1 at 383.5 W kg−1 (30.8 Wh kg−1 at 19.0 kW kg−1) based on the active materials. Considering that the zinc foil can be simultaneously used as both current collector and active material, the unnecessary weight and volume of the devices can potentially be reduced to some extent. Additionally, the self-discharge was substantially suppressed in the hybrid supercapacitors compared with the symmetric supercapacitors, which is highly dependent on the initial voltage. Thirdly, safe, low-cost and environmentally benign zinc-ion hybrid supercapacitors based on neutral aqueous electrolytes are promising for large scale and high power energy storage. A key challenge for Zn-ion hybrid supercapacitors is to increase their energy density without sacrificing the high power performance. A Zn-ion hybrid supercapacitor using a polypyrrole/electrochemical graphene oxide (PPy/EGO) composite cathode and aqueous zinc halide electrolyte (i.e., 1 M ZnCl2 or ZnBr2) was investigated. The EGO was used to prepare conducting polymer composite due to its low degree of oxidation and good integrity in conjugated structure, which are favoured for electron conduction. The water dispersible EGO allows one-step co-electrodeposition of PPy/EGO composite that has an interconnected porous structure for fast ion diffusion in pores. Moreover, the small-sized and monovalent anions in the optimized zinc halides electrolytes are highly mobile in bulk PPy for fast anion insertion/de-insertion. Hence, the PPy/EGO composite showed high specific capacitances > 440 F g−1 and good rate capability in either 1 M ZnCl2 or ZnBr2. The as-fabricated Zn-PPy/EGO system that had an operation
Date of Award31 Dec 2021
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorRobert Dryfe (Supervisor) & Mark Bissett (Supervisor)


  • zinc-ion batteries
  • zinc-ion hybrid supercapacitors
  • mechanism
  • high performance

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