Yan Wu, supervised by Professor P. Chen will complete his MASc oral exam on November 15th.
Abstract:
With the increasing demand for energy storage systems, rechargeable batteries are receiving significant attentions. Lithium-ion batteries (LIBs) have been dominating the market over the decades because of their superior energy density. However, shortages of LIBs such as safety issues, low storage in earth’s crust restrict their applications. Therefore, seeking for alternative battery systems is becoming one of the most urgent topics.
Rechargeable aqueous zinc-ion batteries (RAZIBs) are considered as promising candidates for large-scale energy storage due to high specific capacity from zinc metal (820 mAhg-1), high safety, low cost, and environmental friendliness. Among the choices for cathode materials such as vanadium oxides, sodium manganese oxides, and Prussian blue analogue, MnO2 receives significant attention for its high theoretical capacity (308 mAhg-1 for 1 e- transfer; 615 mAhg-1 for 2 e- transfer) and non-toxicity. Although rechargeable aqueous Zn/MnO2 batteries have been researched for decades, the mechanism of the cathode reaction is still controversial, and the lifespan of the batteries are not satisfactory. Hence, a series of experiments are conducted in this work to investigate the reaction mechanism in Zn/MnO2 batteries and a promising method on proton regulating to enhance the cycling performance was proposed.
Firstly, the main cathode reactions are addressed by characterization methods such as inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), transmission electron microscopy (TEM). It is demonstrated that conversion reactions (MnO2 ↔ MnOOH) and dissolution/deposition reactions (MnO2 ↔ Mn2+) are the 2 types of main reactions happening in Zn/MnO2 batteries. Zinc intercalations are not observed according iv to the characterization results. Afterwards, the irreversible side reactions which cause capacity loss are researched. It is reported in this work that side products from discharge process, zinc sulphate hydroxide hydrates (Zn4(OH)6SO4∙5H2O, or ZHS), can react with Mn2+ when charged over 1.55V (vs. Zn/Zn2+) to form irreversible products, a low crystallized woodruffite ((Zn,Mn)2Mn5O12∙4H2O, or ZMO). This reaction is considered as the direct reason for short cycle life of Zn/MnO2 batteries.
Herein, strongly acidic cation exchange resin is used to regulate the proton concentration in the electrolyte and suppress the growth of ZHS side products during discharge process. The use of ion exchange resin (IER) can eliminate the ZHS side products at 1.55V (vs. Zn/Zn2+) during charge process and hence inhibit the generation of ZMO irreversible products. At a relatively small current density of 50 mAg-1, the proposed Zn/MnO2 battery with IER can deliver a reversible specific capacity of 170 mAhg-1 over 200 cycles. The capacity retention of Zn/MnO2 battery with IER after 200 cycles is ~85% and ~83% higher than that of Zn/MnO2 battery in 2 M ZnSO4 electrolyte and 2 M
ZnSO4 + 0.2 M MnSO4 electrolyte, respectively. Also, the columbic efficiency of the Zn/MnO2 battery in the proposed IER electrolyte is improved to be ~99.84%, while the columbic efficiency of the Zn/MnO2 battery in 2 M ZnSO4 electrolyte and 2 M ZnSO4 + 0.2 M MnSO4 electrolyte is only 98.74% and 98.84%, respectively. It is believed that regulating proton concentration is a promising and efficient method to improve the cycling performance of
Zn/MnO2 battery.