An Investigation into the Charge Storage Mechanism and Cycling Performance of Mn2O3 as the Cathode Material for Zinc-ion Batteries
Creators: Hou, Qingping
Publisher: name: University of Alberta Library
Year: 2022

<p>There has been increasing research interest to develop next generation energy storage devices to replace or at least compliment lithium-ion batteries (LIBs), because of their potential safety issues and resource deficiencies. Aqueous rechargeable zinc-ion batteries (ZIBs) are becoming one of the most promising alternatives because of their safe operation without risk of catching fire or exploding, cost competitiveness, eco-friendliness, high theoretical capacity, impressive long term cycling stability, and superior rate capability. Mn is one of the most abundant metals in the earth’s crust. Mn oxide is widely used in various applications, such as deoxidization and desulfurization, catalysts and battery materials due in part to the multiple oxidization states (+2, +3, and +4) of Mn. For battery materials, the diversity of Mn oxide crystal structures and crystal phases allow Mn oxide to combine with other metal ions and store energy easily.  Mn-based ZIB electrode materials have been widely studied and utilized for many years.<br>
The purpose of this work is to synthesize high purity and highly crystalline Mn oxide and to use the synthesized Mn oxide to develop high performing cathode materials for ZIBs and investigate the discharge/charge mechanism behind the Mn oxide cathode materials. The first study is focused on synthesis of desired Mn oxide material. The synthesis method is based on the method used to fabricate cathode materials for LIBs and sodium-ion batteries (SIBs), established in the author’s previously study. The Mn oxide precursor was obtained through precipitation of an acidic Mn sulfate solution reacting with a basic aqueous solution consisting of sodium hydroxide and ammonia with a fixed molar ratio under controlled temperature and pH conditions. The final product was formulated through high temperature calcination of the precursor precipitate. The final product was characterized using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Depending on the calcination temperature, the final product was identified as hausmannite Mn3O4, bixbyite Mn2O3, or a mixture of Mn3O4 and Mn2O3. Since the electrochemical performance and reaction mechanism of Mn3O4 as the cathode material in ZIBs was already studied by a previous group member, Arjun Dhiman. Mn2O3 was chosen for further study in this thesis.<br>
The second study used the synthesized Mn2O3 as the cathode material for ZIBs with an emphasis on investigating the electrochemical performance and charge storage mechanism behind the material. The electrodes were prepared by uniformly spreading a slurry, containing Mn2O3 (70 wt%), acetylene carbon (20 wt%), and polyvinylidene fluoride (pVdF, 10 wt%) as the binder, onto a graphite carbon current collector. CR2032 coin cells were assembled in air using the Mn2O3 composite cathode, Zn foil as the anode, Whatman glass fiber paper (GF/D) as the separator, and 2 M ZnSO4 with 0.2 M MnSO4 aqueous solution as the electrolyte. Cyclic voltammetry (CV) and galvanostatic discharge–charge (GCD) profiles were obtained from 1 V to 1.9 V vs Zn/Zn2+ at multiple scan rates and current densities. The electrode delivered discharge capacities of 375, 341, 289, 205, 113, and 65 mAh/g at 50, 100, 200, 500, 1000, and 2000 mA/g, respectively. Through a combination of transmission electron microscopy (TEM), selected area electron diffraction (SAED), rotating ring-disk electrode (RRDE) measurements, atomic absorption spectroscopy (AAS), XRD, and multiple electrochemical testing methods, a mechanism involving conversion reactions is proposed for the Mn2O3 electrode.</p>