Date of Award

Fall 2018

Project Type

Dissertation

Program or Major

Chemical Engineering

Degree Name

Doctor of Philosophy

First Advisor

Xiaowei Teng

Second Advisor

Russell Carr

Third Advisor

Young Jo Kim

Abstract

Aqueous electrochemical energy storage (EES) has been investigated as a potential alternative to commonly used non-aqueous lithium-ion batteries since it can achieve high power performance, long cycle life and good safety. Manganese oxide based materials using Na-ions as charge carriers for aqueous storage have been studied with the advantages of low cost, high earth abundance, and environmental friendliness. However, the applications of oxide materials in aqueous Na-ion storage are limited by low energy density due to low specific capacity and the narrow potential window (~ 1.23 V). To address these two issues, this research work have been focused on the design of manganese oxide based materials, including the cobalt doped manganese oxides nanoparticles/nanolayers hybrid materials and the manganese oxides nanoparticles with a hydroxylated interphase on surface, with objectives of increasing specific storage capacity and expanding the workable potential window to improve energy density for aqueous Na-ion storage. The research results show that an enhancement of specific capacity as well as good rate performance can be achieved by using cobalt-manganese oxides (Co-Mn-O) via doping cobalt into manganese oxide. Electro-kinetics and in-situ X-ray diffraction (XRD) demonstrate that both larger capacitive and diffusion-limited redox capacities compared with pure manganese oxides, and very distinct redox activity with obvious crystalline structure change upon intercalation and de-intercalation of Na-ions. On the other hand, bivalence layered Mn5O8 material with a hydroxylated interphase shows stable electrochemical performance within a potential window of 2.5 V in half-cell and 3.0 V in a symmetric full-cell without the significant generation of hydrogen and oxygen gas in an aqueous electrolyte. Soft X-ray absorption spectroscopy (sXAS) and computational modeling demonstrate a well-ordered hydroxylated interphase is formed on the surface of Mn5O8 electrode during cycling which kinetically inhibits the gas evolution reactions. In summary, our works on manganese oxide based electrode materials show that Co-Mn-O electrode enhances the specific capacity and Mn5O8 with a hydroxylated interphase electrode expands the potential window for aqueous Na-ion storage, and therefore, provides new strategies for designing innovative electrode materials for increasing the energy density in aqueous electrolyte comparable to that of non-aqueous Li-ion batteries with better safety and lower cost.

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