Interlayer Chemistry and Energy Storage Mechanisms of Manganese-Rich Oxides in Aqueous Electrolytes

• Main Author: Boyd, Shelby Katherine
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2020
• Subjects: Activated carbon; Aluminum; Biology; Chemistry; Collaboration; Computer Assisted Instruction; Conflict of Interest; Conflicts of interest; Continuing education; Crystal structure; Departments; Education; Electrodes; Electrolytes; Energy; Energy storage; Engineering; Engineering schools; Ethics; Fellowships; Graduate Students; Graduate Study; Hemodialysis; Internship Programs; Internships; Literary Devices; Manufacturing; Metal oxides; Microscopy; Morphology; Organic Chemistry; Oxidation; Particle size; Philosophy; Professional development; Scholarships & fellowships; School Holding Power; School Support; Scientific Research; Scientists; Spectroscopy; Storage; Teaching; Water
• ISBN: 9798494449511

Electrochemical alkali metal cation insertion from aqueous electrolytes into transition metal (TM) oxides is appealing for low-cost and safe electrochemical energy storage (EES), desalination, and element recovery. The strong interactions between water, electrolyte salts, and TM oxide surfaces dictate the material stability and EES mechanisms. However, these are not yet understood well enough to enable large scale long-life neutral-pH aqueous EES. To advance this understanding I synthesized micron-sized and nanostructured layered manganese-rich oxides and determined how their interlayer environment effects their structural and electrochemical behavior in aqueous electrolytes. First, by tuning the TM content. Second, by tuning the amount of interlayer water. And third, by determining the mechanisms of pseudocapacitive behavior in nanostructured birnessite MnO2The effect of TM composition on P2 layered Na+ manganese-rich oxides has been extensively investigated for non-aqueous electrolytes, but not aqueous electrolytes. In Chapter 2, I synthesize the following series of P2 oxides and characterize their structural stability upon aqueous electrochemistry: Na0.62Ni0.22Mn0.66Fe0.10O2, Na0.61Ni0.22Mn0.66Co0.10O2, Na0.64Ni0.22Mn0.66Cu0.11O2, and Na0.64Mn0.62Cu0.31O2. Electrochemistry and ex situ X-ray diffraction (XRD) show that water intercalation upon interlayer Na+ removal causes an irreversible phase transformation in all compositions, although transformation extent depends on TM composition and the maximum anodic potential. The 25% c-axis expansion causes eventual electrode failure due to loss of electronic connectivity and particle delamination. These first studies on the structural effects of aqueous electrochemistry in P2 oxides show the significance of TM composition on interlayer water affinity.The capacitive electrochemical behavior of hydrated P2 oxides suggested that these particles have potential for high power EES. Chapter 3 describes my in situ synchrotron XRD investigation of the water insertion mechanism, and the scalable “top-down” strategy I subsequently developed to electrochemically expand micron-scale P2 oxides. I hypothesize that the electrochemical expansion produces a promising electrochemical capacitor material by two changes: (1) interlayer hydration, which improves interlayer diffusion kinetics and buffers intercalation-induced structural changes, and (2) particle expansion, which significantly improves electrode integrity and volumetric capacitance. Compared with commercially available activated carbon, the expanded materials have higher volumetric capacitance at charge/discharge timescales ≥40 seconds. Therefore hydrating the interlayer of large manganese-rich oxide particles makes them viable for high power electrodes.To conclude my study of the effect of interlayer environment on the electrochemical and structural behavior of layered manganese-rich oxides, I investigated the nature of capacitive charge storage in nanostructured birnessite δ-MnO2. Whether capacitance in birnessite is due to nonfaradaic (double layer) or faradaic (pseudocapacitive) behavior is a topic of debate. In Chapter 4, I used ex situ XRD, electrochemical quartz crystal microbalance (EQCM), in situ Raman spectroscopy, and operando atomic force microscope (AFM) dilatometry to observe the electronic, vibrational, structural, gravimetric, and mechanical response of birnessite during electrochemical cycling in an aqueous electrolyte. XRD and Raman spectroscopy showed significant interlayer contributions likely due to faradaic processes. EQCM showed that a combination of ions and water molecules were involved in the complex energy storage mechanism. AFM showed that while the interlayer expands upon ion removal, the electrode as a whole contracts. In addition, this was the first electrochemical AFM dilatometry study on a material undergoing interlayer ion insertion. Overall, this study showed that capacitive energy storage in birnessite MnO2 involves ion intercalation into the interlayer in a capacitive charge storage process. Based on experimental and simulation results, we hypothesize that nanoconfinement within the hydrated interlayer blurs the distinction between pseudocapacitive and electrical double layer processes.Overall, my dissertation showed how to tune the electrochemical response and interlayer chemistry of manganese-rich oxides and their mechanism of charge storage in aqueous electrolytes. These results are promising for obtaining high power EES with low cost, sustainable metal oxides as well as fundamental understanding of electrochemical behavior under confinement. I discovered the role of transition metal composition on the stability of sodium manganese-rich oxides in aqueous electrolytes. I developed a scalable synthesis to electrochemically expand and hydrate large transition metal oxide particles for high volumetric capacity electrodes. Finally, I conducted a multi-modal study to address a major fundamental question of aqueous EES, determining that capacitive charge storage in birnessite originates from a complex mechanism involving (de)insertion of both ions and water molecules in the interlayer, where nanoconfinement blurs the distinction between pseudocapacitive and double layer capacitive mechanisms.