MAGNETIC FIELD APPLICATIONS FOR LOW-COST ENERGY STORAGE

William McLeod

Global demand for electrochemical energy storage (EES) is rising, driven by integration of intermittent renewable energy sources like solar and wind, demand for electric vehicles with long ranges and quick charging times, and increasing ubiquity of portable electronics such as smartphones, laptops, power tools, and more. This diverse range of applications requires a diverse body of EES devices to meet the different needs of each; however, the current EES market primarily consists of only a select few technologies. In particular, the battery market is dominated by the lithium-ion battery (LIB), which exhibits excellent electrochemical performance but is expensive, has a poor safety record, relies on materials with limited and ethically questionable sourcing, requires toxic and environmentally hazardous organic electrolytes, and cannot currently be practically recycled. Alternative technologies must be developed which can address these concerns within suitable corners of the market. A leading candidate is the aqueous zinc-ion battery (AZIB), which is dramatically cheaper and safer, and utilizes abundant and environmentally benign materials including nontoxic and nonflammable water-based electrolytes. Despite these advantages, the AZIB has yet to achieve commercial success primarily due to poor energy density. This poor energy density is partly a result of the narrow electrochemical stability window of water but also partly a result of inefficient utilization of incorporated active materials, namely the zinc metal anode, only a small fraction of which participates in cycling in the standard configuration. Therefore, two main strategies are employed in this dissertation to improve the energy density of AZIBs: increasing the utilization of active material through the anode-free configuration and improving the capacity through novel synthesis of cathode materials. Both strategies utilize magnetic fields, the former through permanent magnets incorporated into the device for operation and the latter via permanent magnets applied during electrochemical synthesis of active material. In Chapter 2, a fundamental investigation is performed on the effect of magnetic fields on the electroplating/stripping of aqueous zinc electrolytes. Magnetic fields of all tested strengths are shown to lower the charge transfer resistance of zinc stripping/plating in three different aqueous zinc-based electrolytes, and as has been previously reported, magnetic fields are shown to affect the morphology of electrodeposited zinc on zinc sheets. Furthermore, this effect is also demonstrated on other substrates, namely single-walled carbon nanotubes and copper. Interestingly, the effect of the magnetic field on the deposition morphology on copper substrates is shown to be dependent on the macroscale geometry of the copper surface. In Chapter 3, copper is used as the current collector for anode-free AZIBs, and the surface area of the current collector relative to the capacity of the cathode is shown to have a significant impact on capacity retention, where smaller surface area current collectors permit the formation of a complete zinc layer that promotes reversibility of zinc stripping/plating. Additionally, the interplay of alternating roles of limiting reagent between the cathode and anode is demonstrated as a significant source of capacity fading, and a low potential hold at the end of each discharge is shown to mitigate this effect. The highest capacity retention was achieved when both of these strategies were combined with an internally applied magnetic field. In Chapter 4, graphene oxide is investigated as a current collector in AZIBs, constituting the first ever use of a carbon-based material as a current collector for an anode-free AZIB. Magnetic fields are shown to alter the morphology of electrodeposited zinc on graphene oxide electrodes, as well as to reduce the polarization of asymmetric Zn//graphene oxide cells during cycling. In AZIBs with MnO2 cathodes, magnetic fields internally applied via permanent magnets raised the open circuit voltage, improved the discharge capacity and Coulombic efficiency at low C rate, and reduced the charge transfer resistance. In Chapter 5, the effect of a magnetic field on the electropolymerization of aniline is investigated for applications in supercapacitors and AZIBs. A pulsed potential polymerization protocol was developed for growing polyaniline (PANI) films, and the application of a moderate magnetic field resulted in PANI films with significantly improved capacitance. Furthermore, this improvement is demonstrated to be beyond what is achievable through traditional mechanical stirring. This protocol was translated to the copolymerization of aniline and metanilic acid for use in AZIBs, and the magnetic field is shown to alter the ratios of the monomers. In Chapter 6, magnetic fields were utilized to improve the electrodeposition of MnO2 for binder-free cathodes. The morphology of the deposited MnO2 is shown to be more homogeneous, and the deposits are shown to have higher content of δ-phase crystallinity. These alterations resulted in improved areal capacity and capacity retention, as well as greatly reduced charge transfer resistance. These strategies demonstrate multiple ways that magnetic fields can be utilized to improve the performance of low-cost energy storage devices.

MAGNETIC FIELD APPLICATIONS FOR LOW-COST ENERGY STORAGE

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