Hierarchical Oxide Nanostructures for High Performance Energy Storage

The future of space exploration requires energy storage technologies that are reliable, high capacity, lightweight, and have long lifetimes. The overall goals of this Ph.D. research project were to develop and characterize nanostructured materials that improve upon the current state of art. The initial focus was on using electrospinning and controlled annealing techniques to synthesize electrode materials for lithium-ion batteries. Over time, it became apparent that as research into nanostructured materials has progressed, characterization techniques for localized material study has lagged behind. To fill this gap, the research project evolved toward characterization of nanostructured materials using electrochemical strain microscopy (ESM), which can relate nanostructured morphologies to electrode electrochemistry as well as to device-level properties. However, experimentally observed behavior under ESM can be difficult to explain, and required a set of customized experiments to be developed to distinguish among different possible mechanisms. These techniques were applied to lithium iron phosphate, an electrode for lithium-ion batteries and to samarium-doped ceria, an electrolyte for solid oxide fuel cells, relating ESM results to material morphologies and electrochemical properties. In addition, as part of the on-site experiences, three summers were spent at the Jet Propulsion Laboratory, investigating materials that can act as bifunctional catalysts for metal-air batteries. The next few paragraphs will briefly describe a portion of the research project and highlight a few key accomplishments. An early goal was to synthesize lithium iron phosphate (LiFePO4) fibers for lithium ion batteries, using electrospinning and controlled annealing techniques. LiFePO4 is advantageous compared to the more commonly used lithium cobalt oxide due to its low toxicity, high thermal stability, abundant sources, and relatively lower costs. In its olivine structure, lithium ions can only travel through one dimensional channels, which leave the material susceptible to lowered capacity when the channels are blocked by ionic disorder, foreign phases, or stacking faults. For practical purposes, these issues can be resolved by forming nanostructured materials with a conductive coating introduce to improve electronic mobility. LiFePO4 fibers were fabricated by electrospinning precursor ingredients, mixed with an organic compound to increase viscosity, and then annealed using a multi-step thermal treatment process. Although this technique proved to be successful in fabricating porous lithium iron phosphate fibers, it required many steps and a long processing time, making it difficult for mass production and limited in practical use. The research project then evolved toward the characterization of energy storage materials using a recently developed tool termed electrochemical strain microscopy (ESM). ESM is a scanning probe microscopy (SPM) technique, in which a series of alternating current and direct current electric fields are applied to the surface of a material of interest using an electrically conductive probe. These electric fields induce strain changes in the material, which can be measured as vertical or lateral displacements using the SPM probe. This technique was based on a similar one that measured the piezoelectric properties of ferroelectric materials, in which electromechanical coupling is due to the motion of cations and anions within an asymmetrical crystal lattice. The underlying mechanisms that cause observed strain changes in ESM are not nearly as clear as that in ferroelectric materials. Electromechanical coupling observed in energy storage materials can be due to ion concentration changes through Vegard’s Law, electrostrictive strain, electrostatic effects, dipolar motion, and valence changes. In order to distinguish among the possible mechanisms, a series of experiments were devised to determine, for example, if the material expands or contracts under a positive bias, which can serve to determine the sign of the active ionic species. Another series of experiments compared the linear versus quadratic responses to applied bias, which can separate out Vegard related strain from electrostrictive behavior. In addition, direct current electric fields could be applied to alter the surface state of the material; the effects of these surface state changes can further illustrate the underlying mechanisms in play. The series of experimental protocols developed involved software customization of the atomic force microscope system, and has become standard in the research group while investigating new materials. These ESM experiments were applied to two particular energy storage systems, a cathode material for lithium-ion batteries and an electrolyte material for solid-oxide fuel cells. The cathode material, lithium iron phosphate, was prepared using a thin-film deposition process that created both nanocrystalline regions and microcrystalline regions within a single sample. This effect, though not ideal for actual battery performance, was advantageous for this particular project. Since the response was hypothesized to be due to the rate of lithium ion concentration change, nanocrystalline regions evidenced higher responses due to the higher diffusivity of lithium ions in that region. A similar sample, de-lithiated in a half-cell battery, showed much lower response, due to its lower concentration of lithium ions. The lithium ion concentration could also be locally altered, by applying positive and negative direct current biases. As expected, a drop in response to a positive bias and an increase in response to a negative bias were observed. From these results, it is evident that electrochemical strain microscopy can be used as a nondestructive localized measurement system to characterize lithium ion behavior on electrodes, which is very promising for the further improvement of lithium-ion batteries. A similar set of experiments were performed on samarium doped ceria (SDC), an electrolyte material for solid-oxide fuel cells. SDC had been the topic of some controversy in the fuel cell community, as it was believed that nanostructured SDC could show high ionic conductivities at low temperatures, even though oxygen ions were not expected to be mobile at such temperatures. However, more recent research suggested that the improved conductivity arose from an electronic component rather than ionic. Through a series of ESM experiments on SDC, the sign of the active species was measured, corresponding to the sign of small polarons, that is, electrons and their associated lattice distortions. The observed displacements in ESM can be attributed to the motion of the small polarons, as the valence change from Ce4+ to Ce3+ leads to an expansion of the crystal lattice. The observation of larger responses close to grain boundaries and at higher temperatures are compatible with the space charge theory, as regions closer to grain boundaries have a higher small polaron concentration, and its mobility increases with temperature, causing higher responses. This project resulted in the first direct imaging of space charge regions on doped ceria, suggested experimental evidence for the long theorized small polaron model. This area of study will be continued and developed further by other researchers in the group, expanding upon the temperature range of experimentation and working to directly control the environment in which the experiment is carried out. Over the course of the Ph.D. research, three internships were completed at the Jet Propulsion Laboratory, under the supervision of the NSTRF mentor, Dr. Andrew Kindler. Although the internships encompassed a number of different tasks, the overall focuses were similar: the development of a robust, low-cost bifunctional air cathode. The first system studied centered around nanostructured whisker supports fabricated through thermal evaporation, and then covered with a catalytic material by sputtering. These nanostructured catalysts could then be characterized using a rotating disc electrode setup. Sputtered thin film catalysts were also characterized using the rotating disc electrode, with thin films of noble metals such as platinum and platinum-ruthenium alloys showing superior performance compared to that of the bulk materials. This implies that small loadings of noble metals can be used as bifunctional catalysts while reducing the overall cost. With the same goal in mind, another project involved depositing nickel cobalt oxide thin films through reactive sputtering. Through rotating disc electrode experiments, it was determined that while nickel cobalt oxide films were adequate oxygen reduction and evolution catalysts, they deteriorated quickly after exposure to oxygen. The results from these experiments indicated that thin films of noble metals can be promising low-cost bifunctional catalysts for aqueous metal-air systems such as iron-air and metal hydride-air. The move toward nanostructured materials for energy storage has the potential for greatly improving the performance of energy storage devices. Many challenges relating to nanostructured materials remain, as they can be time-consuming and difficult to fabricate in an economical and efficient manner. From a more basic science point of view, many aspects of nanostructured material behavior remain poorly understood, as they can differ greatly from their bulk counterparts. Electrochemical strain microscopy has been shown to be a versatile new tool that can relate localized behavior to macroscopic performance. Increasing understanding of behavior at nanoscale is crucial for the future development of energy storage solutions in applications on earth or in space.