Development of Nanostructured Nickel-Rich Cathode Materials for Fast-Charging Lithium-Ion Batteries and of High-Conductivity Doped Semiconducting Polymers for Energy Applications
The development of new materials for energy applications is necessary to create new solutions to minimize our use of fossil fuels. This dissertation is composed of two separate projects that use different materials to address energy challenges. The first part focuses on nanostructured nickel-rich cathode materials for use in fast-charging lithium-ion batteries. Fast-charging batteries are desired for use in electric vehicles to shorten charging times from hours to minutes, which could help with their larger scale implementation and reduction of fossil fuel use. Fast-charging can be achieved by nanostructuring certain battery materials, which decreases lithium-ion diffusion lengths and can help suppress slow discontinuous, first-order phase transitions, while retaining high capacity. This behavior has been termed pseudocapacitance. While a number of pseudocapacitive anodes have been produced, there are few examples of high-capacity pseudocapacitive cathodes. Here, we studied the nickel-rich cathode materials LiNi0.80Co0.15Al0.05O2 (NCA) and LiNi0.xMn0.yCo0.zO2 (NMCxyz), both of which are high-capacity materials that show suppressed discontinuous phase transitions. Because of this favorable continuous phase transition behavior, we hypothesized that only modest decreases in particle sizes would be needed to develop pseudocapacitive behavior. We used polymer templating with a sol-gel synthesis to synthesize nanoporous NCA and NMC materials with decreased particle sizes. We then studied the effect of the particle size on the electrochemical kinetic properties of the material and cycling behavior at fast-charging rates. The results showed improved (dis)charge kinetics compared to the bulk and identified characteristics of pseudocapacitance. Nanostructured NCA cathodes were also paired with a fast-charging pseudocapacitive anode to demonstrate their potential for commercial full-cell fast-charging devices.The second part of this dissertation studies semiconducting polymers, which have potential applications in organic electronics, such as solar cells and thermoelectrics. These materials are interesting for energy applications because they are flexible, low-cost, and solution-processable. While semiconducting polymers show low conductivity, molecular doping can improve conductivity by adding mobile charge carriers. Here a novel redox-tunable dodecaborane-based dopant was introduced into a semiconducting polymer network and the resulting electronic, structural, and optical properties were studied. Large and strongly-oxidizing dopants were found to dramatically increase conductivity by producing more and higher-mobility charge carriers.