Abstract:

Secondary batteries are becoming commonplace in commercial applications such as fully electric vehicles in the automotive industry. Therefore, battery materials delivering high electrochemical output over extended periods of time beyond what the current state-of-the-art can produce is needed. Lithium-ion batteries (i.e. Li-ion batteries) are a tremendous advancement in secondary battery devices even being awarded the Nobel Prize in 2019, but over time and repeated charging, it physically degrades until it can no longer power devices. Its electrode components are complicated matrices of active material, conductive carbon binders, and additives therefore, elucidating a single cause of battery failure is non-trivial, often being a marriage of multiple degradation modes operating synergistically. This defense aims at simplifying electrode architectures by removing any sort of binder or additive and assessing the degradation modes resulting from extended cycling. This approach yields more meaningful results about processes occurring at the electrode interfaces. In this defense I assess the degradation of Nb2O5 thin films, a model intercalation material for Li-ion batteries, fabricated from a simple, binder-free electrophoretic deposition route directly on a FTO current collector. The nanostructured Nb2O5 remarkably (de)intercalated Li+ for as long as 10,000 cycles, however its capacity attenuated to ~50%. Origins of the capacity loss were investigated with various ex situ and in situ characterization methods and the attenuation was determined to stem from a synergistic effect of crystallinity loss due to repeated Li+ (de)insertion from long-term cycling with active material film delamination off the current collector. The fundamental insights gained from probing simple interfaces of the binder-free Nb2O5 electrode prompted an investigation into examining an emerging class of secondary battery materials for Na-ion batteries promising significantly higher electrochemical output operating via conversion reactions. I pursued manganese(ii) sulfide, MnS, as a model conversion Na-ion battery anode because it was demonstrated to be fabricated on Au thin films through a facile, binder-free electrodeposition route. Three questions are of interest: 1) Does increasing the MnS thin film thickness influence realized capacity? 2) Does nanostructuring the MnS thin film yield improved cycle life? 3) How severe is the electro-chemo-mechanical degradation on the material after Na+ cycling? To this end I utilized some of the various ex situ tools utilized in the Nb2O5 investigation. I discovered that MnS film thickness did not impart significant differences in specific capacities and all three thicknesses investigated reported significant capacity fade in <100 cycles. The origin of the capacity loss was due to active material loss operated via two major degradation modes: first, severe particle fracture imparted by the large lattice expansion/contraction of repeated conversion cycling. Second, polysulfide generation and dissolution led to active material leaching into the electrolyte.