Surface Engineering of Electrospun Fibers to Optimize Ion and Electron Transport in Li + Battery Cathodes
This report documents research into the fabrication of Lithium Manganese Oxide (Spinel) (LMO) cathode material in the form of fibrous mats, the degradation process of this material in a battery electrolyte, the formation of core-shell coated fibers and the electrochemical properties of the active cathode core fiber. In practice, LMO experiences several degradation mechanisms ranging from capacity fading, the Jahn-Teller distortion phenomenon leading to dissolution in the electrolyte phase, and the formation of surface films by degradation of the electrolyte, and/or phase transitions at the surface to form Mn 2 O 3 . Density Functional Theory (DFT) was applied to model from first principles the interactions between the electrolyte and the surface leading to Mn ion dissolution. Thermodynamic values of solvation energy were calculated and related to the applied potential on the material during electrochemical cycling. Direct in situ observation of the degradation process was conducted on fiber structures using AFM and coin cell measurements, showing the rapid formation of theorized surface films. This provides the first direct evidence of processes occurring at the interface, such as dissolution of the active material, facet development, decomposition of the electrolyte to form a surface electrolyte interface (SEI) layer, or the formation of Li alcoxides/carboxylates. Methods for fabricating fiber structures of LMO were researched using two techniques; electrospinning and forcespinning. Chemical precursors were evaluated and optimized to lead to polycrystalline, hollow fibers of LMO, as well as tetragonal zirconia fiber mats. Sol-gel chemistries for the synthesis of LMO and zirconia have been implemented to generate these fiber morphologies. Core-shell methods were attempted using both techniques, proving partial success and insight into the physics of coherent core-shell fibers. The knowledge developed in this program has increased understanding of the degradation mechanisms of the active energy storage material, and aided development of a protective interface which resists these processes while permitting rapid Li + transport, good e - conductivity, a stable interface, and prevention of the formation of a solid-electrolyte interphase (SEI) layer.