| Rechargeable lithium--ion batteries (LIBs) have attained widespread use in small consumer electronic devices, as a consequence of their high energy densities that originate from the small atomic mass and the low standard reduction potential of metallic lithium. However, their modest storage capacities and limited deliverable power restrict their high power applications in long-range electric vehicles and large scale batteries for electrical energy storage and utilization for electrical power grid applications. High voltage LiNi xMnyCo1--x--yO2 (NMC) cathode materials offer an attractive route for improving the power deliverable by LIBs, yet NMCs exhibit substantial charge-discharge capacity fade and poor discharge rate performances that currently limit their utilization.;In this thesis, we describe the development and application of various organic phosphonate coatings to high voltage NMC cathode materials in order to improve their electrochemical performance characteristics. First, we developed a thiophene-based coating that may be grafted to NMC cathodes surfaces, which electropolymerizes during LIB charging to generate a thin, electrically conductive organic cathode coating that improves NMC cycling efficiency and fast charge/discharge capacity rate performance. We also studied the impact of electrically insulating, linear alkylphosphonate surface coatings on the electrochemical performances of NMC cathodes. These studies revealed that thicker and more crystalline surface coatings protect the electrolyte from unwanted degradation reactions at the cathode surface, at the cost of impeding Li+ permeation to NMC surface for cathode intercalation in a manner that severely curtails their discharge rate performance. Finally, we developed a kinetically controlled, template-assisted strategy for generating spatially heterogeneous mixed alkylphosphonate monolayers on NMC cathode surfaces. By studying the electrochemical behaviors of variously coated NMCs, we determined that surface Ni-rich surface domains are the primary species that cause electrolyte decomposition, while the Mn -and Co- surface domains exhibit the lowest kinetic barrier to Li-ion intercalation during LIB discharging. Thus, this study thus provides new molecular-level insights for developing new coatings that enhnace the performance characteristics of high voltage NMC cathode materials. |
