Tuning Nanoparticle Synthesis to Enable Anionic and Cationic Redox in Cathode Materials

With an ever-growing number of applications, from portable electronics to electric vehicles, that rely on Li-ion battery technologies, advancements in this field are now shaping the convenience, efficiency, and sustainability of modern society. Further performanceimprovements are still highly anticipated, pushing the scientific community to develop a novel strategy to design a battery material with enhanced properties. An in-depth understanding of the synthesis process is critical to achieve this goal, since it can provideinsights towards controlling the material’s chemical and physical properties, such as crystal structure, morphology, and defects. In this study, a polyol method has been developed as a versatile technique for the synthesis of high-performance, dispersive cathode nanoparticles. Although the polyol method is a promising synthetic process that offers many advantages such as low cost, ease of use, and proven scalability for industrial applications, the scope of previous studies has been mainly limited to simple metals and metal oxides. As a novel method to synthesize battery materials, a detailed reaction mechanism study was conducted using a combination of in situ and ex situ characterization. Nanoscale dynamics that occur during the synthesis process were reported with a focus on the material’s structural and chemical transformation. The detailed knowledge of the reaction mechanism helped provide an insight towards finding the optimum synthesis conditions for a variety of cathode materials - including layered LiNi0.4Mn0.4Co0.2O2, spinel LiNi0.5Mn1.5O4, and olivine LiCoPO4. Using scanning transmission electron microscopy, polyol-synthesized LiNi0.4Mn0.4Co0.2O2 was further revealed to be enriched with highly coherent twin boundary defects. The positive role of twin boundary defects was thus proposed, as mitigated anisotropy and volume expansion were observed in the polyol sample during the charge anddischarge process. Lastly, in order to increase the capacity, the scope of the studied material was expanded beyond the conventional transitional metal redox and towards high capacity anionic redox materials, referred to as cation-disordered rock salt cathode materials. The strategies to develop the synthesis method that can control the morphology of this material have been proposed, along with the precautions that need to be taken when studying this type of cathode nanoparticles