Development of Functionalized Carbon Materials for Sustainable Energy Storage via Mechanochemical Solid-state Reaction

Abstract

Mechanochemistry induces chemical reactions by transferring kinetic energy directly to reactants through mechanical actions such as abrasion, collision, friction, and impact. Mechanochemistry has attracted the attention of many researchers because it enables reactions that are not possible in traditional chemistry, such as thermochemistry, photochemistry, and electrochemistry. Solvent-based methods for synthesizing functionalized materials yield hundreds to thousands of times more byproducts than actual products. To produce sustainable functionalized materials, limiting the use of fossil fuel-based organic solvents and reducing chemical byproducts is essential. Mechanochemical-based synthesis not only circumvents the need for organic solvents but also requires only mild reaction conditions and electricity for kinetic energy. Emerging at the intersection of mechanical engineering and chemistry, mechanochemistry offers an eco-friendly approach to replacing conventional solvent-based chemistry and developing new materials. This thesis focuses on active materials for enhancing energy storage performance by functionalizing carbon materials through mechanochemical processes. All mechanochemical experiments were conducted using solid-phase reactions. The first chapter discusses the synthesis of fluorocarbons using polytetrafluoroethylene (PTFE) and graphite. Traditionally, fluorination of carbon materials involves hydrofluoric acid (HF), fluorine gas (F2), or fluorine-containing plasma, all of which are highly toxic and corrosive, hindering mass production. Mechanochemical fluorination with PTFE overcomes these disadvantages and produces fluorocarbons without additional purification or heat treatment. Fluorocarbon materials (FCMs) have been employed as cathodes in lithium-ion batteries, exhibiting an lithium-ion storage capacity approximately 2.6 times higher than pure graphite, along with excellent electrochemical stability. In the second chapter, building on the reactivity of fluorocarbons and graphite, a mechanochemical atom substitution method was investigated to induce new reactions in the precursor. Ball milling of Na2SiF6 (SFS) and graphite (G) in ZrO2 material induced a mechanochemical atomic substitution reaction in which the central atom was substituted from Si to Zr. The as-prepared Na3ZrF7-graphite (SFZ-G) was used as the anode of a sodium ion battery for performance evaluation, and the mechanism of the performance enhancement was demonstrated by in-situ XRD measurement. The SFZ-G, which was made electrochemically active by the central atom substitution, was not only increased in the storage capacity of SFZ-G by the sodium resource provided from SFZ, but also greatly enhanced its electrochemical stability by promoting the formation of C-F bonds on the fragmented graphite edges and basal plane.