Halogen functionalization of the edges of the graphene sheets can improve processability, add new properties, and enhance its interactions with other materials. Through functionalization, improved materials can be realized. Typically, halogenated graphenes are produced from pure or reactive halogen sources. Current approaches present significant safety challenges. By generating reactive dichlorine monoxide (Cl2O) in situ, a chlorinated graphene with the nominal composition C17Cl2OH can be realized safely and scalably. Chlorinated graphene can be used as a precursor for an array of functionalized materials by mechanically driven solid-state metathesis reactions. For example, nearly 75% of the chlorine in chlorinated graphene can be exchanged with fluorine by using the safer fluorine-containing compound ammonium fluoride (NH4F) as a reagent. A material with the composition C34Cl3F(OH)(2) is realized. Preliminary work shows that F-graphene has oxygen reduction properties and Cl-graphene can improve existing zinc-air fuel cells. A scalable production of chlorinated and fluorinated graphenes and graphites will accelerate their adoption in fuel cells, batteries, polymer composites, and catalysts.