NOVELTY - Chemically functionalized submicron graphitic fibril (I) having a diameter or thickness greater than 100 nm and less than 1 mu m and a length-to-diameter or length-to-diameter ratio no less than 5, where (I) is obtained by splitting a micron-diameter carbon fiber or graphite fiber and (I) has a chemical functional group attached to a surface of (I), is claimed.    USE - (I) is useful for preparing several products: paper, thin-film, mat, and web products; rubber or tire products; energy conversion or storage devices, such as fuel cells, lithium-ion batteries, and supercapacitors; adhesives, inks, coatings, paints, lubricants, and grease products; heavy metal ion scavenger; absorbent (e.g. to recover spill oil); sensors; friction and brake components; radiation-shield components; catalyst carrier; and composite materials. (I) is useful as reinforcement filler in a structural composite or as a conductive additive in an electrically conductive composite for static charge dissipation, lightning strike protection, and shielding against electromagnetic interference or radio frequency interference; and good electrode materials for energy conversion or storage devices.    ADVANTAGE - (I) is free of continuous thermal carbon overcoat, free of continuous hollow core, and free of catalyst. (I) is pure and exhibits much more surface areas for interactions with a chemical species or a matrix material. (I) exhibits high electrical conductivity, high thermal conductivity, high elastic modulus, high strength and good interfacial bonding with a matrix resin in a composite. (I) has improved solubility in a liquid medium and improved dispensability in a matrix material. (I) on tiers exhibits a good stiffening effect and improved heat-dissipating capability. (I) can be readily mass-produced at low cost. (I) has a well-controlled, consistent, and stable structure to ensure consistent properties and performance.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) the preparation of (I) comprising: (a1) introducing an intercalating agent into inter-crystallite spaces or imperfections of a micron-diameter carbon fiber or graphite fiber to form an intercalated fiber, (b1) activating the intercalating agent to split the intercalated fiber into an aggregate of multiple interconnected submicron graphitic fibrils having interconnections between fibrils, (c) mechanically severing or separating the interconnections to obtain the submicron graphitic fibrils and (d) chemically functionalizing the fibril before, during, or after the mechanical separation step;    (2) a composite material comprising multiple functionalized graphitic fibrils where the fibrils are dispersed in a matrix material where the graphitic fibrils have a weight fraction of 0.01-90% of the nanocomposite; and    (3) a composite comprising a polymerized mixture of functionalized graphitic fibrils and a monomer which can react with (I).