NOVELTY - Submicron-scaled graphitic fibril having a diameter or thickness of 100 nm to 1 mu m, where the fibril is obtained by splitting a micron-diameter carbon fiber or graphite fiber, is claimed.    USE - The fibril is useful for preparing several products, which is: 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; and nanocomposite materials. The fibril is useful as: a 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 a electrode active materials, conductive additive, or a current collector ingredient in a supercapacitor, lithium ion battery, lithium metal battery, and/or lithium-air battery.    ADVANTAGE - The fibril is free of continuous thermal carbon overcoat, free of continuous hollow core, and free of catalyst. The fibril is pure and exhibits much more surface areas for interactions with a chemical species or a matrix material. The fibril can be readily mass-produced at low cost. The fibril has well-controlled, consistent, and stable structure to ensure consistent properties and performance. The fibril exhibit high electrical conductivity, high thermal conductivity, high elastic modulus, high strength and good interfacial bonding with a matrix resin in a composite. The fibril on tiers exhibits a good stiffening effect and improved heat-dissipating capability.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a method of producing the submicron graphitic fibrils, comprising (a) introducing an intercalating agent into inter-crystallite spaces or imperfections of a micron-diameter carbon fiber or graphite fiber to form an intercalated fiber, (b) activating the intercalating agent to split the intercalated fiber into an aggregate of multiple interconnected submicron graphitic fibrils having interconnections between fibrils, and (c) mechanically severing the interconnections to obtain the submicron graphitic fibrils; and    (2) a nanocomposite material comprising multiple graphitic fibrils, where the fibrils are dispersed in a matrix material and where the graphitic fibrils have a weight fraction of 0.1-90% of the nanocomposite.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a carbon or graphite fiber containing multiple graphite fibrils.