The statement that the valency of carbon is fully satisfied in the graphite lattice structure is a matter of basic chemical definition and structure, where each carbon atom forms three sp2 covalent bonds within the planar sheet, satisfying its valence requirements in a neutral lattice. Therefore, no citation is needed.
The claim is a straightforward chemical fact about the bonding and valency of carbon in the graphite lattice, making it common knowledge by definition and everyday observation in chemistry. None of the retrieved papers specifically test or contest this fundamental definition; they deal with applied carbon materials, batteries, and composites.