Interfacial Spacing Tunes π–d Charge Transfer to Co‐Optimize Conductivity and Strength in Cu–Graphene Composites

Simultaneously enhancing electrical conductivity and tensile strength in copper–graphene composites remains challenging because microstructural strategies that strengthen the metal typically impede carrier transport. Here we establish a spacing‐controlled interfacial mechanism: at a Cu–graphene gap of ∼2.5 Å, the interface forms a metallocovalent π–d charge‐transfer bond in which graphene's C‐2p z orbital hybridizes with Cu‐3d (a “doorway” covalency) while Cu‐4s/4p states donate charges into the graphene's π manifold. Density‐functional theory combined with nonequilibrium Green's functions (DFT+NEGF) shows that the spacing pins a hybrid interfacial resonance near the Fermi level and maximizes electrode coupling (lead broadenings), producing a peak in the transmission at E F , T(E F ). The same hybridization stabilizes occupied bonding states, generating a co‐peak in the work of separation, Wsep. Projected density of states and Mulliken analysis further reveal a maximum Cu→C charge transfer at ∼2.5 Å, with C‐2p gain balanced by Cu‐4s/4p depletion. Experiments on pressure‐sintered Cu–graphene validate this mechanism, delivering 128% IACS conductivity (vs. ∼100% IACS for identically processed Cu) together with a tensile strength of 589 MPa at 0.4 wt.% graphene addition. These results identify interfacial spacing as a prescriptive knob that simultaneously optimizes transport and adhesion, enabling copper–graphene composites that are both stronger and more conductive.