Grain Boundary Character Governs Lithium Deposition Uniformity on Polycrystalline Copper

In anode-free Li metal batteries, a bare current collector must be converted into a reversible Li metal anode during the first charge, making Li nucleation decisive for reversibility. However, how current collector microstructure governs Li nucleation and growth remains poorly understood, since prior studies focused on single-crystal surfaces to identify favorable crystallographic orientations, leaving grain boundaries unexplored. Here, we identify grain boundary density and character in copper current collectors as key regulators of Li deposition uniformity. Systematic annealing decouples grain size from surface chemistry, revealing that deposition uniformity improves with copper grain coarsening but saturates as disordered boundaries are eliminated during grain growth, leaving predominantly low-energy configurations. Atomistic simulations show disordered boundaries impose high barriers to Li adatom migration, whereas ordered boundaries and adjacent low-index terraces provide low-barrier pathways for redistribution. Copper foils engineered beyond this threshold sustain uniform Li plating across diverse electrolytes and improve cyclability in anode-free cells.