Atomic‐Scale Tracing of Lithium Trapped in Copper Current Collectors

Zero-excess lithium-metal batteries (LMBs) are expected to provide the maximum energy density in combination with classic lithium-containing cathode materials. However, the lithium plating process on a blank current collector suffers from huge irreversibility, thus, causing rapid capacity fading. Herein, we employ atom probe tomography to track the occurring lithium losses. The results unambiguously reveal that lithium is trapped at grain boundaries and junctions in the copper foil after the first cycle—not only on the copper foil as commonly assumed – with a local concentration up to ∼28 at.% at the boundary junctions. After continuous plating and stripping, the copper foil subsurface regions (∼100 nm) become nanocrystalline, creating additional defects. These defects facilitate further trapping of lithium, along with oxygen and carbon, in the subsurface region of the copper foil. Such compositional and structural changes in the copper foil surface may contribute to the degradation of the current collector during cycling, which has so far been largely overlooked in discussions of LMB performance decay.