Unveiling the Parasitic Lithium Deposition Puzzle: Implications for Symmetric Coin Cell Testing and Full‐Cell Stability

Lithium metal is a prime anode candidate for next‐generation high‐energy batteries owing to its ultrahigh specific capacity and lowest redox potential. Lithium symmetric coin cell (Li||Li) is widely used to study Li depositing/stripping, and metrics such as voltage polarization and cycle life are interpreted as intrinsic properties of Li metal anodes. However, under the prevalent cell architectures, we found that besides the Li foil, the stainless steel (SS) component inside the cell also undergoes parasitic Li plating/stripping during cycling. This parasitic process (i) significantly enlarges the reaction area and dilutes the true current density, thereby confounding the above metrics and (ii) exhibits poorer reversibility on SS. By different current density, areal capacity, and Li foil thickness, the directional bias of symmetric coin cell lifetime assessment caused by parasitic depositing/stripping is revealed: high depth‐of‐cycling leads to an apparent underestimation of cycle life, whereas shallow cycling can overestimate it. The similar artifacts are also observed in full cells, indicating the broad relevance of the parasitic lithium deposition puzzle. Therefore, we advocate that future studies explicitly control and report current density, areal capacity, and Li excess, and employ advanced cell‐design remedies to suppress SS‐mediated Li deposition. Our findings provide methodological guidance for more reliable evaluation of lithium‐metal anodes.