Ultrahigh‐Efficiency and Long‐Calendar‐Life Aqueous Cadmium Metal Batteries Under Extremely Harsh Conditions

ABSTRACT Aqueous multivalent metal batteries (AMMBs) hold great promise for non‐flammable, cost‐effective, and scalable energy storage. However, the parasitic hydrogen evolution reaction (HER) has severely plagued the metal plating efficiency and calendar life, particularly under realistic stress conditions, including low current densities, extended storage periods, and harsh temperatures. Herein, we leverage the inherent HER resistance of cadmium metal and the water‐confining solvation structures of concentrated electrolytes to synergistically tackle the HER challenge, and we successfully demonstrated ultrahigh‐efficiency and long‐calendar‐life cadmium metal batteries under strict conditions (0.1 mA cm −2 , 99.75% efficiency, 21.4 months’ life). Even under extreme conditions, such as ultralow current (0.01 mA cm −2 ), long rest periods (up to 60 days), and wide temperature ranges (−50°C to +80°C), Cd maintains a high efficiency of 90%–99.9%. In stark contrast, zinc suffers from drastic performance degradation and loses 27%–73% efficiency. The superior performance is correlated with the distinct solvation structure in the concentrated electrolyte, which transforms the hydration form of Cd 2+ cations and strengthens water molecules via a strong cation‐coordination effect. Our work establishes a new benchmark for AMMBs and highlights the decisive role of electrode selection and electrolyte design in advancing AMMB performance.