Reversible Multiple Cation Storage in High‐Entropy MXenes for Durable Seawater Batteries

Rechargeable seawater batteries (SWBs) offer compelling advantages for sustainable energy storage, owing to the natural abundance and intrinsic safety of seawater electrolytes. However, the presence of multiple cations in seawater poses challenges for the reversibility and stability of conventional storage materials. Herein, a high‐entropy strategy is introduced to unlock the electrochemical activity of MXenes toward multiple cations in seawater. Mechanistic studies reveal that the unique “cocktail effect” of high‐entropy MXene accelerates ion transport kinetics and stabilizes the layered structure, enabling reversible storage of monovalent (Na + , K + ) and divalent (Mg 2+ , Ca 2+ ) ions with zero‐strain characteristic. As a proof‐of‐concept, a novel SWB is constructed based on an MXene anode with multiple‐cation storage capability and a polytriphenylamine cathode with Cl − storage chemistry. This SWB achieves a high specific capacity of 155 mAh g −1 , excellent rate capability at 15 A g −1 , and impressive cycling durability over 11000 cycles at 25°C. Furthermore, this SWB functions even at −30°C, broadening the application field of seawater‐based energy devices. Overall, this study provides fundamental insights into the cocktail effect in high‐entropy materials and offers a new strategy for designing advanced electrodes for high‐performance SWBs.