Current‐Controlled Inactive Aluminum Loss: Quantification, Mechanism, and Mitigation via Stack Pressure for Long‐Cycle Al‐Ion Batteries

ABSTRACT Aluminum‐ion batteries are a promising energy storage technology for high resource abundance, safety, and cost‐effectiveness advantages, but practical implementation is hindered by two degradation mechanisms in Al metal anode: electrically isolated metallic aluminum (iAl 0 ) and electrochemical side‐reaction products (iSEI Al 3+ ). Precise quantification of these inactive species and relationships with cycling reversibility have rarely been established, limiting rational anode design. Here, we develop a basic titration gas chromatography technology to accurately quantify iAl 0 and iSEI Al 3+ , revealing a current density‐dependent Al loss mechanism: iSEI Al 3+ loss dominates at low current, while metallic iAl 0 loss prevails at high current. We discover that an inorganic‐dominated interphase favors significant suppression of inactive Al formation. Applying an optimal stack pressure strategy (780 kPa) constructs tip‐localized stack pressure/electric field, effectively reducing inactive Al loss by 81.99% and prolonging battery cycling life by ≥500%. These advancements offer fundamental theory and techniques for Al anode reaction mechanism research and provide design principles for high‐performance aluminum‐ion battery.