Li–B–Mg Composite Foils With a LiB 0.88 Microrod Array Enable Ultrathin Self‐Supporting Lithium Metal Anodes

Ultrathin self‐supporting lithium foils (USSLFs, ≤50 µm) are highly desirable for high‐energy‐density batteries, as eliminating current collectors enhances energy density and reduces material costs. However, the intrinsic mechanical vulnerability of metallic Li remains a critical barrier to both the fabrication and long‐term cycling of USSLFs. Herein, we develop a Li‐rich Li–B–Mg composite foil (LBM) through sequential melt alloying followed by post‐solidification thermomechanical processing, yielding a dense and interconnected LiB 0.88 microrod array with LiMg and MgB 2 lithiophilic phases within the Li‐rich matrix. This architecture enables large‐area (>170 cm 2 ) self‐supporting foils down to 15 µm while preserving a high Li content. During cycling, the microrod array redistributes local stress and restrains deformation, preserving the internal architecture and accessibility of lithiophilic sites. These sites, in turn, favor Li accommodation within the electrode interior, suppressing surface Li accumulation and alleviating local stress concentration. This self‐sustained coupling between stress regulation and lithiophilic guidance supports stable cycling for over 5000 h in symmetric cells using 50 µm LBM foils with about 10% thickness expansion, while 15 µm foils cycle for over 500 h at a depth of discharge of about 37.4%. This work advances the design of practical USSLFs for high‐energy lithium metal batteries.