Stable High‐Rate Sodium Metal Batteries at Room and Low Temperature Enabled by Mixed Ion/Electron‐Conducting Interphase Engineering

Sodium metal batteries (SMBs) with high energy density showed great promise for various applications, but they are plagued by uneven and low‐reversibility sodium plating/stripping, particularly when fast‐charging capability at subzero temperatures is pursued. Here, we present a facile method to modulate the Al foil by constructing a mixed ion/electron‐conducting (MIEC) interfacial layer, which takes Na ₁₅ Sn ₄ /NaF MIEC interphase as an example, for stable and high‐rate SMBs at room and low temperature. The Na ₁₅ Sn ₄ /NaF MIEC interfacial layer can homogenize interfacial Na ⁺ /e ⁻ distribution, enhance the sodiophilicity, and facilitate interfacial ion transport kinetics, thus inducing favorable Na deposition behavior. Therefore, the asymmetric cells display an average Coulombic efficiency of 99.5% over 800 cycles at 3 mA cm ⁻² /3 mAh cm ⁻² ; the symmetric cells achieve a small overpotential (20 mV at 3 mA cm ⁻² ) for 1800 h. Moreover, the Na ₁₅ Sn ₄ /NaF@Al/Na‖Na ₃ V ₂ (PO ₄ ) ₃ SMBs can deliver extraordinary rate capability and superb stability (95.7% capacity retention after 1800 cycles at 10 C). More impressively, at the harsh condition of −40 °C, the full cells display outstanding cycling stability with a capacity retention of 97.4% at 1 C for 1700 cycles. This work envisions new perspectives on manipulating interfacial properties of current collectors for high‐performance SMBs.