Regulating Metallic Deposition Behavior by Gradient Alloy/Solid Electrolyte Interphase for Durable Na/K Anodes

Abstract Na/K metal batteries (NMBs; KMBs), featuring intrinsic high theoretical capacity, energy density, and resource availability, represent an ideal choice for sustainable energy storage. However, the unstable solid electrolyte interface (SEI) and uncontrollable Na/K dendrite growth have seriously impeded their widespread application. Herein, an in situ conversion strategy is proposed for constructing a gradient interphase layer (NT/NBS) on the Na metal surface, which consists of a Na 2 Te‐rich (denoted as NT) outer layer and a sodiophilic Na 3 Bi–Na 3 Sb alloy (denoted as NBS) inner layer. This innovative design reduces nucleation overpotential, promotes Na + diffusion kinetics, homogenizes the Na + flux, ensures high interfacial and mechanical stability, thereby realizing the dendrite‐free Na deposition. As a result, the optimized Na symmetric cell demonstrates remarkable durability of 1600 h at 0.5 mA cm −2 . Furthermore, the Na full cell (NVP||NT/NBS@Na) coupling with Na 3 V 2 (PO 4 ) 3 (NVP) cathode achieves 82.8% capacity retention after 2600 cycles at 20C rate. Notably, NVP||NT/NBS@Na still delivers excellent rate capability and superior cyclic stability at −40 °C and 60 °C, respectively. More impressively, both modified K‐half and K‐full cells achieve superior cycle life. These findings provide a paradigm for the design of a multifunctional gradient interphase layer that enables wide‐temperature operation of stable NMBs/KMBs.