Engineering a stable solid electrolyte interphase (SEI) is critical for suppression of lithium dendrites. However, the formation of a desired SEI by formulating electrolyte composition is very difficult due to complex electrochemical reduction reactions. Here, instead of trial-and-error of electrolyte composition, we design a Li-11 wt % Sr alloy anode to form a SrF2-rich SEI in fluorinated electrolytes. Density functional theory (DFT) calculation and experimental characterization demonstrate that a SrF2-rich SEI has a large interfacial energy with Li metal and a high mechanical strength, which can effectively suppress the Li dendrite growth by simultaneously promoting the lateral growth of deposited Li metal and the SEI stability. The Li–Sr/Cu cells in 2 M LiFSI-DME show an outstanding Li plating/stripping Coulombic efficiency of 99.42% at 1 mA cm–2 with a capacity of 1 mAh cm–2 and 98.95% at 3 mA cm–2 with a capacity of 2 mAh cm–2, respectively. The symmetric Li–Sr/Li–Sr cells also achieve a stable electrochemical performance of 180 cycles at an extremely high current density of 30 mA cm–2 with a capacity of 1 mAh cm–2. When paired with LiFePO4 (LFP) and LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes, Li–Sr/LFP cells in 2 M LiFSI-DME electrolytes and Li–Sr/NMC811 cells in 1 M LiPF6 in FEC:FEMC:HFE electrolytes also maintain excellent capacity retention. Designing SEIs by regulating Li-metal anode composition opens up a new and rational avenue to suppress Li dendrites.