Abstract Calcium‐metal batteries have received growing attention recently after several studies reporting successful metal plating and stripping with organic electrolytes. Given the low redox potential of metallic calcium, its surface is commonly covered by a passivation layer grown by the accumulation of electrolyte decomposition products. The presence of borate species in this layer has been shown to be a key parameter allowing for Ca 2+ migration and favoring Ca electrodeposition. Here, boron‐based additives are evaluated in order to tune the SEI composition, morphology, and properties. The decomposition of a BF 3 ‐based additive is studied at different potentiostatic steps and the resulting SEI layer was thoroughly characterized. SEI growth mechanism is proposed based on both experimental data and DFT calculations pointing at the formation of boron‐crosslinked polymeric matrices. Several boron‐based adducts are explored as SEI‐forming additives for calcium‐metal batteries paving the way to very rich chemistry leading to Ca 2+ conducting SEI.