The uncontrollable dendritic growth of metallic lithium during repeated cycling in carbonate electrolytes is a crucial obstacle hindering the practical use of Li‐metal batteries (LMBs). Among numerous approaches proposed to mitigate the intrinsic constraints of Li metal, the design of a functional separator is an attractive approach to effectively suppress the growth of Li dendrites because direct contact with both the Li metal surface and the electrolyte is maintained. Here, a newly designed all‐in‐one separator containing bifunctional CaCO3 nanoparticles (CPP separator) is proposed to achieve the flattening of Li deposits on the Li electrode. Strong interactions between the highly polar CaCO3 nanoparticles and the polar solvent reduces the ionic radius of the Li+‐solvent complex, thus increasing the Li+ transference number and leading to a reduced concentration overpotential in the electrolyte‐filled separator. Furthermore, the integration of CaCO3 nanoparticles into the separator induces the spontaneous formation of mechanically‐strong and lithiophilic CaLi2 at the Li/separator interface, which effectively decreases the nucleation overpotential toward Li plating. As a result, the Li deposits exhibit dendrite‐free planar morphologies, thus enabling excellent cycling performance in LMBs configured with a high‐Ni cathode in a carbonate electrolyte under practical operating conditions. CPP separator is developed. The one‐body structure minimizes the sacrifice in energy density even with the inorganic particle introduction. Polar CaCO3 alters the solvation structure of Li+, enhancing Li+ mobility and reducing concentration overpotential. Concurrently, CaCO3 reacts with Li to form lithiophilic CaLi alloy spontaneously, reducing the nucleation overpotential. Because of the overall overpotential control, dendrite‐free Li deposition is achieved.