vs SHE) and tenfold increased theoretical specific capacity (3860 mA h g −1 ) [6,7] than graphite anode (376 mA h g −1 ). [8][9][10] Despite these fundamental advantages, Li metal anodes still suffer from the inevitable formation of Li dendrites, resulting in reduced long-term performance and undesirable internal short circuits. [11][12][13][14] Thus, it is essential to develop a novel strategy to effectively suppress the formation of Li dendrites by manipulating the nucleation and growth behavior of Li deposits during cycling.According to Sand's time model, increasing the Li + transference number is a feasible strategy to reduce the concentration overpotential in the electrolyte and to suppress the diffusion-limited growth of Li dendrites. [15][16][17][18][19] The most intuitive approach to achieve this is the modification of the electrolyte composition; however, there are few electrolyte compositions that can fulfill the various requirements of LMBs in terms of ionic conductivity, chemical/electrochemical stability, and safety. Instead, introducing functional materials with high polarity onto the separator provides a practical approach for improving the Li + transference number because it allows a direct interaction between functional materials and electrolyte solvents. [20][21][22][23][24] However, it would be difficult to completely alter the inherent properties of the separator via a conventional coating process, whichThe 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 CaCO 3 nanoparticles (CPP separator) is proposed to achieve the flattening of Li deposits on the Li electrode. Strong interactions between the highly polar CaCO 3 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 CaCO 3 nanoparticles into the separator induces the spontaneous formation of mechanically-strong and lithiophilic CaLi 2 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.