The strategies for achieving a high cationic transport polymer electrolyte (HTPE) have mostly focused on developing single-ion conducting polymer electrolytes, which is far from being practical due to sluggish ion transport. Herein, we present an unprecedented approach on designing an HTPE via in situ copolymerization of regular ionic conducting and single-ion conducting monomers in the presence of a lithium salt. The HTPE, i.e., poly(VEC10-r-LiSTFSI), exhibits a combination of impressive properties, including high cationic transport number (0.73), high ionic conductivity (1.60 mS cm^–1), tolerance of high current density (10 mA cm^–2), and high anodic stability (5 V). A lithium-metal battery constructed with the developed HTPE retains 70% capacity after 1200 cycles at 1 C, and it also operates in a wide temperature range and with a high mass loading of the cathode. Advanced characterizations and computations reveal that the high t_Li⁺ and high ionic conductivity effectively suppress Li⁰-dendrite growth by circumventing concentration polarizations that plague most polymer electrolytes.