Solid lithium batteries (SLBs) have received much attention due to their potential to achieve secondary batteries with high energy density and high safety. The solid electrolyte (SE) is believed to be the essential material for SLBs. Among the recent SEs, composite electrolytes have good interfacial compatibility and customizability, which have been broadly investigated as promising contenders for commercial SLBs. The high Li+ transference number (t Li+ ${{_{{\rm Li}{^{+}}}}}$ ) of composite electrolytes is critically important concerning the power/energy density and cycling life of SLBs, however, which is often overlooked. This Review presents a current opinion on the key factors in high t Li+ ${{_{{\rm Li}{^{+}}}}}$ composite electrolytes, including polymers, Li‐salts, inorganic fillers, and additives. Various strategies concerning providing a continuous pathway for Li‐ions and immobilizing anions via component interaction are discussed. This Review highlights the major obstacles hindering the development of high t Li+ ${{_{{\rm Li}{^{+}}}}}$ composite electrolytes and proposes future research directions for developing composite electrolytes with high t Li+ ${{_{{\rm Li}{^{+}}}}}$ .