The practical application of Li metal anodes has been hindered by severely irreversible side reactions for low Coulombic efficiency, uncontrollable growth of Li dendrites, and large volume change. Herein, we report subnanopore-rich carbon spheres encapsulated with Sn single atoms (Sn/CS@SC) as a Li host to address these challenges. Owing to the high Li affinity of Sn single atoms, Sn/CS@SC can promote storage of quasi-metallic Li within the inner void space other than direct plating of metallic Li on the outer surface. Moreover, the subnanopores with a strong spatial confinement effect can prevent the penetration of ester electrolyte for reduced side reactions. As expected, the Sn/CS@SC host demonstrates a high Coulombic efficiency of 99.8% over 600 cycles. Moreover, a full cell using a prelithiated Sn/CS@SC anode and LiNiCoMnO cathode shows high capacity retention (~80%) over 500 cycles at high current density.