An ultrafine tin dioxide/N, P-doped porous carbon (SnO2/NPPC) nanocomposite is prepared through in-situ growth of tin dioxide (SnO2) nanoparticles in N, P-doped porous carbon (NPPC). Owing to the in-situ growth method, the size of SnO2 nanoparticles in SnO2/NPPC is quite small and uniform (generally less than 5.0 nm). NPPC provides a support and a conductive carbon skeleton for the SnO2 nanoparticles. The small SnO2 nanoparticles are less likely to aggregate during the discharge-charge process due to the presence of Sn-O-C bonding and nanoconfinement effect of SnO2 nanoparticles in carbon matrix. The N and P doping can provide abundant defects to facilitate the penetration of Li+ or Na+ into the interior of the electrode. In addition, the presence of Sn-N bonding can further improve the electrochemical properties of the electrodes. Thus, as an anode material for lithium-ion batteries, SnO2/NPPC possesses an enhanced rate performance, an excellent cycling stability, and a high initial Coulombic efficiency. The structure of the ultrafine SnO2 nanoparticles is well maintained in cycled SnO2/NPPC. Meanwhile, SnO2/NPPC also possesses good electrochemical performance as an anode for sodium-ion batteries. The good electrochemical properties for SnO2/NPPC materials can be ascribed to the synergetic effect between small SnO2 nanoparticles and NPPC. (C) 2020 Elsevier Inc. All rights reserved.