▎ 摘　　要

Tin dioxide (SnO2) has been widely implemented as an advanced anode material for lithium or sodium ion batteries (LIBs/SIBs) owing to its high capacity and moderate potential. However, conventional synthetic approaches often yield large-sized SnO2, which suffers from low conductivity, huge volume expansion and Sn coarsening issues, hampering its practical implementation. Herein, a unique solvothermal-driven solid-to-solid transition (SDSST) strategy is developed to craft tartaric acid (TA) capped ultrafine SnO2 nanoparticles (NPs) in situ on sacrificial SnC2O4 microrods. Ball-milling combined with solvent evaporation treatment realizes the homogeneous composition and precise mass ratio control of TA-capped SnO2 NPs and reduced graphene oxide (rGO). Remarkably, the SnO2 NPs-rGO nanocomposite manifests outstanding lithium and sodium storage capacities of 1775 and 463 mA h g(-1) after 800 and 100 cycles at 1000 and 20 mA g(-1), respectively, and an ultralong lifespan of 4000 cycles for LIBs. Notably, systematic electrochemical and componential characterization of the cycled electrodes reveals that SnO2 NPs-rGO manifests fully reversible three-step lithiation-delithiation reactions of SnO2 and a primary highly reversible sodiation-desodiation conversion reaction between Sn and SnO combined with a secondary partially reversible alloying-dealloying reaction between Sn and NaxSn (0